The Evidence
Over the past century, as a result of burning fossil fuels and other human activities, atmospheric concentrations of greenhouse gases—including CO2, methane, nitrous oxide, and halocarbons—have risen to levels unprecedented in at least the last 800,000 years. Extensive observations document that the global average surface temperature in the atmosphere and ocean has increased by about 1°C (1.8°F) from 1880 to 2018. The current decade is now the hottest in the history of modern civilization. Based on extensive scientific evidence, it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. There is no alterative explanation supported by convincing evidence.
Many other changes related to heating have been documented: more frequent heat waves on land and in the ocean; reductions in Arctic sea ice, the Northern Hemisphere’s snow cover, the Greenland and West Antarctic ice sheets, and mountain glaciers; changes in the global water cycle, including intensifying
precipitation events; and rising sea levels. Greater CO2 concentrations in the atmosphere are also affecting the growth and nutritional value of land plants and are directly acidifying ocean waters.
The Predictions
Realistic and continually improving computer simulations of the global climate predict that global temperatures will continue to rise as a result of past and future greenhouse gas emissions, with growing risks to natural and human systems. The amount of warming predicted in the coming decades depends primarily on the choices society makes and how those choices affect future emissions.
The Consequences
Human-caused climate change is occurring more rapidly than has been typical in Earth’s history, disrupting communities and ecosystems adapted to previous, relatively stable climatic conditions. The changing climate will increase heat-related deaths, various mental and physical illnesses, and some infectious diseases. It will accentuate hazards such as flooding, wildfire, and dryland water scarcity, and toxic algae. Economic disruption and additional health impacts will result from shifts in agricultural and fisheries productivity, diminished worker productivity, damages to critical infrastructure, and more severe weather disasters, including expected increases in drought and hurricane intensities. Economic or social disruption is likely to drive migration and compound risks of conflict and global insecurity. Climate change will continue to severely stress the world’s ecosystems, including threatened coral reefs, permafrost landscapes and the Arctic; decrease biodiversity; and cause extraordinary numbers of extinctions on land and in the oceans. The severity of these and other impacts will worsen with more warming.
Climate change is manifest in myriad ways that exacerbate many existing challenges, stressing every region of the world and every sector of the economy. Some populations, communities, regions, and ecosystems are especially vulnerable. As lower-income and other marginalized populations are likely to be more affected and generally have less capacity to adapt to changes, climate change is expected to worsen pre-existing inequalities.
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