Climate change can have natural and human causes, but the dominant cause of the current long term global warming observed since the nineteenth century is human activity, particularly the increase in atmospheric greenhouse gas concentrations. Natural climate influences include changes in solar output, volcanic eruptions, Earth's orbital configuration, internal ocean atmosphere variability and long term geological processes. These mechanisms operate over different timescales and can produce warming or cooling.
Human activities change atmospheric composition and land surface properties on timescales of decades to centuries. Burning fossil fuels is the largest source of anthropogenic carbon dioxide emissions, while land use change, cement production, agriculture, industrial processes and waste contribute additional greenhouse gases. Greenhouse gases alter the balance between incoming solar energy and outgoing infrared energy, causing a positive radiative forcing that raises global temperature until the climate system reaches a new energy balance. The modern warming pattern is therefore explained by the combined effects of physical climate forcings, with human greenhouse gas emissions providing the dominant persistent forcing.
Carbon dioxide is especially important because it is released in large quantities by fossil fuel combustion and remains influential in the climate system for a long time. When coal, oil or natural gas is burned, carbon combines with oxygen and forms carbon dioxide. Some emitted carbon dioxide is absorbed by oceans and terrestrial ecosystems, but a substantial fraction accumulates in the atmosphere.
Atmospheric carbon dioxide concentrations have risen substantially above pre industrial levels and are now far outside the range associated with the relatively recent pre industrial climate. Carbon dioxide absorbs infrared radiation at particular wavelengths and contributes to the greenhouse effect. Because carbon dioxide is chemically and physically coupled to the carbon cycle, its influence is not determined solely by its concentration at one instant. Some carbon remains in the atmosphere for centuries or longer, while other portions are transferred among oceans, vegetation, soils and geological reservoirs. The long residence and cumulative nature of carbon dioxide emissions make it a central driver of long term climate change.
Methane and nitrous oxide are additional important anthropogenic greenhouse gases. Methane is emitted by sources including fossil fuel extraction and distribution, livestock digestion, manure management, rice cultivation, landfills and wetlands, with the relative contribution varying by region. Methane has a much shorter atmospheric lifetime than carbon dioxide but is considerably more effective at trapping infrared radiation during the period in which it remains in the atmosphere.
Oxidation of methane eventually produces carbon dioxide and water, linking methane to broader atmospheric chemistry and climate processes. Nitrous oxide is emitted from agricultural soils, particularly through the use of nitrogen fertilisers and manure, as well as from industrial and other processes. Fluorinated gases are emitted in smaller quantities but can have very high warming potentials and long atmospheric lifetimes. Different greenhouse gases therefore differ in concentration, lifetime, radiative efficiency and sources, which is why climate assessments use measures such as carbon dioxide equivalent to compare their approximate warming effects over specified time horizons.
Human activities also change climate through aerosols and land use. Aerosols are tiny particles or droplets suspended in the atmosphere. Some, such as sulphate aerosols produced from sulphur containing fuel combustion, reflect incoming sunlight and can exert a cooling influence. Aerosols can also affect clouds, changing their reflectivity and lifetime. Other particles, such as black carbon, can absorb sunlight and produce warming effects in the atmosphere while also contributing to snow and ice darkening when deposited on bright surfaces.
Air pollution therefore has complex and sometimes competing climate effects. Land use change can alter surface reflectivity, vegetation cover, evapotranspiration, soil carbon and regional energy and water cycles. Deforestation can release carbon dioxide while reducing future carbon uptake, although the overall climate effect of land use depends on location, vegetation, soil and timescale. These influences do not overturn the dominant greenhouse gas explanation for modern global warming but help explain regional and short term variations in observed climate change.
Natural climate forcing has played an important role throughout Earth's history. Volcanic eruptions can inject sulphate forming gases into the stratosphere, where resulting aerosols reflect sunlight and can cause temporary global cooling lasting months to several years. Large volcanic eruptions can therefore produce noticeable short term changes in global temperature. Solar activity also varies over time, but measured changes in solar output are too small to explain the magnitude and pattern of modern long term warming.
Earth's orbital parameters change over thousands to hundreds of thousands of years and influence the distribution of sunlight across seasons and latitudes, contributing to natural ice age cycles. Ocean circulation and atmospheric variability can redistribute heat without necessarily changing the total energy entering the Earth system. These natural processes remain active today, but their timescales, magnitudes and characteristic patterns differ from the sustained forcing produced by anthropogenic greenhouse gas increases.
Climate feedbacks can amplify or reduce an initial forcing. A major positive feedback involves water vapour because warmer air can contain more water vapour, and water vapour itself is a greenhouse gas. Another important feedback involves snow and ice. As bright snow and ice surfaces shrink, darker land or ocean absorbs more solar radiation, increasing warming.
Clouds can exert both warming and cooling effects depending on their altitude, optical properties and location, making cloud feedback a significant source of uncertainty in climate projections. Carbon cycle feedbacks also matter. Warming can alter vegetation productivity, soil decomposition, ocean carbon uptake and other processes, potentially changing how much carbon remains in the atmosphere. Permafrost thaw can expose previously frozen organic matter to decomposition and release carbon dioxide and methane. Feedbacks do not create the original forcing but modify the response of the climate system to that forcing.
The physical evidence for the causes of modern climate change comes from multiple independent observations. Atmospheric greenhouse gas concentrations have increased alongside fossil fuel use and other human activities. The chemical characteristics of atmospheric carbon indicate a strong fossil carbon contribution to the observed carbon dioxide increase. The atmosphere and ocean are warming, sea level is rising, glaciers are retreating and ocean heat content is increasing.
Climate models reproduce major features of the observed warming when human influences are included, whereas simulations using only natural factors generally cannot reproduce the observed long term warming pattern. The vertical structure of atmospheric temperature changes also provides diagnostic information because greenhouse gas forcing produces characteristic patterns that differ from some alternative explanations. Attribution studies examine these observations together with physical models and statistical methods. This body of evidence allows scientists to distinguish the dominant drivers of modern climate change from natural variability and shorter lived forcings.
Understanding causes is essential because different causes require different responses. Reducing carbon dioxide emissions addresses the long lived forcing responsible for much of modern warming, while reducing methane can produce relatively rapid climate benefits because methane has a shorter atmospheric lifetime. Controlling aerosol pollution has important health benefits but can remove some temporary cooling effects, meaning that climate responses can change as air quality improves.
Protecting forests and other ecosystems can reduce carbon emissions while maintaining carbon storage and other ecological functions. Adaptation remains necessary because some warming and associated impacts are already locked in by past emissions and because climate variability will continue. A common misconception is that identifying natural climate change disproves human caused climate change.
Natural climate change is well established, but the relevant scientific question for the modern period is which physical forcings explain the observed magnitude, timing and pattern of current changes. Evidence from atmospheric chemistry, physics, observations and modelling consistently identifies human greenhouse gas emissions as the dominant cause of recent global warming.
