"Global warming" redirects here. For other uses, see Climate change

Solar and volcanic activity
Further information: Solar activity and climate
As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the climate system.^[98] Solar irradiance has been measured directly by satellites,^[108] and indirect measurements are available from the early 1600s onwards.^[98] Since 1880, there has been no upward trend in the amount of the Sun's energy reaching the Earth.^[109]
Explosive volcanic eruptions represent the largest natural forcing over the industrial era. When the eruption is sufficiently strong (with sulfur dioxide reaching the stratosphere), sunlight can be partially blocked for a couple of years. The temperature signal lasts about twice as long. In the industrial era, volcanic activity has had negligible impacts on global temperature trends.^[110] Present-day volcanic CO₂ emissions are equivalent to less than 1% of current anthropogenic CO₂ emissions.^[111]
Physical climate models are unable to reproduce the rapid warming observed in recent decades when taking into account only variations in solar output and volcanic activity.^[112] Further evidence for greenhouse gases causing global warming comes from measurements that show a warming of the lower atmosphere (the troposphere), coupled with a cooling of the upper atmosphere (the stratosphere).^[113] If solar variations were responsible for the observed warming, the troposphere and stratosphere would both warm.^[67]
Climate change feedback
Main articles: Climate change feedback and Climate sensitivity
Sea ice reflects 50% to 70% of incoming sunlight, while the ocean, being darker, reflects only 6%. As an area of sea ice melts and exposes more ocean, more heat is absorbed by the ocean, raising temperatures that melt still more ice. This is a positive feedback process.^[114]
The response of the climate system to an initial forcing is modified by feedbacks: increased by "self-reinforcing" or "positive" feedbacks and reduced by "balancing" or "negative" feedbacks.^[115] The main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and the net effect of clouds.^[116][117] The primary balancing mechanism is radiative cooling, as Earth's surface gives off more heat to space in response to rising temperature.^[118] In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of CO₂ on plant growth.^[119] Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.^[120]
As air warms, it can hold more moisture. Water vapour, as a potent greenhouse gas, holds heat in the atmosphere.^[116] If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. If clouds become higher and thinner, they act as an insulator, reflecting heat from below back downwards and warming the planet.^[121] The effect of clouds is the largest source of feedback uncertainty.^[122]
Another major feedback is the reduction of snow cover and sea ice in the Arctic, which reduces the reflectivity of the Earth's surface.^[123] More of the Sun's energy is now absorbed in these regions, contributing to amplification of Arctic temperature changes.^[124] Arctic amplification is also melting permafrost, which releases methane and CO₂ into the atmosphere.^[125] Climate change can also cause methane releases from wetlands, marine systems, and freshwater systems.^[126] Overall, climate feedbacks are expected to become increasingly positive.^[127]
Around half of human-caused CO₂ emissions have been absorbed by land plants and by the oceans.^[128] Climate change increases droughts and heat waves that inhibit plant growth, which makes it uncertain whether this carbon sink will continue to grow.^[129] Soils contain large quantities of carbon and may release some when they heat up.^[130] As more CO₂ and heat are absorbed by the ocean, it acidifies, its circulation changes and phytoplankton takes up less carbon, decreasing the rate at which the ocean absorbs atmospheric carbon.^[131] Overall, at higher CO₂ concentrations the Earth will absorb a reduced fraction of our emissions.^[132]
Modelling
Further information: Carbon budget, Climate model, Climate change scenario, and Earth's energy budget
Projected global surface temperature changes relative to 1850–1900, based on CMIP6 multi-model mean changes
A climate model is a representation of the physical, chemical and biological processes that affect the climate system.^[133] Models also include natural processes like changes in the Earth's orbit, historical changes in the Sun's activity, and volcanic forcing.^[134] Models are used to estimate the degree of warming future emissions will cause when accounting for the strength of climate feedbacks,^[135][136] or reproduce and predict the circulation of the oceans, the annual cycle of the seasons, and the flows of carbon between the land surface and the atmosphere.^[137]
The physical realism of models is tested by examining their ability to simulate contemporary or past climates.^[138] Past models have underestimated the rate of Arctic shrinkage^[139] and underestimated the rate of precipitation increase.^[140] Sea level rise since 1990 was underestimated in older models, but more recent models agree well with observations.^[141] The 2017 United States-published National Climate Assessment notes that "climate models may still be underestimating or missing relevant feedback processes".^[142] Additionally, climate models may be unable to adequately predict short-term regional climatic shifts.^[143]
Simplified model: Energy flows between space, the atmosphere, and Earth's surface, with greenhouse gases in the atmosphere absorbing and emitting radiant heat, affecting Earth's energy balance. Data as of 2007.
A subset of climate models add societal factors to a simple physical climate model. These models simulate how population, economic growth, and energy use affect—and interact with—the physical climate. With this information, these models can produce scenarios of future greenhouse gas emissions. This is then used as input for physical climate models and carbon cycle models to predict how atmospheric concentrations of greenhouse gases might change.^[144][145] Depending on the socioeconomic scenario and the mitigation scenario, models produce atmospheric CO₂ concentrations that range widely between 380 and 1400 ppm.^[146]
The IPCC Sixth Assessment Report projects that global warming is very likely to reach 1.0 °C to 1.8 °C by the late 21st century under the very low GHG emissions scenario. In an intermediate scenario global warming would reach 2.1 °C to 3.5 °C, and 3.3 °C to 5.7 °C under the very high GHG emissions scenario.^[147] These projections are based on climate models in combination with observations.^[148]
The remaining carbon budget is determined by modelling the carbon cycle and the climate sensitivity to greenhouse gases.^[149] According to the IPCC, global warming can be kept below 1.5 °C with a two-thirds chance if emissions after 2018 do not exceed 420 or 570 gigatonnes of CO₂. This corresponds to 10 to 13 years of current emissions. There are high uncertainties about the budget. For instance, it may be 100 gigatonnes of CO₂ smaller due to methane release from permafrost and wetlands.^[150] However, it is clear that fossil fuel resources are too abundant for shortages to be relied on to limit carbon emissions in the 21st century.^[151]
Even though the temperature will need to stay at or above 1.5 °C for 20 years to pass the threshold defined by the Paris agreement, a temporary rise above this limit also can have severe consequences. According to the World Meteorological Organization, there is a 66% chance that global temperature will rise temporarily above 1.5 °C in the years 2023–2027.^[152][153]
Impacts
Main article: Effects of climate change
The sixth IPCC Assessment Report projects changes in average soil moisture that can disrupt agriculture and ecosystems. A reduction in soil moisture by one standard deviation means that average soil moisture will approximately match the ninth driest year between 1850 and 1900 at that location.