Placing our current ‘hyperthermal’ in the context of rapid climate change in our geological past

It is widely recognized that anthropogenic climate change and ocean acidification resulting from the emission of vast quantities of CO2 and other greenhouse gases pose a considerable threat to ecosystems and modern society. Global temperatures are already warmer today than at any time in at least the last 2000 years [1], and unabated use of fossil fuel will cause continued warming and sea-level rise potentially for millennia as the climate system slowly adjusts to the enhanced greenhouse effect [2]. Exactly how the climate will respond to this anthropogenic forcing is currently uncertain because our understanding of the climate system is incomplete. There are the things, however, we know we know. For instance, that as we increase the concentration of CO2 in the atmosphere the climate will warm [3]. Then there are the things we know we do not know, such as the exact value of climate sensitivity and the extent to which it depends on background climate state (e.g. [4]). Then there are the things we know nothing about—the unknown unknowns—which have the potential to take future climate into unimagined directions. Much of the research into predicting future warming involves the use of complex numerical models, climate models, which encapsulate the state-of-the-art understanding of the modern climate system. While these tools can inform on the ‘known unknowns’, albeit imperfectly as these are often too uncertain to parametrize directly or are emergent properties of the model that are hard to test against observations, they are completely blind to the unknown unknowns as these are by definition not quantitatively represented in any model. Consequently, we urgently need to find alternative tools other than models to investigate them.

The climate has often changed in the past, and these ancient climate events naturally included the response of the climate system to all the feedbacks in operation—even if we currently do not know anything about what these mechanisms might be. Studying the behaviour of the system during these natural climate cycles is therefore one of the only ways to perform a true test of the climate models and whether the system they encapsulate behaves like the real climate system during large CO2 emission events. Although there are few, if any, actual analogues of anthropogenic climate change in the geological record, there are a number of examples of abrupt climate change that are particularly useful in testing the limits of our best climate models. These include the hyperthermal events of the Phanerozoic. What these events are, what caused them, what effects they had and what they can teach us about our climate system, are the subject of this special issue.

Foster G. L., Hull P., Lunt D. J. & Zachos J. C., 2018. Placing our current ‘hyperthermal’ in the context of rapid climate change in our geological past. Philosophical Transactions of the Royal Society A. 376 (2130): 20170086. doi:10.1098/rsta.2017.0086. Article.

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