What causes climate change?

What causes climate change?

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Summary

See Video 1 for an plain English summary and Video 2 for a comprehensive (also plain English) explanation.
The causes of climate change are often called ‘climate forcings’. This term comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy (heat) reaching Earth’s atmosphere and the amount that escapes.
If more solar energy escapes than arrives (negative RFs) the planet cools. Conversely, if less energy escapes than gets in (positive RFs) the planet warms (Fig. 1). This is because of the Law of Conservation of Energy, a basic law of thermodynamics, which states that:

Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.

Earth is taking in more energy than it releases back to space—a growing “energy imbalance” that is fueling global warming. – Udel et al, Phys.org December 2025

In recent decades the imbalance has risen dramatically, and in 2023 it reached 1.8 Wm-2, or twice as much as expected. – Thorsten, European Geosciences Union General Assembly April 2025

There are many climate forcings, each type contributing to how much solar energy arrives from the sun, and how much escapes.
Natural Forcings happen through natural changes; these were slowly cooling the planet over the past few thousand years due primarily to Milankovitch Cycles.
Anthropogenic Forcings due to human activities are far more powerful. They are causing temperatures to increase much more than natural cooling, and the pace is accelerating (Figs. 3 & 4).

Video 1: Earth's temperature has changed wildly, so what's the big deal about a few degrees?

Video 2: What causes climate change? A fascinating journey into the past, present, and future

Home > Climate wiki > What causes climate change?

Summary

See Video 1 for an plain English summary and Video 2 for a comprehensive (also plain English) explanation.
The causes of climate change are often called ‘climate forcings’. This term comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy (heat) reaching Earth’s atmosphere and the amount that escapes.
If more solar energy escapes than arrives (negative RFs) the planet cools. Conversely, if less energy escapes than gets in (positive RFs) the planet warms (Fig. 1). This is because of the Law of Conservation of Energy, a basic law of thermodynamics, which states that:

Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.

Earth is taking in more energy than it releases back to space—a growing “energy imbalance” that is fueling global warming. – Udel et al, Phys.org December 2025

In recent decades the imbalance has risen dramatically, and in 2023 it reached 1.8 Wm-2, or twice as much as expected. – Thorsten, European Geosciences Union General Assembly April 2025

There are many climate forcings, each type contributing to how much solar energy arrives from the sun, and how much escapes.
Natural Forcings happen through natural changes; these were slowly cooling the planet over the past few thousand years due primarily to Milankovitch Cycles.
Anthropogenic Forcings due to human activities are far more powerful. They are causing temperatures to increase much more than natural cooling, and the pace is accelerating (Figs. 3 & 4).

Video 1: Earth's temperature has changed wildly, so what's the big deal about a few degrees?

Video 2: What causes climate change? Journey into the past, present, and future

Why the energy imbalance is increasing (Radiative Forcing: RF)
Some incoming radiation from the sun is reflected back into space due to the albedo effect of the cryosphere (ice caps, permafrost, glaciers) and atmospheric aerosols much of which, paradoxically, come from burning fossil fuels. About 89% of the retained excess heat is being absorbed by the oceans. The land takes up ~6%, the cryosphere absorbs ~4%—which is why it’s rapidly melting—and the remaining 1% is in the atmosphere.

The imbalance leads to energy accumulation in the atmosphere, oceans and land, and melting of the cryosphere, resulting in increasing temperatures, rising sea levels, and more extreme weather. – Mauritsen et al 2025

The role of the land, ocean, and cryosphere in taking up most of the heat is included in climate models. However:

Worryingly, the observed energy imbalance is rising much faster than expected, reaching 1.8 W/m2 in 2023—or twice that predicted by climate models—after having more than doubled within just two decades. – op. cit.

While the land and oceans together absorbed 95% of the excess heat, they’re also doing double duty by absorbing (mostly via plants) 38% of the excess CO₂ we’re putting into the atmosphere. If not for them, temperatures would be much higher (see Greenhouses gases and how they work).

