The scientific understanding of how Earth's atmosphere regulates planetary temperature began in the early nineteenth century.¹⁵ In 1824, French mathematician and physicist Joseph Fourier calculated that a planet of Earth's size and distance from the Sun ought to be significantly colder than observed if warmed solely by incoming solar radiation.¹⁶¹⁷ Fourier proposed that the atmosphere acts as an insulator, allowing visible light to pass through while retarding the escape of invisible heat rays emitted from the surface. This concept was further developed by French physicist Claude Pouillet in 1827 and 1838. In 1856, American scientist Eunice Newton Foote conducted experiments using glass cylinders filled with different gases exposed to sunlight, demonstrating that moist air warmed more than dry air and that a cylinder filled with carbon dioxide trapped the greatest amount of heat. Foote concluded that an atmosphere with a higher proportion of carbon dioxide would result in a higher planetary temperature.

Three years later, in 1859, Irish physicist John Tyndall independently constructed a ratio spectrophotometer to measure the precise absorption of infrared radiation by various gases. Tyndall showed that the primary constituents of the atmosphere, nitrogen and oxygen, are virtually transparent to infrared radiation, whereas trace gases such as water vapor, carbon dioxide, ozone, and hydrocarbons absorb radiant heat strongly.¹⁸ In 1896, Swedish chemist Svante Arrhenius produced the first quantitative calculation of how changes in atmospheric carbon dioxide could alter surface temperatures. Accounting for the amplifying feedback of water vapor, Arrhenius estimated that halving atmospheric carbon dioxide could trigger an ice age, whereas doubling it would raise global temperatures by several degrees Celsius. He also recognized that industrial coal combustion was adding carbon dioxide to the air, though he anticipated that significant warming would take many centuries to materialize and might prove beneficial to colder regions.

During the first half of the twentieth century, many scientists doubted that human emissions could measurably alter the climate, assuming either that water vapor already absorbed radiation in the same spectral bands as carbon dioxide or that the vast oceans would quickly absorb any excess gas. In 1938, English steam engineer and amateur meteorologist Guy Stewart Callendar challenged this view by compiling global temperature records from the preceding fifty years alongside historical gas measurements, arguing that fossil fuel combustion had already increased atmospheric carbon dioxide and initiated a warming trend. Meanwhile, Serbian mathematician Milutin Milanković formulated the astronomical theory of paleoclimate between 1920 and the 1940s, demonstrating that long-term glacial and interglacial cycles are paced by regular variations in Earth's orbital eccentricity, axial tilt, and precession.

In the 1950s, improved infrared spectroscopy by Gilbert Plass confirmed that carbon dioxide and water vapor absorption bands do not completely overlap, especially in the upper atmosphere. In 1957, oceanographer Roger Revelle and chemist Hans Suess demonstrated that the chemical buffering capacity of seawater limits the rate at which the upper ocean can absorb atmospheric carbon dioxide, meaning a substantial fraction of industrial emissions remains in the air for centuries. To test this, American geochemist Charles David Keeling initiated continuous, high-precision measurements of atmospheric carbon dioxide at the South Pole and at Mauna Loa Observatory in Hawaii in 1958.¹⁹ The resulting record, known as the Keeling Curve, provided definitive proof that concentrations were rising steadily year after year alongside seasonal fluctuations driven by Northern Hemisphere vegetation. In the 1960s and 1970s, researchers including Syukuro Manabe and James Hansen developed computer-based general circulation models that simulated the three-dimensional physics of the atmosphere and oceans, confirming the sensitivity of global temperatures to rising greenhouse gas levels.

Earth's climate is governed by the balance between incoming solar radiation and outgoing thermal infrared radiation returned to space. When external factors alter this balance, the resulting perturbation is measured as radiative forcing, expressed in watts per square meter.²⁰ Positive radiative forcing traps additional energy within the climate system and warms the planet, whereas negative forcing cools it. Natural external forcings include cyclical eleven-year fluctuations in solar irradiance and episodic explosive volcanic eruptions, which inject sulfur dioxide into the stratosphere to form reflective sulfate aerosols that cool the surface for one to three years. Over the industrial era, however, human activities have generated a positive radiative forcing that far outweighs natural variations.

