Scientists go to extremes to understand climate change
Scientists have traditionally been reluctant to attribute individual extreme weather events such as floods, heat waves and heavy storms to climate change, but that could be about to change.
The Independent reports that researchers from the Met Office, the US National Centre for Atmospheric Research (NCAR) and the US National Oceanic and Atmospheric Organisation (NOAA) are joining forces.
The new coalition is called the Attribution of Climate-Related Events and will investigate individual extreme weather events to determine the probability that they have been caused or exacerbated by manmade climate change – known as “climate attribution”.
The group is currently preparing a report on climate attribution for a meeting of the World Climate Research Programme in Denver later this year.
This emerging branch of climate science has already had some success. For example, the European heat-wave in 2003, which is thought to have hastened the deaths of around 35,000 people, was found to have been made much more probable by the global warming brought about by manmade greenhouse gas emissions.
Another example to consider is the record-breaking spate of tornados that hit the US in April this year. They were closely followed by the sixth most deadly US tornado on record, which killed 151 people in Joplin, Missouri in May.
Media coverage of the event asked the inevitable question: Can this apparent increase in tornado activity be put down to climate change? In an attempt to clarify the issue the Climate Science Rapid Response Team (CSRRT) has produced a report that sums up the current scientific knowledge about tornados and climate change.
Interestingly, records show that whilst US tornado activity has increased, the increase is largely confined to the weakest class of tornados, whilst the number of violent tornados has decreased over the same period.
Trying to quantify how much of this increase is down to manmade global warming as opposed to natural climate variability is not easy, however. This is because observational data may not always be reliable, computer models struggle to simulate tornados, and we simply do not have a good understanding of how human actions can affect tornado activity.
Climate models provide perhaps the best means of determining a link between warming and tornado activity. This might seem strange given that the models are so poor at simulating tornados. However, they can provide information about changes in the large-scale environmental conditions that govern tornado behaviour.
The two key factors are convective available potential energy (CAPE), which represents low-lying moist warm air, and vertical wind shear, which is the difference between wind strength at ground level and higher levels in the atmosphere. High CAPE and vertical shear cause thunderstorms, from which tornados can develop. Vertical shear is also linked to the intensity of a tornado. The tornado spate in April/May this year was linked to high CAPE and vertical shear.
Models project that CAPE is likely to increase over the coming century, but vertical shear is likely to decrease. The influence of higher CAPE is likely to win out over the lessened vertical shear causing more thunderstorms to form. However, it is not yet clear what impact this will have on tornado frequency and intensity. It is clear that further research in this area is needed, to provide better data and higher resolution models.
It is vital that we can determine quantitatively whether the risk of extreme weather events has increased due to human activity given the level of human suffering and destruction caused. As Dr Peter Stott, UK Met Office, puts it:
“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.”
The science of “climate attribution” clearly still has some way still to go, but the outlook is good.