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Carbon Brief Staff

Carbon Brief Staff

08.07.2013 | 10:25am
ScienceWMO report finds a decade of climate extremes
SCIENCE | July 8. 2013. 10:25
WMO report finds a decade of climate extremes

From hotter days and heat waves to droughts and floods – the first 10 years of the millenium have been described as “a decade of climate extremes” in a new report from the World Meteorological Organisation (WMO).

Although extremes occur naturally as the climate fluctuates, greenhouse gas emissions are likely changing things too, the report finds. But how has this nuanced message been communicated in the media? And what about the other factors that affect how costly these extremes are – both in economic and human terms?

A decade of extremes

The WMO’s report documents how rainfall and temperature trends between 2000 and 2010 compare to previous decades, and highlights some of the most extreme individual weather events over the period.

It states, for example, that “the first decade of the 21st century was the warmest decade recorded since modern measurements began around 1850” and that rainfall was “above average.” In many countries, the record for the hottest day was broken.

There were a number of other important messages in the report – the WMO said that droughts affected more people than any other disaster, while floods are the extreme which occurred most frequently. The number of deaths associated with heat waves rose, while those linked to flooding fell.

What’s natural and what’s not?

When it comes to the causes of these extremes, the WMO describes a mix of natural processes and human influence:

“Many of these events and trends can be explained by the natural variability of the climate  system. Rising atmospheric concentrations of greenhouse gases, however, are also affecting the climate.”

Some of the media coverage of the report avoided digging into this issue of attribution entirely. Many reports implied that climate change was to blame, even if it wasn’t stated. Some mentioned the fact that natural variability was a factor too, but by and large it was glossed over.

A Reuters’ article stood out as one of the few which gave a balanced view on the causes of extreme weather. In the second paragraph, it neatly summarised:

“[The WMO report] said many extremes could be explained by natural variations – freak storms and droughts have happened throughout history – but that rising emissions of man-made greenhouse gases also played a role.”

But it’s not just climate change complicating the picture of what’s natural and what’s not. There’s another human element determining just how severe extremes are.

Making matters worse (or better)

Climate extremes – and the impacts they have – may well alter as climate change interferes with temperature and rainfall patterns. But how humans prepare for and deal with temperature and weather extremes also makes a big difference to how deadly and costly they are.

A good example of this is the case of flooding. The WMO report finds that although rainfall was above average between 2000 and 2010, the number of deaths associated with flooding fell. The reason? People are better prepared, and there are better early warning systems in place.

But it’s not all good news. More properties and businesses are at risk of flooding, the report says. That’s a function of population growth and a direct consequence of concreting over surfaces, which means rain can’t drain into the soil.

Are things changing?

Talking about extreme weather events isn’t plain sailing – with a diversity of causes and a diversity of types of extremes, there’s no simple satisfying answer as to what’s causing them, whether they are getting worse and why.

It is logical that extremes like hottest days would get worse as greenhouse gases trap more warmth. Some research suggests heat waves like the one which affected Europe in 2003 are now twice as likely, thanks to humans’ greenhouse gas emissions. And there are laws of physics which say a warmer atmosphere should hold more moisture and lead to more rainfall.

It’s harder to say for certain how climate change is affecting other extremes, such as storms, droughts and floods. Here, the processes are more complex, and there’s less data to look for trends in.

As ever, mapping the causes of extremes simply remains challenging. The WMO report makes the case that for some types of extreme weather, more measurements may be needed before attribution can improve.

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