Climate change includes both human-induced global warming and its large-scale impacts on weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known events in Earth's history. The main cause is the emission of greenhouse gases, mostly carbon dioxide (CO
2) and methane. Burning fossil fuels for energy use creates most of these emissions. Agriculture, steel making, cement production, and forest loss are also significant sources. Temperature rise is affected by climate feedbacks as well, such as the loss of sunlight-reflecting snow cover, and the release of carbon dioxide from drought-stricken forests. Collectively, these amplify global warming.
On land, where temperatures have risen about twice as fast as the global average, deserts are expanding and heat waves and wildfires are becoming more common. Temperature rise is also amplified in the Arctic, where it has contributed to melting permafrost, glacial retreat and sea ice loss. Warmer temperatures are increasing rates of evaporation, causing more intense storms and weather extremes. Impacts on ecosystems include the relocation or extinction of many species as their environment changes, most immediately in coral reefs, mountains, and the Arctic. Climate change threatens people with food insecurity, water scarcity, flooding, infectious diseases, extreme heat, economic losses, and displacement. These human impacts have led the World Health Organization to call climate change the greatest threat to global health in the 21st century. Even if efforts to minimise future warming are successful, some effects will continue for centuries, including rising sea levels, rising ocean temperatures, and ocean acidification.
Many of these impacts are already felt at the current level of warming, which is about 1.2 °C (2.2 °F). The Intergovernmental Panel on Climate Change (IPCC) has projected significant increases in these impacts as warming continues to 1.5 °C (2.7 °F) and beyond. Additional warming increases the risk of triggering critical thresholds called tipping points. Responding to these impacts involves both mitigation and adaptation. Mitigation – limiting climate change – consists of reducing greenhouse gas emissions and removing them from the atmosphere. Methods to achieve this include the development and deployment of low-carbon energy sources such as wind and solar, a phase-out of coal, enhanced energy efficiency, and forest preservation. Adaptation consists of adjusting to actual or expected climate, 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.
Under the 2015 Paris Agreement, nations collectively agreed to keep warming "well under 2.0 °C" (3.6 °F) through mitigation efforts. However, with pledges made under the Agreement, global warming would still reach about 2.8 °C by the end of the century. Limiting warming to 1.5 °C would require halving emissions by 2030 and achieving net-zero emissions by 2050.