Aggregate impacts
Main article: Economic impacts of climate change
Aggregating impacts adds up the total impact of climate change across sectors and/or regions.[126] Examples of aggregate measures include economic cost (e.g., changes in gross domestic product (GDP) and the social cost of carbon), changes in ecosystems (e.g., changes over land area from one type of vegetation to another),[127] human health impacts, and the number of people affected by climate change.[128] Aggregate measures such as economic cost require researchers to make value judgements over the importance of impacts occurring in different regions and at different times.

Observed impacts
Global losses reveal rapidly rising costs due to extreme weather-related events since the 1970s.[129] Socio-economic factors have contributed to the observed trend of global losses, e.g., population growth, increased wealth.[130] Part of the growth is also related to regional climatic factors, e.g., changes in precipitation and flooding events. It is difficult to quantify the relative impact of socio-economic factors and climate change on the observed trend.[130] The trend does, however, suggest increasing vulnerability of social systems to climate change.[130]

Projected impacts
The total economic impacts from climate change are highly uncertain.[131] With medium confidence, Smith et al. (2001)[132] concluded that world GDP would change by plus or minus a few percent for a small increase in global mean temperature (up to around 2 °C relative to the 1990 temperature level). Most studies assessed by Smith et al. (2001)[132] projected losses in world GDP for a medium increase in global mean temperature (above 2-3 °C relative to the 1990 temperature level), with increasing losses for greater temperature increases. This assessment is consistent with the findings of more