In 2024, the global annual growth rate of atmospheric CO₂ surged to a record of 3.73 ppm year—the highest since 1959—exceeding the 1.5°C climate threshold for the first time. – Dang et al April 2026

Human emissions were still climbing, but not nearly enough to account for all that extra CO_2. Dang et al all found that from 2014-2023 the land had been absorbing and average 3.22Gt/year of carbon. But in 2024—the year that temperatures spiked above 1.5°C (Fig. 1)—the land suddenly stopped absorbing about a third of the CO₂ that it had done over the previous ten years of measurements.
Figure 1: Temperatures 1940-2025
This explained the increasing energy imbalance (Fig. 2) and corresponding sudden increase in temperatures. The 6% of heat that the land had been taking up was driving up ground surface temperatures, increasing soil respiration, and triggering decomposition of soil organic matter—which released CO₂. Together with melting permafrost and an increase in wildfires and ongoing destruction of forests, the land has not only stopped absorbing as much CO₂, it’s now releasing some of the long-held CO₂ back into the atmosphere. A tipping point has been crossed.

….the underlying mechanism for the reduction was caused by hotter and drier conditions leading to a larger increase in respiration than photosynthesis….These results challenge previous assumptions about the long-term stability of the terrestrial carbon sink. – op cit.
Figure 2: Earth's increasing energy imbalance

Fig. 9: The imbalance between the incoming radiation (black line) and outgoing infrared radiation (red line). The imbalance leads to energy accumulation in the atmosphere, oceans, and land, leading to rising temperatures (Image: Leon Simmons, February 2025).
This had already been raised in 2021 at COP 26, but was ignored:

The only reason why the IPCC provides the world with the remaining carbon budget* of roughly 4 – 500 gigatonnes, is that the models assume that the carbon sink capacity of intact nature will continue. So not only are we assuming that biological systems will not cross tipping points, we’re also assuming that the stocks of carbon in forests in soils, in wetlands and permafrost, remain reasonably intact over the next 50 years. That is quite an optimistic assumption because it means we need to invest in conserving the remaining intact ecosystems. – Rockström, COP 26 (see Tipping points).

* The ‘carbon budget’ was the assumed amount of carbon that we could keep releasing into the atmosphere to keep temperatures under 1.5°C , on the assumption that the land and ocean would keep absorbing it in perpetuity.

The main climate forcings
Greenhouse gases (Fig. 3) change the chemistry of the atmosphere through natural (very slowly) and anthropogenic (human; very fast) processes.
Figure 3: The three dominant greenhouse gases

Fig. 3: The three dominant greenhouse gases in Earth’s atmosphere—carbon dioxide, methane, and nitrous oxide (Image credit: Zack Labe /NOAA March 2026)
Land use changes destroying biodiversity (anthropogenic; replacing natural ecosystems with livestock and agriculture)

Black carbon (soot) and ash (anthropogenic: forest fires & industrial pollution) and (natural: volcanoes)

Albedo (anthropogenic feedback effect) Ice caps, glaciers, and sea ice reflect solar radiation back into space (albedo effect) but these are melting (or in the case of sea ice, not forming) so more heat is retained. This leads to even less ice, and more heat being retained (feedback effect).

The Milankovitch Cycle (natural): how Earth orbits the Sun

Sunspots and solar activity (natural): variations in solar energy

Plate tectonics (natural): the position of continents

Ocean currents (natural): distributing heat and nutrients

Iron flux (natural): fertilising life in the oceans

Life (natural)

Aerosols (natural and anthopogenic)

Rocks from space (natural): not often, but dramatic!
Figure 4: How the primary forcings have contributed to temperature changes since 1850

Fig. 4: The climate forcings that have contributed to climate change from 1850 to 2023.The black line shows the observed global surface temperatures since 1850. ‘Natural’ forcings (green line) include volcanic eruptions (such as Mt. Pinatubo in 1991) which together generally cooled the atmosphere for short periods. It’s clear that the cooling effect of volcanic eruptions and even greater cooling effect of aerosol emissions during this period offset some of the warming. When added together, however (amber line), all of the cooling forcings aren’t enough to offset the main warming forcing: greenhouse gases (grey line) (Image: Zeke Hausfather, January 2025).

How do forcings work?
If the strength of cooling = warming, the forcings balance one another so the climate stays the same. But when several cooling forcings happen at the same time, they can push Earth into an ‘ice house’ cold state. Conversely, if several warming forcings compound one another, Earth is forced into a hot ‘greenhouse’ state.

