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Climate change includes both global warming driven by human emissions of greenhouse gases, and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century, humans have had unprecedented impact on Earth's climate system and caused change on a global scale.[2]
The Global map shows sea temperature rises of 0.5 to 1 degree Celsius; land temperature rises of 1 to 2 degree Celsius; and Arctic temperature rises of up to 4 degrees Celsius.
Average surface air temperatures from 2011 to 2020 compared to a baseline average from 1951 to 1980 (Source: NASA)
Observed temperature from NASA versus the 1850–1900 average as a pre-industrial baseline. The main driver for increased global temperatures in the industrial era is human activity, with natural forces adding variability.[1]
The largest driver of warming is the emission of greenhouse gases, of which more than 90% are carbon dioxide (CO
2) and methane.[3] Fossil fuel burning (coal, oil, and gas) for energy consumption is the main source of these emissions, with additional contributions from agriculture, deforestation, and industrial processes.[4] The human cause of climate change is not disputed by any scientific body of national or international standing.[5] Temperature rise is accelerated or tempered by climate feedbacks, such as loss of sunlight-reflecting snow and ice cover, increased water vapour (a greenhouse gas itself), and changes to land and ocean carbon sinks.
Temperature rise on land is about twice the global average increase, leading to desert expansion and more common heat waves and wildfires.[6] Increasing rates of evaporation cause more intense storms and weather extremes.[7] Temperature rise is amplified in the Arctic, where it has contributed to melting permafrost and the retreat of glaciers and sea ice.[8] Additional warming also increases the risk of triggering critical thresholds called tipping points.[9] Impacts on ecosystems include the relocation or extinction of many species as their environment changes, most immediately in coral reefs, mountains, and the Arctic.[10] Human impacts include undernutrition and hunger from reduced crop yields,[11] declining fish stocks,[12] increases in vector-borne diseases,[13] potentially severe economic impacts,[14] increased global economic inequality,[15] more people living in uninhabitable climate zones,[16] and increased migration.[17] Effects such as these have led the World Health Organization to declare climate change the greatest threat to global health in the 21st century.[18] Even if efforts to minimize future warming are successful, some effects will continue for centuries, including rising sea levels, rising ocean temperatures, and ocean acidification.[19]
Many of these impacts are already felt at the current level of warming, which is about 1.1 °C (2.0 °F).[21] The Intergovernmental Panel on Climate Change (IPCC) has issued a series of reports that project significant increases in these impacts as warming continues to 1.5 °C (2.7 °F) and beyond.[22] Responding to climate change involves mitigation and adaptation.[23] Mitigation – limiting climate change – consists of reducing greenhouse gas emissions, and removing them from the atmosphere;[24] methods include the development and deployment of low-carbon energy sources such as wind and solar, a phase-out of coal, enhanced energy efficiency, reforestation, and forest preservation. Adaptation consists of adjusting to actual or expected climate,[25] such as through improved coastline protection, better disaster management, and the development of more resistant crops. Adaptation alone cannot avert the risk of "severe, widespread and irreversible" impacts.[26]
Under the Paris Agreement, nations agreed to keep warming "well under 2.0 °C (3.6 °F)" through mitigation efforts. However, under those pledges, global warming would reach about 2.8 °C (5.0 °F) by the end of the century, and current policies will result in about 3.0 °C (5.4 °F) of warming.[27] Limiting warming to 1.5 °C (2.7 °F) would require halving emissions by 2030, then reaching near-zero emissions by 2050.[28]
