As the Earth’s temperature rises, extreme weather events that once were a rarity are now becoming common occurrences around the world. Incidences of dangerous heatwaves, major hurricanes, floods and other natural disasters are on the rise, influenced by new temperature records and the effect this has on weather patterns globally. A key question for scientists is understanding the connection between extreme weather and climate change, to inform our expectations of the weather we are experiencing today – and what dangers may lie ahead in the future.
‘There is very solid evidence that heat extremes are occurring more often than they used to,’ says Dr Karsten Haustein, a researcher in the World Weather Attribution team at Oxford’s Environmental Change Institute (ECI). ‘Temperature extremes, which could be defined as hot days over 30°C in northern Europe, are definitely increasing. This is the root cause for heatwaves, with knock-on effects for the water systems responsible for droughts and flooding.’
Dr Friederike Otto, acting director of ECI and leader of the World Weather Attribution team, has been working to analyse the effects of climate change on weather for almost a decade in order to find out how much of the increase in extreme weather can be attributed to climate change. ‘We use weather observations and climate models, the same models used to do the weather forecast,’ explains Dr Otto. ‘By running two scenarios, one that includes the current amount of greenhouse gases in the atmosphere, and one with the human-caused fraction of the greenhouse gases removed, we can see what the world might have been like without man-made climate change.’
This process makes it clear how much our weather is affected by climate change. ‘For example, we looked at the UK heatwave in July 2019, which would have been a once-in-40-years event,’ says Dr Otto. ‘However, with the current level of greenhouse gases in our atmosphere, we are looking at a one in eight likelihood – thus, climate change made that heatwave five times more likely.’
While some people may enjoy the increasingly hot summers in the UK, it is not only northern Europe that is experiencing rising temperatures. ‘The duration and severity of heatwaves are expected to get worse almost everywhere in Europe, North America, Russia, Australia, Southern Africa and parts of South America,’ says Dr Haustein. ‘Unfortunately, for tropical regions, this also means that the hot and moist conditions are exacerbated, with poorer – mostly Central African – countries to bear the brunt of climate change with very limited resources to adapt.’
While heatwaves are a danger in themselves, they also play a role in increasing the likelihood of other extreme weather events like flooding and droughts. ‘Climate models and theoretical underpinning suggests an increase in both flood and drought risk, in many cases – counterintuitively – at the same time,’ says Dr Haustein. ‘Since warmer air has a higher capacity to hold water, it takes longer before it rains. If it eventually does rain, the rainfall has the tendency to be more torrential than it would have been without human-induced climate change.’
This effect will not be spread evenly around the world, however. ‘Another way rainfall pattern can be influenced is by altered atmospheric circulation – basically, where and how weather systems develop. In contrast to the rising temperatures observed everywhere, this effect is very uneven from place to place and season to season,’ explains Dr Otto.
Unpredictable weather systems present a danger of their own. ‘We need to think about vulnerability and exposure,’ says Dr Otto. ‘Droughts in East Africa are a good example. Climate change is not playing a big role there yet, but the region is affected by a strong natural variability in rainfall year-to-year and a highly vulnerable population. Many regions in the world today are already struggling to adapt to the present climate; thus small additional changes can have dramatic consequences. Whether an extreme weather event turns into a disaster depends on how well prepared we are. And in every society, it is the most vulnerable that pay the price of a changing climate.’
So how can rising temperatures be stopped? ‘Even if we decrease emissions, climate change will continue to occur, including the trend for more extreme weather events,’ says Dr Haustein. ‘This is due to the greenhouse gases already in the atmosphere. That said, every avoided tonne of carbon is reducing warming and therefore counts. The more and the faster we decrease global emissions, the fewer additional instances of extreme weather we are going to have in the future.’