{"id":19390,"date":"2021-03-30T10:05:30","date_gmt":"2021-03-30T13:05:30","guid":{"rendered":"http:\/\/www.cr2.cl\/eng\/?p=19390"},"modified":"2021-05-04T10:21:06","modified_gmt":"2021-05-04T14:21:06","slug":"mapped-how-proxy-data-reveals-the-climate-of-the-earths-distant-past-carbon-brief","status":"publish","type":"post","link":"https:\/\/www.cr2.cl\/eng\/mapped-how-proxy-data-reveals-the-climate-of-the-earths-distant-past-carbon-brief\/","title":{"rendered":"Mapped: How \u2018proxy\u2019 data reveals the climate of the Earth\u2019s distant past (Carbon Brief)"},"content":{"rendered":"<p><strong>At any one moment in time, thousands of measurements are being taken of the world\u2019s weather. Across land, sea and sky, data is being gathered manually and automatically using a range of technologies, from the humble thermometer to the latest\u00a0<a href=\"https:\/\/www.carbonbrief.org\/interactive-satellites-used-monitor-climate-change\">multi million-pound satellite<\/a>.<\/strong><\/p>\n<p><span class=\"title-highlight\">By Robert McSweeney and Zeke Hausfather.\u00a0<\/span><span class=\"title-highlight\">Design by Tom Prater.<\/span><\/p>\n<p>Put together over many years, these measurements provide\u00a0<a href=\"https:\/\/www.carbonbrief.org\/explainer-how-do-scientists-measure-global-temperature\">a record of the Earth\u2019s climate<\/a>\u00a0and how it is changing.<\/p>\n<p>But even the world\u2019s longest climate archive \u2013 the\u00a0<a href=\"https:\/\/www.metoffice.gov.uk\/hadobs\/hadcet\/\" target=\"_blank\" rel=\"noopener noreferrer\">central England temperature record<\/a>\u00a0\u2013 only goes back to 1659. This is a mere snapshot in time considering the hundreds of thousands of years that humans have roamed the planet.<\/p>\n<p>Fortunately, the Earth has been keeping its own records. Tucked away in an assortment of unlikely places \u2013 from shells and stalactites to pollen and seal pelts \u2013 the natural world has recorded the ebb and flow of the climate for millions of years.<\/p>\n<p>This is known as \u201cproxy data\u201d \u2013 indirect records of climate imprinted on different parts of the biosphere.<\/p>\n<p>In the same way that something \u201cprehistoric\u201d relates to a time before written history, proxy data provides an insight into the climate before dedicated records. It forms a fundamental part of the study of past climates, known as \u201cpalaeoclimatology\u201d, while also helping underpin scientists\u2019 understanding of how the climate will change in the future.<\/p>\n<p>In this in-depth Q&amp;A, Carbon Brief explores what proxy data is, the different types, how scientists draw climate data from them, and what they can tell us about the Earth\u2019s climate in the past, present and future.<\/p>\n<p>In addition, Carbon Brief has produced an interactive map of the US\u00a0<a href=\"https:\/\/www.noaa.gov\/\" target=\"_blank\" rel=\"noopener noreferrer\">National Oceanic and Atmospheric Administration<\/a>\u00a0(NOAA) archive of\u00a0<a href=\"https:\/\/www.ncdc.noaa.gov\/data-access\/paleoclimatology-data\/datasets\" target=\"_blank\" rel=\"noopener noreferrer\">more than 10,000 proxy datasets<\/a>.<\/p>\n<div class=\"row central\">\n<p id=\"one\"><strong>What is proxy data?<\/strong><\/p>\n<p>In 1714, German physicist\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Daniel_Gabriel_Fahrenheit\" target=\"_blank\" rel=\"noopener noreferrer\">Daniel Gabriel Fahrenheit<\/a>\u00a0invented what is considered to be the\u00a0<a href=\"https:\/\/www.thoughtco.com\/the-history-of-the-thermometer-1992525\" target=\"_blank\" rel=\"noopener noreferrer\">first example of the modern thermometer<\/a>. It enclosed mercury in a glass tube and had a standardised scale running up the side. A decade later, he would add the temperature scale that bears his name. (Swedish astronomer\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Anders_Celsius\" target=\"_blank\" rel=\"noopener noreferrer\">Anders Celsius<\/a>\u00a0would not devise his alternative scale for another two decades.)<\/p>\n<p>The thermometer, along with other instruments such as the\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Barometer\" target=\"_blank\" rel=\"noopener noreferrer\">barometer<\/a>\u00a0for measuring air pressure and\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Hygrometer\" target=\"_blank\" rel=\"noopener noreferrer\">hygrometer<\/a>\u00a0for humidity, went on to become a key part of the formal weather station. These stations \u2013 shielded behind the shutters of a\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Stevenson_screen\" target=\"_blank\" rel=\"noopener noreferrer\">Stevenson screen<\/a>\u00a0\u2013 were first installed in Europe and the US in the 1800s, and spread around the world throughout the century and beyond.<\/p>\n<\/div>\n<p>By the middle of the 19th century, there were sufficient weather stations and enough observations being recorded on land \u2013 and\u00a0<a href=\"https:\/\/www.carbonbrief.org\/explainer-how-data-adjustments-affect-global-temperature-records\">sea<\/a>\u00a0\u2013 to produce a reliable measurement of global temperature. The\u00a0<a href=\"https:\/\/www.carbonbrief.org\/explainer-how-do-scientists-measure-global-temperature\">longest record of global temperature<\/a>\u00a0\u2013\u00a0<a href=\"https:\/\/www.metoffice.gov.uk\/hadobs\/hadcrut4\/\" target=\"_blank\" rel=\"noopener noreferrer\">produced jointly<\/a>\u00a0by the\u00a0<a href=\"https:\/\/www.metoffice.gov.uk\/weather\/climate-change\/organisations-and-reports\/met-office-hadley-centre\" target=\"_blank\" rel=\"noopener noreferrer\">UK Met Office Hadley Centre<\/a>\u00a0and the\u00a0<a href=\"http:\/\/uea.ac.uk\/\" target=\"_blank\" rel=\"noopener noreferrer\">University of East Anglia<\/a>\u2019s\u00a0<a href=\"http:\/\/www.cru.uea.ac.uk\/\" target=\"_blank\" rel=\"noopener noreferrer\">Climatic Research Unit<\/a>\u00a0\u2013 begins in 1850. Others, such as those produced by\u00a0<a href=\"https:\/\/data.giss.nasa.gov\/gistemp\/\" target=\"_blank\" rel=\"noopener noreferrer\">NASA<\/a>\u00a0and the\u00a0<a href=\"https:\/\/www.ncdc.noaa.gov\/data-access\/marineocean-data\/mlost\" target=\"_blank\" rel=\"noopener noreferrer\">National Oceanic and Atmospheric Administration<\/a>\u00a0(NOAA), start in 1880.<\/p>\n<p class=\"row central\"><iframe loading=\"lazy\" id=\"highchart\" src=\"https:\/\/cbhighcharts2021.s3.eu-west-2.amazonaws.com\/proxy-explainer\/global-surface-temp-records.html\" name=\"myiFrame\" width=\"800px\" height=\"550px\" frameborder=\"0\" marginwidth=\"0px\" marginheight=\"0px\" scrolling=\"no\" allowfullscreen=\"allowfullscreen\" data-mce-fragment=\"1\"><\/iframe>Annual global average surface temperatures from 1850-2020. Data from\u00a0<a href=\"https:\/\/data.giss.nasa.gov\/gistemp\/\" target=\"_blank\" rel=\"noreferrer noopener\">NASA GISTEMP<\/a>,\u00a0<a href=\"https:\/\/www.ncdc.noaa.gov\/monitoring-references\/faq\/anomalies.php\" target=\"_blank\" rel=\"noreferrer noopener\">NOAA GlobalTemp<\/a>,\u00a0<a href=\"https:\/\/www.metoffice.gov.uk\/hadobs\/hadcrut5\/\" target=\"_blank\" rel=\"noreferrer noopener\">Hadley\/UEA HadCRUT5<\/a>,\u00a0<a href=\"http:\/\/berkeleyearth.org\/data\/\" target=\"_blank\" rel=\"noreferrer noopener\">Berkeley Earth<\/a>\u00a0and Carbon Brief\u2019s raw temperature record. 1979-2000 temperatures from\u00a0<a href=\"https:\/\/climate.copernicus.eu\/2020-warmest-year-record-europe-globally-2020-ties-2016-warmest-year-recorded\" target=\"_blank\" rel=\"noreferrer noopener\">Copernicus ERA5<\/a>\u00a0(as the reanalysis record starts in 1979). Anomalies plotted with respect to a 1880-1899 baseline to show warming since the preindustrial period.<\/p>\n<div class=\"row central\">\n<p>This means that scientists have a robust account of how global temperatures have changed over the past century and a half, or so. But, of course, the Earth is much older than that.<\/p>\n<p>To look back even further \u2013 and for places that did not have instrumentation until relatively recently \u2013 scientists need to cast their eyes beyond direct observations to indirect evidence that is locked up in various forms across the Earth. This is \u201cproxy\u201d data.<\/p>\n<p>The word \u201cproxy\u201d is typically defined as an intermediary or substitute \u2013 often in reference to a person given the authority to vote or speak on behalf of someone else. Proxy data, therefore, is information that is a substitute for direct observations of the Earth\u2019s climate.