Attributing extreme weather to climate change in real-time

Scientists know that an increase in average temperature as the climate changes will lead to an increase in the number or magnitude of some extreme events, while others will get less likely.But the chaotic nature of weather means it's generally impossible to say, for any particular event, that it only happened because of climate change.

Such studies compare how often a particular extreme weather event occurs in model experiments representing the "world as it is" (with human influence on climate) with how often it occurs in experiments representing the "world that might have been", where the estimated impact of human influence on climate is removed.

Flood risk

Take the example of flooding.

The figure below shows a series of model runs for peak flow in English and Welsh rivers during Autumn 2000. Each circle represents a model run. The blue circles are the world as it is - with climate change. The green circles are the world that might have been - without climate change. 

Public perception of extreme weather and climate

At the moment these studies require months to complete and are published well after public attention to a weather event has peaked.

This can lead to situations where extreme weather events are listed as examples of the impacts of anthropogenic climate change before  attribution studies have shown that it is indeed an event made more likely by human greenhouse gas emissions.

In part, this is because immediately after an extreme weather event the only available statement is that all events are now happening in a changing climate.

While this is true, it doesn't actually say anything about the likelihood of the event occurring, and might imply that all events are made more likely. Given that often we find that climate change played no role in changing the likelihood of an event, and sometimes we find climate change has made an extreme weather event less likely, this is not ideal.

To avoid such issues, we are working on developing ways to assess the links between a particular extreme weather event and climate change that work a lot faster.

Real-time event attribution

Many uncertainties remain in attribution studies. It's also easier to work with some types of weather events  than others - there is generally more confidence in studies focusing onheatwaves than those focusing on extreme  precipitation. Investigation of hurricanes and typhoons is currently limited by the ability of global climate models to simulate these events.

Models also tend to produce more reliable results in certain regions - for example, the mid-latitudes are easier to model because their weather is much more random and does not rely on getting large scale processes exactly right. Monsoon regions, on the other hand, are trickier.

So far the events we've examined have been picked rather randomly, leading to a very patchy picture of how climate change is affecting us already. The next stage of our work is to begin to fill the gaps and systematically investigate how anthropogenic climate change is affecting us today.

We won't be able to give a robust answer in every case, due to limited data and limits to our understanding. But we are working towards developing a more complete picture, that can hopefully lead to better understanding of what climate change means for extreme weather in different parts of the world.

http://www.carbonbrief.org/blog/2014/10/attributing-extreme-weather-to-climate-change-in-realtime/