This will be of no comfort to the people of Somerset, but California is experiencing its worst drought for 500 years. David Horsey of the Los Angeles Times points out that the last comparable drought in the region lasted for 300 years and led to the collapse of previously flourishing Native American cultures:

“The question facing us today is how much disruption our more complex society can handle.

“In the agricultural regions of California, where half of the nation’s fruits, nuts and vegetables are grown, many farmers are not planting crops this year because there is no water. Cattle and sheep are being sold off by ranchers because there is no grass. More than 25 million people who rely on dwindling local water sources are being told not to expect rescue because state and federal water reserves are quickly running out.”

As the ghosts of the Anasazi could tell you, climate catastrophes can take place independently of any man-made cause. Nevertheless, with all the weird weather we’ve been having lately, it’s not surprising that climate change is rising up the agenda again.

But, hang on, do I mean ‘climate change’ or ‘global warming’? Those who downplay or dismiss the issue – such as the Global Warming Policy Foundation (GWPF) – prefer the latter term. This is for two main reasons: Firstly, the temperature trend over the last decade or so has been pretty flat – the GWPF website features a graph depicting this fact on its masthead. Secondly, it is argued that a broad term like ‘climate change’ allows just about any unusual weather pattern to be blamed on carbon emissions when, supposedly, the only criterion on which we ought to judge the seriousness of the greenhouse effect is elevated temperature.

However, to place so much emphasis on a single decade of temperature records is misleading. After all, there was a longer pause in the middle of the 20th century, which was followed by several decades of rapid warming. Moreover, a focus on surface temperatures alone ignores the fact that most warming takes place in the ocean – meaning that variations in the rate at which the seas absorb excess heat can overwhelm the land-based record. Obviously what matters to us most in the short-term is what happens on the surface, but in the long-term what we should really worry about is the amount of energy going into the whole system – land, sea and sky.

As for the relationship between global warming and other climatic phenomena – there’s a very good reason why we might expect the greenhouse effect to result not just in higher temperatures, but extreme weather events of all kinds. In just about any context, a burst of additional energy provides a jolt to the system – things start moving faster and are more likely to become disordered.

It was a point made earlier this week by Dame Julia Slingo, the Met Office chief scientist, when she was asked to comment on the causes of the recent floods. According to the BBC report, she said it was impossible to identify a definitive cause:

“‘But all the evidence suggests there is a link to climate change,’ she added.

“‘There is no evidence to counter the basic premise that a warmer world will lead to more intense daily and hourly rain events.’”

More energy in the water cycle means more evaporation and, therefore, precipitation – it also means that existing weather systems will be disrupted. So more rain overall, but not in ways we’re used to having it.

Though we still don’t know whether we can blame the West Country floods or Californian drought on man-made global warming, we need to keep a weather eye on the frequency of all extreme climatic events and not just the thermometer.

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