Climate change ranks among the most serious problems facing the world today. There is now a strong scientific consensus that emissions of carbon dioxide and other greenhouse gases into the atmosphere have changed, and will continue to change, the world climate, increasing average temperatures more rapidly than has been seen since long before humans existed. The main source of carbon dioxide emissions is the production and consumption of fossil fuels, but there are many other contributing factors associated with industrial activity and agriculture. In addition, land use changes, including the destruction of forests for farmland, have reduced natural sources of carbon dioxide absorption, further increasing the concentration of carbon dioxide in the atmosphere.
The most optimistic forecast is that climate change will be mild and the changes will happen slowly. Even in this case, local variations in the climate will disrupt traditional economic activities such as agriculture, resulting in the wasting of capital investments, significant migration, and so forth. Even if the sea level rises very little, the dangers from storms will increase, and people will need to build seawalls, to move farther from the coast, and to face other burdens and incur other costs. Warm-weather diseases such as malaria will spread, kill many people, and consequently will need to be seriously addressed.
The median forecast is that under business-as-usual scenarios, global temperature changes will be substantial and the effects of climate change will include severe disruption, with millions of otherwise avoidable deaths caused by flooding, disease, and other hazards, and trillions of dollars in costs. As we shall emphasize, the impacts will probably be worst in the most vulnerable places—poor nations like India and countries in Sub-Saharan Africa. There is also a genuine risk of a truly catastrophic outcome—for example, significant