Climate change

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enhouse gases are trapping heat near the Earth’s surface and preventing it from radiating into space.

Regional aspects to temperature rises

See also: Climate variability and change § Variability between regions

Regions of the world warm at differing rates. The pattern is independent of where greenhouse gases are emitted, because the gases persist long enough to diffuse across the planet. Since the pre-industrial period, the average surface temperature over land regions has increased almost twice as fast as the global-average surface temperature. This is because of the larger heat capacity of oceans, and because oceans lose more heat by evaporation. The thermal energy in the global climate system has grown with only brief pauses since at least 1970, and over 90% of this extra energy has been stored in the ocean.The rest has heated the atmosphere, melted ice, and warmed the continents.

The Northern Hemisphere and the North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but also more seasonal snow cover and sea ice. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start absorbing more heat.Local black carbon deposits on snow and ice also contribute to Arctic warming.Arctic temperatures are increasing at over twice the rate of the rest of the world.Melting of glaciers and ice sheets in the Arctic disrupts ocean circulation, including a weakened Gulf Stream, further changing the climate.

Drivers of recent temperature rise

Main article: Attribution of recent climate change

Drivers of climate change from 1850–1900 to 2010–2019. There was no significant contribution from internal variability or solar and volcanic drivers.

The climate system experiences various cycles on its own which can last for years (such as the El Niño–Southern Oscillation (ENSO)), decades or even centuries. Other changes are caused by an imbalance of energy that is “external” to the climate system, but not always external to the Earth. Examples of external forcings include changes in the concentrations of greenhouse gases, solar luminosity, volcanic eruptions, and variations in the Earth’s orbit around the Sun.

To determine the human contribution to climate change, known internal climate variability and natural external forcings need to be ruled out. A key approach is to determine unique “fingerprints” for all potential causes, then compare these fingerprints with observed patterns of climate change.For example, solar forcing can be ruled out as a major cause. Its fingerprint would be warming in the entire atmosphere. Yet, only the lower atmosphere has warmed, consistent with greenhouse gas forcing. Attribution of recent climate change shows that the main driver is elevated greenhouse gases, with aerosols having a dampening effect.

Greenhouse gases

Main articles: Greenhouse gas, Greenhouse gas emissions, Greenhouse effect, and Carbon dioxide in Earth’s atmosphere

CO₂ concentrations over the last 800,000 years as measured from ice cores (blue/green) and directly (black)^{[citation needed]}

Greenhouse gases are transparent to sunlight, and thus allow it to pass through the atmosphere to heat the Earth’s surface. The Earth radiates it as heat, and greenhouse gases absorb a portion of it. This absorption slows the rate at which heat escapes into space, trapping heat near the Earth’s surface and warming it over time.Before the Industrial Revolution, naturally-occurring amounts of greenhouse gases caused the air near the surface to be about 33 °C warmer than it would have been in their absence. While water vapour (~50%) and clouds (~25%) are the biggest contributors to the greenhouse effect, they increase as a function of temperature and are therefore feedbacks. On the other hand, concentrations of gases such as CO₂ (~20%), tropospheric ozone,CFCs and nitrous oxide are not temperature-dependent, and are therefore external forcings.

Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels (coal, oil, and natural gas), has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. In 2019, the concentrations of CO₂ and methane had increased by about 48% and 160%, respectively, since 1750. These CO₂ levels are higher than they have been at any time during the last 2 million years. Concentrations of methane are far higher than they were over the last 800,000 years.

The Global Carbon Project shows how additions to CO₂ since 1880 have been caused by different sources ramping up one after another.

Global anthropogenic greenhouse gas emissions in 2019 were equivalent to 59 billion tonnes of CO₂. Of these emissions, 75% was CO₂, 18% was methane, 4% was nitrous oxide, and 2% was fluorinated gases.CO₂ emissions primarily come from burning fossil fuels to provide energy for transport, manufacturing, heating, and electricity. Additional CO₂ emissions come from deforestation and industrial processes, which include the CO₂ released by the chemical reactions for making cement, steel, aluminum, and fertiliser.Methane emissions come from livestock, manure, rice cultivation, landfills, wastewater, and coal mining, as well as oil and gas extraction. Nitrous oxide emissions largely come from the microbial decomposition of fertiliser.

Despite the contribution of deforestation to greenhouse gas emissions, the Earth’s land surface, particularly its forests, remain a significant carbon sink for CO₂. Land-surface sink processes, such as carbon fixation in the soil and photosynthesis, remove about 29% of annual global CO₂ emissions.The ocean also serves as a significant carbon sink via a two-step process. First, CO₂ dissolves in the surface water. Afterwards, the ocean’s overturning circulation distributes it deep into the ocean’s interior, where it accumulates over time as part of the carbon cycle. Over the last two decades, the world’s oceans have absorbed 20 to 30% of emitted CO₂.

