Extreme seasons

Seasons in the UK and Ireland have always been stable and mild compared to many parts of the world. Is climate change transforming the outlook?

Last year, dust storms ravaged China, cyclones tore through Fiji, mainland Europe and India experienced catastrophic flooding whilst an extreme heatwave across western North America sparked droughts and wildfires.

Extreme weather events like these are becoming more frequent and more intense. Human-caused climate change is now accepted as the most likely cause, but students may ask: what of the natural ebb and flow of our climate? The Earth has gone through multiple Ice Ages in the past – could we just be heading into the other natural extreme?

Earth’s long-term climate is driven primarily by Milankovitch cycles. These determine the amount of solar radiation absorbed by Earth and are driven by the shape of Earth’s orbit (eccentricity), the tilt of Earth’s axis with respect to its orbital plane (obliquity) and the wobble of Earth’s axis of rotation (precession). The obliquity cycle gives us seasons – the more extreme the tilt, the more extreme the seasons we get. The more extreme the seasons at the poles, the more likely any ice which forms in the winter is to melt in the summer. If polar ice doesn’t persist through the year, then we can’t cool into a glacial period. The eccentricity cycle can have a similar effect, controlling how close the Sun and Earth are at differing times of the year.

But Milankovitch cycles lead to changes in our climate over tens of thousands to hundreds of thousands of years. They cannot account for the current period of accelerated warming Earth has experienced since the Industrial Revolution (1.19 ˚C).

Over the past 60 years, physicists have refined computer simulations of the Earth’s climate to understand this rapid and continuing change. From this, a clear cause has emerged: carbon dioxide. Doubling carbon dioxide in any of these simulations leads to a 2.5–4 °C global temperature increase. More difficult has been teasing apart whether rising levels of carbon dioxide are due to natural changes (like volcanic activity) or human actions. To extract the ‘fingerprints’ of climate change, many theories, observations and simulations have been used with sobering results. We now know humanity’s emissions are having a significant impact on the world’s climate. But does this explain the extreme weather we are witnessing? Can the relatively slowly changing patterns of weather in an area (climate) affect the ferocity of specific events (weather), from hot days to hurricanes?

Attribution studies aim to answer this question. They feed a wide range of data and observations into two different computer simulations: one simulating the climate as it is today, and another simulating the climate as it would have been before the Industrial Revolution. Comparing the models lets researchers see the effect climate change had on a weather event.

For example, one study looked at the 2019 European heatwave, which saw a record 38.725 °C recorded in Cambridge. The researchers concluded that climate change made the heatwave about 10 times more likely, causing temperatures 1.5–3°C higher than they would have been without human influence.

Attribution studies also help shape our understanding of climate change and its impacts more generally. The planet warming up means more record-breaking hot days in more places. Higher temperatures lead to drier conditions, with moisture evaporating from water bodies and soil, leading to drought. This enhanced evaporation drives a more active water cycle: in the summer, more active thunderstorms can lead to flash floods when rain falls on dry ground whilst drought-stricken areas lose out on evaporative cooling, effectively making hot spells even hotter.

How does all this affect us? In summer (June–August), we should expect more hotter, drier weather with flash flooding and extreme summer heat. Warmer, wetter winters will be generally stormier, with depressions (low pressure weather systems) bringing heavier rainfall. Through continuing attribution studies of weather events and more sophisticated climate simulations, physicists will be able to better project future changes to our seasons and weather, helping us adapt to our changing weather - as well as motivating us to reach net zero faster.

Dr Benjamin Skuse Freelance science writer