Multiple independent instrumental datasets—maintained by organizations including the European Centre for Medium-Range Weather Forecasts (Copernicus ERA5), NASA, the US National Oceanic and Atmospheric Administration (NOAA), the UK Met Office Hadley Centre, the Japan Meteorological Agency, and Berkeley Earth—confirm a rapid, sustained rise in global surface temperatures.²³²⁴ The eleven years from 2015 to 2025 are the eleven warmest years in the 176-year instrumental record. The year 2024 stands as the warmest individual year on record, with global surface air temperatures reaching approximately 1.60 °C above the 1850–1900 pre-industrial average in the ERA5 dataset (and 1.55 °C in WMO's consolidated analysis), boosted by a strong El Niño event in 2023–2024.²⁵ In 2025, despite the onset of cooler La Niña conditions, the global average temperature remained at 14.97 °C in ERA5 (1.47 °C above pre-industrial levels) and 1.44 °C ± 0.13 °C in the WMO eight-dataset consolidation, making 2025 the second or third warmest year on record behind 2024 and 2023. Land surfaces warm substantially faster than the ocean; for the 2021–2025 period, average temperatures over land were roughly 2.0 °C above pre-industrial levels, compared to 1.1 °C over the seas.

Because water has a high heat capacity, the global ocean absorbs more than 90 percent of the excess heat trapped in the Earth system. The average rate of ocean heat uptake nearly doubled from 0.6 watts per square meter between 1971 and 2024 to 1.0 watt per square meter between 2005 and 2024, with global ocean heat content reaching an all-time high in 2024 by exceeding the 2023 record by roughly 16 zetajoules.⁴ Simultaneously, the uptake of anthropogenic carbon dioxide alters seawater chemistry by increasing hydrogen ion concentrations and lowering carbonate ion availability, a process known as ocean acidification that impairs shell formation in corals, mollusks, and calcifying plankton. Warmer waters also hold less dissolved oxygen and increase thermal stratification, hindering nutrient mixing between deep and surface waters.

The cryosphere has experienced widespread contraction across both hemispheres. In the hydrological year 2023–2024, monitored reference glaciers worldwide suffered an observed record mass loss of 1.3 meters of water equivalent. The Greenland ice sheet is losing mass four times faster than in the early 2000s, while warm ocean currents are eroding floating ice shelves in West Antarctica from below. In March 2025, Arctic sea ice reached an annual maximum extent of 13.8 million square kilometers, the lowest maximum in the satellite record dating back to the late 1970s. In February 2025, Antarctic sea ice fell to an annual minimum of 2.1 million square kilometers, the third lowest on record after 2023, bringing combined polar sea ice cover to a historic satellite-era low.

Ocean thermal expansion and meltwater from land ice have driven a sustained rise in global mean sea level of approximately 21 to 24 centimeters since 1880. Between 1960 and 2023, thermal expansion accounted for roughly 43 percent of observed sea level rise, while melting glaciers and ice sheets contributed about 54 percent. The rate of rise has roughly doubled during the satellite altimetry era, accelerating from 2.1 millimeters per year between 1993 and 2002 to 4.1 millimeters per year between 2016 and 2025, with sea levels in 2025 standing about 11 centimeters higher than in 1993. Rising seas exacerbate coastal erosion, salinize freshwater aquifers, and increase the frequency of high-tide and storm-surge flooding for the roughly 770 million people living less than 5 meters above the high-tide line.

Across terrestrial and marine ecosystems, climate change has become a primary driver of biodiversity loss and habitat shifts. Species are migrating poleward and to higher elevations in search of cooler temperatures, while seasonal biological events such as flowering, insect emergence, and bird migration are occurring earlier in the spring, risking ecological mismatches between predators and prey. Tropical coral reefs face recurrent mass bleaching events as marine heatwaves exceed the thermal tolerance of symbiotic algae. For human populations, attribution studies confirm that anthropogenic warming has increased the frequency and intensity of deadly heatwaves, agricultural droughts, and extreme rainfall events. These hazards threaten global food and water security, expand the geographic range of vector-borne diseases such as dengue and malaria, reduce outdoor labor productivity, and drive internal and cross-border human displacement.