Aerosols and clouds

Air pollution, in the form of aerosols, not only puts a large burden on human health, but also affects the climate on a large scaleFrom 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth’s surface was observed, a phenomenon popularly known as global dimming,typically attributed to aerosols from biofuel and fossil fuel burning.Globally, aerosols have been declining since 1990, meaning that they no longer mask greenhouse gas warming as much.

Aerosols scatter and absorb solar radiation. They also have indirect effects on the Earth’s radiation budget. Sulfate aerosols act as cloud condensation nuclei and lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger dropletsThey also reduce the growth of raindrops, which makes clouds more reflective to incoming sunlight.Indirect effects of aerosols are the largest uncertainty in radiative forcing.

While aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or ice can contribute to global warming. Not only does this increase the absorption of sunlight, it also increases melting and sea-level rise. Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.

Land surface changes

Humans change the Earth’s surface mainly to create more agricultural land. Today, agriculture takes up 34% of Earth’s land area, while 26% is forests, and 30% is uninhabitable (glaciers, deserts, etc.). The amount of forested land continues to decrease, which is the main land use change that causes global warming. Deforestation releases CO₂ contained in trees when they are destroyed, plus it prevents those trees from absorbing more CO₂ in the future. The main causes of deforestation are: permanent land-use change from forest to agricultural land producing products such as beef and palm oil (27%), logging to produce forestry/forest products (26%), short term shifting cultivation (24%), and wildfires (23%).

Land use changes not only affect greenhouse gas emissions. The type of vegetation in a region affects the local temperature. It impacts how much of the sunlight gets reflected back into space (albedo), and how much heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface lighter, causing it to reflect more sunlight. Deforestation can also affect temperatures by modifying the release of chemical compounds that influence clouds, and by changing wind patterns.In tropic and temperate areas the net effect is to produce significant warming, while at latitudes closer to the poles a gain of albedo (as forest is replaced by snow cover) leads to a cooling effect. Globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface albedo.

Solar and volcanic activity

Further information: Solar activity and climate

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. As the Sun is the Earth’s primary energy source, changes in incoming sunlight directly affect the climate system. Solar irradiance has been measured directly by satellites,and indirect measurements are available from the early 1600s onwards. There has been no upward trend in the amount of the Sun’s energy reaching the Earth. 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). If solar variations were responsible for the observed warming, the troposphere and stratosphere would both warm.

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.Present-day volcanic CO₂ emissions are equivalent to less than 1% of current anthropogenic CO₂ emissions.

Climate change feedback

Main articles: Climate change feedback and Climate sensitivity

Sea ice reflects 50% to 70% of incoming solar radiation while the dark ocean surface only reflects 6%, so melting sea ice is a self-reinforcing feedback.

The response of the climate system to an initial forcing is modified by feedbacks: increased by self-reinforcing feedbacks and reduced by balancing feedbacks.The main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and the net effect of clouds. The primary balancing mechanism is radiative cooling, as Earth’s surface gives off more heat to space in response to rising temperature In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of CO₂ on plant growth.Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.

As the air is warmed by greenhouse gases, it can hold more moisture. Water vapour is a potent greenhouse gas, so this further heats the atmosphere.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.effect of clouds is the largest source of feedback uncertainty.

Another major feedback is the reduction of snow cover and sea ice in the Arctic, which reduces the reflectivity of the Earth’s surface. More of the Sun’s energy is now absorbed in these regions, contributing to amplification of Arctic temperature changes.amplification is also melting permafrost, which releases methane and CO₂ into the atmosphere.Climate change can also cause methane releases from wetlands, marine systems, and freshwater systems. Overall, climate feedbacks are expected to become increasingly positive.

Around half of human-caused CO₂ emissions have been absorbed by land plants and by the oceans. On land, elevated CO₂ and an extended growing season have stimulated plant growth. Climate change increases droughts and heat waves that inhibit plant growth, which makes it uncertain whether this carbon sink will continue to grow in the future.^[Soils contain large quantities of carbon and may release some when they heat up. 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.Overall, at higher CO₂ concentrations the Earth will absorb a reduced fraction of our emissions.

Future warming and the carbon budget

Further information: Carbon budget, Climate model, and Climate change scenario

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.Models are used to calculate the degree of warming future emissions will cause when accounting for the strength of climate feedbacks.Models also include natural processes like changes in the Earth’s orbit, historical changes in the Sun’s activity, and volcanic forcing. In addition to estimating future temperatures, they 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.

The physical realism of models is tested by examining their ability to simulate contemporary or past climates.Past models have underestimated the rate of Arctic shrinkage and underestimated the rate of precipitation increase. Sea level rise since 1990 was underestimated in older models, but more recent models agree well with observations.The 2017 United States-published National Climate Assessment notes that “climate models may still be underestimating or missing relevant feedback processes”.

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 in the future. Depending on the socioeconomic scenario and the mitigation scenario, models produce atmospheric CO₂ concentrations that range widely between 380 and 1400 ppm.

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.These projections are based on climate models in combination with observations.

The remaining carbon budget is determined by modelling the carbon cycle and the climate sensitivity to greenhouse gases.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. 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.