<\/p>\n<p>\u201cA climate proxy is something we use to reconstruct variations of climatically relevant factors in the past, such as temperature, precipitation, CO2 levels \u2013 or whatever else is of interest,\u201d explains\u00a0<a href=\"https:\/\/www.cardiff.ac.uk\/people\/view\/1719366-pearson-paul\" target=\"_blank\" rel=\"noopener noreferrer\">Prof Paul Pearson<\/a>\u00a0from the School of Earth and Ocean Sciences at\u00a0<a href=\"https:\/\/www.cardiff.ac.uk\/\" target=\"_blank\" rel=\"noopener noreferrer\">Cardiff University<\/a>. He tells Carbon Brief:<\/p>\n<p class=\"long-quote\">\u201cObviously, these things can\u2019t be measured directly without a time machine, so we need to find something that survives from the past that is dateable and contains something we can measure that would have responded to the variable we are interested in \u2013 hence, the name \u2018proxy\u2019.\u201d<\/p>\n<p>Scientists, therefore, look for ways that the climate \u201chas left a mark in the environment\u201d, says\u00a0<a href=\"http:\/\/www.dgf.uchile.cl\/~maisa\/Maisa_english\/Maisa_Rojas.html\" target=\"_blank\" rel=\"noopener noreferrer\">Dr Maisa Rojas<\/a>, associate professor in the Department of Geophysics at the\u00a0<a href=\"https:\/\/www.uchile.cl\/english\" target=\"_blank\" rel=\"noopener noreferrer\">University of Chile<\/a>\u00a0and a lead author on the\u00a0<a href=\"https:\/\/www.ipcc.ch\/site\/assets\/uploads\/2018\/02\/WG1AR5_Chapter05_FINAL.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">palaeoclimate chapter<\/a>\u00a0(pdf) of the\u00a0<a href=\"https:\/\/www.ipcc.ch\/\" target=\"_blank\" rel=\"noopener noreferrer\">Intergovernmental Panel on Climate Change\u2019s<\/a>\u00a0(IPCC)\u00a0<a href=\"https:\/\/www.ipcc.ch\/report\/ar5\/wg1\/\" target=\"_blank\" rel=\"noopener noreferrer\">fifth assessment report<\/a>\u00a0(AR5). She tells Carbon Brief:<\/p>\n<p class=\"long-quote\">\u201cThe living part of our world \u2013 the biosphere \u2013 responds to the climate and, as such, it leaves marks in a number of environmental indicators that we can then use to reconstruct back the climate.\u201d<\/p>\n<p>These clues to past climate are scattered across the Earth, from the layers of vast ice sheets and sediments at the bottom of lakes to rings of tree growth and towering stalagmites in caves. (See\u00a0<a href=\"https:\/\/interactive.carbonbrief.org\/how-proxy-data-reveals-climate-of-earths-distant-past\/#three\">later section<\/a>\u00a0for more on the different sources of proxy data.)<\/p>\n<\/div>\n<div class=\"img-md-container\">\n<div class=\"row central\">\n<p>This information allows scientists to \u201cstudy the climate over the past centuries and millennia and, thus, to go further back in time than by using instrumental climate data alone\u201d, explains\u00a0<a href=\"https:\/\/www.valerietrouet.com\/\" target=\"_blank\" rel=\"noopener noreferrer\">Dr Valerie Trouet<\/a>, an associate professor in the\u00a0<a href=\"https:\/\/ltrr.arizona.edu\/\" target=\"_blank\" rel=\"noopener noreferrer\">Laboratory of Tree-Ring Research<\/a>\u00a0at the\u00a0<a href=\"https:\/\/www.arizona.edu\/\" target=\"_blank\" rel=\"noopener noreferrer\">University of Arizona<\/a>\u00a0and author of\u00a0<a href=\"https:\/\/www.valerietrouet.com\/tree-story.html\" target=\"_blank\" rel=\"noopener noreferrer\">Tree Story<\/a>, a book about tree-rings. She adds:<\/p>\n<p class=\"long-quote\">\u201cBy studying the climate prior to the 20th century, when meteorological station data become available, we can put current climate change in a longer-term context and study natural, non-anthropogenically driven, climate variability.\u201d<\/p>\n<p>The way that the climate can leave its mark on the Earth\u2019s surface has long been observed. In the 15th century, for example, Italian artist and inventor Leonardo da Vinci\u00a0<a href=\"https:\/\/repository.arizona.edu\/handle\/10150\/254848\" target=\"_blank\" rel=\"noopener noreferrer\">documented<\/a>\u00a0that the thickness of tree-rings \u2013 the concentric circles found running through a tree\u2019s trunk \u2013 varied with rainfall.<\/p>\n<p>The scientific discipline of tree-ring dating \u2013 known as \u201cdendrochronology\u201d \u2013 was later pioneered by American astronomer\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/A._E._Douglass\" target=\"_blank\" rel=\"noopener noreferrer\">A E Douglass<\/a>\u00a0in the early 20th century.\u00a0<a href=\"https:\/\/link.springer.com\/chapter\/10.1007\/978-1-4757-4249-7_4\" target=\"_blank\" rel=\"noopener noreferrer\">His research<\/a>\u00a0attempted to connect the pattern of sunspot cycles with fluctuations in climate and tree-ring patterns. From this earliest work, Douglass went on to found the Laboratory of Tree-Ring Research mentioned above.<\/p>\n<p>Another proxy with a long history is \u201cthe oxygen isotope composition of\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Calcite\" target=\"_blank\" rel=\"noopener noreferrer\">calcite<\/a>\u00a0shells\u201d in marine organisms, says Pearson:<\/p>\n<div class=\"long-quote\">\u201cThis method was pioneered by [American chemist]\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Harold_Urey\" target=\"_blank\" rel=\"noopener noreferrer\">Harold Urey<\/a>\u00a0in the immediate post-war years and helped launch the entire field of\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Paleoclimatology\" target=\"_blank\" rel=\"noopener noreferrer\">palaeoclimatology<\/a>.\u201d<\/div>\n<p>Urey showed that the chemical composition of these shells (see the\u00a0<a href=\"https:\/\/interactive.carbonbrief.org\/how-proxy-data-reveals-climate-of-earths-distant-past\/#isotope\">next section<\/a>\u00a0for more on isotopes) varied depending on the temperature of the water. Extracting this information, thus provided information on the climate when the organisms were alive \u2013 going back many millions of years.<\/p>\n<p>Urey described his discovery as \u201csuddenly [finding] myself with a\u00a0<a href=\"https:\/\/www.cambridge.org\/core\/journals\/the-paleontological-society-papers\/article\/oxygen-isotopes-in-foraminifera-overview-and-historical-review\/CD34EFBFFB6AC31DB85A61BB7E6FA64F\" target=\"_blank\" rel=\"noopener noreferrer\">geologic thermometer<\/a>\u00a0in my hands\u201d, explains Pearson.<\/p>\n<p><strong>Where in the world is proxy data found?<\/strong><\/p>\n<p>From the ice sheets of Antarctica and the seabed of the Atlantic, to the boreal forests of Europe and corals of southeast Asia, proxy data is found across the Earth\u2019s land and ocean.<\/p>\n<p>NOAA holds an archive of\u00a0<a href=\"https:\/\/www.ncdc.noaa.gov\/data-access\/paleoclimatology-data\/datasets\" target=\"_blank\" rel=\"noopener noreferrer\">more than 10,000 proxy datasets<\/a>\u00a0covering more than a dozen categories. With its permission, Carbon Brief has mapped this data.<\/p>\n<p>Use the categories in the legend on the left to select a particular proxy or archive type, and the buttons in the top-right hand corner to zoom in and out. Clicking on an individual data point will reveal the period covered by the data, the site name and a link to NOAA\u2019s reference webpage for further information.<\/p>\n<div class=\"row central\">\n<p id=\"two\"><strong>What information do proxies capture?<\/strong><\/p>\n<p>Proxy data can provide insights on a range of climate-relevant changes. These include sudden events \u2013 such as\u00a0<a href=\"https:\/\/www.nature.com\/articles\/30943\" target=\"_blank\" rel=\"noopener noreferrer\">volcanic eruptions<\/a>\u00a0or\u00a0<a href=\"https:\/\/hess.copernicus.org\/articles\/19\/3047\/2015\/\" target=\"_blank\" rel=\"noopener noreferrer\">floods<\/a>\u00a0\u2013 and gradual, long-term trends \u2013 such as\u00a0<a href=\"https:\/\/science.sciencemag.org\/CONTENT\/339\/6124\/1198.abstract\" target=\"_blank\" rel=\"noopener noreferrer\">warming and cooling<\/a>,\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41561-021-00698-0\" target=\"_blank\" rel=\"noopener noreferrer\">drought<\/a>,\u00a0<a href=\"https:\/\/advances.sciencemag.org\/content\/6\/20\/eaaz1346\" target=\"_blank\" rel=\"noopener noreferrer\">changing sea levels<\/a>,\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41561-020-00656-2\" target=\"_blank\" rel=\"noopener noreferrer\">cyclone patterns<\/a>,\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41598-020-66001-0\" target=\"_blank\" rel=\"noopener noreferrer\">monsoon seasons<\/a>,\u00a0<a href=\"https:\/\/www.sciencedirect.com\/science\/article\/abs\/pii\/S0012821X16000388\" target=\"_blank\" rel=\"noopener noreferrer\">fluctuating atmospheric CO2<\/a>\u00a0or\u00a0<a href=\"https:\/\/www.nature.com\/articles\/nature08355\" target=\"_blank\" rel=\"noopener noreferrer\">thinning ice sheets<\/a>.