[Reader-list] Extreme events

[Nagraj Adve]

Will this summer of extremes be a wake-up call?

This decade has been marked by a number of weather extremes – which
show how vulnerable our societies are

• 2010 could be among warmest years recorded by man


Stefan Rahmstorf
guardian.co.uk, Monday 16 August 2010 13.19 BST


This summer has been one of weather-related extremes in Russia,
Pakistan, China, Europe, the Arctic – you name it. But does this have
anything to do with global warming, and are human emissions to blame?

While it cannot be scientifically proven (or disproven, for that
matter) that global warming caused any particular extreme event, we
can say that global warming very likely makes many kinds of extreme
weather both more frequent and more severe.

For weeks, central Russia has been in the grips of its worst-ever
heatwave, which has caused probably thousands of fatalities. As a
result of drought and heat, more than 500 wildfires have raged out of
control, smothering Moscow in smoke and threatening several nuclear
facilities. Russia's government has banned wheat exports, sending
world grain prices soaring.

Meanwhile, Pakistan is struggling with unprecedented flooding that has
killed more than a thousand people and affected millions more. In
China, flash floods have so far killed more than a thousand people and
destroyed more than a million homes. On a smaller scale, European
countries like Germany, Poland, and the Czech Republic have also
suffered serious flooding.

Meanwhile, global temperatures in recent months have been at their
highest levels in records that go back 130 years. Arctic sea-ice cover
reached its lowest ever recorded average level for the month of June.
In Greenland two huge chunks of ice broke off in July and August.

Are these events connected? Looking only at individual extreme events
will not reveal their cause, just like watching a few scenes from a
movie does not reveal the plot. But, viewed in a broader context, and
using the logic of physics, important parts of the plot can be
understood.

This decade has been marked by a number of extremes. In 2003, the most
severe heatwave in living memory broke temperature records by a large
margin and caused 70,000 deaths in Europe. In 2005, the most severe
hurricane season ever witnessed in the Atlantic devastated New Orleans
and broke records in terms of the number and intensity of storms.

In 2007, unprecedented wildfires raged across Greece, nearly
destroying the ancient site of Olympia. And the Northwest Passage in
the Arctic became ice-free for the first time in living memory. Last
year, more than a hundred people were killed in bush fires in
Australia, following drought and record-breaking heat.

This cluster of record-breaking events could be merely an streak of
bad luck. But that is extremely unlikely. This is far more likely to
be the result of a warming climate – a consequence of this decade
being, worldwide, the hottest for a thousand years.

All weather is driven by energy, and the sun ultimately provides this
energy. But the biggest change in Earth's energy budget by far over
the past hundred years is due to the accumulation in our atmosphere of
greenhouse gases, which limit the exit of heat into space. Owing to
fossil-fuel emissions, there is now one-third more carbon dioxide in
the atmosphere than at any time in at least a million years, as the
latest ice drilling in Antarctica has revealed.

The changes in the planet's energy budget caused by solar variations
are at least ten times smaller in comparison. And they go in the wrong
direction: in recent years, the sun has been at its dimmest since
satellite measurements began in the 1970's. So, when unprecedented
extreme weather events occur, the prime suspect is naturally the
biggest atmospheric change that has happened over the past hundred
years – one that has been caused by human emissions.

The fact that heatwaves like the one in Russia become more frequent
and extreme in a warmer world is easy to understand. Extreme rainfall
events will also become more frequent and intense in a warmer climate,
owing to another simple fact of physics: warm air can hold more
moisture. For each degree celsius of warming, 7% more water is
available to rain down from saturated air masses. Drought risk also
increases with warming: even where rainfall does not decline,
increased evaporation dries out the soils.

The carbon-dioxide effect can also change the preferred patterns of
atmospheric circulation, which can exacerbate extremes of heat,
drought, or rainfall in some regions, while reducing them in others.
The problem is that a reduction in those extremes to which we are
already well-adapted provides only modest benefits, whereas the new
extremes to which we are not adapted can be devastating, as recent
events in Pakistan show.

The events of this summer show how vulnerable our societies are to
weather-related extremes. But what we see now is happening after only
0.8C of global warming. With swift and decisive action, we can still
limit global warming to a total of 2C or a bit less. Even that much
warming would require a massive effort to adapt to weather extremes
and rising sea levels, which needs to start now.

With weak action, like that promised by governments in Copenhagen last
December, we will be on course for 3-4C of global warming. This is
bound to outstrip the ability of many societies and ecosystems to
adapt. And, with no action at all, the planet could even heat up by
5-7C by the end of this century – and more thereafter. Knowingly
marching down that road would be insane.

We must face the facts: our emissions of greenhouse gases probably are
at least partly to blame for this summer of extremes. Clinging to the
hope that it is all chance, and all natural, seems naive. Let us hope
that this summer of extremes is a last-minute wake-up call to policy
makers, the corporate world, and citizens alike.

• Stefan Rahmstorf is Professor of physics of the oceans at Potsdam
University, and a member of the German Advisory Council on Global
Change. His latest book is The Climate Crisis (co-written with David
Archer).