One way to think of it is what happens when two people from opposite directions push a stool. You might both be pushing really hard, but if you’re both applying the same exact force, the stool won’t move. Humans are pushing so hard that we can see the climate tipping, overwhelming natural cooling forces (Fig. 4). But we can’t be certain when the climate will crash and break, so we just keep pushing. Once certain tipping points are reached, the geological record shows that the climate will become unstable for several thousand years until a new stable state is reached.
Figure 4: How each has contributed to temperature changes since 1850

Fig. 4: The climate forcings that have contributed to climate change from 1850 to 2023.The black line shows the observed global surface temperatures since 1850. ‘Natural’ forcings (green line) include volcanic eruptions (such as Mt. Pinatubo in 1991) which together generally cooled the atmosphere for short periods. It’s clear that the cooling effect of volcanic eruptions and even greater cooling effect of aerosol emissions during this period offset some of the warming. When added together, however (amber line), all of the cooling forcings aren’t enough to offset the main warming forcing: greenhouse gases (grey line) (Image: Zeke Hausfather, January 2025).

More information

Explainer: The Albedo Effect
Clean ice and snow have a very high albedo, that is, they reflect up to 90% of solar radiation back into space The ocean is much darker, so it has a very low albedo, reflecting only about 6% of the incoming solar radiation and absorbing the other 94%, warming it much faster than the snow and ice (Fig. 5).

As more ice forms, the water is cooler, leading to more ice forming, and so on, in a feedback effect. However,

Recent global temperature surge intensified by record-low planetary albedo – Science, 05 Dec. 2024 (Figs. 6 & 7)
Figure 5: The albedo of ice versus open water

Image: Nathan Kurtz / NASA
Figure 6: The loss of sea ice
Figure 7: Earth's declining albedo

Explainer: Timeline of events - Earth's changing climate
4.6 billion years ago Earth formed and the sun was only 70% as bright as it is today. Earth would have frozen, but the atmosphere was composed of hydrogen sulphide and the greenhouse gases methane and carbon dioxide, but no oxygen.

~4 billion years ago life appeared as blue-green algae called cyanobacteria. Using sunlight, they took carbon dioxide from the air & used water to convert it into energy, just as plants do today.

Over the next 1.7 billion years, the blue-green algae spread across the entire planet. They took so much carbon dioxide from the atmosphere and released so much oxygen (as a waste product) that by

2.4 billion years ago they had changed the chemistry of the atmosphere and caused a mass-extinction event (see the next Explainer for details).

The methane haze cleared, the skies turned blue, and although the sun was getting brighter (about 7% every billion years) the temperature slowly declined, until Earth became so cold that it may have been covered in ice (Video 3).

Volcanoes erupted, the planet warmed, but more ‘Snowball Earth’ events followed (although some may have been more ‘slushball’ than ‘snowball’) as the climate see-sawed between warm and cold.
Video 3: Snowball Earth
635 million years ago the last ‘Snowball Earth’ event ended

600 – 100 million years ago complex life evolved, continents collided, and natural climate forcings shifted Earth’s climate several times between cool ‘icehouse’ and warm ‘hothouse’ or ‘greenhouse’ climates.

358.9 – 298.9 million years ago the warmest ‘greenhouse’ event during this time was the Carboniferous when atmospheric carbon dioxide (CO₂₎ was ~800ppm (twice as much as today) and sea levels were 80-120m higher.

~100 million years ago multiple natural forcings set events in motion that led to our present day ice age.

See also: 2024: Judd et al; A 485-million-year history of Earth’s surface temperature, Science 385 | 6715

Explainer: Life caused the first major climate change - and mass extinction event
Around 2.4 – 2 billion years ago during the Paleoproterozoic era, the earliest life forms were anaerobic cyanobacteria, which produced oxygen as a waste product. So many produced so much oxygen that that they changed the chemistry of the atmosphere…and poisoned themselves in the process.This is known as the Great Oxidation Event, also called the Great Oxygenation Event, the Oxygen Crisis and the Oxygen Catastrophe (Video 4).
Video 4: That Time Oxygen Almost Killed Everything
While almost all of them died out, some were engulfed by eukaryotes to become endosymbiotic cyanobacteria. Over hundreds of millions of years, they evolved into chloroplasts: the green parts inside of plants that we see today, responsible for photosynthesis. They still produce the oxygen we need to survive and now play an essential role in the carbon cycle.

Explainer: Why coal, oil and gas are called 'fossil' fuels

Fossil fuels are literally the carbon in plants and animals that died millions of years ago. As the conditions that made them no longer exist, they cannot be replaced (at least in anything remotely relevant to human lifespans), so they are non-renewable.