<\/p>\n<p>Proxies generally falls into one of three categories \u2013 physical, biological or chemical \u2013 explains Prof Pearson:<\/p>\n<p class=\"long-quote\">\u201cProxies can be something physical like the amount of silt in sea floor mud which can be a proxy for the speed of the current, something biological like the width of a tree-ring or growth band in a marine shell, or something chemical like the elemental or isotopic composition of a substance that we can measure in the lab.\u201d<\/p>\n<p>These different types of proxies have been captured by an array of palaeoclimate \u201c<a href=\"https:\/\/www2.usgs.gov\/landresources\/lcs\/paleoclimate\/archives.asp\" target=\"_blank\" rel=\"noopener noreferrer\">archives<\/a>\u201d, such as sediments, ice cores and cave formations. These are the \u201cmedia that the [proxy] data is being recorded in\u201d, explains\u00a0<a href=\"https:\/\/usoceandiscovery.org\/allison-cluett\/\" target=\"_blank\" rel=\"noopener noreferrer\">Allison Cluett<\/a>, a PhD candidate at the\u00a0<a href=\"https:\/\/www.buffalo.edu\/\" target=\"_blank\" rel=\"noopener noreferrer\">University at Buffalo<\/a>.<\/p>\n<p>For example, Cluett\u2019s research analyses leaf waxes (the proxy) in ocean sediments (the archive) to reconstruct the climate of southern Greenland.<\/p>\n<p>Each type of proxy is reflecting a change in conditions, but they are not simply capturing temperature or rainfall or some other single variable. Instead, they often reflect a combination of several. Thus, the field of palaeoclimatology involves \u201cdisentangling\u201d specific climate information, says Dr Rojas.<\/p>\n<p><strong>Biological<\/strong><\/p>\n<p>Take tree-rings, which provide a biological record of how a tree has added new layers of wood over time. Each ring comprises a light and dark part \u2013 with the pale part signifying the fast growth of spring and early summer and the dark part indicating the slower growth of late summer and autumn. Taken together, each ring indicates a year of a tree\u2019s life.<\/p>\n<p>Scientists can \u201c<a href=\"https:\/\/www.ncdc.noaa.gov\/news\/picture-climate-how-can-we-learn-tree-rings\" target=\"_blank\" rel=\"noopener noreferrer\">core<\/a>\u201d a tree to extract a cross-section through its trunk. This allows them to analyse its rings without damaging the tree.<\/p>\n<p>The rate of tree growth can \u201crespond to both precipitation and temperature \u2013 and will depend on the tree and where it is located\u201d, explains Rojas. Rings will be wider in warm, wet years where the tree is getting sufficient sunshine and rainfall to support growth, and rings will be narrower during a drought or if the tree is hit by pests, disease or fire.<\/p>\n<div class=\"row central\">\n<p>It is, therefore, the job of scientists to draw out the climate data from the information the proxy provides. In the case of tree-rings, this will first involve \u201c<a href=\"https:\/\/www.ltrr.arizona.edu\/lorim\/basic.html\" target=\"_blank\" rel=\"noopener noreferrer\">cross-dating<\/a>\u201d rings between a number of trees to identify the correct year for each ring.<\/p>\n<p>Then, using records of local weather data, scientists can calibrate the rings against an observed climate record. Simpler relationships can be calibrated using a straightforward equation, but scientists use models for those that are more complex.<\/p>\n<p class=\"container\">If the two data sources are well matched, the tree-rings can be used to cast further back \u2013 before the observed record began \u2013 to analyse the climate during the tree\u2019s full lifetime. (See\u00a0<a href=\"https:\/\/interactive.carbonbrief.org\/how-proxy-data-reveals-climate-of-earths-distant-past\/#four\">later section<\/a>\u00a0for more on calibrating proxy data.)<\/p>\n<p>Tree-ring records can go back a very long way, notes Dr Trouet:<\/p>\n<p class=\"long-quote\">\u201cThe longest continuous tree-ring record \u2013 that includes a measurement for each year \u2013 is the German oak-pine chronology that dates back to 10,461BC\u2026But for palaeoclimate purposes, tree-rings are mostly used to study the past ~500 to 2,000 years.\u201d<\/p>\n<p>And there are subtleties around which locations are best suited to certain analyses, explains Trouet:<\/p>\n<p class=\"long-quote\">\u201cTrees grow a lot \u2013 and form wide rings \u2013 under favorable climate conditions. These can be wet conditions in dry regions, such as the American southwest, or warm conditions in cold regions, such as the European Alps or Scandinavia\u2026To reconstruct past temperature, we use tree-rings from cold regions. To reconstruct past drought conditions, we use tree-rings from dry regions.\u201d<\/p>\n<p>There are other complications, of course. Rings are most prominent in trees that experience clearly defined seasons throughout the year. This means that \u201ctrees in midlatitudes are more responsive to climate than trees in the tropics\u201d, says Rojas:<\/p>\n<p class=\"long-quote\">\u201cSo Europe is good, North America, northern Asia, South America as well \u2013 and along the Andes there are lots of trees you can use.\u201d<\/p>\n<p>In contrast, tropical trees are more of a challenge for dendrochronology, although some species do still form annual rings.<\/p>\n<p>This can be seen in the map above \u2013 the majority of tree-ring data comes from the temperate and boreal forests of the northern hemisphere. Tree-ring data is not impossible in the tropics, adds Trouet \u2013 there are hundreds of records available and there is \u201cdefinitely potential for more\u201d.<\/p>\n<p id=\"isotope\"><strong>Chemical<\/strong><\/p>\n<p>Moving onto chemical proxies, one example is\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Isotope\" target=\"_blank\" rel=\"noopener noreferrer\">isotopes<\/a>. And that provides an opportunity to talk about one of the most well-known types of palaeoclimate archive \u2013 ice cores.<\/p>\n<p>These cylinders of ice are drilled out of ice sheets and glaciers, and can run to several kilometres. The Earth\u2019s ice sheets and glaciers are built up from snowfall over thousands of years, with each layer \u2013 compacted over time \u2013 trapping tiny bubbles of air. Taking a cross-section through the ice thus provides a timeline of these air bubbles over centuries.<\/p>\n<div class=\"row central\">\n<p>The air bubbles are tiny samples of the atmosphere through the life of the ice sheet or glacier.<\/p>\n<p>While scientists can analyse the bubbles directly to ascertain the makeup of the atmosphere in the distant past, they also contain a proxy to\u00a0<a href=\"https:\/\/www.carbonbrief.org\/factcheck-what-greenland-ice-cores-say-about-past-and-present-climate-change\">estimate past temperatures<\/a>\u00a0\u2013 the oxygen isotope \u201c<a href=\"https:\/\/en.wikipedia.org\/wiki\/Oxygen_isotope_ratio_cycle\" target=\"_blank\" rel=\"noopener noreferrer\">18O<\/a>\u201d.<\/p>\n<p>Isotopes are forms of the same element that are identical except for a different number of\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Neutron\" target=\"_blank\" rel=\"noopener noreferrer\">neutrons<\/a>\u00a0within the nucleus of the atom. The most abundant oxygen isotope is\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Oxygen-16\" target=\"_blank\" rel=\"noopener noreferrer\">16O<\/a>, which has eight neutrons, giving it an overall atomic mass of 16 (eight neutrons plus eight\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Proton\" target=\"_blank\" rel=\"noopener noreferrer\">protons<\/a>).