By definition, the counterfactual baseline required for impact attribution cannot be observed. However, it may be approximated by impact model simulations forced by a stationary climate, for example derived by de-trending the observed climate , while other relevant drivers  of changes in the system of interest  evolve according to historical conditions. To attribute to anthropogenic climate forcing, the anthropogenic trends in climate are estimated from a range of different climate models and subtracted from the observed climate  or the 'no anthropogenic climate forcing' baseline is directly derived from a large ensemble of climate model simulations not accounting for anthropogenic forcings . In any case, it has to be demonstrated that the applied impact models are able to explain the observed changes in natural, human or managed systems by, for example, reproducing the observations when forced by observed changes in climate-related systems and other relevant drivers.
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report