Average global temperatures from 2010 to 2019 compared to a baseline average from 1951 to 1978. Source: NASA
The rising average temperature of Earth's climate system, called global warming, is driving changes in rainfall patterns, extreme weather, arrival of seasons, and more. Collectively, global warming and its effects are known as climate change. While there have been prehistoric periods of global warming, observed changes since the mid-20th century have been unprecedented in rate and scale.
The Intergovernmental Panel on Climate Change (IPCC) concluded that "human influence on climate has been the dominant cause of observed warming since the mid-20th century". These findings have been recognized by the national science academies of major nations and are not disputed by any scientific body of national or international standing. The largest human influence has been the emission of greenhouse gases, with over 90% of the impact from carbon dioxide and methane. Fossil fuel burning is the principal source of these gases, with agricultural emissions and deforestation also playing significant roles. Temperature rise is enhanced by self-reinforcing climate feedbacks, such as loss of snow cover, increased water vapour, and melting permafrost.
Land surfaces are heating faster than the ocean surface, leading to heat waves, wildfires, and the expansion of deserts. Increasing atmospheric energy and rates of evaporation are causing more intense storms and weather extremes, damaging infrastructure and agriculture. Surface temperature increases are greatest in the Arctic and have contributed to the retreat of glaciers, permafrost, and sea ice. Environmental impacts include the extinction or relocation of many species as their ecosystems change, most immediately in coral reefs, mountains, and the Arctic. Surface temperatures would stabilize and decline a little if emissions were cut off, but other impacts will continue for centuries, including rising sea levels from melting ice sheets, rising ocean temperatures, and ocean acidification from elevated levels of carbon dioxide.
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Diagram of factors that determine climate sensitivity. After increasing CO
levels, there is an initial warming. This warming gets amplified by the net effect of feedbacks. Self-reinforcing feedbacks
include the melting of sunlight-reflecting ice, and higher evaporation increasing average atmospheric water vapour (a greenhouse gas
Climate sensitivity is a measure of how much the Earth's climate will cool or warm after a change in the climate system, for instance, how much it will warm for doubling in carbon dioxide (CO
2) concentrations. In technical terms, climate sensitivity is the average change in the Earth's surface temperature in response to changes in radiative forcing, the difference between incoming and outgoing energy on Earth. Climate sensitivity is a key measure in climate science, and a focus area for climate scientists, who want to understand the ultimate consequences of anthroprogenic climate change.
The Earth's surface warms as a direct consequence of increased atmospheric CO
, as well as increased concentrations of other greenhouse gases such as nitrogen dioxide
. Increasing temperatures have secondary effects on the climate system
, such as an increase in atmospheric water vapour
, which is itself also a greenhouse gas. Because scientists do not know exactly how strong these climate feedbacks
are, it is difficult to precisely predict the amount of warming that will result from a given increase in greenhouse gas concentrations. If climate sensitivity turns out to be on the high side of scientific estimates, the Paris Agreement
goal of limiting global warming
to below 2 °C (3.6 °F) will be difficult to achieve. Read more...
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