<\/p>\n<p>18O has an extra two neutrons, giving it an atomic mass of 18. As a result, atoms of 18O are very slightly heavier than 16O. This weight difference has implications when water is evaporated from the oceans and falls as snow at the Earth\u2019s poles, explains\u00a0<a href=\"https:\/\/www.bas.ac.uk\/profile\/rmu\/\" target=\"_blank\" rel=\"noopener noreferrer\">Dr Robert Mulvaney<\/a>, a glaciologist at the\u00a0<a href=\"https:\/\/www.bas.ac.uk\/\" target=\"_blank\" rel=\"noopener noreferrer\">British Antarctic Survey<\/a>, in an article for\u00a0<a href=\"https:\/\/www.scientificamerican.com\/article\/how-are-past-temperatures\/\" target=\"_blank\" rel=\"noopener noreferrer\">Scientific American<\/a>:<\/p>\n<p class=\"long-quote\">\u201cSimply put, it takes more energy to evaporate the water molecules containing a heavy isotope from the surface of the ocean, and, as the moist air is transported polewards and cools, the water molecules containing heavier isotopes are preferentially lost in precipitation.\u201d<\/p>\n<p>Both of these processes are temperature dependent, says Mulvaney, which means that measurements of 18O in ice cores can tell scientists how warm the climate was at that time in the past.<\/p>\n<p>Research into oxygen isotopes \u2013 much of it\u00a0<a href=\"https:\/\/science.sciencemag.org\/content\/194\/4270\/1121\" target=\"_blank\" rel=\"noopener noreferrer\">involving marine sediments<\/a>\u00a0\u2013 \u201cwas fundamental to a great discovery of 20th century science that the\u00a0<a href=\"https:\/\/www.carbonbrief.org\/explainer-how-the-rise-and-fall-of-co2-levels-influenced-the-ice-ages\">ice ages<\/a>\u00a0are paced by the Earth\u2019s orbital variables of eccentricity, axial obliquity and precession\u201d, says Pearson.<\/p>\n<p>These orbital variables \u2013 known as \u201c<a href=\"https:\/\/climate.nasa.gov\/news\/2948\/milankovitch-orbital-cycles-and-their-role-in-earths-climate\/\" target=\"_blank\" rel=\"noopener noreferrer\">Milankovitch cycles<\/a>\u201d after Serbian scientist Milutin Milankovitch who developed the theory \u2013 describe how a collection of variations in the Earth\u2019s position relative to the sun can trigger both the beginning and end of ice ages.<\/p>\n<p><strong>Physical<\/strong><\/p>\n<p>Finally, for the last of the three categories, an example of a physical proxy is marine and lake sediments.<\/p>\n<p>Each year, billions of tonnes of sediment are washed into lakes and seas around the world. These sediments build up over time, adding layer upon layer with each year that passes. Drilling a core down through the bed of a sea or lake can, therefore, provide a timeline of how the sediments \u2013 and, hence, the climate \u2013 have changed.<\/p>\n<\/div>\n<div class=\"row central\">\n<p>The size, shape, structure and colour of these sediments can all provide clues about the climate of the time. For example, explains the\u00a0<a href=\"https:\/\/www2.usgs.gov\/landresources\/lcs\/paleoclimate\/proxies.asp\" target=\"_blank\" rel=\"noopener noreferrer\">US Geological Survey website<\/a>:<\/p>\n<p class=\"long-quote\">\u201cScientists use the size and shape of sediment particles to determine if the sediment was transported, how far it was transported, and how energetic the environment of transportation was (for example, waves crashing on a beach leave behind coarse sand particles, whereas very small grains are deposited in very still conditions).\u201d<\/p>\n<p>But sediments are also a very important archive for other proxies. Buried along with layers of sediment are all sorts of fossils that scientists can analyse. \u201c<a href=\"https:\/\/en.wikipedia.org\/wiki\/Foraminifera#:~:text=Foraminifera%20(%2Ff%C9%99%CB%8Cr,test%22)%20of%20diverse%20forms%20and\" target=\"_blank\" rel=\"noopener noreferrer\">Foraminifera<\/a>\u201d\u00a0are a classic example, explains Pearson:<\/p>\n<p class=\"long-quote\">\u201cForaminifera are \u2013 mostly \u2013 microscopic shells secreted by single celled organisms that live as plankton or on the sea bed. In the right burial conditions, their shells can survive in virtually perfect condition indefinitely. These shells build up slowly on the seafloor producing a more or less continuous proxy record.\u201d<\/p>\n<p>Foraminifera build their shells from calcium carbonate extracted from the seawater. Isotope analysis can reveal the conditions in the ocean \u2013 and, hence, the climate \u2013 when those organisms were alive.<\/p>\n<\/div>\n<div class=\"row central\">\n<p><a href=\"https:\/\/diatoms.org\/what-are-diatoms#:~:text=Diatoms%20are%20single%2Dcelled%20algae,and%20striking%20patterns%20of%20silica.\" target=\"_blank\" rel=\"noopener noreferrer\">Diatoms<\/a>\u00a0are another microfossil \u2013 this time with shells made of silicon dioxide. While foraminifera are\u00a0<a href=\"https:\/\/link.springer.com\/chapter\/10.1007\/978-3-030-39212-3_15\" target=\"_blank\" rel=\"noopener noreferrer\">restricted to marine and coastal environments<\/a>, diatoms are also found in inland lakes. Lake sediments are a\u00a0<a href=\"https:\/\/www.nap.edu\/read\/11676\/chapter\/8#60\" target=\"_blank\" rel=\"noopener noreferrer\">key natural archive for reconstructing histories of drought<\/a>\u00a0and the diatoms they contain have been used, for example, to piece together\u00a0<a href=\"https:\/\/link.springer.com\/article\/10.1023\/A:1007929006001\" target=\"_blank\" rel=\"noopener noreferrer\">records of extreme droughts in the US midwest<\/a>.<\/p>\n<p>Leaf waxes found in the marine sediments are also a useful climate proxy, adds Cluett:<\/p>\n<p class=\"long-quote\">\u201cLeaf waxes are a group of simple organic molecules that are widely produced by vegetation, both on the terrestrial landscape and within lakes\u2026 [They] are useful biomarkers because plants incorporate hydrogen atoms into the structure of these molecules from the water \u2013 generally derived from precipitation \u2013 in which they use to grow.\u201d<\/p>\n<p>Isotope analysis of leaf waxes \u201cprovide terrestrial climate records analogous to measurements of stable water isotopes in ice cores\u201d, notes Cluett. This approach has been used, for example, to reconstruct rainfall patterns during the \u201cGreen Sahara\u201d period around 11,000-5,000 years ago when the region supported\u00a0<a href=\"https:\/\/advances.sciencemag.org\/content\/3\/1\/e1601503\" target=\"_blank\" rel=\"noopener noreferrer\">diverse vegetation, permanent lakes and human populations<\/a>.<\/p>\n<p>As proxy data is accumulated naturally, its records can extend back as far as that medium exists. So for the isotopes in ice cores, for example, that is as long as the ice sheet or glacier has been in place. Marine sedimentary records can be millions of years long, going all the way back to the\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Cretaceous\" target=\"_blank\" rel=\"noopener noreferrer\">Cretaceous period<\/a>\u00a0100m years ago \u2013 the time of the dinosaurs. This reflects the fact that the seabed has existed for a lot longer than trees, corals or even ice sheets.<\/p>\n<p>To probe the oldest parts of Earth history, palaeoclimatologists must use rock formations, explains\u00a0<a href=\"https:\/\/www.geo.arizona.edu\/~jesst\/\" target=\"_blank\" rel=\"noopener noreferrer\">Dr Jessica Tierney,<\/a>\u00a0an associate professor at the University of Arizona and a\u00a0<a href=\"https:\/\/apps.ipcc.ch\/report\/authors\/report.authors.php?q=35&amp;p=\" target=\"_blank\" rel=\"noopener noreferrer\">lead author<\/a>\u00a0on the IPCC\u2019s\u00a0<a href=\"https:\/\/www.ipcc.ch\/report\/sixth-assessment-report-working-group-i\/\" target=\"_blank\" rel=\"noopener noreferrer\">sixth assessment report<\/a>. She tells Carbon Brief:<\/p>\n<div class=\"long-quote\">\u201cTo study climate changes before about 100m years ago, we must work on rock formations on land, which contain marine or terrestrial sediments that have been lithified.\u201d<\/div>\n<p><a href=\"https:\/\/www.britannica.com\/science\/lithification\" target=\"_blank\" rel=\"noopener noreferrer\">Lithification<\/a>\u00a0is the process by which sediments are compacted under pressure to form solid rock. These might naturally \u201coutcrop\u201d on the landscape, says Tierney, or scientists might drill into them to get a core. She adds:<\/p>\n<p class=\"long-quote\">\u201cIn these ancient archives, we find evidence of truly extreme climate changes, like the\u00a0<a href=\"https:\/\/www.sciencedirect.com\/science\/article\/abs\/pii\/S0012825214000750\" target=\"_blank\" rel=\"noopener noreferrer\">end-Permian global warming<\/a>\u00a0and mass extinction, and \u2018<a href=\"https:\/\/www.pnas.org\/content\/112\/17\/5337\" target=\"_blank\" rel=\"noopener noreferrer\">Snowball Earth<\/a>\u2019 \u2013 a time when the Earth was totally covered in ice.