A third approach looks at the problem from the perspective of who has contributed most to the problem. Because the industrialized countries have contributed more than two-thirds of the stock of human-induced GHGs in the atmosphere, this approach suggests that they should bear the largest share of the costs. This stock of emissions has been described as an "environmental debt" (Munasinghe "et al.", 1996, p. 167). In terms of efficiency, this view is not supported. This is because efficiency requires incentives to be forward-looking, and not retrospective (Goldemberg et al., 1996, p. 29). The question of historical responsibility is a matter of ethics. Munasinghe "et al." (1996, p. 167) suggested that developed countries could address the issue by making side-payments to developing countries.