Many readers will be returning to work after their summer break. For those of us in the UK, this has been an extraordinary few months, encompassing both only the second royal diamond jubilee celebration in our history and London’s hosting of the Olympic Games (and now, the Paralympics).
It’s also been a fairly extraordinary season for weather across much of the northern hemisphere. After a second dry winter, springtime brought that most English of crises, hosepipe bans, across large areas of the country. Water shortages, we were assured, would be with us for some time. In March, the Met Office was still suggesting that there would be several more months of below-average rainfall.
But instead of this, much of southern Britain experienced much higher rainfall than expected. April was the wettest on record, as was the entire April to June period. Until aquifers and reservoirs had been replenished, summer 2012 was being talked about as the wettest drought ever. Latest reports are that this will have been the wettest summer in the UK for a century. Although this season has been extreme, statistics show that five of the last six summers (including the present one) have had below average sunshine, and all six have been wetter than average. The British Isles are not alone. Much of north western Europe has had a miserable summer and large swathes of the USA are still in the grip of heat waves and drought.
Predictably, there has been considerable debate about what causes these weather patterns and how they are influenced by global warming. Meteorologically, the primary cause of the extremes on both sides of the Atlantic is the route taken by the northern hemisphere jet stream, effectively locking in prevailing weather patterns. It’s not necessarily that more or less rain is falling overall, but that its distribution has been significantly skewed.
Other major influences on weather patterns are recurrent fluctuations in ocean temperature distributions. The best known is El Niño/La Niña (more correctly known as ENSO: the El Niño Southern Oscillation), which refers to a periodic warming or cooling of the surface temperature of the eastern Pacific and linked changes to western Pacific atmospheric pressure in the Tropics. This has a direct impact on temperatures and rainfall, particularly along the eastern seaboard of North and South America.
In a similar way, the North Atlantic Oscillation (NOA) is a cyclic change in the relative atmospheric pressure over this ocean which influences wind patterns and storminess. Much of sub-Saharan Africa is plagued by a recurring cycle of drought, and the monsoon upon which many Asian farmers depend is not as dependable as people would like. It does not always arrive on time and may bring either too much or too little rain.
Climatic patterns on both a local and global scale are subject to these and other influences; factors which have names and whose effect can be measured and recorded, but the root cause of which is poorly understood, if at all. Cause and effect is almost impossible to attribute in a system where so many factors are inter-dependent. Patterns like ENSO look like a sort of dynamic equilibrium, with one position (such as the warm, En Niño phase) representing a point of temporary stability which then decays and moves towards the opposite, La Niña, phase. We know that the warm water in the eastern Pacific is correlated with high pressure over the western Pacific. Similarly, the cold La Niña occurs at the same time as low pressure to the west. But it is too simplistic to talk of cause and effect, at least without a much better understanding of the underlying mechanisms.
Weather records are broken somewhere almost every day. Some temperature or rainfall extremes have stood for many years, others change quite frequently. However, the exceptional attention the present climate is receiving often makes new records headline news. Recently, we have heard of the ‘unprecedented’ surface melting of the Greenland ice sheet and, just this week, the Arctic sea ice having reached its ‘lowest extent ever’. These are records of a sort, but we should bear in mind that our record-keeping only goes back a few decades. How significant these figures are in the longer run of weather patterns is really impossible to say.
Climatic trends, their possible uniqueness and the degree to which they are influenced by human behaviour are the subject of much assertion and debate. Whatever may happen in the meantime, the arguments will intensify this time next year, when the first (and arguably most important) part of the IPCC’s Fifth Assessment report (AR5) is published. Working Group 1 is producing what will be billed as the definitive compilation of material on the scientific basis of climate change.
We can say now that it will generate a range of reactions. Many will hail it as further ‘proof’ that climate change is mainly driven by the enhanced greenhouse effect and that we had better do something about it urgently. The more activist wing of the climate change industry will say that the IPCC is understating the problem and that we’re all doomed. Meanwhile, more sceptical voices will raise questions about everything from the reliability of the temperature record to cherry-picking the range of evidence quoted. There will even be those who accuse some scientists of wilful impropriety or claiming that carbon dioxide cannot have a warming effect on the atmosphere.
The extent to which any of these messages will be heeded will depend to some extent on weather pattern over the next twelve months. In particular, if we have yet another year which fails to show any upward trend in average temperatures this century, mainstream climate scientists will find it hard to ignore the call for some credible explanation.
Whatever the outcome, we should all remember two things. First, the IPCC was set up expressly to find evidence for human impact on climate, rather than being given an open brief to investigate the drivers of change. Second, the ‘evidence’ for anthropogenic global warming is actually the output of sophisticated but incomplete computer models of a global climate system which we truly do not fully understand.
When we look at what will undoubtedly be high-profile media coverage of the dangers of climate change as we return to work next year, we should try to filter all the information via these two incontrovertible facts.
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