What climate change attribution can tell us about extreme weather - and the recent UK floods
- 14 Feb 2014, 15:00
- Dr Peter Stott
A guest post from Dr
Peter Stott, head of the Climate Monitoring and Attribution
team at the Met Office.
Climate change attribution is the science of
determining the causes of unusual climate trends and
climate-related events, and it's an area of research that I've
focused on throughout my career as a climate scientist at the Met
Attribution studies can help us understand how
humans are influencing the climate. Such studies have identified
the 'fingerprints' of change due to human influence on climate in
observed records of temperature, rainfall, and other climate
parameters. Scientists can distinguish these fingerprints from the
effects of natural factors, like changing solar output and natural
climate fluctuations like the El Nino Southern Oscillation.
These studies have shown that human influence on
climate, from increasing greenhouse gas concentrations in the
atmosphere, has been the dominant cause of the global warming
observed since the mid 20th century.
Unusual extremes have always happened in our
variable climate. It can be all too easy to put the entire blame of
catastrophic weather-related disasters onto human-caused climate
change or onto natural climate variability, but such misattribution
can easily lead to bad policymaking around how to adapt to climate
That's why attribution science is now being
applied to single events, such as the extremely hot summers in
Europe in 2003 and Australia in 2013, to see how the probability of
such an event might have changed.
To do this, we want to compare what actually
happened with what might have happened in a world without
anthropogenic climate change. The only way to determine how the
world could have evolved without anthropogenic climate change is to
use a climate model to simulate the conditions we could have
experienced in its absence.
We can then compare the results of climate model
simulations with and without human influence on climate to see how
the odds of particular events such as heatwaves, floods and
droughts have changed.
Attribution studies have shown that human-caused
climate change has significantly increased the chances of the
catastrophic temperatures seen in Europe in 2003, which brought
many thousands of heat-related deaths.
Likewise, the record Australian temperatures of
2013, which brought devastating forest fires and the destruction of
many homes, have become substantially more likely due to human
influence on climate. Given this information, societies may wish to
better protect vulnerable populations and ensure infrastructure is
more resilient to a greater frequency of such extreme weather
events in future.
But not all extreme events are associated with
anthropogenic climate change. If some types of events are becoming
less likely as a result of human interference in the climate,
societies may choose not to direct resources towards adapting for
more unlikely scenarios.
Furthermore, natural climate variability can
complicate the long-term signal of future climate change that's
expected from human activity.
For example, while the expectation is for British
summers to become warmer and drier on the whole, the recent run of
wet summers in the UK from 2007 to 2012 has been shown to be
associated with naturally driven variations in temperatures of the
North Atlantic Ocean.
Extreme weather events challenge our scientific
theories, and provide a stern test of climate models. Where we can
develop our understanding and modelling of the physical processes
at work, we can hope to improve our capability to forecast the
chances of unusual weather in the months ahead, and help society
By disentangling natural from anthropogenic
factors, attribution studies can inform affected societies about
how recent events relate to the long term picture of climate
Wet and stormy UK weather
The current exceptionally wet and stormy British
winter provides a particularly challenging test case for
attribution science. A disturbed and stronger than usual jet stream
has brought a sequence of intense storms on a more southerly track
The precursors for such unusual jet stream
behaviour are described in detail in a recent
briefing note from the Met Office. The report implicates
meteorological conditions far away from Britain, demonstrating the
role of the global circulation in driving our local weather
But while we can build up a detailed qualitative
picture, it is much harder to know exactly how the risk of such
unusual winter conditions has changed quantitatively.
It is clear that global warming has led to an
increase in moisture in the atmosphere - with about four per cent
more moisture over the oceans than in the 1970s - which means that
when conditions are favourable to the formation of storms there is
a greater risk of intense rainfall.
Total rainfall (mm) for January in southern
England from records going back to 1910 (top panel).
Source: "The recent storms and floods in the UK" report (
But an unusual feature of the weather this winter
has been the persistence of weather patterns, with storms lining up
across the Atlantic to batter the country one after the other.
At present we don't know how different drivers
within the global circulation have affected this sequence of
storms, nor do we know for sure if our current generation of
climate models are able to calculate the relevant processes
reliably enough to make an accurate calculation of the changed risk
of such events.
The good news, however, is that recent advances
in climate science are starting to pay dividends. Improved spatial
resolution in models - that just means that they can model weather
and climate in more spatial detail - is allowing the models to
better represent some of the key factors that drive regional
weather patterns in climate models.
Last month we began a new European project called
EUCLEIA, led by the Met Office,
which aims to develop an attribution system for extreme
Such a system would deliver reliable and
user-relevant attribution assessments of floods, droughts,
heatwaves, cold spells and storm surges as they happen. It will be
exciting to see what can be achieved over the three years of this
In the meantime, in the absence of such formal
attribution assessment, we have clear evidence that a warmer
atmosphere has increased our risk of really heavy rain when the
meteorological conditions are favourable.
Quantitative answers on how the risk of such
damaging weather has changed, especially when we factor in complex
dynamical factors such as the unusual persistence of the current
jet stream, require further scientific research.
Dr Peter Stott is head of the Climate
Monitoring and Attribution team at the Met Office. He is an
expert in the attribution of climate change to anthropogenic and
natural causes, climate modelling of past and future climate and