As climate change proceeds - which the record summer melt of Arctic sea-ice suggests it is doing at a worrying pace - nations, communities and individual citizens may begin to seek compensation for losses and damage arising from global warming. Climate scientists should be prepared for their skills one day to be probed in court. Whether there is a legal basis for such claims, such as that brought against the energy company ExxonMobil by the remote Alaskan community of Kivalina, which is facing coastal erosion and flooding as the sea ice retreats, is far from certain, however. So lawyers, insurers and climate negotiators are watching with interest the emerging ability, arising from improvements in climate models, to calculate how anthropogenic global warming will change, or has changed, the probability and magnitude of extreme weather and other climate-related events. But to make this emerging science of 'climate attribution' fit to inform legal and societal decisions will require enormous research effort.
Attribution is the attempt to deconstruct the causes of observable weather and to understand the physics of why extremes such as floods and heatwaves occur. This is important basic research. Extreme weather and changing weather patterns - the obvious manifestations of global climate change - do not simply reflect easily identifiable changes in Earth's energy balance such as a rise in atmospheric temperature. They usually have complex causes, involving anomalies in atmospheric circulation, levels of soil moisture and the like. Solid understanding of these factors is crucial if researchers are to improve the performance of, and confidence in, the climate models on which event attribution and longer-term climate projections depend.
Event attribution is one of the proposed 'climate services' - seasonal climate prediction is another - that are intended to provide society with the information needed to manage the risks and costs associated with climate change. Advocates of climate services see them as a counterpart to the daily weather forecast. But without the computing capacity of a well-equipped national meteorological office, heavily model-dependent services such as event attribution and seasonal prediction are unlikely to be as reliable.
At a workshop last week in Oxford, UK, convened by the Attribution of Climate-related Events group - a loose coalition of scientists from both sides of the Atlantic - some speakers questioned whether event attribution was possible at all. It currently rests on a comparison of the probability of an observed weather event in the real world with that of the 'same' event in a hypothetical world without global warming. One critic argued that, given the insufficient observational data and the coarse and mathematically far-from-perfect climate models used to generate attribution claims, they are unjustifiably speculative, basically unverifiable and better not made at all. And even if event attribution were reliable, another speaker added, the notion that it is useful for any section of society is unproven.
Both critics have a point, but their pessimistic conclusion - that climate attribution is a non-starter - is too harsh. It is true that many climate models are currently not fit for that purpose, but they can be improved. Evaluation of how often a climate model produces a good representation of the type of event in question, and whether it does so for the right reasons, must become integral to any attribution exercise. And when communicating their results, scientists must be open about shortcomings in the models used.
It is more difficult to make the case for 'usefulness'. None of the industry and government experts at the workshop could think of any concrete example in which an attribution might inform business or political decision-making. Especially in poor countries, the losses arising from extreme weather have often as much to do with poverty, poor health and government corruption as with a change in climate. The United Nations is planning to set up a fund with the aim of reducing loss and damage due to climate change, but the complexity of such issues is making negotations difficult.
These caveats do not mean that event attribution is a lost cause. But they are a reminder that designers of climate services must think very clearly about how others might want to use the knowledge that climate scientists produce. That could be a task for social scientists, who have good methods for analysing decision-making and social trans-actions. They need to be more involved in shaping the production and dissemination of climate knowledge.