Extreme weather link ‘can no longer be ignored’

Scientists to end 20-year reluctance with study into global warming and exceptional
weather events

1.7.2011  (Independent)
By Steve Connor, Science Editor

Scientists are to end their 20-year reluctance to link climate change with extreme
weather – the heavy storms, floods and droughts which often fill news bulletins
– as part of a radical departure from a previous equivocal position that many
now see as increasingly untenable.


Climate researchers from Britain, the United States and other parts of the world
have formed a new international alliance that aims to investigate exceptional
weather events to see whether they can be attributable to global warming caused
by greenhouse gas emissions.

They believe that it is no longer plausible merely to claim that extreme weather
is “consistent” with climate change. Instead, they intend to assess each unusual
event in terms of the probability that it has been exacerbated or even caused
by the global temperature increase seen over the past century.

The move is likely to be highly controversial because the science of “climate
attribution” is still in the early stages of development and so is likely to be
pounced on by climate “sceptics” who question any link between industrial emissions
of carbon dioxide and rises in global average temperatures.

In the past scientists have been extremely reluctant to link a single extreme
weather event with climate change, arguing that the natural variability of the
weather makes it virtually impossible to establish any definitive association
other than a possible general consistency with what is expected from studies based
on computer models.

However, a growing number of climate scientists are now prepared to adopt a far
more aggressive posture, arguing that the climate has already changed enough for
it to be affecting the probability of an extreme weather event, whether it is
an intense hurricane, a major flood or a devasating drought.

“We’ve certainly moved beyond the point of saying that we can’t say anything
about attributing extreme weather events to climate change,” said Peter Stott,
a leading climate scientist at the Met Office Hadley Centre in Exeter.

“It’s very clear we’re in a changed climate now which means there’s more moisture
in the atmosphere and the potential for stronger storms and heavier rainfall is
clearly there.”

Kevin Trenberth, a distinguished senior scientist at the US National Centre for Atmospheric Research (NCAR) in Boulder, Colorado, also believes the time has come to emphasise the link
between extreme weather and the global climate in which it develops.

“The environment in which all storms form has changed owing to human activities,
in particular it is warmer and more moist than it was 30 or 40 years ago,” Dr
Trenberth said.

“We have this extra water vapour lurking around waiting for storms to develop
and then there is more moisture as well as heat that is available for these storms
[to form]. The models suggest it is going to get drier in the subtropics, wetter
in the monsoon trough and wetter at higher latitudes. This is the pattern we’re
already seeing.”

The Met Office and NCAR have joined forces with other climate organisations, including the influential
US National Oceanic and Atmospheric Organisation (NOAA), to carry out detailed investigations of extreme weather events, such as the
vast flooding in Pakistan last year, to see whether they can detect a climate
change “signal” as a likely cause.

A group of their researchers has formed a coalition called the Attribution of Climate-Related Events which is preparing a report on the subject to be published later this year at
a meeting of the World Climate Research Programme in Denver. They hope in future
to assess each extreme weather phenomenon in terms of its probability of being
linked with global warming and then to post the result on the internet.

“There is strong evidence if you look across the world that we are seeing an
increase in heatwaves and floods and droughts and extreme rainfall and extreme
temperatures,” Dr Stott said.

“The evidence is clear from looking at the observational records globally that
extreme temperatures and extreme rainfall are changing. But you can’t jump from
that and say that a specific event is straightforwardly attributable because we
know that natural variability could have played a part.

“We’ve been developing the science to be increasingly more quantitative about
the links and make more definitive statements about how the risk has changed.
You look sensibly about these things by talking about changing risk, or changing
probability of these events.”

Dr Stott had his colleagues have already carried out studies of the 2003 heatwave
in Europe, in which up to 35,000 people died of heat-related illnesses, as well
as the devastating UK floods in 2000 which cost £1.3bn in insurance claims and
destroyed 10,000 homes following the wettest autumn in England and Wales since
records began in 1766.

In both cases, the scientists found that the contribution of man-made greenhouse
gases to global warming substantially increased the risk of such extreme events
occurring. The group is also investigating the exceptional warm April in Britain
this year, which was the warmest since central England records were kept in 1659
and 0.5C warmer on average than the previous warmest April.

Also this year, an unprecedented number of tornadoes across the southeastern
US and the flooding of major rivers such as the Mississippi and Missouri led many
people to question whether they were exacerbated by global warming. In the past
scientists would have been reluctant to link single weather events such as these
with climate change, but Dr Trenberth believes this is wrong.

“I will not say that you cannot link one event to these things. I will say instead
that the environment in which all of these storms are developing has changed,”
Dr Trenberth told The Independent.

“It’s not so much the instantaneous result of the greenhouse effect, it’s the
memory of the system and the main memory is in the oceans and the oceans have
warmed up substantially, at depth, and we can measure that. I will assert that
every event has been changed by climate change and the main time we perceive it
is when we find ourselves outside the realms of the previous natural variability,
and because natural variability is so large this is why we don’t notice it most
of the time.

“When we have things that occur usually 4 per cent of the time start to occur
10 per cent of the time, that’s when we begin to notice. The main way we perceive
climate change is in changes in the extremes? this is when we break records.”

A report by the insurance company Munich Re found that 2010 was one of the worst
years on record for natural disasters, nine-tenths of which were related to extreme
weather, such as the floods in Pakistan and eastern Australia and heatwave in
Russia, which is estimated to have killed at least 56,000 people, making it the
most deadly natural disaster in the country’s history.

“This long-term trend can no longer be explained by natural climate oscillations
alone. No, the probability is that climate change is contributing to some of the
warming of the world’s oceans,” said Peter Höppe, author of the Munich Re report.

Making the connection

Tornadoes, US, 2011 More than 220 people were killed by tornadoes and violent storms that ripped
through south-eastern United States in April; 131 were killed in Alabama alone.
Fifteen people died in Tuscaloosa and sections of the city were destroyed.

Heatwave, UK, 2011 April was the warmest since 1659, when records in England began. Sun-lovers flocked
to St Ives, but fears of drought were raised. Rainfall in the UK that month was
only 52 per cent of the long-term average.

Drought, Brazil, 2005 The Amazon region suffered the worst drought in more than a century. The floodplains
dried up and people were walking or using bicycles on areas where canoes and river
boats had been the only means of transport.

Floods, USA, 2005 Katrina was one of the five deadliest hurricanes in the history of the US, and
it caused the destruction of New Orleans when levees were overwhelmed. Some 90
per cent of residents of south-east Louisiana were evacuated.


see also
By Steve Connor  1.7.2011
  • Steve Connor: It has been impossible to say these events were part of a bigger
    picture – until now