Summary

Emissions from greenhouse gases are predicted to cause climate to change. Increased solar radiation gradually warms the oceans, which leads to warmer climates. How much future climates will change depends on the cumulative emissions of greenhouse gases, which in turn depends on the magnitude of future economic growth. The global warming caused by humanmade emissions will likely affect many phenomena across the planet. The future damage from climate change is the net damage that these changes will cause to mankind.

Oceans are expected to expand with warmer temperatures, and glaciers and ice sheets are expected to melt, leading to sea level rise over time (a damage). Crops tend to have a hill-shaped relationship with temperature, implying that some farms will be hurt by warming and some farms will gain, depending on their initial temperature. Cooling expenditures are expected to increase (a damage), whereas heating expenditures are expected to fall (a benefit). Water is likely to become scarcer as the demand for water increases with temperature (a damage). Warming is expected to cause ecosystems to migrate poleward. Carbon fertilization is expected to cause forest ecosystems to become more productive, but forest fires are expected to be more frequent so that it is uncertain whether forest biomass will increase or decrease. The expected net effect of all these forest changes is an increase in timber supply (a benefit). It is not known how ecosystem changes will alter overall enjoyment of ecosystems. Warmer summer temperatures will cause health effects from heat waves (a damage), but even larger reductions in health effects from winter cold (a benefit). Large tropical cyclones are expected to get stronger, which will cause more damage from floods and high winds. Winter recreation based on snow will be harmed, but summer outdoor recreation will enjoy a longer season, leading to a net benefit.

The net effect of historic climate change over the last century has been beneficial. The beneficial effects of climate change have outweighed the harmful effects across the planet. However, the effects have not been evenly distributed across the planet, with more benefits in the mid to high latitudes and more damage in the low latitudes. The net effect of future climate is expected to turn harmful as benefits will shrink and damages will become more pervasive. A large proportion of the damage from climate change will happen in the low latitudes, where temperatures will be the highest.

Measurements of the economic impact of climate change have changed over time. Early studies focused only on the harmful consequences of climate change. Including climate effects that are beneficial has reduced net damage. Early studies assumed no adaptation to climate change. Including adaptation has reduced the net harm from climate change. Catastrophe has been assumed to be a major motivation to do near-term mitigation. However, massive sea level rise, ecosystem collapse, and high climate sensitivity are all slow-moving phenomena that take many centuries to unfold, suggesting a modest present value.