Existing legal regimes dealing with analogous issues provide useful guideposts in considering the degree to which emitters bear responsibility for past carbon emissions. With regard to liability for environmental damage, the European Union but not the United States recognizes an exemption from liability if the harm was not understood at the time of the conduct. States have also been reluctant to accept strict liability for environmental damage under international law. Similarly, in European law, a defendant is usually not responsible if the state of scientific knowledge at the time did not enable the discovery of a product defect at the time the product was made. In the United States, many state governments recognize a similar limitation on products liability, although some do not.
The problem of climate change is unique in some ways, and the legal system is not always the best gauge of ethical judgments. Nevertheless, understanding how societies have handled other cases of historical responsibility can illuminate the problem of climate justice. These analogous legal regimes suggest strongly that the right answer is somewhere between full responsibility for all historic carbon emissions and complete exoneration for those emitters.