Monte Carlo versus blocking formations: why attributing heatwaves to climate change is still a gamble

  • 27 Oct 2011, 16:26
  • Ros Donald
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."

But, inconveniently for the skeptics, this doesn't mean that climate change won't play a role. The study warns that increased background temperatures caused by climate change will make extreme heatwaves like the one last year more likely, with the probability rising "from 1 per cent in 2010 to 10 per cent or more by the end of this century".

A 2003 study of heatwaves in Europe also concludes that human activity is increasing the likelihood of heatwaves, but says it's not possible to make "deterministic" links between climate change and heatwaves.

Or was it?

A new study by Stefan Rahmstorf and Dim Coumou at the Potstam Institute for Climate Impact Research (PIK) differs from previous work. Rahmstorf and Coumou say there is an 80 percent chance that the Russian heat wave wouldn't have occurred without climate warming.

PIK's findings rely on a computational process called the Monte Carlo simulation, which was developed during the 1940s by physicists working on the Manhattan project and named after the city's casino, and designed to apply the laws of probability and statistics to the natural sciences.

These simulations are used to reveal all the possible outcomes of different inputs, and are used for a variety of purposes including calculating business risk. By repeatedly testing a Monte Carlo model, involving both defined and random influences, trends emerge.

According to Rahmstorf, quoted in Wired, the researchers created a simulation made from average July temperatures in Moscow.

"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."

A Russian group is also researching whether the heatwave is connected to climate change. According to an abstract they produced, their work so far appears to agree with PIK's findings.

Different methods

The key difference between the two results is that NOAA found there was no warming trend in Moscow in July over the past century or so, while the Potsdam report found a strong warming trend. Rahmstorf tackles the differences between his study and the NOAA paper in a blog post.

He thinks one of the reasons why the two results are different is correction for the urban heat island effect - which means cities are generally warmer than the countryside.

According to Rahmstorf, the temperature data NOAA used over-compensates for the urban heat island effect in Moscow in July because it's mostly a winter phenomenon. He says: "This unrealistic adjustment turns strong July warming into slight cooling." In contrast, PIK used unadjusted temperature data.

So what does it all mean?

In a nutshell, it's too early to say, as scientists are still trying different techniques. As ACE's report points out, the different messages we receive on attribution can be pretty confusing. Meanwhile, the need for policy responses to extreme weather events that could lead to severe hardship means scientists are under pressure come up with answers.

Jumping to conclusions in an area where scientists are still trying out new ways to measure attribution is risky, however. ACE warns:

"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]."

But one thing we can see from the data so far is that while it's still unclear whether we can attribute specific events to climate change, warming surface temperatures are increasing the likelihood of extreme weather events. Herein, for the moment, is the real policy story.

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