\u201d<\/p>\n<\/div>\n<div class=\"row central\">\n<p>While the potential record from marine sediments is long, the sampling \u201cinterval\u201d that can be derived is more limited. The data might only be able to show climate changes from one century to the next, while data from tree-rings and stalagmites, for example, can show changes from one year to the next. (See the section below for more information on different types of proxies and archives.)<\/p>\n<p>Proxy data can help provide an insight into the way humans have responded to changes in their environment. For example, a\u00a0<a href=\"https:\/\/advances.sciencemag.org\/content\/6\/45\/eabb6376?rss=1\" target=\"_blank\" rel=\"noopener noreferrer\">Science Advances<\/a>\u00a0paper from earlier this year used pollen and charcoal data from 17 sediment cores \u2013 along with archeological surveys \u2013 to show how inhabitants of the Isles of Scilly off the south-western tip of England adapted to changing sea levels during the Bronze Age around 4-5,000 years ago.<\/p>\n<p>Finally, it is worth highlighting another form of palaeoclimate archive \u2013 historical documents. These can be diaries, logbooks, photographs and\u00a0<a href=\"https:\/\/acp.copernicus.org\/articles\/14\/2987\/2014\/acp-14-2987-2014.html\" target=\"_blank\" rel=\"noopener noreferrer\">even paintings<\/a>\u00a0that carry direct and indirect climate information.<\/p>\n<p>For example, English manorial accounts from the Middle Ages \u2013 financial and farming records kept by rural estates \u2013 provide detailed information on harvests and milk production. Scientists have used these records to\u00a0<a href=\"https:\/\/rmets.onlinelibrary.wiley.com\/doi\/abs\/10.1002\/wea.3599\" target=\"_blank\" rel=\"noopener noreferrer\">piece together records of droughts<\/a>\u00a0that hit England hundreds of years ago.<\/p>\n<\/div>\n<div class=\"row central\">\n<p>Another example is how\u00a0<a href=\"https:\/\/link.springer.com\/article\/10.1007\/s00382-011-1145-7\" target=\"_blank\" rel=\"noopener noreferrer\">historical photographs and sketched maps<\/a>\u00a0can be used to reconstruct changes in the lengths of glaciers \u2013 and, hence, fluctuations in the climate.<\/p>\n<p>Other forms include weather descriptions in\u00a0<a href=\"https:\/\/link.springer.com\/article\/10.1023\/A:1005698302572\" target=\"_blank\" rel=\"noopener noreferrer\">personal diaries<\/a>, records of\u00a0<a href=\"https:\/\/agupubs.onlinelibrary.wiley.com\/doi\/full\/10.1029\/2007GL031381\" target=\"_blank\" rel=\"noopener noreferrer\">grape harvest dates<\/a>, and descriptions of wind, weather and sea ice cover\u00a0<a href=\"https:\/\/www.carbonbrief.org\/guest-post-piecing-together-arctic-sea-ice-history-1850\">in ship logbooks<\/a>.<\/p>\n<\/div>\n<div class=\"row central\">\n<p id=\"three\"><strong>What are the different sources of proxy data?<\/strong><\/p>\n<p>The table below summarises the key archives of climate proxies, the data they provide and the typical intervals and time spans that they cover.<\/p>\n<\/div>\n<div id=\"table-chapter\">\n<table id=\"proxy-table\" class=\"table table-bordered table-striped table-responsive-sm\">\n<thead>\n<tr>\n<th scope=\"col\">Proxy Archive<\/th>\n<th scope=\"col\">Types of measurements<\/th>\n<th scope=\"col\">Typical interval<\/th>\n<th scope=\"col\">Typical time span<\/th>\n<th scope=\"col\">Description<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Boreholes<\/td>\n<td>Temperature<\/td>\n<td>Century<\/td>\n<td>Hundreds of years<\/td>\n<td class=\"table-overflow\">Boreholes are narrow shafts drilled into the Earth, typically to extract substances such as water or oil. As heat at the surface slowly diffuses vertically down into the Earth, temperature readings taken a different depths through the borehold can indicate past temperatures at the surface. While borehole measurements are taken directly, they are classed as a proxy as they are used to indirectly measure past temperatures.<\/td>\n<\/tr>\n<tr>\n<td>Corals and sponges<\/td>\n<td>Isotopes, chemical properties, growth rate<\/td>\n<td>Year<\/td>\n<td>Centuries<\/td>\n<td class=\"table-overflow\">Corals build their hard skeletons from calcium carbonate they extract from seawater. The density of those skeletons changes from season to season and year to year with fluctuations in sea temperature, water clarity and available nutrients. These variations are revealed in annual growth rings similar to those in trees. Scientists take small samples from the corals to analyse these rings, which sometimes need x\u2060\u2013rays to identify. Isotope analysis of the oxygen atoms contained in the skeleton can also signal changes in variables such as ocean temperature. Although sponges do not build hard exoskeletons like corals, they grow by putting on layers of calcium carbonate or silicon dioxide, which also produces growth rings.<\/td>\n<\/tr>\n<tr>\n<td>Glaciers<\/td>\n<td>Glacier extent<\/td>\n<td>Year<\/td>\n<td>Hundreds of years<\/td>\n<td class=\"table-overflow\">Mountain glaciers grow and retreat over time in response to the climatic conditions and so records of their length can be used as a climate proxy. Records \u2060\u2013 in the form of measurements, photos and paintings \u2060\u2013 often go back several hundred years. Carbon dating of plants and other organic material that is uncovered by a retreating glacier can also indicate past glacier extent.<\/td>\n<\/tr>\n<tr>\n<td>Historical documents<\/td>\n<td>Historical<\/td>\n<td>Hour to day<\/td>\n<td>Hundreds of years<\/td>\n<td class=\"table-overflow\">Direct and indirect information about the climate can be gleaned from historical documents. These include accounts of weather in newspapers, ship logs, personal diaries and church records, while documented harvest dates \u2060\u2013 for grapes and other crops, for example \u2060\u2013 can also indicate climatic conditions of the past. Photos, maps, charts and paintings can all be sources of data too.<\/td>\n<\/tr>\n<tr>\n<td>Ice cores<\/td>\n<td>Isotopes, dust, accumulation rate, greenhouse gas concentrations<\/td>\n<td>Year<\/td>\n<td>Hundreds of thousands of years<\/td>\n<td class=\"table-overflow\">Ice sheets and glaciers form from the accumulation and compaction of snow over thousands of years. Drilling down through the layers of ice to retrieve a \u00abcore\u00bb provides a cross\u2060\u2013section of that accumulation and hence a timeline of snow build\u2060\u2013up. The information contained in the ice includes dust from volcanic eruptions, air bubbles that provide sample of past atmospheres, and isotopes that offer evidence of past climates.<\/td>\n<\/tr>\n<tr>\n<td>Lake sediments<\/td>\n<td>Physical and chemical properties, shells, pollen, insects, molecular fossils, isotopes<\/td>\n<td>Decades to centuries<\/td>\n<td>Millions of years<\/td>\n<td class=\"table-overflow\">Sediments are washed into lakes and accumulate through time. Like marine sediments, lake sediments offer a variety of climate proxies. Carbon and hydrogen isotope analysis of molecular fossils like leaf waxes (coming from the protective coat on the leaves of plants) can be used to infer changes in the landscape and the water cycle. The fossilised remains of insects can be used to infer past climate. Grains of pollen from flowering plants are preserved in lake sediments, and can be used to infer changes in both vegetation and climate. The amount of charcoal in lake sediments can be used to infer changes in fire frequency and intensity.<\/td>\n<\/tr>\n<tr>\n<td>Loess<\/td>\n<td>Dust accumulation, physical and chemical properties, molecular fossils<\/td>\n<td>Centuries to millennia<\/td>\n<td>Millions of years<\/td>\n<td class=\"table-overflow\">The formation, transport and deposition of wind-blown silt \u2060\u2013 known as \u00abloess\u00bb and \u00abEolian dust\u00bb \u2060\u2013 is closely linked to changes in climate. Large dust deposits are recorded in dry periods \u2060\u2013 for example, during the ice ages when large portions of land were covered with ice sheets and glaciers. Dust deposits on land can reach tens or hundreds of metres thick.