How is Attribution Performed?
To establish the cause(s) of a climate condition requires identifying candidate forcings, determining the response produced by such forcings, and determining the agreement between the forced response and the observed condition. It is also necessary to demonstrate that the observed climate condition is unlikely to have originated from other forcing mechanisms.
The methods are designed to identify signals, and can involve both empirical analysis of past climate relationships and experiments with climate models in which forcing-response relations are evaluated. Both empirical and modeling approaches have limitations. The former is hampered by the relatively short duration of the climate record, the confounding of influences by various forcing mechanisms, and by possible non-physical inhomogeneities in the climate record that can result from changing monitoring techniques and analysis procedures. The latter is hampered by uncertainties in the representation of physical processes numerically and by course spatial resolution (currently on the order of several hundred kilometers) that can produce biases within climate models. In each case, the identified signal (forcing-response relationship) must be robust to these uncertainties. This includes demonstrating that an empirical analysis is both physically meaningful and is robust to sample size, and that a numerical result is replicated when using different climate models. Best attribution practices employ combinations of empirical and numerical approaches using multiple climate models, to minimize the effects of possible biases resulting from a single line of approach.
The specific attribution method can also differ according to the forcing-response relation being probed. Three methods that have been widely employed consider different hierarchical links in causal relationships (i) climate conditions rising from mechanisms internal to the atmosphere, ii) climate conditions forced from changes in atmospheric lower boundary conditions (for example, changes in ocean or land surface conditions), iii) climate conditions forced externally, whether natural or anthropogenic. Note that in some cases, more than one of these links can be involved. For example, changes in greenhouse gas forcing may induce changes in the ocean component of the climate system. These ocean conditions can then force a response in the atmosphere that leads to regional temperature or precipitation changes.