Most pollution is an unequivocal social bad – a negative externality – but the relationship between GHG emissions and human well-being is unusually complex. In the long run, there is a strong scientific consensus that GHG emissions will result in higher temperatures and sea levels, and a disruption of historical weather patterns, from heat waves and droughts to more intense storms. The effect of climate change on human well-being will vary greatly from country to country, but if the increase in global average annual temperature exceeds a 2ºC threshold, total world food supplies will begin to shrink even as the risk of triggering feedback processes that would accelerate warming grows steeply (Stern 2006; IPCC, 2007).