Monte Carlo versus blocking formations: why attributing heatwaves to climate change is still a gamble
Tens of thousands died in Russia last year in a 60-day heatwave, prompting inquiries into whether it could happen again and whether it is possible to predict this kind of event. But the growing body of climate attribution science appears divided over whether climate change caused the heatwave, making it tricky for policymakers to decide on future action based on the findings. Here we examine the different conclusions and the routes scientists have taken to get there.
Climate attribution is a relatively new area of climate science, in which scientists attempt to work out if changes in the climate system can be attributed to “natural or anthropogenic causes, or both”.
The approach has gained momentum in recent years, spurring a group of researchers from different organisations to join forces in 2009 to form the Attribution of Climate-related Events (ACE). ACE prepared a report on climate attribution for a meeting of the World Climate Research Programme in Denver, which is going on right now.
Climate change wasn’t the culprit
In a report released in March this year, the US National Oceanic and Atmospheric Administration (NOAA) say they weren’t able to attribute “the intensity of this particular heat wave to climate change”. Rather, they think natural climate variability caused the Russian heatwave – a conclusion reported with glee by skeptics.
The study concludes that the heatwave was down to a ridge of high pressure that blocked normal cooling phenomena such as storms from reaching the country from the West and allowed warm air to flow in, something we’ve looked at in relation to the UK and Russia.
The group searched through Russian temperature records over the last century and temperatures simulations from a suite of climate models for trends that could help explain the 2010 heat wave. They also ran model simulations based on actual 2010 observations for factors such as sea surface temperature and atmospheric carbon dioxide levels. This allowed the scientists to determine which of these factors might have contributed to the heat wave.
It concludes:
“Despite [the] strong evidence for a warming planet, greenhouse gas forcing fails to explain the 2010 heat wave over western Russia. The natural process of atmospheric blocking, and the climate impacts induced by such blocking, are the principal cause for this heat wave. It is not known whether, or to what extent, greenhouse gas emissions may affect the frequency or intensity of blocking during summer. It is important to note that observations reveal no trend in a daily frequency of July blocking over the period since 1948, nor is there an appreciable trend in the absolute values of upper tropospheric summertime heights over western Russia for the period since 1900.”
Or was it?
“These [temperatures] provided a baseline temperature trend. Parameters for random variability came from the extent to which each individual July was warmer or cooler than usual.
“After running the simulation 100,000 times, ‘we could see how many times we got an extreme temperature like the one in 2010,’ said Rahmstorf. ‘After that, the researchers ran a simulation that didn’t include the warming trend, then compared the results.’
“‘For every five new records observed in the last few years, one would happen without climate change. An additional four happen with climate change,’ said Rahmstorf. ‘There’s an 80 percent probability’ that climate change produced the Russian heat wave.”
Different methods
So what does it all mean?
“Mistakenly attributing an increased risk of an extreme event to climate change could, if natural variability is playing the major role, lead to poor adaptation decisions; for example, through allocating expensive resources toward preparing for a greater frequency of such events when they are set to become less likely [ – that is, if climate change trends mean frequency of some events decreases].”