Climate change has multiple possible causes, but the causes relevant to the present period include both natural processes and human activities. Natural drivers include changes in Earths orbital configuration, variations in solar energy, volcanic eruptions, changes in ocean circulation, and long term geological processes. Human activities have become the dominant cause of the observed global warming since the middle of the twentieth century. The principal human influence is the emission of greenhouse gases, especially carbon dioxide, methane, and nitrous oxide. These gases alter the radiative balance of the planet by increasing the amount of infrared energy retained within the Earth system.
Carbon dioxide is the most important long lived greenhouse gas emitted by human activities. Large quantities are released when fossil fuels are burned for electricity generation, heating, transport, industry, and other purposes. Cement production also releases carbon dioxide through chemical reactions involved in converting limestone into clinker. Deforestation and land use change can release carbon stored in vegetation and soils while simultaneously reducing biological carbon uptake. Because carbon dioxide can remain influential in the climate system for a very long time, cumulative carbon dioxide emissions are particularly important for determining long term warming.
Methane is another important human influenced greenhouse gas. Major sources include livestock digestion, manure management, rice cultivation, landfills, fossil fuel extraction and distribution, and leaks from oil and gas systems. Methane has a much shorter atmospheric lifetime than carbon dioxide but is more effective per molecule at absorbing infrared radiation over particular time periods. Methane oxidation also produces carbon dioxide and affects atmospheric chemistry. Reducing methane emissions can therefore influence near term warming while reductions in carbon dioxide are central to limiting long term cumulative warming.
Nitrous oxide is another significant greenhouse gas and is strongly associated with agricultural activity, particularly the use of nitrogen fertilisers and manure management. Industrial processes and combustion also contribute. Fluorinated gases used in refrigeration, air conditioning, electronics, and industrial applications can have very high global warming potentials, even though their atmospheric concentrations are much lower than those of carbon dioxide. Changes in land use can additionally alter surface reflectivity, water cycles, carbon storage, and energy exchanges. Human influence on climate is therefore broader than fossil fuel combustion alone.
Natural factors continue to affect climate but do not explain the magnitude and pattern of recent global warming. Volcanic eruptions can temporarily cool global temperatures because sulfur containing gases can produce reflective aerosols in the stratosphere. Solar output varies naturally, but the observed recent warming pattern is not consistent with changes in solar energy being the dominant cause.
Orbital variations operate over thousands to hundreds of thousands of years and are important in explaining major past ice age cycles, but they cannot explain the rapid modern warming. Natural variability, including ocean atmosphere interactions, can affect year to year and decade to decade temperature patterns without accounting for the long term trend.
Climate feedbacks can amplify or moderate an initial climate forcing. A warming climate can reduce snow and ice cover, exposing darker surfaces that absorb more sunlight and therefore produce additional warming. Warmer air can hold more water vapour, and water vapour is itself a greenhouse gas, producing an important positive feedback. Changes in clouds can have both warming and cooling effects depending on their properties, altitude, and location. Ocean and terrestrial ecosystems can absorb some human emitted carbon dioxide, creating natural carbon sinks, but their capacity is not unlimited and can change as climate conditions and human land use change.
Climate change can also be influenced by changes in aerosols and atmospheric particles. Some human produced aerosols reflect sunlight or modify clouds and therefore exert a cooling influence that partly offsets greenhouse gas warming. Other particles, such as black carbon deposited on snow and ice, can contribute to warming by reducing surface reflectivity. Atmospheric pollution can therefore have competing climate effects. This complexity means that climate change cannot be reduced to one single pollutant or mechanism. Scientists quantify different radiative forcings and feedbacks to determine how various influences contribute to observed changes.
The strongest evidence for the causes of modern climate change comes from the agreement among multiple independent lines of evidence. Atmospheric greenhouse gas concentrations have risen substantially because of human activities, the chemical characteristics of atmospheric carbon show a fossil carbon contribution, observed warming follows the expected physical effects of increased greenhouse gases, oceans are gaining heat, and the observed pattern is inconsistent with natural factors alone. Climate models reproduce observed historical changes more accurately when human influences are included. The causes of climate change are therefore best understood as a combination of natural climate variability and external drivers, with human greenhouse gas emissions providing the dominant forcing behind current long term global warming.