Take coal for example. This comes from trees that grew in vast lowland swamp forests during the (appropriately named) Carboniferous Period, 358.9 million years ago (Mya), to 298.9 Mya. The high levels of carbon dioxide and even higher levels of oxygen in the atmosphere during that period plus the collision of continents that created low lying land and a hot wet climate, were the perfect conditions over millions of years for dead trees to fall into swampy ground. Here, they couldn’t be decomposed through normal processes. Instead, they turned into peat and eventually became the ‘fossil’ fuel coal. (See the carbon cycle on this website for more details and how oil and gas was made).

Today, burning this coal is releasing the carbon back into the atmosphere as carbon dioxide (CO₂₎_.

Explainer: How plate tectonics can release carbon dioxide
Where two plates are separating at a region where the rocks contain a lot of carbon, it can belch out large quantities of carbon dioxide (CO₂₎. For example, the East Africa rift releases about 20 megatonnes of CO₂ every year from magma below the crust. See this Univ ersity of Auckland paper, and these research papers:

2022: Muller et al; Evolution of Earth’s tectonic carbon conveyor belt, Nature volume 605, pages 629–63

2020: Muirhead et al; Displaced cratonic mantle concentrates deep carbon during continental rifting, Nature 582, pp 67–72

Explainer: How many tonnes of carbon does it take to add 1ppm of carbon dioxide to the atmosphere?
Carbon already in the atmosphere

(ppm = parts per million; Gt = one gigatonne or one billion tonnes)
- 2.13 Gt of carbon = 1 ppm currently in the atmosphere
- To convert carbon (C) to carbon dioxide (CO₂), first divide the atomic mass of carbon (12) by the atomic mass of CO₂(44) = 3.67.
- Then multiply this by 2.13 Gt carbon: 3.67 x 2.13 = 7.8 Gt carbon dioxide
- So, 7.8Gt carbon dioxide = 1ppm of CO₂currently in the atmosphere
- As there is currently around 415ppm* of CO₂ in the atmosphere, that’s 415 x 7.8 Gt = 3,373Gt CO₂.
* The amount of CO₂ in the atmosphere varies seasonally because plants accumulate carbon in the spring and summer and release some back to the air in autumn and winter. As the northern hemisphere has more land and plants, carbon dioxide levels go up in winter because plants become less productive. Annual measurements of carbon dioxide are an average of these ups and downs. On April 11, 2021, CO₂ in the atmosphere peaked at 420ppm

Calculations for adding carbon to the atmosphere from emissions

Emissions are NOT the same as concentrations! This is because the ocean and biosphere absorb around 55% of emissions. Currently, about 45% stays in the atmosphere.
- To calculate each additional ppm, divide 7.8 Gt / 0.45 = 17.3Gt
- So, it takes about 17.3Gt of CO₂emissions to add 1ppm to the atmosphere
But that number is changing because in the future, the oceans will not be able to absorb as much of this extra CO₂future, which means that fewer emissions will add more CO₂and thus more warming to the atmosphere.

Moreover, “Additional ecosystem responses to warming not yet fully included in climate models, such as CO₂ and CH₄ [methane] fluxes from wetlands, permafrost thaw and wildfires, would further increase concentrations of these gases in the atmosphere (high confidence).” – IPCC 2021 p41.

Explainer: Rocks from space
Earth has periodically been hit by asteroids, comets, and other space debris large enough to abruptly change the climate. The best-known event was the Cretaceous–Paleogene (K–Pg) extinction event ~65 million years ago when an asteroid impact blew dust, soil, and rocks not only into the atmosphere but also out into space, where it fell back into the upper atmosphere, creating a dust shroud for weeks to months. This blocked out sunlight, which led to a cool ‘impact winter’ for years.

Worse, the asteroid slammed into rocks rich in carbonates, that is, they were full of carbon. This, along with global-scale wildfires that also released huge quantities of CO₂ into the atmosphere from burning pretty much everything, led to a rapid global warming of ~5°C soon after the relatively brief ‘impact winter’ cooling period.

Evidence of the impact event can be seen in a layer of iridium (dust from the comet) in the Waipara River, North Canterbury.

There is also evidence that large scale volcanic eruptions in what is now Northern India also contributed to the warming.

Dust from an extraterrestrial impact event near (but not into) Earth ~466 million years has been implicated in an an ‘impact winter’ that led to an Ice Age.

