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
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
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
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
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
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
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.
"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
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".
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
Or was it?
by Stefan Rahmstorf
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
, 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
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
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
He thinks one of the reasons why the two results are different is
correction for the urban heat
- which means cities are generally warmer than
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
"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