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What the Geological Record Tells Us About Our Present and Future Climate

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What the Geological Record Tells Us About Our Present and Future Climate Downloaded from http://jgs.lyellcollection.org/ by guest on October 2, 2021 Perspective Journal of the Geological Society https://doi.org/10.1144/jgs2020-239 | Vol. 178 | 2020 | jgs2020-239 Geological Society of London Scientific Statement: what the geological record tells us about our present and future climate Caroline H. Lear1*, Pallavi Anand2, Tom Blenkinsop1, Gavin L. Foster3, Mary Gagen4, Babette Hoogakker5, Robert D. Larter6, Daniel J. Lunt7, I. Nicholas McCave8, Erin McClymont9, Richard D. Pancost10, Rosalind E.M. Rickaby11, David M. Schultz12, Colin Summerhayes13, Charles J.R. Williams7 and Jan Zalasiewicz14 1 School of Earth and Environmental Sciences, Cardiff University, Main Building, Park Place, CF10 3AT, UK 2 School of Environment Earth and Ecosystem Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK 3 National Oceanography Centre Southampton, University of Southampton, European Way, Southampton, SO14 3ZH, UK 4 Geography Department, Swansea University, Swansea, SA2 8PP, UK 5 Lyell Centre, Heriot Watt University, Research Avenue South, Edinburgh, EH14 4AP, UK 6 British Antarctic Survey, British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK 7 School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, UK 8 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK 9 Department of Geography, Durham University, Lower Mountjoy, South Road, Durham, DH1 3LE, UK 10 School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, BS8 1RJ Bristol, UK 11 Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK 12 Department of Earth and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK 13 Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge CB2 1ER, UK 14 School of Geography, Geology and the Environment, University of Leicester, University Road, Leicester, LE1 7RH, UK CHL, 0000-0002-7533-4430 * Correspondence: [email protected] Received 7 December 2020; revised 17 December 2020; accepted 17 December 2020 Executive Statement often called climate tipping points. The geological record can be used to calculate a quantity called Equilibrium Climate Sensitivity, Geology is the science of how the Earth functions and has evolved and, which is the amount of warming caused by a doubling of as such, it can contribute to our understanding of the climate system atmospheric CO2, after various processes in the climate system and how it responds to the addition of carbon dioxide (CO2)tothe have reached equilibrium. Recent estimates suggest that global atmosphere and oceans. Observations from the geological record show mean climate warms between 2.6 and 3.9°C per doubling of CO2 that atmospheric CO2 concentrations are now at their highest levels in once all slow Earth system processes have reached equilibrium. at least the past 3 million years. Furthermore, the current speed of The geological record provides powerful evidence that atmospheric human-induced CO2 change and warming is nearly without precedent CO2 concentrations drive climate change, and supports multiple lines in the entire geological record, with the only known exception being of evidence that greenhouse gases emitted by human activities are the instantaneous, meteorite-induced event that caused the extinction altering the Earth’s climate. Moreover, the amount of anthropogenic of non-bird-like dinosaurs 66 million years ago. In short, whilst greenhouse gases already in the atmosphere means that Earth is atmospheric CO2 concentrations have varied dramatically during the committed to a certain degree of warming. As the Earth’sclimate geological past due to natural processes, and have often been higher changes due to the burning of fossil fuels and changes in land-use, the than today, the current rate of CO2 (and therefore temperature) change planet we live on will experience further changes that will have is unprecedented in almost the entire geological past. increasingly drastic effects on human societies. An assessment of past The geological record shows that changes in temperature and climate changes helps to inform policy decisions regarding future greenhouse gas concentrations have direct impacts on sea-level, the climate change. Earth scientists will also have an important role to play hydrological cycle, marine and terrestrial ecosystems, and the in the delivery of any policies aimed at limiting future climate change. acidification and oxygen depletion of the oceans. Important climate – phenomena, such as the El Niño Southern Oscillation (ENSO) and