Why the UK's unique weather makes understanding rain patterns harder
- 04 Jan 2013, 12:30
- Ros Donald
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.
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.
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:
So is climate change making the UK wetter?
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
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
"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