To the Editor

Parmesan and colleagues1 criticize a guidance paper that was produced following an Intergovernmental Panel on Climate Change (IPCC) expert meeting on detection and attribution2. This paper includes methods that seek to establish links between observed changes and external drivers of climate change, including greenhouse gases. Parmesan and co-authors argue that attempting to attribute ecological impacts to rising greenhouse gases is 'misguided' and instead propose concentrating on assessing the interacting roles of climate and other environmental factors, regardless of their underlying causes.

The guidance paper — of which most of us are co-authors — does not advocate one particular type of research over another. Rather, it attempts to bring clarity and uniformity to the diverse set of methods associated with the detection and attribution of climate change and its impacts. We contend, however, that detection and attribution is both possible and advisable.

We agree that it is important to carefully account for confounding drivers of change, and this is indeed stressed in the guidance paper. It seems near-sighted, however, to suggest that the difficulty of attributing a species' extinction to the human influence on climate makes any such attempt 'misguided' in principle. Parmesan and co-authors observe that it is difficult to attribute the extinction of a species known to have been caused by a single event to human-induced climate change with high confidence. It is incorrect, however, to suggest that this means it is 'inappropriate' even to try. If human influence on climate doubles or quadruples the probability of a given event occurring — as has been estimated in a few well-studied cases — then there is a clear sense in which its causal role can be quantified, albeit probabilistically3.

Attributing events to natural versus anthropogenic causes may not always be the most important research goal, particularly in the case of some conservation challenges. However, being able to identify changes that are due to greenhouse-gas forcing has important implications for what lies ahead. A change associated with greenhouse-gas forcing is likely to continue, while changes due to internal climate variability may be more likely to reverse. Quantifying the impacts of anthropogenic climate change in this way is also important in guiding the allocation of resources available for adaptation.