Anthropogenic climate change brings many challenges for society, and for science. But without knowledge of past climate changes and their impacts, we are flying blind. Climate changes, with diverse causes, magnitudes, patterns and rates, have occurred throughout \u00a0Earth history. Rapid climate change today, caused by human modification of the atmosphere on an unprecedented scale, has no previous analogue; but this fact does not absolve us from the need to understand how climate has changed \u2013 and living organisms and communities have responded \u2013 in the past.<\/p>\n
Palaeoclimate science and palaeoecology are substantial fields of study, largely ignored by ecologists and policy makers alike. They are absent from most academic curricula in biology or meteorology. In the UK at least, it is mainly geography departments that teach these subjects. Few ecologists are knowledgeable about them. I have encountered incredulity when, for example, pointing out\u00a0 (as has been known for over a century!) that much of northern Europe was around two degrees warmer in the mid-Holocene (about 6000 years ago) than it has been in recent years; or to the well-documented Dansgaard-Oeschger (D-O) warming events during the last ice age, when Greenland warmed by as much as 10-15\u02daC (and Europe by 5-8\u02daC) over the course of a few decades.<\/p>\n
The treatment of past climates in IPCC reports has been patchy. For example, the IPCC Fifth Assessment report (AR5) actually documented D-O events (in a specific chapter on past climates in the WG1 Report), but they were not mentioned in the Technical Summary and were ignored altogether in WG2. The IPCC Sixth Assessment Report (AR6) \u201cmainstreamed\u201d palaeoclimate, which is thus mentioned in several chapters \u2013 but attention was confined to reconstructed changes in mean annual temperature, a one-dimensional view of climate.<\/p>\n
I have heard many (bogus) excuses for not<\/em><\/strong> studying past climates.<\/p>\n \u201cPast climate changes were much slower\u201d. One paper in Science<\/em> in 2013 even claimed that they were \u201corders of magnitude\u201d slower, based on a diagram (deeply buried in the paper\u2019s supplementary material) that shows unrealistically small magnitudes and rates of change \u2013 for example, at the end of the last ice age, when the global (unglaciated) land surface warmed by 5\u02daC or more during about 20 years.<\/p>\n \u201cNot enough information is available.\u201d This might have been true half a century ago; it is not true now. Moreover, major efforts are underway to synthesize information from multiple data sources on all continents.<\/p>\n \u201cThe time resolution of the records is insufficient\u201d. Not all palaeoclimate archives are highly resolved. However some \u2013 especially those from ice cores, laminated lake sediments, and speleothems \u2013 can resolve individual decades, and these records provide the detailed chronology that is essential for reconstructing rates of change.<\/p>\n \u201cDating control is inadequate\u201d. Various dating methods are used, and none is perfect. However, many techniques are used to synchronize records within a region, including tephras (markers of volcanic eruptions) and dynamic time warping (matching the shapes of time series).<\/p>\n \u201cThe data are questionable\u201d. Many indirect methods, including isotopic analyses, are used to infer past climates and environments from sedimentary, speleothem and ice-core records. Continual progress is being made in refining these methods. In addition, some data sources are closely linked to the system they record: pollen assemblages to surrounding vegetation, and charcoal abundances to fires in the catchment.<\/p>\n I was invited to write a review for Annual Review of Environment and Resources <\/em>on the relevance of studying past climates for present and future climates. The journal agreed that Sandy Harrison (Reading University) should take the lead author position, reflecting her role as a leading researcher on the reconstruction and modelling of Quaternary climate change and ecosystems on land. The final author list includes both highly experienced scientists straddling palaeoclimate science, modelling and ecology, and early-career scientists from our groups and elsewhere. Some of the key points we make in the review are as follows.<\/p>\n We noted that palaeoclimate science has made a few notable contributions to mainstream climate science, including providing one data source (the contrast in climate between the last glacial maximum and the present (Holocene) interglacial) that has helped to constrain the value of the equilibrium climate sensitivity. Palaeodata have also been used in a few recent publications to provide additional constraints on climate model parameters. However, there is scope for far more work along these lines.<\/p>\n The review also contains some warnings.<\/p>\n A supplement to the review provides readers with information on important compilations of data on past environments, presented with hopes of facilitating such collaboration.<\/p>\n There are many outstanding questions in this field. Climate models fail to represent some aspects of past environments well (including, famously, the full extent of the \u201cgreen Sahara\u201d during the mid-Holocene); we do not know why. However, if palaeodata are only used as a test of climate models \u2013 they have often been presented in this way \u2013 then it is not clear what to do when the test fails (an all-too-common response is to question the data!). Instead, we advocate the use of palaeodata as a test of specific causal hypotheses \u2013 which will also require running bespoke model \u201cexperiments\u201d. The literature to date provides relatively few examples; much more could be done.<\/p>\n I would like to end (lest my words be taken out of context \u2013 a common hazard) by re-iterating that nothing in the palaeorecord gives the slightest grounds for climate \u201cscepticism\u201d. Analogies from the past can only be taken so far. Conservation in today\u2019s landscapes cannot simply rely on species\u2019 ability to adjust to changing conditions in a less fragmented past. Climate-induced changes in fire regimes now impact settled human populations in many ways, and could lead to extinction of species that have been confined to reserves or marginal habitats. However, we do need realism \u2013 in order to act on what is actually known to science. Palaeodata tell us, for example, that ecosystems have continually changed. Nowhere is there a prelapsarian state to which \u201crewilding\u201d would return us. Rising CO2<\/sub> does bring increased primary production and greening of ecosystems, alleviating some effects of climate change. And the organisms at the greatest risk of extinction due to a changing climate are probably large mammals \u2013 not trees.<\/p>\n <\/p>\n You can read the full paper here:<\/p>\n Harrison, S.P., Bartlein, P.J., Cruz-Silva, E., Haas, O., Jackson, S.T., Kaushal, N., Liu, M., Magri, D., Robson, D.T., Vettoretti, G. & Prentice, I.C. (2025). Paleoclimate perspectives on contemporary climate change. Annual Review of Environment and Resources, 50,\u00a0https:\/\/doi.org\/10.1146\/annurev-environ-112922-110121<\/a><\/p>\n\n
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