Responses to climate change are divided into two complementary categories: mitigation, which seeks to limit the magnitude and rate of warming by reducing greenhouse gas emissions and enhancing carbon sinks, and adaptation, which involves adjusting ecological, social, and economic systems to moderate harm from current and projected climatic impacts. Under the IPCC's assessment framework, halting global warming requires achieving net-zero carbon dioxide emissions, a state in which any residual human emissions are fully counterbalanced by durable removals of carbon dioxide from the atmosphere.

Mitigation centers on the transition away from unabated fossil fuels toward low-carbon energy sources. According to the International Energy Agency (IEA), global electricity generation from renewable sources reached approximately 9,900 terawatt-hours (TWh) in 2024—accounting for 32 percent of global power—and is projected to expand by 60 percent to 16,200 TWh by 2030, surpassing coal-fired generation between late 2025 and mid-2026 to become the world's largest source of electricity.²⁶ Solar photovoltaics and wind power account for the vast majority of this expansion, having already surpassed global hydropower output in 2024.²⁷ Beyond power generation, decarbonization relies on the direct electrification of end-use sectors, notably through the deployment of battery electric vehicles in road transport and electric heat pumps for space and water heating in buildings.²⁸ In heavy industries such as steel, cement, and chemicals, where high-temperature heat and process emissions are harder to abate, mitigation strategies incorporate energy efficiency, bioenergy, green hydrogen produced via renewable electrolysis, and carbon capture, utilization, and storage.²⁹³⁰ Land-based mitigation includes halting tropical deforestation, reforestation, afforestation, and regenerative agricultural practices that sequester carbon in soils and reduce fertilizer-driven nitrous oxide and livestock methane emissions. Some climate pathways also model large-scale carbon dioxide removal technologies, such as bioenergy with carbon capture and storage and direct air capture, alongside controversial research into solar radiation modification techniques that would reflect sunlight without addressing ocean acidification or greenhouse gas accumulation.

Because substantial warming and multi-century sea level rise are already locked in by past emissions, adaptation is required across all regions. Infrastructure adaptation includes constructing seawalls, storm-surge barriers, and elevated buildings, as well as implementing nature-based solutions such as restoring coastal mangroves, salt marshes, and urban green spaces to absorb floodwaters and mitigate the urban heat island effect. In agriculture and water management, adaptation involves breeding drought- and heat-tolerant crop varieties, expanding drip irrigation, and building desalination and water-recycling facilities. A major priority of the WMO and the United Nations is expanding multi-hazard early warning systems to protect vulnerable populations from extreme weather events.

Despite growing policy attention—with at least 56 countries having submitted formal National Adaptation Plans to the UN by mid-2024—a substantial adaptation finance gap persists. Developing nations, which have contributed the smallest share of historical emissions, often face the highest exposure and vulnerability to climatic shocks.³¹³² International assessments estimate that adaptation financing needs in developing countries range from $215 billion to $387 billion annually through 2030, which is ten to eighteen times higher than tracked public adaptation finance flows.

Because greenhouse gases mix globally in the atmosphere regardless of where they are emitted, addressing climate change requires multilateral cooperation. In 1988, the WMO and the United Nations Environment Programme established the Intergovernmental Panel on Climate Change (IPCC) to provide governments with regular, comprehensive scientific assessments of climate change, its impacts, and mitigation options. The IPCC's First Assessment Report in 1990 laid the scientific foundation for the United Nations Framework Convention on Climate Change (UNFCCC), an international treaty adopted at the Earth Summit in Rio de Janeiro in 1992 that entered into force in 1994.³³ Ratified by 197 parties, the UNFCCC established the objective of stabilizing greenhouse gas concentrations at a level that would prevent dangerous anthropogenic interference with the climate system, guided by the principle of "common but differentiated responsibilities and respective capabilities" between industrialized and developing nations.