Introduction the monsoons, which today affect the socio-economic stability and food and water security of billions of people, have varied markedly Geological processes influence the climate system over a wide range of with past changes in climate. time-scales, from years to billions of years, and across all aspects of the Climate reconstructions from around the globe show that climate Earth system, including the atmosphere (from the troposphere to the change is not globally uniform, but tends to exhibit a consistent exosphere), cryosphere (encompassing snow, ice and permafrost), pattern, with changes at the poles larger than elsewhere. This polar hydrosphere (encompassing oceans, streams, rivers, lakes and amplification is seen in ancient warmer-than-modern time intervals groundwater), biosphere (including soils), lithosphere (e.g. the rock like the Eocene epoch, about 50 million years ago and, more cycle) and global carbon cycle. For example, on long time-scales, recently, in the Pliocene, about 3 million years ago. The warmest plate-tectonic processes control rates of volcanism (a key factor intervals of the Pliocene saw the disappearance of summer sea ice affecting the rate of supply of CO2 to the atmosphere), and also the size from the Arctic. The loss of ice cover during the Pliocene was one of and position of continents, mountains and ocean gateways, which the many rapid climate changes observed in the record, which are influence oceanic and atmospheric circulation patterns, silicate © 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/ licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://jgs.lyellcollection.org/ by guest on October 2, 2021 2 C. H. Lear et al. weathering (which extracts CO2 from the atmosphere by reacting it Earth’s climate has remained within habitable bounds over this time, with rocks), biological productivity (plants extract CO2 from the despite large changes in the Sun’senergyoutput.Formostofits atmosphere for growth during photosynthesis) and weathering of history, Earth has been in a greenhouse state, significantly warmer organic rich material which returns CO2 to the ocean–atmosphere than the present, with no polar ice caps and high sea-levels. This system. Geological processes also impact climate on much shorter greenhouse state is sometimes referred to as a hothouse state instead, time-scales, including the cooling effects of major volcanic eruptions to distinguish it from the greenhouse effect that is always present on such as the 1991 eruption of Mount Pinatubo. Earth’s climate is also Earth because of the greenhouse gases present in the atmosphere (e.g. influenced by extraterrestrial processes. On the longest time-scale, this H2O, CO2,CH4,N2O), which maintain Earth’s temperature at a level includes the gradual progression of the Sun through its life cycle, as it capable of supporting life. For example, the Cretaceous chalk slowly brightens over hundreds of millions of years. On medium time- forming the cliffs at Dover was deposited when the oceans flooded scales, these include changes in the Earth’s orbit (eccentricity) and continental shelves during a greenhouse interval. Earth’s greenhouse changes in the tilt of the Earth’s axis (obliquity). These operate on states have been punctuated by colder icehouse states, such as our cycles of multiple periods, including 100 000, 40 000 and 20 000 current state and/or colder than today, with ice caps on both poles. years and work, together with the long-term carbon cycle and uneven (The icehouse state refers to a state of reduced, but not zero, distribution of Earth’s land masses between the Northern and Southern greenhouse effect.) Other notable icehouse states include the first Hemisphere, to control large-scale climate cycles such as glaciations. Snowball Earth event around 2.4 billion years ago, when ice Shorter time-scale changes in the Sun’s output include the 11- expanded from the poles down to low latitudes (Evans et al. 1997). A year sunspot cycle and variations in that cycle with frequencies of further pair of Snowball Earth events occurred during the 88, 208 and 2300 years among others. Crucially, for the majority of Neoproterozoic between 717 and 635 million years ago (Hoffman Earth’s history, natural geological and extraterrestrial processes et al. 2017), and another less-extreme icehouse state characterized by have directly affected climate, not least by producing major polar ice sheets occurred during the Carboniferous about 300 million variations in atmospheric greenhouse gas concentrations. The years ago, before the present icehouse state (Fig. 1). long-term geological perspective on climate change makes it
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