The Met office have provisionally put April as the hottest in the UK since records began. The month was also unusually dry, and with the warm, dry weather continuing into May, forecasters suggest that it is set to continue, prompting Caroline Spelman, the Environment Secretary, to voice concerns over the impacts of water stress on British crop production.
So what has caused the warm, dry weather? According to a BBC Weather Centre Spokesperson, it’s persistant high pressure systems which have dominated the weather pattern over the past months. High pressure ‘blocks’ the weather pattern, keeping it locked in place.
When high pressure blocking systems disrupt more typical weather patterns, it can have dramatic effects. When blocking systems prevent rain-bearing weather fronts from the Atlantic reaching Britain, this tends to lead to warm, dry weather – and heatwaves. Heatwaves over Europe in 2003 and over Russia in 2010 resulted from similar types of blocking high systems.
How abnormal is this, and should we be drawing direct links to the climate changing? Not yet, according to Dr Peter Stott – head of climate monitoring and attribution at the Met Office – who points out:
“The difficulty comes in attributing variability and changes in blocking activity and this is a subject of current active research, and it may well be the case that studies find that there is no evidence for changes in blocking frequency.”
This was certainly the case for the Russian blocking system during 2011 – researchers concluded that, whilst they could not entirely rule out a contribution from climate change, naturally occurring processes were the main cause of the heatwave.
On the other hand, a team of scientists including Dr Stott found that human influence had impacted the European heatwave of 2003, more than doubling its likelihood.
So the same kinds of events don’t necessarily have the same causes, as Dr Stott notes:
“This illustrates an important point about this so-called “event attribution”, namely that scientific evidence that establishes a human influence or the lack of a human influence on a particular weather event does not mean that this applies to other types of event.”
The IPCC AR4 report (2007) found that it is very likely that there will be more warm spells over most land areas over the 21st century due to manmade climate change.
April’s record temperatures are certainly in line with that trend. However, this does not mean that April’s warmth can or should be attributed to climate change. As Dr Stott puts it:
“Regionally it depends much more on the interplay of variability and climate change, so that while the global pattern of extreme weather emerges more and more clearly over the globe during the course of the 21st century, in any one place there could well be changes contradictory to the overall trend or associated much more with natural climate variability.”
The attribution of a single weather event to a specific cause such as manmade greenhouse gas emissions is not an easy task. It requires comprehensive records of long-term trends in observational data, and the use of climate models to determine how the climate is changing and how it might have behaved without any manmade greenhouse gas emissions.
Scientists are trying to develop the capability to routinely attribute weather events to climate change. However, as it stands, the conclusions we can draw from a warm spring, or a cold winter, are limited.