Global warming, also known as climate change, is the increase observed at the scale of the century in the average temperature of the Earth's climate system and its related effects. Several lines of scientific evidence show that the climate system is heating up. Many of the changes observed since the 1950s are unprecedented in the record of instrumental temperature dating back to the mid-nineteenth century, and in proxy paleoclimatic records for thousands of years.
In 2013, the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC) concluded that "human influence is very likely to have been the main cause of the warming observed since the mid-twentieth century." The greatest human influence has been the emission of greenhouse gases such as carbon dioxide, methane and nitrous oxide. The projections of the climate model summarized in the report indicated that during the 21st century global surface temperature is likely to increase from 0.3 to 1.7 ° C for its lower emissions scenario and from 2.6 to 4.8 C (4.7 to 8.6 ° F) for the highest emission scenario. These findings have been recognized by the national academies of science of the major industrialized nations and are not disputed by any scientific organization of national or international rank.
Future climate change and associated impacts differ from region to region around the world. Predicted effects include global warming, rising sea levels, changing precipitation, and expanding subtropical deserts. Warming is expected to be higher on land than on the oceans and higher in the Arctic, with the continuous retreat of glaciers, permafrost and sea ice. Other likely changes include more frequent extreme weather events, such as heat waves, droughts, heavy rains with floods and heavy snowfalls; Oceanic acidification; And species extinctions due to changes in temperature regimes. Significant effects on humans include the threat to food security of declining crop yields and the abandonment of populated areas due to rising sea levels. Because the climate system has great "inertia" and greenhouse gases will remain in the atmosphere for a long time, many of these effects will not only exist for decades or centuries but will persist for tens of thousands of years.
Possible social responses to global warming include mitigation through reduction of emissions, adaptation to their effects, construction of systems resistant to their effects and possible future climate engineering. Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC), whose ultimate goal is to prevent dangerous anthropogenic climate change. The 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 above pre-industrial levels, with efforts to limit warming to 1,5 ° CF).
Public reactions to global warming and concern about its effects are also increasing. A global report from the 2015 Pew Research Center showed that a median of 54% considered it to be "a very serious problem". There are significant regional differences between the least affected Americans and Chinese (whose economies are responsible for the largest annual CO2 emissions).
