Global warming, also referred to as climate change, is the observed century-scale rise in the average temperature of the earth’s climate system and its related effects.Multiple lines of scientific evidence show that the climate system is warming.
Many of the observed changes since the 1950s are unprecedented in the instrumental temperature which extends back to the mid-19th century, and in paleoclimate proxy records covering thousands of years.
In 2013, the intergovernmental panel on climate change (IPCC) Fifth Assesment report concluded that “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”The largest human influence has been the emission of green house gases such as carbon dioxide,methane,and nitrous oxide. Climate model projections summarized in the report indicated that during the 21st century, the global surface temperature is likely to rise a further 0.3 to 1.7 °C (0.5 to 3.1 °F) in the lowest emissions scenario, and 2.6 to 4.8 °C (4.7 to 8.6 °F) in the highest emissions scenario. These findings have been recognized by the national science academies of the major industrialized nations[a] and are not disputed by any scientific body of national or international standing.
Future climate change and associated impacts will differ from region to region.
Anticipated effects include increasing global temperatures, risisng sea levels, changing precipitation, and expansion of deserts in the subtropics.Warming is expected to be greater over land than over the oceans and greatest in the Arctic, with the continuing retreat of glaciers,permafrost and sea ice.
Other likely changes include more frequent extreme weather events such as heat waves,droughts,heavy rainfall with floods and heavy snowfall;ocean acidification; and species extinctions due to shifting temperature regimes. Effects significant to humans include the threat to food security from decreasing crop yields and the abandonement of populated areas due to rising sea levels.
Because the climate system has a large “inertia” and greenhouse gases will remain in the atmosphere for a long time, many of these effects will persist for not only decades or centuries, but for tens of thousands of years to come.
Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, building systems resilient to its effects, and possible future climate engineering. Most countries are parties to the United Nations Framework Convention on Clinate Change (UNFCCC),whose ultimate objective is to prevent dangerous anthropogenic climate change.Parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2.0 °C (3.6 °F) compared to pre-industrial levels,[b] with efforts made to limit warming to 1.5 °C (2.7 °F).
Public reactions to global warming and concern about its effects are also increasing. A global 2015 Pee Research Center report showed that a median of 54% of all respondents asked consider it “a very serious problem”. Significant regional differences exist, with Americans and Chinese (whose economies are responsible for the greatest annual CO2 emissions)among the least concerned.