In recent years some extreme weather events have arisen from global warming because of human activities, with studies indicating an increasing threat from extreme weather in the future. Scientists were certain that global warming is being caused mostly by increasing concentrations of greenhouse gases and other human activities.
Its effects include warming global temperature, rising sea levels and expansion of deserts in the subtropics(亚热带). Warming is expected to be greater over land than over the oceans and greatest in the Arctic, with the continuing loss of glaciers, permafrost and sea ice. Other likely changes include more frequent extreme weather events including heat waves, droughts, heavy rainfall with floods, heavy snowfall and species extinction due to great change of temperature. Bad effects on humans include the threat to food security from decreasing crop yields and the loss of populated(住人的) areas because of rising sea levels.
Possible responses to global warming are reduction of emissions(排放), 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 objective is to prevent dangerous climate change. It has adopted a range of policies designed to reduce greenhouse gas emissions and to assist in adaptation to global warming. It is agreed that future global warming should be limited to below 2.0 ℃ (3.6 °F) relative to the pre-industrial level.
On 12 November 2015, NASA scientists reported that human-made carbon dioxide continues to increase above levels that have not been seen in hundreds of thousands of years: currently, about half of the carbon dioxide released from the burning of fossil fuels remains in the atmosphere and is not absorbed by vegetation and the oceans.