<\/td>\n<\/tr>\n<tr>\n<td>Marine sediments<\/td>\n<td>Physical and chemical properties, shells, pollen, molecular fossils, isotopes<\/td>\n<td>Centuries to millennia<\/td>\n<td>Tens of millions of years<\/td>\n<td class=\"table-overflow\">Each year sees billions of tonnes of sediment accumulate on the beds of seas and oceans across the world. These sediments capture tiny clues to the climate of the time \u2060\u2013 including microfossils, such as foraminifera shells, and molecular fossils, such as leaf waxes. Isotope analysis of these clues can reveal information about the climate. Properties of the sediment itself \u2060\u2013 such as its size, shape, structure and colour \u2060\u2013 can also change with the climate.<\/td>\n<\/tr>\n<tr>\n<td>Pack rat middens<\/td>\n<td>Pollen, insects, plant remnants, bones, teeth, isotopes<\/td>\n<td>Decades<\/td>\n<td>Tens of thousands of years<\/td>\n<td class=\"table-overflow\">Pack rats, also known as woodrats, construct debris piles that can be crystallised over time by their urine, preserving the contents for thousands of years. These \u00abmiddens\u00bb contain remnants of plants, bones, teeth, insects, shells and seeds that can be dated and analysed for their isotope content, providing climatic information.<\/td>\n<\/tr>\n<tr>\n<td>Rock outcrops<\/td>\n<td>Physical and chemical properties, shells, pollen, insects, teeth, plant fossils, molecular fossils, isotopes<\/td>\n<td>Millennia<\/td>\n<td>Hundreds of millions of years<\/td>\n<td class=\"table-overflow\">Ancient sedimentary rocks provide the oldest archives of Earth\u2019s climate. As with marine and lake sediments, a number of different proxies can be measured in them. Fossils of plants offer an opportunity to reconstruct CO2 levels, through the analysis of their stomata. The shape and condition of the teeth of fossilised mammals can provide information on past climate conditions. For example, worn surfaces on the teeth of herbivores can indicate the vegetation \u2060\u2013 and, hence, the climate \u2060\u2013 of the past.<\/td>\n<\/tr>\n<tr>\n<td>Seal pelts<\/td>\n<td>Isotopes<\/td>\n<td>Decades<\/td>\n<td>Centuries to millennia<\/td>\n<td class=\"table-overflow\">The fur\/skin of seals have been used for thousands of years to make warm and waterproof clothing and footwear. These skins contain isotopes of elements, such as carbon and nitrogen, that the seals accumulated from the prey they consumed. The concentrations of the different isotopes can, thus, indicate the structure of the food chain in the past and, hence, the environmental conditions of the time.<\/td>\n<\/tr>\n<tr>\n<td>Speleothems<\/td>\n<td>Isotopes, chemical properties<\/td>\n<td>Decades to centuries<\/td>\n<td>Tens of thousands of years<\/td>\n<td class=\"table-overflow\">Speleothems are cave formations, such as stalactites (hanging from the cave ceiling) and stalagmites (rising from the floor). They are formed from the build up of mineral deposits \u2060\u2013 primarily, calcium carbonate \u2060\u2013 carried by groundwater percolating through the rock. Changes in isotopes and trace elements can be used to determine past climates.<\/td>\n<\/tr>\n<tr>\n<td>Tree rings<\/td>\n<td>Ring width, wood density, isotopes<\/td>\n<td>Year<\/td>\n<td>Thousands of years<\/td>\n<td class=\"table-overflow\">The annual growth of a tree is typically recorded in rings through the trunk. Each ring has a light part (fast growth in spring\/early summer) and a dark part (slow growth in late summer\/autumn) for each year of growth. The amount of growth \u2060\u2013 and, thus, the width of the rings \u2060\u2013 reflects the climatic conditions of the time. For example, trees tend to grow faster in warm, wet conditions, and slower in cold and dry. Trees can be \u00abcored\u00bb to remove a small cross\u2060\u2013section of the trunk in order to access the rings without damaging the tree. Isotopes within the wood can also be analysed to provde climate information.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p class=\"row central\">Table produced with the help of Dr Jessica Tierney.<\/p>\n<div class=\"row central\">\n<p id=\"four\"><strong>How is proxy data calibrated and used?<\/strong><\/p>\n<p>As proxies do not directly measure climate variables, a conversion is needed to turn an oxygen isotope value, tree-ring width or other proxy measurement into a climate variable, such as temperature or rainfall. The conversion is known as \u201c<a href=\"https:\/\/www.metlink.org\/wp-content\/uploads\/2018\/05\/Advanced-Guide-to-Calibration-and-Natural-Climate-Proxies.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">calibration<\/a>\u201d and usually takes one of two forms.<\/p>\n<p>Many more recent high-resolution proxies can be \u201c<a href=\"https:\/\/agupubs.onlinelibrary.wiley.com\/doi\/full\/10.1029\/2009GC002604\" target=\"_blank\" rel=\"noopener noreferrer\">calibrated-in-time<\/a>\u201d. This is where researchers look to see what the relationship is between the proxy values and direct observations during the recent period in which climate observations exist, and use that relationship to infer values in the more distant past.<\/p>\n<p class=\"container\">For example, if tree-ring width is closely related to temperature during the 1850-2000 period, scientists can use the tree-ring record before 1850 \u2013\u00a0say, from 1500 to 1850 \u2013\u00a0to reconstruct the temperature of that period.<\/p>\n<p>A second approach is \u201c<a href=\"https:\/\/link.springer.com\/article\/10.1007\/s10933-012-9582-9\" target=\"_blank\" rel=\"noopener noreferrer\">calibration-in-space<\/a>\u201d. This involves measuring the proxy across a wide spatial range of modern environments, where the controlling factors \u2013 such as temperature \u2013 are known. This technique is used when direct comparison with observational records is not possible.<\/p>\n<p class=\"container\">For example, different kinds of pollen might be measured in modern lake sediments that span a range of temperatures in order to produce a calibration. In some cases, calibrations can also be made in the lab, by culturing organisms \u2013 such as foraminifera or algae \u2013 under different temperatures.<\/p>\n<\/div>\n<div class=\"row central\">\n<p>In some cases, however, more than one factor can affect proxy estimates. For example, tree-ring width may depend both on rainfall and temperature for a location, and researchers want to ensure that they do not misinterpret a drought as a period of low temperature. Researchers may make use of sophisticated statistical models to distinguish between different factors affecting proxy measurements.<\/p>\n<p class=\"container\">In addition, the relationship between the proxy value and the climate variable may not hold steady over time. For example, if a changing climate makes tree-ring width \u2013\u00a0and associated tree growth \u2013 more rainfall-dependent than temperature-dependent, the proxy may cease being useful. This has occurred in\u00a0<a href=\"https:\/\/agupubs.onlinelibrary.wiley.com\/doi\/10.1029\/2018GL080981\" target=\"_blank\" rel=\"noopener noreferrer\">some Bristlecone pine records<\/a>.<\/p>\n<p>An example of this is the \u201c<a href=\"https:\/\/en.wikipedia.org\/wiki\/Divergence_problem\" target=\"_blank\" rel=\"noopener noreferrer\">divergence problem<\/a>\u201d \u2013 a tendency of some tree-ring wood density records to \u201c<a href=\"https:\/\/link.springer.com\/article\/10.1007%2Fs10584-009-9594-2\" target=\"_blank\" rel=\"noopener noreferrer\">decouple<\/a>\u201d from observed temperatures after 1950. While wood density and tree-ring width in some regions of the world matched observed temperatures well prior to 1950, some \u2013 though not all \u2013 tree-ring records failed to capture the rapid observed warming after that point.<\/p>\n<p>These sorts of events are problematic, however, as researchers have to try and ensure that similar divergences in the relationship between the proxy value and climate variable did not occur during other periods prior to the availability of observational records.<\/p>\n<p>In some cases, limited historical observations prior to the advent of modern climate monitoring networks\u00a0<a href=\"https:\/\/science.sciencemag.org\/content\/328\/5977\/486.abstract\" target=\"_blank\" rel=\"noopener noreferrer\">can be used<\/a>\u00a0for providing a cross-check on reconstructions based on other proxy records and for validating past model simulations.<\/p>\n<\/div>\n<div class=\"row central\">\n<p id=\"five\">How does proxy data inform climate science?