Causes of climate change
The scientific community has been investigating the causes of current climate change for decades. After thousands of studies, the scientific consensus is that it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." This consensus is supported by around 200 scientific organizations worldwide. The scientific principle underlying current climate change is the greenhouse effect, which provides that greenhouse gases pass sunlight that heats the earth, but trap some of the resulting heat that radiates from the planet's surface. Large amounts of greenhouse gases such as carbon dioxide and methane have been released into the atmosphere through burning of fossil fuels since the industrial revolution. Indirect emissions from land use change, emissions of other greenhouse gases such as nitrous oxide, and increased concentrations of water vapor in the atmosphere, also contribute to climate change.
The warming from the greenhouse effect has a logarithmic relationship with the concentration of greenhouse gases. This means that every additional fraction of and the other greenhouse gases in the atmosphere has a slightly smaller warming effect than the fractions before it as the total concentration increases. However, only around half of emissions continually reside in the atmosphere in the first place, as the other half is quickly absorbed by carbon sinks in the land and oceans. Further, the warming per unit of greenhouse gases is also affected by feedbacks, such as the changes in water vapor concentrations or Earth's albedo.
As the warming from increases, carbon sinks absorb a smaller fraction of total emissions, while the "fast" climate change feedbacks amplify greenhouse gas warming. Thus, the effects counteract one another, and the warming from each unit of emitted by humans increases temperature in linear proportion to the total amount of emissions. Further, some fraction of the greenhouse warming has been "masked" by the human-caused emissions of sulfur dioxide, which forms aerosols that have a cooling effect. However, this masking has been receding in the recent years, due to measures to combat acid rain and air pollution caused by sulfates.
Factors affecting Earth's climate
A forcing is something that is imposed externally on the climate system. External forcings include natural phenomena such as volcanic eruptions and variations in the sun's output. Human activities can also impose forcings, for example, through changing the composition of Earth's atmosphere. Radiative forcing is a measure of how various factors alter the energy balance of planet Earth. A positive radiative forcing will lead towards a warming of the surface and, over time, the climate system. Between the start of the Industrial Revolution in 1750, and the year 2005, the increase in the atmospheric concentration of carbon dioxide led to a positive radiative forcing, averaged over the Earth's surface area, of about 1.66 watts per square metre.Climate feedbacks can either amplify or dampen the response of the climate to a given forcing.
There are many feedback mechanisms in the climate system that can either amplify or diminish the effects of a change in climate forcing.
The climate system varies in response to changes in external forcings. The climate system also has internal variability both in the presence and absence of external forcings. This internal variability is a result of complex interactions between components within the climate system, such as the coupling between the atmosphere and ocean. An example of internal variability is the El Niño–Southern Oscillation.
Human-caused influences
Factors affecting Earth's climate can be broken down into forcings, feedbacks and internal variations. Four main lines of evidence support the dominant role of human activities in recent climate change:- A physical understanding of the climate system: greenhouse gas concentrations have increased and their warming properties are well-established.
- There are historical estimates of past climate changes suggest that the recent changes in global surface temperature are unusual.
- Advanced climate models are unable to replicate the observed warming unless human greenhouse gas emissions are included.
- Observations of natural forces, such as solar and volcanic activity, show that solar activity cannot explain the observed warming. For example, an increase in solar activity would have warmed the entire atmosphere, yet only the lower atmosphere has warmed.
Greenhouse gases
es 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. While water vapour and clouds are the biggest contributors to the greenhouse effect, they primarily change as a function of temperature. Therefore, they are considered to be feedbacks that change climate sensitivity. On the other hand, gases such as, tropospheric ozone, CFCs and nitrous oxide are added or removed independently from temperature. Hence, they are considered to be external forcings that change global temperatures.Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels, has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. Over the past 150 years human activities have released increasing quantities of greenhouse gases into the atmosphere. By 2019, the concentrations of and methane had increased by about 48% and 160%, respectively, since 1750. These 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.
This has led to increases in mean global temperature, or global warming. The likely range of human-induced surface-level air warming by 2010–2019 compared to levels in 1850–1900 is 0.8 °C to 1.3 °C, with a best estimate of 1.07 °C. This is close to the observed overall warming during that time of 0.9 °C to 1.2 °C. Temperature changes during that time were likely only ±0.1 °C due to natural forcings and ±0.2 °C due to variability in the climate.
Global anthropogenic greenhouse gas emissions in 2019 were equivalent to 59 billion tonnes of. Of these emissions, 75% was, 18% was methane, 4% was nitrous oxide, and 2% was fluorinated gases.
Carbon dioxide
emissions primarily come from burning fossil fuels to provide energy for transport, manufacturing, heating, and electricity. Additional emissions come from deforestation and industrial processes, which include the released by the chemical reactions for making cement, steel, aluminum, and fertiliser.is absorbed and emitted naturally as part of the carbon cycle, through animal and plant respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of carbon to the atmosphere, causing concentrations in the atmosphere to rise.
The high-accuracy measurements of atmospheric concentration, initiated by Charles David Keeling in 1958, constitute the master time series documenting the changing composition of the atmosphere. These data, known as the Keeling Curve, have iconic status in climate change science as evidence of the effect of human activities on the chemical composition of the global atmosphere.
Keeling's initial 1958 measurements showed 313 parts per million by volume. Atmospheric concentrations, commonly written "ppm", are measured in parts-per-million by volume. In May 2019, the concentration of in the atmosphere reached 415 ppm. The last time when it reached this level was 2.6–5.3 million years ago. Without human intervention, it would be 280 ppm.
In 2022–2024, the concentration of in the atmosphere increased faster than ever before according to National Oceanic and Atmospheric Administration, as a result of sustained emissions and El Niño conditions.
In November, 2025 Global Carbon Budget predicted emissions from burning coal, oil and gas would be a record 38.1 billion tonnes in 2025, up 1.1 percent from the prior year.
Methane and nitrous oxide
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.Methane and to a lesser extent nitrous oxide are also major forcing contributors to the greenhouse effect. The Kyoto Protocol lists these together with hydrofluorocarbon, perfluorocarbons, and sulfur hexafluoride, which are entirely artificial gases, as contributors to radiative forcing. The chart at right attributes anthropogenic greenhouse gas emissions to eight main economic sectors, of which the largest contributors are power stations, industrial processes, transportation fuels, and agricultural by-products.
Aerosols
Air pollution, in the form of aerosols, affects the climate on a large scale. Aerosols scatter and absorb solar radiation. From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth's surface was observed. This phenomenon is popularly known as global dimming, and is primarily attributed to sulfate aerosols produced by the combustion of fossil fuels with heavy sulfur concentrations like coal and bunker fuel. Smaller contributions come from black carbon, organic carbon from combustion of fossil fuels and biofuels, and from anthropogenic dust. Globally, aerosols have been declining since 1990 due to pollution controls, meaning that they no longer mask greenhouse gas warming as much.Aerosols also have indirect effects on the Earth's energy 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 droplets. They 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.