Britain, as you may have noticed if you’re reading from there, has experienced some extreme weather in recent weeks. Floods have followed drought, with new records for rainfall in May and June. We’re not the only place facing bizarre weather. The US is halfway through another summer of extremes, with over 3,000 new record temperatures for June and 2,000 for July so far.
It’s natural to ask if the extremes that we’re experiencing are just freak incidents or part of climate change. Until recently, the answer would have been ‘it’s impossible to say’. We know that, in theory, warmer climates would see more precipitation and more extremes. In reality, it’s very hard to attribute specific weather events to climate change when there are so many other factors.
That’s beginning to change. As the body of science around climate change grows, scientists have been able to identify key areas that aren’t well understood and focus on them. The interplay of weather patterns and longer climate trends has been one such area of focus, bringing together meteorology and climate science to work across disciplines. That research is now paying off, and we’re beginning to see new studies with a much more sophisticated understanding of weather and climate.
As an example, a briefing from NOAA (pdf) this week looks at six extreme weather events in 2011 and assesses whether climate change was a factor or not. The floods in Thailand, it concludes, were not driven by climate change. Neither was Britain’s cold winter in 2010/11 – that was simply “a rare weather event”. On the other hand, the failed rains and resulting famine in East Africa was consistent with warming trends and climate change was probably a cause.
Some events aren’t so easy to call, such as the drought in Texas. “While we can provide evidence that the risk of hot and dry conditions has increased,” say the authors, “we cannot say that the 2011 Texas drought and heat wave was “extremely unlikely” to have occurred before this recent warming.”
This sort of language is important – ‘likely’ or ‘probable’ is still all we can say, because this is a matter of probabilities. As the climate warms, the odds of warm or cold temperatures shorten or lengthen accordingly. The Met Office describes the difference between two months in Britain’s winter conditions: “The extreme warm average temperature in November 2011 is 60 times more likely to have occurred than in the 1960s. The change in odds of the extremely cold December was considerably less, however, being only about half as likely. Even without climate change, unusual circulation patterns can still bring very cold winter months.”
Climate change isn’t progressing in a linear fashion, but in a complex dance of trends and patterns that sometimes reinforce and sometimes compete with the gradual warming of the earth. We’re understanding that better all the time.