Global Warming

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Current Change in Earth's climate.
Global warming refers to the rising average temperature of Earth's atmosphere and oceans since the late 19th century.

Since the early 20th century, Earth's average surface temperature has increased by about 1.4 °F, with about two thirds of the increase occurring since 1980.

Warming of the climate system is unequivocal, and scientists are more than 90% certain that most of it is caused by increasing concentrations of greenhouse gases produced by human activities such as deforestation and burning fossil fuels.

These findings are recognized by the national science academies of all major industrialized nations.

Climate model projections are summarized in the 2007 fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC).

An increase in global temperature will cause sea levels to rise and change the amount and pattern of precipitation, as well as cause a probable expansion of subtropical deserts.

Warming is expected to be strongest in the Arctic and would be associated with the continuing retreat of glaciers, permafrost and sea ice.

Other likely effects of warming include a more frequent occurrence of extreme-weather events including heat waves, droughts and heavy rainfall, species extinctions (due to shifting temperature regimes), and changes in crop yields.

Warming and related changes will vary from region to region around the globe, with projections being more robust in some areas than others.

If global mean temperature increases to 7.2 °F above pre-industrial levels, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits of adaptation for natural systems would largely be exceeded throughout the world.

Hence, the ecosystem services upon which human livelihoods depend would not be preserved.

Human responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possible future geoengineering.

Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC), whose ultimate objective is to prevent dangerous anthropogenic (i.e., human-induced) climate change.

Parties to UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions and to assist in adaptation to global warming.

Parties to UNFCCC have agreed that deep cuts in emissions are required, and that future global warming should be limited to below 3.6°F relative to the pre-industrial level.

A 2011 report of analyses by the United Nations Environment Program and International Energy Agency suggest that efforts in the early 21st century to reduce emissions may be inadequately stringent to meet the UNFCCC's 2 °C target.