The Eocene–Oligocene extinction event 35 million years ago may have been triggered in part by up to five impact events including Popigai, Siberia and Chesapeake Bay, US.

References and further reading
2026: Dang et al; Respiration-Induced Weakening of Land Sink Contributed to the Largest CO2 Increase Global Change Biology 32 |4: 01 April

2026: Marks-Peterson et al; Broadly stable atmospheric CO2 and CH4 levels over the past 3 million years, Nature 651 pp647-652

2025: Mauritsen et al; Earth’s Energy Imbalance More Than Doubled in Recent Decades, AGU Advances (Open access)

2025: Thorsten; Earth’s energy imbalance rising faster than expected, and we must keep a close watch European Geosciences Union General Assembly 2025 (EGU25), 27 April-2 May, 2025 in Vienna, Austria.

2025: Park et al; Negligible contribution from aerosols to recent trends in Earth’s energy imbalance, Science 11 | 48

Plain English: Earth’s growing heat imbalance driven more by clouds than air pollution, study finds

2025; Udel et al: Earth’s growing heat imbalance driven more by clouds than air pollution, study finds, Phys.org

2025: Quilcaille et al; Systematic attribution of heatwaves to the emissions of carbon majors, Nature (Open access) 645 pp392-398

2025: State of the Cryosphere Report, International Cryosphere Climate Initiative (ICCI), Stockholm, Sweden

2025: Copernicus: Global climate highlights 2024

2025: WMO confirms 2024 as warmest year on record at about 1.55°C above pre-industrial level

2024: Duspayev et al; Earth’s Sea Ice Radiative Effect from 1980 to 2023, Geophysical Research Letters 51 | 14 (Open access)

2024: State of the Climate 2024, Update for COP29

2024: State of the Climate 2023, Special Supplement to the Bulletin of the American Meteorological Society 105 | 8, August 2024

2024: Watson-Parris et al; Weak surface temperature effects of recent reductions in shipping SO2 emissions, with quantification confounded by internal variability (pre-print 09 July) EGUsphere/Copernicus

2024: Hodenbrog et al; Recent reductions in aerosol emissions have increased Earth’s energy imbalance, Nature Communications Earth & Environment 5 | 166 (Open access)

2024: Judd et al; A 485-million-year history of Earth’s surface temperature, Science 385 | 6715

Carbon Brief: Why scientists think 100% of global warming is due to humans

BBC: The event that transformed Earth

BBC: Earth was a frozen snowball when animals first evolved

NASA: Snowball Earth may have been slushy

The Conversation: Hothouse Earth: our planet has been here before—and here’s what it looks like

Wikipedia: Evolution of chloroplasts

National Geographic: The Carboniferous

2024: Global Climate Highlights 2023; Copernicus

2023: Lelieveld et al; Air pollution deaths attributable to fossil fuels: observational and modelling study, BMJ 383

2023: Aerosols: are SO2 emissions reductions contributing to global warming? Copernicus

2022: Muller et al; Evolution of Earth’s tectonic carbon conveyor belt, Nature volume 605, pages 629–63

2021: Denton et al; The Zealandia Switch: Ice age climate shifts viewed from Southern Hemisphere moraines: Quaternary Science Reviews 257.

2021: Kramer et al; Observational evidence of increasing global radiative forcing, Geophysical Research Letters, 25 March

2020: Muirhead et al; Displaced cratonic mantle concentrates deep carbon during continental rifting, Nature 582, pp 67–72

2019: Ayuso-Fernández et al; Peroxidase evolution in white-rot fungi follows wood lignin evolution in plants; PNAS 116 (36) 17900-17905

2019: Schmitz et al; An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body. Science Advances 5/9

2016: Gernon et al; Snowball Earth ocean chemistry driven by extensive ridge volcanism during Rodinia breakup, Nature Geoscience 9, 242-248

2015: Rene et al; State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact: Science 350 (6256) 76-78

2013 IPPC: Chapter 8: Anthropogenic and Natural Radiative Forcing in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change

2009: Koeberl; Late Eocene impact craters and impactoclastic layers—an overview in The Late Eocene Earth—Hothouse, Icehouse, and Impacts: The Geological Society of America Vol 452

2004: Pierrehumbert; High levels of atmospheric carbon dioxide necessary for the termination of global glaciation Nature 429, 646–649

Causes