<\/p>\n<p>As discussed above, observed records of global temperature only go back to 1850 and, in many regions, the temperature record is even shorter. Other climate records \u2013\u00a0such as the composition of the Earth\u2019s atmosphere, streamflow, hurricanes, wildfires or solar output \u2013\u00a0may have much shorter historical observational records.<\/p>\n<p>To understand changes across different aspects of the Earth system prior to the start of instrumental records, scientists, therefore, have to rely on proxy measurements.<\/p>\n<p>Proxies have been instrumental in many ways to the development of modern climate science. For example, proxy records of both greenhouse concentrations and temperatures over the past 800,000 years have helped scientists understand the drivers of ice age cycles \u2013 including the\u00a0<a href=\"https:\/\/www.carbonbrief.org\/explainer-how-the-rise-and-fall-of-co2-levels-influenced-the-ice-ages\">critical role of CO2<\/a>\u00a0in the process.<\/p>\n<p>While many studies have examined individual proxy types and locations \u2013\u00a0such as a specific coral reef or cave \u2013\u00a0over the past few decades there has been a focus on using a number of different proxies in conjunction to get a better understanding of regional or global changes.<\/p>\n<p>This is important, as a single location such as Antarctica may exhibit much larger swings in temperature or other climate variables over time than the globe as a whole. These \u201c<a href=\"https:\/\/www.ncdc.noaa.gov\/data-access\/paleoclimatology-data\/datasets\/climate-reconstruction\" target=\"_blank\" rel=\"noopener noreferrer\">multiproxy climate reconstructions<\/a>\u201d have examined changes in air temperatures, drought, precipitation, sea surface temperatures, sea level, sea ice and vegetation among other factors.<\/p>\n<p class=\"container\">For example, a recent\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41586-020-2617-x\" target=\"_blank\" rel=\"noopener noreferrer\">Nature<\/a>\u00a0study pulled together \u201ca large collection of geochemical proxies for sea surface temperature\u201d in an effort to reconstruct global temperatures during the most recent ice age, known as the\u00a0<a href=\"http:\/\/www.antarcticglaciers.org\/2017\/06\/global-last-glacial-maximum\/\" target=\"_blank\" rel=\"noopener noreferrer\">Last Glacial Maximum<\/a>. The researchers then validated their results against 18O isotope records derived from ice cores and speleothems.<\/p>\n<p>One of the largest multiproxy reconstructions is from the Past Global Changes (<a href=\"http:\/\/www.pastglobalchanges.org\/\" target=\"_blank\" rel=\"noopener noreferrer\">PAGES<\/a>) project, a collaboration between thousands of palaeoclimatologists from 125 different countries that began back in 1991.<\/p>\n<p>In 2019, they\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41586-019-1401-2\" target=\"_blank\" rel=\"noopener noreferrer\">published<\/a>\u00a0a thorough analysis of global surface temperatures over the past 2,000 years \u2013\u00a0called the\u00a0<a href=\"http:\/\/pastglobalchanges.org\/science\/wg\/2k-network\/intro\" target=\"_blank\" rel=\"noopener noreferrer\">PAGES 2K<\/a>\u00a0project. The figure below shows their resulting reconstructions across all the different methods the team examined. The yellow line shows the median across all of the palaeoclimate proxy reconstructions they examined, while the yellow shaded area shows the uncertainty range (2.5th percentile to 97.5th percentile) of the proxy reconstructions. The red line shows observed global surface temperatures after 1850.<\/p>\n<p><iframe loading=\"lazy\" id=\"highchart\" src=\"https:\/\/cbhighcharts2021.s3.eu-west-2.amazonaws.com\/proxy-explainer\/2000_yr_proxy.html\" name=\"myiFrame\" width=\"800px\" height=\"550px\" frameborder=\"0\" marginwidth=\"0px\" marginheight=\"0px\" scrolling=\"no\" allowfullscreen=\"allowfullscreen\" data-mce-fragment=\"1\"><\/iframe>Global mean surface temperature reconstruction (yellow line) and uncertainties (yellow range) for the years 0-2000 period from the\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41561-019-0400-0\" target=\"_blank\" rel=\"noopener noreferrer\">PAGES 2k Consortium<\/a>\u00a0along with observations from Cowtan and Way from 1850-2017. Data available in the\u00a0<a href=\"https:\/\/www1.ncdc.noaa.gov\/pub\/data\/paleo\/pages2k\/neukom2019temp\/recons\/Full_ensemble_median_and_95pct_range.txt\" target=\"_blank\" rel=\"noopener noreferrer\">NOAA Paleoclimate Archive<\/a>.Researchers have also produced palaeoclimate proxy reconstructions over the full Holocene \u2013 the modern geologic epoch spanning the past 12,000 years. The figure below shows a range of reconstructions (grey band) as well as the median estimate (yellow line) of global average temperature.<\/p>\n<p><iframe loading=\"lazy\" id=\"highchart\" src=\"https:\/\/cbhighcharts2021.s3.eu-west-2.amazonaws.com\/proxy-explainer\/12k_proxy.html\" name=\"myiFrame\" width=\"800px\" height=\"550px\" frameborder=\"0\" marginwidth=\"0px\" marginheight=\"0px\" scrolling=\"no\" allowfullscreen=\"allowfullscreen\" data-mce-fragment=\"1\"><\/iframe>Global mean surface temperature reconstruction (yellow line) and uncertainties (grey range) for the period from 10,050BC to AD1950 from the\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41597-020-0530-7\" target=\"_blank\" rel=\"noopener noreferrer\">Temperature 12k Database<\/a>. Recent observations are not shown due to the low temporal resolution of the underlying data. Data available in the\u00a0<a href=\"https:\/\/www.ncdc.noaa.gov\/paleo-search\/study\/27330\" target=\"_blank\" rel=\"noopener noreferrer\">NOAA Paleoclimate Archive<\/a>.<\/p>\n<\/div>\n<div class=\"row central\">\n<p>(It is worth noting that there is some disagreement between different methods of Holocene temperature reconstructions, with\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41586-020-03155-x\" target=\"_blank\" rel=\"noopener noreferrer\">one recent paper<\/a>\u00a0suggesting that the holocene maximum temperatures may have been considerably lower than other proxy reconstructions have estimated.)<\/p>\n<p>The further back in time reconstructions go, the\u00a0<a href=\"https:\/\/eo.ucar.edu\/staff\/rrussell\/climate\/paleoclimate\/proxy_span_resolution.html\" target=\"_blank\" rel=\"noopener noreferrer\">lower temporal resolution<\/a>\u00a0they tend to have. In other words, records from 10,000 years ago might only represent an average temperature value over a 100-year period or more, while more recent proxy data tends to be closer to 20-year averages.<\/p>\n<p>This somewhat limits the ability of scientists to compare earlier proxy reconstructions to the modern temperature record without applying similar long-term averaging, though researchers\u00a0<a href=\"http:\/\/shpud.com\/Science-2013-Marcott-1198-201.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">have found<\/a>\u00a0some ways to get around this problem.<\/p>\n<p>In addition to understanding how temperatures and other climate variables have changed in the past, proxy data also gives scientists hints at how it might change in the future. Proxy data provides one of three\u00a0<a href=\"https:\/\/www.carbonbrief.org\/guest-post-why-low-end-climate-sensitivity-can-now-be-ruled-out\">key lines of evidence<\/a>\u00a0that scientists have used to better estimate the range of\u00a0<a href=\"https:\/\/www.carbonbrief.org\/explainer-how-scientists-estimate-climate-sensitivity\">climate sensitivity<\/a>\u00a0\u2013\u00a0which determines how much the Earth will warm in the future if CO2 concentrations double.<\/p>\n<p>For example, explains\u00a0<a href=\"http:\/\/www.bris.ac.uk\/geography\/people\/dan-j-lunt\/index.html\" target=\"_blank\" rel=\"noopener noreferrer\">Prof Dan Lunt<\/a>, professor of climate science at the\u00a0<a href=\"https:\/\/www.bristol.ac.uk\/\" target=\"_blank\" rel=\"noopener noreferrer\">University of Bristol<\/a>, \u201c50m years ago, CO2 concentrations were greater than today and the planet was substantially warmer. Proxies for CO2 allow us to quantify the former and proxies for temperature allow us to estimate the latter\u201d. Scientists can use this information to estimate climate sensitivity.