To operationalize the convention, the parties adopted the Kyoto Protocol in December 1997, which entered into force in February 2005. The Kyoto Protocol established legally binding emission reduction targets exclusively for developed (Annex I) countries during an initial commitment period from 2008 to 2012, followed by the Doha Amendment covering 2013 to 2020. However, the protocol's effectiveness was constrained when the United States—then the world's largest cumulative emitter—declined to ratify it, and because rapidly industrializing economies such as China and India were exempt from binding reduction obligations.

Following years of negotiations to create a universal framework, 195 parties adopted the Paris Agreement at the 21st Conference of the Parties (COP21) on December 12, 2015, which entered into force on November 4, 2016. The Paris Agreement commits all participating nations to hold the increase in global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C. Rather than imposing top-down targets, the treaty requires every country to submit and update Nationally Determined Contributions (NDCs) every five years, outlining domestic climate action plans designed to become progressively more ambitious over time.

Subsequent annual UN climate summits have focused on rulebooks, fossil fuel phase-downs, and climate finance. At COP28 in Dubai in December 2023, negotiators completed the first Global Stocktake of the Paris Agreement, concluding with a consensus decision calling on countries to transition away from fossil fuels in energy systems and to triple global renewable power capacity by 2030. At COP29 in Baku, Azerbaijan, in November 2024, parties negotiated a New Collective Quantified Goal on climate finance to replace a previous $100 billion annual pledge, agreeing that developed countries would take the lead in mobilizing at least $300 billion per year for developing nations by 2035. At COP30, held in Belém, Brazil, in November 2025, countries agreed to triple funding aimed at helping developing nations adapt to climate impacts and to increase support for workers and communities navigating the clean energy transition, though UN officials noted that aggregate national pledges remained insufficient to keep warming within the 1.5 °C limit.

While the physical reality of anthropogenic climate change is supported by an overwhelming consensus among climate scientists and national scientific academies worldwide, public perception and political action have been shaped by decades of organized controversy and economic debate.³⁴ Beginning in the late 1980s and 1990s, as international negotiations moved toward binding emissions limits, fossil fuel corporations, industry trade associations such as the Global Climate Coalition, and conservative think tanks launched public relations and lobbying campaigns in the United States and other Western nations to challenge climate policies.³⁵

Investigative reporting in 2015 and subsequent academic analyses—including a 2023 study in the journal Science evaluating internal corporate documents—demonstrated that scientists employed by Exxon (later ExxonMobil) between 1977 and 2003 had developed climate models that accurately projected the trajectory of fossil-fuel-driven global warming, with 63 to 83 percent of their internal projections matching subsequent observed temperature increases.³⁶³⁷ Despite this internal scientific knowledge, the company and allied industry groups spent millions of dollars funding advocacy organizations and publishing regular newspaper advertorials from 1989 to 2010 emphasizing scientific uncertainty and opposing the ratification of the Kyoto Protocol.³⁸ Analysts, including the Union of Concerned Scientists, documented that these campaigns employed strategies and personnel previously used by the tobacco industry to dispute the health risks of smoking. These revelations prompted congressional hearings and numerous lawsuits by state and municipal governments in the United States alleging that major oil companies misled shareholders and the public regarding climate risks, though several early investor-fraud suits, such as a 2018 case brought by the New York Attorney General, were dismissed by courts.

As empirical evidence of warming became increasingly unmistakable in the twenty-first century, public controversy largely shifted from outright denial of rising temperatures toward disputes over the pace, cost, and equity of the energy transition. In domestic politics across Europe, North America, and Australia, policies such as carbon taxes, phasing out internal combustion vehicles, and eliminating fossil fuel subsidies have periodically faced voter backlash over energy affordability, inflation, and job losses in coal-, oil-, and gas-producing regions. Internationally, geopolitical friction centers on historical responsibility and climate justice: developing nations and small island states emphasize that industrialized countries generated the majority of cumulative historical emissions and therefore bear a moral and legal obligation to finance both mitigation and compensation for "loss and damage" suffered by vulnerable populations, whereas developed nations have sought broader financial contributions from high-emitting emerging economies.