It is a crucial question: is the climate crisis to blame for the extreme weather disasters taking lives and destroying homes around the world. But it has not been an easy one to answer. How much is due to global heating, how much is just the severe weather that has always happened?
The good news is that the scientific techniques used to untangle that question – called climate attribution – are now well established. The bad news is what they reveal: the studies show that the burning of fossil fuels has changed the climate so dramatically that heatwaves, floods and storms are now hitting communities with a severity and frequency never seen during the entire development of human civilisation.
What are attribution studies?
There are three approaches, used in combination to give the most reliable results. All compare the present with the past in order to calculate the increased frequency or severity of events. Weather data from the overheated present and the cooler past, when available, can be compared to see how many times more an extreme event happens today. Climate models can be used in a similar way to compare modern and preindustrial climates. Thirdly, climate models can also simulate the climate from, say, 1900 to the modern day, with slowly rising human-caused emissions. This enables scientists to detect trends in extreme weather, as well as the overall change in likelihood.
What have the studies found?
The most striking findings are of extreme heatwaves that would have been impossible without global heating, because they have no historical precedent and do not happen in model simulations without the added heat from human-caused climate change. That is as close to saying global heating caused the heatwave as makes no difference. At least 24 previously impossible heatwaves have already struck around the world, from Europe to North America, Africa and east Asia.
What about other events?
Many more extreme weather events have been made significantly worse, or more likely, by global heating. That means hotter heatwaves, more intense rains, stronger gale-force winds. Of the 744 attribution studies in a database produced by Carbon Brief, the most comprehensive that exists, three-quarters found that global heating had a significant impact.
Which extreme weather events are most supercharged by the climate crisis?
Heatwaves have been the most studied extreme weather events, with more than 200 analyses, and 95% were made more severe or more likely. Not a single one was made less likely.
What about floods and droughts?
Deluges of rain can be analysed relatively easily but floods are more complex, because their occurrence is also affected by human-built defences and the topography of the land. Nonetheless, of the 177 assessments of rain and flooding events, more than 60% were worsened by global heating, while 11% were made less likely, and the rest showed no influence or were inconclusive. Almost 70% of the 106 drought events were made more likely, with only one made less likely.
Can attribution analyses look at death tolls from extreme weather?
Yes. It is more complicated but a growing number of studies are estimating the influence of global heating on the impacts of extreme weather events, not just the events themselves. Of 33 such studies, 91% showed impacts made worse by global heating.
One found that one in three newborn babies who died due to heat in some countries would have survived if global heating had not pushed temperatures beyond normal bounds. Another found approximately 100,000 heat-related deaths in summer every year due to the climate crisis.
Other studies found that Hurricane Harvey would not have flooded 30%-50% of the US properties that it did in 2017 without global heating, and that four major floods in the UK would have caused only half the $18bn (£14.3bn) of wrecked buildings were it not for human-caused climate change.
The pre-existing vulnerability of communities is also a critical factor in how damaging climate impacts are. People in poorer nations with less resilient homes and fewer resources suffer far more than those in rich nations.
What about changes to snow storms and freezing snaps?
Almost 60 of these have been assessed, finding unsurprisingly that in most cases that global heating made such weather milder or less likely.
Can attribution studies be used to hold polluters to account?
Yes. Such studies are increasingly being used as evidence of responsibility for climate damage in landmark legal cases, such as Juliana v United States and Lluiya v RWE. Attribution science is also being used to inform negotiations over funding for the UN’s “loss and damage” fund, which would finance the rebuilding of communities after climate disasters.
What can be said if no attribution study has been done?
Scientists have the capacity to assess only a small fraction of all extreme weather events. Researchers at the World Weather Attribution initiative use criteria to decide which to analyse, including how many people are affected and how much damage has been caused, as well as the availability of data.
However, enough research has now been done to make very confident statements about most extreme weather events. Scientific methods can also give a real-time assessment of how much more likely a temperature was made by global heating anywhere in the world.
Extreme rainfall is also more common and more intense across most of the world, particularly in Europe, most of Asia, central and eastern North America, and parts of South America, Africa and Australia. For coastal flooding, every event is more likely than it would otherwise have been, because human-caused climate breakdown is driving sea level rise. Droughts and wildfires are also becoming more common and severe in many places.
Continuing attribution research will reveal even more about how much global heating has changed the world of extreme weather. But it is already very clear that the climate crisis is here and causing immense damage today.
