global warming

Global warming and climate change can both refer to the observed century-scale rise in the average temperature of the Earth’s climate system and its related effects, although climate change can also refer to any historic change in climate. Multiple lines of scientific evidence show that the climate system is warming. More than 90% of the additional energy stored in the climate system since 1970 has gone into ocean warming; the remainder has melted ice, and warmed the continents and atmosphere. Many of the observed changes since the 1950s are unprecedented over decades to millennia. Scientific understanding of the cause of global warming has been increasing. In its fourth assessment (AR4 2007) the Intergovernmental Panel on Climate Change (IPCC) reported that scientists were more than 90% certain that most of global warming was being caused by increasing concentrations of greenhouse gases produced by human activities (anthropogenic). In 2010 that finding was recognized by the national science academies of all major industrialized nations. Climate model projections were summarized in the 2013 Fifth Assessment Report (AR5) by the IPCC. They indicated that during the 21st century the global surface temperature is likely to rise a further for their lowest emissions scenario using stringent mitigation and for their highest. Future climate change and associated impacts will be different from region to region around the globe. The effects of an increase in global temperature include a rise in sea levels and a change in the amount and pattern of precipitation, as well as a probable expansion of subtropical deserts. Warming is expected to be strongest in the Arctic, with the continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include more frequent extreme weather events including heat waves, droughts, heavy rainfall, 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 loss of habitat from inundation. Possible 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 Climate Change (UNFCCC), whose ultimate objective is to prevent dangerous anthropogenic climate change. The UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions and to assist in adaptation to global warming. Parties to the UNFCCC have agreed that deep cuts in emissions are required, and that future global warming should be limited to below relative to the pre-industrial level.