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 Office.

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 Niño 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 20^th century.

Single events

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 change.

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 results

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.

Natural variability

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 plan ahead.

By disentangling natural from anthropogenic factors, attribution studies can inform affected societies about how recent events relate to the long term picture of climate change.

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 than usual.

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 patterns.

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 ( Met Office)

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.

Recent advances

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 weather.

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 project.

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.