Climate change in the geological record

Human-induced climate change affects us all and has economic, sociological, political, and ecological consequences. In 2019, the Geological Society and the UK Palaeoclimate Society jointly convened an expert working group to produce a statement on climate change. Led by Carrie Lear (Cardiff University, UK), the statement entitled “What the geological record tells us about our present and future climate” was published in the Journal of the Geological Society at the end of 2020. With the aim of exploring the themes laid out in the statement, which were centred around nine key questions, the Geological Society hosted a conference, Climate Change in the Geological Record, in May 2021.

What does the geological record of climate change look like?
The geological record provides evidence of huge swings in past climate (as discussed in a presentation by Jess Tierney, University of Arizona, USA). The extremes range from the cold Snowball Earth conditions that were prevalent about 700 million years ago, evidenced by glacially derived dropstones and striations found in many places around the globe, to the super-warmth of the early Eocene about 50 million years ago, evidenced by crocodile fossils in the Arctic, for example. However, even the fastest natural climatic transitions of the past, such as the Paleocene-Eocene Thermal Maximum about 55 million years ago, were slow compared with the current anthropogenic aberration.

Why has climate changed in the past?
On geological timescales, tectonic, orbital, solar and greenhouse gas forcings, determine climate change, as well as feedbacks between the climate, ice sheets and CO₂. However, when these processes are incorporated into climate models with quantitative representation, it becomes apparent that the observed geological record cannot be explained without a significant role for CO₂ (Paul Valdes, University of Bristol, UK).

Is our current warming unusual?
Analysis of the geological record indicates that it has been a long time since our planet has been as warm as it is today. Recent decades were likely warmer than all other decades during the past 2,000 years and probably warmer than any sustained period since the peak of the Last Interglacial, 125,000 years ago. Darrell Kaufman (Northern Arizona University, USA) showed that the Earth system is changing rapidly: since 1900, observed global mean sea level has risen at a rate unprecedented in at least the last 2,500 years. Unlike previous naturally-occurring climate change, the effects of recent warming are occurring on top of other human-induced stressors, such as deforestation.

What does the geological record indicate about global versus regional change?
Drawing on examples from tropical Indian oceans during the Last Glacial Maximum 21,000 years ago, Kau Thirumalai (University of Arizona, USA) showed that clear non-linearities exist between global and regional climate, and that the geological record of past climates can provide perspective on future regional climate change and impacts. However, there is still much to learn about the mechanisms associated with past cold and warm climate states.

When Earth’s temperature changed in the past, what were the impacts?
Focusing primarily on marine ecosystem change, Daniela Schmidt (University of Bristol, UK) showed that, in general, species have migrated to maintain their optimum temperature during episodes of past climate change, for example, moving towards the poles during periods of global warming. Extreme events and compound events of warming, acidification, and oxygen depletion have resulted in extinction and loss of ecosystems.

There is hope to be gained in the recent rapid increase in the level of engagement with climate-change issues by the business community, corporate leaders and foundations, as well as governments

How does the geological record inform our quantification of how sensitive Earth is to CO₂?
Past climates can provide important information to quantify Climate Sensitivity – a key policy-relevant metric (Anna von der Heydt, Utrecht University, Netherlands). However, some uncertainties make this challenging, such as the role of fast versus slow feedbacks, the dependence of sensitivity on background state, the degree of equilibrium in models versus the real world, and the need to account
for tipping points.

Are there past climate analogues for the future?
Defining a palaeoclimate analogue as “An interval in Earth history that shares similar climatic conditions/characteristics to model simulations of future climate change”, Alan Haywood (University of Leeds, UK) presented examples where past climates can inform our understanding of future processes, for example, those associated with ice-sheet and sea-level change during the last deglaciation, atmospheric-circulation change during the Pliocene (~3 million years ago), and temperature changes during the Eocene. 

How can the geological record be used to evaluate climate models?
Bette Otto-Bliesner (National Center for Atmospheric Research, USA) showed how climate models are usually evaluated by their performance compared to historical meteorological records from the last 150 years. However, comparison to the historical record does not “stress-test” the model over more extreme changes in CO₂.

Using west-east gradients in tropical sea-surface temperatures during the Pliocene, and global mean temperatures during the Late Glacial Maximum, Bette showed how data from the geological record can test model outputs under more extreme climate conditions. Bette also highlighted that geological data have perhaps their greatest potential when used to develop models, not just evaluate them.

