A pre-IPCC recap of the links between extreme weather and climate change
With just over a week to go until the UN’s new climate report is released, media stories are beginning to be written anticipating the report, and a series of leaks from the notoriously leaky review process have provided ample fodder for those who want to put their own spin on things.
Today’s Telegraph gets a few things wrong in examining what scientists know and don’t know about the link between extreme weather and climate change.
Yesterday, the paper uncritically reproduced the argument of a piece in the Mail on Sunday which claimed that the IPCC’s estimate of how much the world has warmed since 1951 has halved since the last report in 2007.
That claim appears to be wrong, but the Telegraph piece this morning repeats some of the arguments – as well as some new ones about the link between extreme weather and climate change.
Hurricanes, droughts and floods
Let’s focus on what the new IPCC report apparently says about rising global temperatures and hurricanes, droughts and floods. Writing in the Telegraph, journalist Bruno Waterfield says:
“The EU has often linked extreme weather events to global warming after the IPCC said six years ago that it was more than 50 per cent sure that hurricanes, flooding and droughts were being caused by manmade global warming. That figure is expected to be revised down to less than a 21 per cent certainty that natural disasters are caused by climate change.”
Looking at hurricanes and droughts specifically, the last IPCC report in 2007 said it’s likely there has been an increase in both since 1970, and that it was at least 50 per cent certain that humans had contributed to the change.
The IPCC’s 2007 assessment of recent trends in extreme, human influence on the trend and projections to 2100. Note “more likely than not” corresponds to a likelihood of between 50 and 100 per cent. Source: IPCC 4th Assessment Report ( AR4)
Judging by the draft chapters of the new report leaked online last year, more up to date information and new ways to analyse data mean scientists are now less certain there has been a worldwide increase in either type of event.
It’s fair to say, then, that scientists have less confidence about the role of humans in droughts and hurricanes in the last few decades. We’ve searched extensively, but it’s not clear where the 21 percent figure cited in the Telegraph comes from – which it suggests applies to “all natural disasters”. The journalist told us that it came from a piece by Bjorn Lomborg, available here – but there’s no more information. We’ll update if we find out.
It’s worth noting the new IPCC report is not new science as such, but a mega-summary of the state of the scientific literature on climate. A large part of the research on extreme weather since the last report in 2007 has already been documented in the IPCC’s Special Report on Extreme Events ( SREX).
Back in 2012, SREX stated that scientists had only medium confidence that droughts had increased in some areas in the last few decades, dropping to low confidence for hurricanes.
While a lack of measurements in some parts of the world has made it difficult to spot trends in the past, drafts of the new report suggest a warmer climate is still likely to mean more droughts and a shift towards more intense hurricanes in the coming century.
Extreme to extreme
Turning to flooding, the picture is more complicated. At the moment scientists don’t have enough data to make conclusive statements about changes in the last few decades or about future flooding.
But the same was true in the 2007 report, in which the IPCC said there had been no robust global trend in the occurrence of floods, although there are large differences from region to region. The 2011 SREX report added that there was no detectable increase in the magnitude either. If AR5 repeats similar statements, they will be in line with what scientists are saying already.
What about other types of extreme events? The drafts of the new report – which it’s worth noting is still subject to change – pull together strong evidence that temperature extremes, like warm days and heatwaves, and heavy rainfall events have become more common since 1950, and that we can expect to see more in the coming century.
Alister Doyle from Reuters noted the distinction between different types of extreme event in the draft report when he broke the story of the leaked summary last month, saying:
“The new study will state with greater confidence than in 2007 that rising manmade greenhouse gas emissions have already meant more heatwaves. But it is likely to play down some tentative findings from 2007, such as that human activities have contributed to more droughts.”
Understanding changes in extreme weather, such as heatwaves, storms, floods and droughts, is important because of their impact on society. Part of that is being clear that rising temperatures affect different types of extreme events in different ways – it’s not a one size fits all situation.