Heat waves: How the geological record helps us improve climate predictions
Written by: Basil Davis
The current heat wave affecting the southwestern USA this week is just one of many that have taken place across the Northern Hemisphere this year, including China, India, Pakistan, Europe and mid-western USA. What has been particularly worrying for climate scientists is not only the size of the areas over which records have been broken, but also the size of the margin with which the new records have exceeded the old.
The heatwave that affected the UK in July established an all-time temperature record of 40.3C that exceeded the old record by 1.6C, set just 3 years ago, and records were broken at many locations by an extraordinary 3-4C. These new records were set at a comparable latitude to the record-breaking heat that affected the Pacific Northwest last year when the previous all-time temperature record in Canada was exceeded by an incredible 4.4C.
Climate model simulations indicate that as the planet warms so the frequency and intensity of heat waves can be expected to increase, but what is more difficult to predict is where and when these heat waves will occur. For instance, last year’s record Canadian temperature in Canadas occurred in June, which is not even a summer month, and at 49.4C it also exceeded all-time records for ‘hotter’ regions much further south such as Las Vegas, Nevada whose record is 47.2C. In the UK, prior to the record heat wave, climate models had already suggested that temperatures would eventually exceed 40C but not for at least another decade, while even short-term weather models had only given it a 10% chance in forecasts made just a week before the heat wave arrived.
Climate models provide the only way to predict future heat waves, but when these models are compared to observations, we find that they tend to underestimate temperature extremes. The reason for this is not entirely clear, but is likely to be the result of models failing to fully reproduce processes within the climate system that drive extremes as our planet warms, such as changes in the atmospheric and ocean circulation.
One way that climate scientists are trying to improve these models is to look at how the climate system organized itself during warm climates in the Earth’s past. These warm climates are not exact analogues of our future Earth because the warming was not driven by the rapid release of greenhouse gases from burning fossil fuels, but they do represent very different climates from recent modern history, which is the period normally used to test and develop climate models.
The Past Global Changes (PAGES), an international paleo-science co-ordinating body, is at the forefront of this work, co-ordinating research into past warm climates through a number of different working groups.
The PAGES-PMIP Working Group on Quaternary Interglacials (QUIGS) is investigating climates during past Interglacials. These relatively short periods of warm climate occurred with regular frequency between longer, cooler, glacial periods over the Quaternary period of the last 2.6 million years. This working group is looking at how and why these warm Interglacials occurred by using compilations of palaeo-climate records from marine, terrestrial and ice archives to compare with climate model simulations.
The Climate Variability Across Timescales (CVAS) working group is looking primarily at climate over the most recent Holocene Interglacial warm period, and particularly how climate variability has changed throughout this time at different locations. This work will help us understand shorter-term natural climate changes and the ability of climate models to reproduce them. These natural changes are likely to occur on top of the future global warming trend, helping amplify or mitigate rising temperatures from anthropogenic greenhouse gases.
The Pliocene and Miocene climate variability over glacial-interglacial timescales
(PlioMioVAR) working group is examining the warm climate of the period from 2.6-23 million years ago. This was a time when temperatures and greenhouse gas concentrations reached levels higher than the present day, and global ice cover was much reduced. The group is working with a combination of climate modelling and reconstructions of the climate at this time using terrestrial and marine records to understand the main drivers of climate change over this period.
As we travel further back in time, these past climates look less and less familiar to us, just as our future climate is likely to become less and less familiar to us as we leave the climate conditions that we have enjoyed over the last 100 years or more. PAGES supports researchers who study these past global changes, helping to improve our ability to foresee future global change, including the heatwaves that we will inevitably increasingly experience as our planet warms.
Climate model experiments show that the inter-annual variability in temperature is likely to be much higher in a future warmer world driven by higher CO2 than it was during the mid-Holocene thermal maximum that occurred around 6000 years ago (Rehfeld et al. 2020 https://pastglobalchanges.org/publications/129475).