Climate change includes both the 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 the rate of human impact on Earth's climate system and the global scale of that impact have been unprecedented.That human activity has caused climate change is not disputed by any scientific body of national or international standing. The largest driver has been the emission of greenhouse gases, of which more than 90% are carbon dioxide (CO2) and methane. Fossil fuel burning for energy consumption is the main source of these emissions, with additional contributions from agriculture, deforestation, and industrial processes. 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.
Because land surfaces heat faster than ocean surfaces, deserts are expanding and heat waves and wildfires are more common. Surface temperature rise is greatest in the Arctic, where it has contributed to melting permafrost, and the retreat of glaciers and sea ice. Increasing atmospheric energy and rates of evaporation cause more intense storms and weather extremes, which damage infrastructure and agriculture. Rising temperatures are limiting ocean productivity and harming fish stocks in most parts of the globe. Current and anticipated effects from undernutrition, heat stress and disease have led the World Health Organization to declare climate change the greatest threat to global health in the 21st century. Environmental effects include the extinction or relocation of many species as their ecosystems change, most immediately in coral reefs, mountains, and the Arctic. 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 from elevated levels of CO2.
Many of these effects are already observed at the current level of warming, which is about 1.1 °C (2.0 °F). 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. Under the Paris Agreement, nations agreed to keep warming "well under 2.0 °C (3.6 °F)" by reducing greenhouse gas emissions. 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. Limiting warming to 1.5 °C (2.7 °F) would require halving emissions by 2030, then reaching near-zero levels by 2050.Mitigation efforts include the research, development, and deployment of low-carbon energy technologies, enhanced energy efficiency, policies to reduce fossil fuel emissions, reforestation, and forest preservation. Climate engineering techniques, most prominently solar radiation management and carbon dioxide removal, have substantial limitations and carry large uncertainties. Societies and governments are also working to adapt to current and future global-warming effects through improved coastline protection, better disaster management, and the development of more resistant crops.