What is the role of geology in dealing with the climate emergency for a sustainable future?
To meet targets set by the Paris Agreement, it may be necessary to actively remove CO₂ from the atmosphere. Rachael James (University of Southampton, UK) emphasised that geology can play a key role in speeding up natural processes of CO₂ removal, for example enhancing carbonate formation by injecting CO₂ into rocks containing high quantities of calcium and magnesium ions, or by enhancing rock weathering by applying calcium and magnesium-rich rocks to agricultural soil (see page 16). Rachael also noted that to meet the demand for battery storage and photovoltaics requires extraction of metals such as selenium, neodymium, and lithium – geologists clearly have an important role to play here.

Optimism for the future
Five upcoming scientists, Aidan Starr, Rebecca Orrison, Pam Vervoot, Rachel Brown, and Margot Cramwinckel, presented their exciting doctoral and postdoctoral research, supporting the Geological Society’s commitment to early career scientists.

Finally, in a plenary talk, Maureen Raymo (Colombia University, USA) presented the geological evidence for variations in the stability of the polar ice sheets and past sea-level change. During the Last Interglacial, for example, relatively small increases in temperature led to large changes in sea level. Future sea-level changes will not be the same everywhere, but the impacts will be felt from the largest cities to the smallest villages. Maureen emphasised the benefits of co-production of knowledge, whereby scientists work closely with local communities, for example when studying sea-level rise in Greenland, to aid progress and improve mitigation against the impacts of climate change. Despite the challenges we face, Maureen finished with some optimistic words for the future: Rates of change are crucial for understanding climate change in the geological record. There is hope to be gained in the recent rapid increase in the level of engagement with climate-change issues by the business community, corporate leaders and foundations, as well as governments – let’s hope that this is one rate that does continue to rise!

Authors

Dan Lunt is a Professor of Climate Science at Bristol University, UK

Mary Gagen is a Professor of Geography at Swansea University, UK

Babette Hoogakker is an Associate Professor of Geoscience at Heriot-Watt University, UK

Charlie Williams is Research Fellow at the University of Bristol, UK

(Read the extended conference discussions and watch the presentations below.)

Climate change and the geological record: Extended discussions

During the symposium on climate change and the geological record, everyone who submitted an abstract was given the opportunity to present their work and opinions via posters and one-minute flash presentations.

The fascinating discussions are synthesised here under two broad themes: 1) What climatic processes are included in climate models? and 2) How well are CO₂ and temperature correlated in the geological record?

What climatic processes are included in climate models?

Climate models represent, in numerical form, our theoretical understanding of the physics, chemistry and biology of our planet. At their core, the models are based on the fundamental equations of fluid mechanics. These equations are solved in a “matrix” that covers the whole Earth at a typical resolution of tens of kilometres, and extends from the ocean floor to the upper atmosphere (See IPCC AR5, Chapter 9, Section 9.1). As a result of including these fundamental equations (which are simple to write down, but much harder to solve!), many complex properties and phenomena spontaneously emerge in the models, including, for example, El Nino events (Brown et al., 2020), tropical storms, Rossby waves, and a thermohaline ocean circulation. See, for example, this beautiful animation of the atmospheric system in a climate model, which is essentially indistinguishable from a satellite image of Earth:

The clouds animation is created using output from UK Met Office flagship Global Climate Model, HadGEM3. The model resolution is about 10 km. Full details are available here: https://uip.primavera-h2020.eu/clouds_animation_flagship_gcm

Climate models also include representations of flows of radiation (shortwave radiation emitted by the Sun and longwave radiation emitted by Earth and the atmosphere; see for example Edwards and Slingo, 1996), and account for effects such as “band saturation” that decreases the efficacy of greenhouse gases such as CO₂ to warm the planet as their concentration increases. By including these radiation flows, models can represent “Earth system heating” that is primarily driven by ocean warming in response to changes in radiation flux, which has been about 350 ZJ (1 ZJ = 10²¹ Joules) in the period 1971-2018 (e.g. von Schuckmann et al., 2020).

The latest climate models also include some very long-timescale processes, such as changes in the Antarctic and Greenland ice sheets in response to climate change (for example, the CISM model, which is part of the NCAR CESM climate model).

Properties of the system that depend on processes that occur at scales smaller than the resolution of the model, such as clouds and atmospheric turbulence, are also accounted for in a way that is informed by meteorological observations and by higher-resolution models that can fully represent these processes. Without inclusion of the cooling effect of clouds, climate models would be much too warm; similarly, all climate models include the effect of water vapour and the full hydrological cycle (evaporation, transpiration, condensation, precipitation, runoff, and ocean circulation), without which the models would be much too cold! Water vapour is associated with a strong positive feedback, in which an initial warming, caused, for example, by increased CO₂ concentration, is amplified because warming leads to more water vapour in the atmosphere, and water vapour is itself a greenhouse gas. A key difference between water vapour and CO₂ is that water vapour is a condensing greenhouse gas, unlike CO₂ (Lacis et al., 2010) – again, an effect that is included in all models.

