Scientists in the 19th century, such as Alexander von Humboldt, began to foresee the effects of climate change. In 1856, Eunice Newton Foote demonstrated that air with carbon dioxide absorbed more heat than dry air, suggesting a warming effect. John Tyndall later established in 1859 that gases like water vapour and carbon dioxide absorb radiated heat, potentially causing climatic changes. Svante Arrhenius, in 1896, published the first climate model, calculating that doubling CO2 levels could increase temperatures by 5–6 °C.
The climate system experiences natural cycles, such as El Niño and La Niña, which can cause short-term temperature fluctuations. However, the current warming trend is primarily attributed to an imbalance of energy caused by external forcings, particularly the increase in greenhouse gas concentrations. Scientists have developed “fingerprints” to identify the causes of climate change, ruling out solar activity as the primary driver, as only the lower atmosphere has warmed.
Greenhouse gases, including water vapour, carbon dioxide, methane, and nitrous oxide, trap heat in the Earth’s atmosphere. Before the Industrial Revolution, naturally occurring greenhouse gases kept the air near the surface about 33 °C warmer than it would have been without them. Since then, human activities have increased the concentration of these gases, with CO2 levels now higher than they have been in 14 million years, and methane levels higher than in 800,000 years. In 2022, CO2 concentrations had increased by about 50% and methane by 164% since 1750.
Different regions of the world warm at different rates, with land regions warming almost twice as fast as the global average. The Arctic is warming three to four times faster than the rest of the world, due to the loss of reflective ice and snow. Changes in land surface, such as deforestation, also impact climate, although to a lesser extent. Deforestation releases CO2, and alters the amount of sunlight reflected back into space.
The effects of climate change are already being observed, including more frequent heat waves, droughts, and intense storms. Global sea level is rising due to thermal expansion and melting glaciers and ice sheets, increasing at a rate of 4.8 cm per decade between 2014 and 2023. These changes are impacting ecosystems, forcing species to relocate or face extinction, and threatening human health and livelihoods. The World Health Organization considers climate change one of the biggest threats to global health in the 21st century.
Mitigation of climate change requires reducing greenhouse gas emissions and increasing carbon dioxide removal. The United Nations Environment Programme estimates that countries need to triple their pledges under the Paris Agreement to limit global warming to 2 °C. Strategies include transitioning to renewable energy sources like wind and solar, improving energy efficiency, and restoring forests. Carbon dioxide removal technologies, such as reforestation and carbon capture and storage, may also play a role, but carry risks.
Adaptation to climate change involves adjusting to current and expected changes. This includes measures like flood control, drought-resistant crops, and managed retreat from rising sea levels. However, adaptation has limits, and some impacts will continue for centuries, including ocean heating, acidification, and sea level rise. The impacts of climate change are unevenly distributed, with poorer communities being disproportionately vulnerable.
Public awareness of climate change has grown since the late 1980s, but has been affected by denial and misinformation, often originating from fossil fuel interests. Climate protests and litigation are increasingly used to demand action from governments and corporations. International agreements, such as the 1997 Kyoto Protocol and the 2015 Paris Agreement, aim to coordinate global efforts to address climate change, though implementation remains a challenge. The Intergovernmental Panel on Climate Change (IPCC) provides regular assessments of the science, impacts, and potential solutions to climate change.