New study linking hot summers to climate change: scientists react
- 08 Aug 2012, 15:00
- Verity Payne
New research linking hot summers to climate change from climate
scientist and activist James Hansen has prompted quite a response
from the media and other scientists. So what does the paper really
tell us, and what are the arguments about?
You've may have heard the so-called loaded dice analogy for
climate change. The idea comes from a 1988
research paper by Hansen and colleagues, where they posited the
notion of a die with two red faces representing hot summer
temperatures. As climate change progresses, they wrote, there might
be three or even four red faces on the dice, loading the odds in
favour of more frequent hot summers and heatwaves.
research from Hansen and colleagues provides evidence that the
climate dice have indeed become loaded over the last 30 years. The
research comes in the wake of a summer of unusual weather events -
a US summer heatwave, warm weather over Greenland, not to mention
the wettest British June on record. So
this research has provided
material for a media and blogosphere already pretty fixated on
extreme weather and its precise links to climate change.
In this latest research paper Hansen and colleagues studied
average summer temperatures over land areas from the last 30 years
as archived in the NASA Goddard Institute for Space Studies (GISS)
temperature record, compared to the average summer temperature for
land areas for the previous 30 year period (1951 to 1980).
We take a look at how Hansen's new paper compares to other
research on climate change and heatwaves, and the responses of
other climate scientists to this new research.
Heatwaves becoming more frequent
Hansen and colleagues find that Earth's land areas have become
more likely to be subject to hot summers over the last 30 years,
and that extremely hot summers - experienced by less than one per
cent of Earth's land area between 1951 and 1980 - now cover as much
as ten per cent of Northern Hemisphere land area each year.
These basic findings are very similar to previous research in
this area. The IPCC's Special Report into Managing the Risks of
Extreme Events and Disasters to Advance Climate Change Adaptation
(SREX) - considered to be
best current synthesis on extreme weather and climate change -
found from observations since 1950:
"It is very likely that there has been
[...] an overall increase in the number of warm days and nights, at
the global scale, that is, for most land areas with sufficient data
[...] In many (but not all) regions over the globe with sufficient
data, there is medium confidence that the length or number of warm
spells or heat waves has increased."
The SREX review also suggests that in the future:
"It is virtually certain that increases
in the frequency and magnitude of warm daily temperature extremes
and decreases in cold extremes will occur in the 21st century at
the global scale. It is very likely that the length, frequency,
and/or intensity of warm spells or heat waves will increase over
most land areas."
So Hansen and colleagues' paper agrees with previous research
suggesting that warmer summers are becoming more common and this
trend is likely to continue.
Would recent heatwaves have occurred in the absence of
Hansen and colleagues also conclude in their paper that the
heatwaves in Texas 2011, Moscow 2010, and France 2003 "almost
certainly would not have occurred in the absence of global
warming". They argue that the occurrence of such extremely hot
summers was negligible between 1951 and 1980, so that sort of
weather is essentially the emergence of a new category.
This interpretation is the point on which Hansen and colleagues
seem to differ from most previous research in this area, and that
has prompted criticism from other climate attribution experts.
Martin Hoerling, research meteorologist at the US National
Oceanographic and Atmospheric Administration's Earth Systems
Research Laboratory, tells
Live Science blog that Hansen and colleagues' interpretation is
Peter Stott, Head of Climate Monitoring and Attribution at the
UK Met Office seems to agree:
"While we can provide evidence that the
risk of heatwaves has increased, we cannot say that the chances of
such heatwaves were negligible before global warming set in."
This, Stott says, is because the heatwaves in Moscow 2010 and
Texas 2011 were both associated with unusual large-scale weather
patterns. The Moscow heatwave involved a blocking high pressure
system, and Texas was affected by a La Niña in 2011.
"[R]ecent research, not cited by
Hansen's paper, has shown that anthropogenic climate change has
increased the odds of record breaking temperatures when such
unusual weather patterns are set up. Further research is needed to
understand how climate change could be affecting such aspects of
climate variability as the position of the jet stream. Therefore it
is the interplay of variability and climate change that
Hansen and colleagues do mention the effect of large-scale
weather patterns in their paper, saying:
"Certainly the locations of extreme
anomalies in any given case depend on specific weather patterns.
However, blocking patterns and La Niñas have always been common,
yet the large areas of extreme warming have come into existence
only with large global warming. Today's extreme anomalies occur as
a result of simultaneous contributions of specific weather patterns
and global warming."
So Hansen and colleagues concede that weather patterns play a
role in extreme heat events, but they argue that the extreme heat
events wouldn't have happened in the absence of climate change.
Hoerling disagrees, telling Live Science:
"The weather patterns responsible for
most of today's heat waves would have happened regardless of
human-induced climate change"
Hoerling also suggests that reducing atmospheric carbon dioxide
to the level Hansen says is safe "would not eradicate heat
Another problem with Hansen and colleagues' interpretation,
according to Myles Allen of the University of Oxford, is that
"[I]mplies that until we do see weather
events that would have been exceedingly unlikely without global
warming, we are not seeing the consequences of global warming in
the weather. This is incorrect. The odds of getting a six with an
unloaded dice are not exceedingly small. But this doesn't mean we
can't increase them by loading the dice, and if you roll a six with
a loaded dice, there is clearly a sense in which that outcome was
affected by the loading even though there was a good chance that it
might have happened anyway."
Once again the range in scientific expert opinion on the links
of extreme weather to climate change has been thrust into the
spotlight in something of a media circus. But is it really
appropriate to have these sorts of discussions playing out in the
media? After all, what is the public to think about this?
"I understand there are many 'climate
communicators' who feel probability is too abstract, and that we
need to be able to point to damaging weather events that would not
have occurred without human influence on climate for the public to
take this issue seriously, but I think this underestimates the
public. People understand risk much better than we give them credit
Realistically, all that scientists are able to ask is whether
the odds of a particular weather event have been altered over the
last century as the world's average temperature has risen.