Without the incorporation of such processes, climate models cannot correctly reconstruct the observed global warming (of about 1^oC) over the last 150 years (as recorded, for example in the GISS surface temperature analysis). Experiments in which models are run with and without CO₂ changes indicate that this warming is primarily caused by an increase in atmospheric CO₂, which has caused a decrease in the outgoing longwave radiation. This forcing (and combined with other forcings, such as changes in atmospheric aerosols, and land-use and land-cover change) leads to a response in temperature that is mediated by changes in clouds, water vapour, lapse rate, and the Planck feedback (see, for example, IPCC AR5, Technical Summary, Box TFE.6, p82).

How well are CO₂ and temperature correlated in the geological record?

To answer this question, we must consider two key timescales—hundreds of thousands of years compared to millions of years.

Over the last 800,000 years, ice cores provide an accurate record of atmospheric CO₂ concentrations, while both ice and marine sediment cores provide an archive of global temperature. Over this timescale, changes in atmospheric CO₂ concentrations, global temperature and sea level are ultimately driven by Milanković forcing – changes in Earth’s orbit and axial tilt (Berger, 1988). Because CO₂ and temperature affect each other through complex feedback loops, they are tightly coupled together in time, with some periods of leads and lags (we refer the reader to the section of the Geological Society of London Scientific Statement “Which came first—the CO₂ or the temperature?”, and the studies of Parrenin et al., 2013 and Shakun et al., 2012). However, modelling studies show that without changes in atmospheric CO₂ concentrations, sea level changes would be diminished compared to those suggested by the geological record, implying that variations in atmospheric CO₂ concentration must have played a role (Abe-Ouchi et al., 2013).

During some periods in the past, such as the Last Interglacial (~125,000 years ago), global temperature was warmer (and sea levels higher) than modern levels, but atmospheric CO₂ concentration was only 280 ppmv (similar to preindustrial concentrations). This apparent anomaly can be explained by the fact that during intense “interglacials”, warmth was driven by changes in Earth’s orbit, not by CO₂. Climate models can simulate this warmth when driven by these orbital changes (Otto-Bliesner et al., 2021).

On longer timescales of the Cenozoic (the last 65 million years), the relationship between CO₂ concentrations and temperature has not always been clear. In particular, despite a general cooling and decrease in CO₂ through the Cenozoic, early studies highlighted some periods when the CO₂ and temperature appeared to diverge (e.g. Pagani et al., 2005). However, reconstructing ancient CO₂ concentrations further back in time than is recorded by the ice-core archive relies on indirect proxies that have large uncertainties. Recent improvements in the reconstruction of palaeo CO₂, in particular through the use and improved understanding of the boron CO₂ proxy, have led to a much clearer correlation between CO₂ and temperature (Rae et al., 2021). There are still some periods, such as the Oligocene (~30 million years ago), for which there are relatively few new and robust CO₂ data; this is an area of ongoing research. The palaeo CO₂ community (www.paleo-CO2.org and www.p-co2.org) are working on community-curated databases of CO₂ that will get to the root of some of these issues.

Others have tried to find correlations between temperature and other phenomena, such as galactic cosmic rays (e.g. Svensmark et al., 2009; Svensmark et al., 2017). However, the mechanism by which such phenomena could affect climate is unclear, and several studies have challenged these links, including studies that use experiments carried out at the CERN CLOUD chamber (e.g. Calogovic et al., 2010; Lee et al., 2019; Pierce et al., 2017; Dunne et al., 2016; Laken, 2016). Another forcing on climate is associated with changes in the output of the Sun. Overall, the IPCC has assessed the magnitude of such solar forcing over the recent period, and concluded that its impact on climate is very small compared with anthropogenic forcings, such as CO₂ concentrations, other greenhouse gases, aerosols, and land-use change (see IPCC AR5, Technical Summary, Figure TS.6). Only models that include these human-induced forcings can accurately reproduce the observed record of temperature over the last 150 years (see IPCC AR5, Technical Summary, Figure TS.9).

The Geological Society of London Conference ‘Climate Change in the Geological Record’, May 2021

Watch the conference presentations and discussions here:

Day 1

Day 2

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