<\/p>\n<p>However, he adds, there are considerable uncertainties associated with these estimates. This is because \u201cwe do not have complete geographical coverage of the whole planet, and because the correlation between proxy and climate is not perfect\u201d. In addition, the\u00a0<a href=\"https:\/\/www.carbonbrief.org\/explainer-how-scientists-estimate-climate-sensitivity\">\u201csensitivity\u201d of the climate<\/a>\u00a0(how much the temperature of the Earth changes when CO2 in the atmosphere increases or decreases) when the Earth was in a different state \u2013 such as an ice age \u2013 may not be \u201ca reliable indicator\u201d for its sensitivity in the future, Lunt says.<\/p>\n<p class=\"container\">Proxy data can also help determine changes to\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41561-018-0195-4?proof=t\" target=\"_blank\" rel=\"noopener noreferrer\">sea level<\/a>,\u00a0<a href=\"https:\/\/www.nature.com\/articles\/s41467-017-00552-1\" target=\"_blank\" rel=\"noopener noreferrer\">ice cover<\/a>\u00a0and\u00a0<a href=\"https:\/\/onlinelibrary.wiley.com\/doi\/pdf\/10.1111\/j.1502-3885.2005.tb01016.x\" target=\"_blank\" rel=\"noopener noreferrer\">vegetation<\/a>\u00a0during past warm periods. One example is the previous interglacial period around 125,000 years ago \u2013\u00a0called the\u00a0<a href=\"https:\/\/www.ncdc.noaa.gov\/global-warming\/penultimate-interglacial-period\" target=\"_blank\" rel=\"noopener noreferrer\">Eemian<\/a>\u00a0that was likely as warm or warmer than today \u2013 which can provide evidence on how all of these factors may change as the world warms.<\/p>\n<p>Another way that proxies inform climate science is through modelling. Proxy records can be used to help evaluate\u00a0<a href=\"https:\/\/www.carbonbrief.org\/qa-how-do-climate-models-work\">climate models<\/a>. Scientists run \u201c<a href=\"https:\/\/www.carbonbrief.org\/qa-how-do-climate-models-work#experiments\">hindcast<\/a>\u201d simulations of models to see how well they reproduce the past climate and proxy data is needed to test against the climate of the distant past.<\/p>\n<p>The benefits of this are two-fold, explains Lunt:<\/p>\n<p class=\"long-quote\">\u201cA model that agrees well with the proxy data can then be used with confidence to improve our understanding of that time period\u2026[and] if a model agrees well with proxy evidence of the past, then in some cases this can give increased confidence in its simulation of the future.\u201d<\/p>\n<p>Lunt leads an international modelling project called\u00a0<a href=\"https:\/\/www.deepmip.org\/\" target=\"_blank\" rel=\"noopener noreferrer\">DeepMIP<\/a>\u00a0(Deep-Time Model Intercomparison Project). The project was conceived in recognition of the fact that climate models are typically evaluated against the Earth\u2019s relatively recent past, while global temperatures might be heading to levels not seen for tens of millions of years (\u201cdeep-time\u201d), he explains.<\/p>\n<p>DeepMIP uses proxy data to\u00a0<a href=\"https:\/\/cp.copernicus.org\/articles\/17\/203\/2021\/cp-17-203-2021.html\" target=\"_blank\" rel=\"noopener noreferrer\">evaluate models<\/a>\u00a0and learn about the past climate system, Lunt says \u2013 \u201cin particular of the super-warm early Eocene Climatic Optimum (EECO, ~50m years ago) and Paleocene-Eocene Thermal Maximum (PETM, ~55m years ago)\u201d.<\/p>\n<\/div>\n<div class=\"row central\">\n<p id=\"six\"><strong>What are the limitations of proxy data?<\/strong><\/p>\n<p>As the sections above highlight, the process of identifying, extracting, interpreting and calibrating proxy data to produce climate records is anything but straightforward. Such complicated techniques, therefore, have their pitfalls and limitations.<\/p>\n<p>Proxy data is, by definition, indirect, says Cluett. So while proxies are recording changes in the environment, scientists need to take account of both the local conditions and any wider influences that might be at play.<\/p>\n<p>On a local scale, for example, \u201cthe relationship between proxies and their environments may differ in different systems, so understanding the system you are working in is critical to accurately interpreting proxy data\u201d, she says.<\/p>\n<p>For example, notes Pearson, the overall amount of the isotopes in the world\u2019s ocean can change over time. This affects the analysis of an individual record, he explains:<\/p>\n<p class=\"long-quote\">\u201cChanges in the size of the world\u2019s ice sheets changes the whole ocean isotopic ratio and so imprints itself on the [proxy] record.\u201d<\/p>\n<p>Not all proxy records are made equal, points out\u00a0<a href=\"https:\/\/www.usgs.gov\/media\/images\/justin-martin\" target=\"_blank\" rel=\"noopener noreferrer\">Dr Justin Martin<\/a>, an ecologist at the\u00a0<a href=\"https:\/\/www.usgs.gov\/\" target=\"_blank\" rel=\"noopener noreferrer\">US Geological Survey<\/a>. An ideal record is typically \u201clong, accurately dated and of sufficiently high resolution over time to provide useful information\u201d, he tells Carbon Brief. However, this is not always what scientists get, he says:<\/p>\n<p class=\"long-quote\">\u201cSome proxies may not cover a very long time period, or may only provide a relative estimate of climate variability over time that lacks certain dates. Others may only provide very coarse estimates of variability measured in decades, centuries or longer.\u201d<\/p>\n<p>And all proxies \u201care subject to the vagaries of preservation one way or another\u201d, adds Pearson.<\/p>\n<p>With historical documents, for example, older records are generally less useful. A book on temperature reconstructions by the\u00a0<a href=\"https:\/\/www.nap.edu\/read\/11676\/chapter\/6\" target=\"_blank\" rel=\"noopener noreferrer\">US National Research Council<\/a>\u00a0notes that there are \u201cweather records preserved in Irish and Norse annals back to the middle of the first millennium AD\u201d, but says \u201ctheir dating is imprecise and descriptions of weather and climate often are exaggerated\u201d.<\/p>\n<p>All of these complications mean that \u201cthere may be quite large error bars that come not just from our analytical precision, but also how well calibrated the proxy is or can be to the target variable of interest\u201d, says Pearson. He adds:<\/p>\n<p class=\"long-quote\">\u201cFor these reasons, there is a lot of technical literature on the subject and, in general, we like to use multiple proxies together, if we can.\u201d<\/p>\n<p>These obstacles also make palaeoclimate a \u201csomewhat maddening science at times\u201d, Pearson notes, and \u201ccertain knowledge about the past is very hard to come by\u201d. Nonetheless, he says:<\/p>\n<p class=\"long-quote\">\u201cIt is also very exciting because the timescales are potentially huge and the proxies we use are limited only by our scientific imagination and ability to measure things of interest.\u201d<\/p>\n<p>Read at\u00a0<a href=\"https:\/\/interactive.carbonbrief.org\/how-proxy-data-reveals-climate-of-earths-distant-past\/\">Carbon Brief<\/a>.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>At any one moment in time, thousands of measurements are being taken of the world\u2019s weather. Across land, sea and sky, data is being gathered manually and automatically using a range of technologies, from the humble thermometer to the latest\u00a0multi million-pound satellite. By Robert McSweeney and Zeke Hausfather.\u00a0Design by Tom Prater. Put together over many [&hellip;]<\/p>\n","protected":false},"author":3,"featured_media":19391,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[12,25],"tags":[32,111,1049,1050],"_links":{"self":[{"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/posts\/19390"}],"collection":[{"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/comments?post=19390"}],"version-history":[{"count":1,"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/posts\/19390\/revisions"}],"predecessor-version":[{"id":19392,"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/posts\/19390\/revisions\/19392"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/media\/19391"}],"wp:attachment":[{"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/media?parent=19390"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/categories?post=19390"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.cr2.cl\/eng\/wp-json\/wp\/v2\/tags?post=19390"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}