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Why the UK's unique weather makes understanding rain patterns harder

  • 04 Jan 2013, 12:30
  • Ros Donald

Stephen McKay

We're not just imagining it - the UK really is wet. 2012 was the second-wettest year on record, according to provisional measurements from the Met Office, which also finds that the country may be suffering more intense bursts of rainfall. But while it's easy to fixate on the UK's current soggy state, climate models predict drier summers for the UK in the future.

The UK has chalked up its second-wettest year ever, according to Met Office measurements out yesterday. But scientists predict that the country will eventually have drier summers due to climate change. A Met Office spokesperson told Carbon Brief:

"In the long term, most climate models project drier UK summers - but it is possible there could be other influences of a changing climate which could override that signal on shorter timescales."

While scientists are making progress in working out how climate change influences the odds of extreme weather, the notoriously unpredictable UK weather makes it difficult to do UK-specific analysis. The Met Office told us:

"It may take many years before we could confirm how the odds of this summer's wet weather happening have been altered by greenhouse gases".

A wetter UK

Looking at the year as a whole, 2000 was the wettest year on record, with 1337.3 millimetres of rain. Although monthly records for precipitation in England and Wales started as far back as 1766, daily measurements for the whole of the UK started in in 1931. The Met Office's most recent statistics show that four of the five wettest years in the UK occurred since the millenium.
Screen Shot 2013-01-03 At 12.01.38
Source: Met Office

In 2012 the UK saw less rain - but only just, with 6.6mm less over the whole year. 2012 was England's wettest year ever, and Wales's third-wettest. In Scotland, however, it was the 17th wettest on record, and just the 40th wettest in Northern Ireland.

Overall, the Met Office says the UK's been getting wetter in recent decades, with a five per cent increase in rainfall from 1961-1990 to 1981-2010.

Extreme rain

The Met Office has also recorded an increase in the frequency of days of extreme rainfall in the UK since 1960. More intense days of rainfall that occur on average about once in every one hundred days over the late twentieth century have been becoming gradually more frequent, nudging closer to once in every 70 days over the past decade or so, as the graph below shows:

Screen Shot 2013-01-03 At 12.17.38

So is climate change making the UK wetter?

Increasing global temperature means the atmosphere can hold more moisture now than at the start of the century. Basic physics suggests that more water in the atmosphere overall means that when it rains, the volume of rainfall may increase. The Met Office told us:

"We do know that the warmer air is, the more moisture it can hold. We have seen a global temperature increase of more than 0.7 deg C (since pre-industrial times) and this has led to an increase of about 4-5% in atmospheric moisture. This means that when we do get unusual weather patterns such as we're seeing now, it's likely there will be more rainfall than the same patterns might have produced in the past. In short, it seems when it does rain, it is heavier."

But it's not easy to link more frequent rainfall to climate change - especially in the UK. Complex weather patterns govern how much rain will fall, where, and when, which explains why the UK might get wetter overall but still expect drier summers in the long term.

The complicated nature of UK weather patterns means trends can be more easily masked. Trends in rainfall severity are easier to detect in countries such as China or India where weather is less variable. Meanwhile, in tropical countries where the air is warmer, a small change in temperature has a much greater impact on the atmosphere's ability to hold moisture and give heavier rainfall, so the relationship is more pronounced.

The Met Office also points out that changes in sea surface temperatures due to shrinking Arctic sea ice and natural climate cycles could play a part in increased rainfall, but warns more research is needed before we can say how great an influence they exert:

"If low levels of Arctic sea ice were found to be affecting the track of the jet stream, for example, this could be seen as linked to the warming of our climate - but this is currently an unknown."

Climate change may also affect atmospheric circulations like the jet stream, which controls how weather systems move over the UK, according to research out this year in the journal Climate Dynamics. According to these early findings - which focus on western and central Europe - climate change's influence on these movements could lead to a greater increase in extreme rainfall than climate models have previously projected.

Unique UK weather

All this goes to show that the UK's unpredictable climate complicates more than the Met Office's predictions for the summer ahead. Although long term models project that the UK will experience drier summers with wetter seasons over the rest of the year, UK scientists have a tricky job on their hands to try to work out how climate change is affecting the country's weather patterns.

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