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Human activity is the dominant cause of the long-term global warming observed since the 19th century. Natural variability still produces much of the year-to-year movement and can raise or lower temperatures for a decade or more, but it is too small to explain the sustained, worldwide warming by itself. Scientists reach that conclusion by comparing observations with the distinct “fingerprints” expected from greenhouse gases, solar changes, volcanic eruptions and internal climate variability.
The Intergovernmental Panel on Climate Change (IPCC) states that human influence has warmed the atmosphere, ocean and land since pre-industrial times. That assessment rests on several independent tests: measured greenhouse-gas increases, the size and pattern of warming, changes observed in different parts of the climate system, and climate-model experiments that reproduce the observed trend only when human influences are included.
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What scientists mean by “caused by humans”
Climate attribution is not a search for one event or one culprit. It is a comparison of competing explanations for a measured change. Researchers ask whether the timing, size, geographic pattern and vertical structure of the observed change match what a particular driver should produce, while allowing for measurement uncertainty and natural fluctuations.
For the current global warming trend, the leading candidates are human influences—especially emissions of carbon dioxide, methane and other well-mixed greenhouse gases, plus human-made aerosols and land-use changes—and natural influences, including volcanic eruptions, variations in solar output and internal ocean-atmosphere variability.
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Climate has always changed. Ice-age cycles, volcanic episodes and ocean circulation shifts demonstrate that natural causes can produce substantial changes. That history establishes that natural change is possible; it does not establish that natural causes are responsible for the warming measured today. Attribution depends on whether a proposed cause has the right fingerprint and enough strength to account for the observations.
The competing explanations and their expected fingerprints
| Candidate driver | Typical time scale | Expected signature | What observations show |
|---|---|---|---|
| Human greenhouse-gas increases | Decades to centuries, with warming continuing while concentrations and other conditions change | Warming of the surface and lower atmosphere, stronger warming over land and in the Arctic, ocean heat gain, a cooling stratosphere and a consistent energy imbalance | The observed multi-decadal pattern and cross-system changes match this combination |
| Other human forcings, including reflective aerosols and land-use changes | Years to decades, with regional and global effects | Cooling that partly offsets greenhouse-gas warming, with uneven regional patterns | IPCC assessments find a net cooling contribution from some anthropogenic forcings, but not enough to cancel greenhouse-gas warming |
| Solar variability | About 11-year cycles and longer variations | A broadly coherent response through the atmosphere, including warming rather than cooling at higher layers when solar output rises | Recent solar changes are too small and have the wrong overall pattern to explain the sustained recent warming |
| Volcanic eruptions | Months to a few years after major eruptions | Temporary global cooling from sunlight-reflecting aerosols in the stratosphere | Eruptions cause short-lived dips, not the persistent warming trend |
| Internal variability | Months to several decades | Redistribution of heat, especially between the ocean and atmosphere, producing temporary regional or global departures from the long-term trend | It explains many short-term ups and downs, but its estimated long-term contribution is small compared with observed warming |
A fingerprint is not a single graph. It can include when warming occurs, how it differs between day and night, how the atmosphere changes with altitude, and whether the ocean, land ice and ecosystems respond in physically consistent ways.
How the attribution test works
- Measure the change. Scientists combine surface thermometers, ocean observations, satellites, weather balloons, ice measurements and other records. They quantify trends over stated periods rather than treating one unusually hot or cold year as decisive.
- Estimate each driver. Physical models and observations provide the expected response to greenhouse gases, aerosols, solar changes, volcanic particles and internal variability. These responses include both temperature and non-temperature indicators.
- Compare patterns, not just averages. A cause must fit the spatial distribution, the seasonal cycle and the vertical profile of the change. A driver that produces the wrong pattern can be rejected even if it happens to produce a similar global average.
- Account for uncertainty. Instrument errors, incomplete historical coverage and the chaotic nature of weather are included in statistical estimates. The result is a range of likely contributions rather than a falsely precise number.
- Test combinations of causes. Simulations with natural drivers alone are compared with simulations that include human influences. Agreement across many independent observations is stronger evidence than success on one indicator.
This method is called detection and attribution. “Detection” means that a change is unusual relative to an appropriate estimate of natural variability. “Attribution” means that the change is consistent with a particular set of drivers and inconsistent with plausible alternatives.
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The figures below come from the IPCC’s 2021 assessment and refer to different periods and temperature metrics. They are assessed ranges, not exact readings from a single thermometer.
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| Assessment | Period and metric | Estimated contribution |
|---|---|---|
| Observed warming | Global surface warming from 1850–1900 to 2010–2019 | About 1.1°C (reported for the longer 1850–2020 record in the same FAQ) |
| Natural variability | Contribution to global surface warming over 1850–2020 | −0.23°C to +0.23°C |
| Observed warming | Global surface air temperature, 2010–2019 relative to 1850–1900 | 0.9°C to 1.2°C |
| Human-induced warming | Same 2010–2019 comparison | 0.8°C to 1.3°C |
| Internal variability | Same 2010–2019 comparison | −0.2°C to +0.2°C |
| Natural external forcings | Same 2010–2019 comparison | −0.1°C to +0.1°C |
| Best estimate of human-induced warming | 1850–1900 to 2010–2019 | 1.07°C |
| Well-mixed greenhouse gases | 2010–2019 relative to 1850–1900 | Likely increased global surface air temperature by 1.0°C to 2.0°C |
| Other anthropogenic forcings | 2010–2019 relative to 1850–1900 | Likely decreased global surface air temperature by 0.0°C to 0.8°C |
These ranges overlap because the components are estimated with uncertainty, but their relative sizes are clear: the assessed human contribution is large enough to account for essentially all of the observed warming, while internal variability and natural external forcings are much smaller over the same comparison.
Why greenhouse gases have the right physical mechanism
Carbon dioxide and other greenhouse gases absorb infrared radiation emitted by Earth’s surface. Increasing their concentration changes the altitude from which heat escapes to space. The planet must warm until outgoing energy again balances incoming energy, with much of the extra heat entering the ocean.
The atmospheric increase itself has an identifiable human signature. Carbon released from fossil fuels has a characteristic mix of carbon isotopes, and fossil-carbon emissions are accompanied by a decline in atmospheric oxygen. Measurements of these and other tracers, together with emission inventories, show that the increase in well-mixed greenhouse gases since around 1750 is caused by human activities. The IPCC describes that conclusion as unequivocal.
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Physics also predicts that greenhouse-gas warming will not be uniform. Land generally warms faster than the ocean because water mixes heat downward and evaporative cooling is stronger. The Arctic is expected to warm faster than the global average because of snow and ice feedbacks. Nights often warm faster than days in many regions. These patterns are present in observations and are difficult to produce with a change in the Sun alone.
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Evidence across the climate system
The atmosphere
Global surface records show a persistent rise in average temperature, superimposed on short-term fluctuations. The vertical pattern matters: the lower atmosphere has warmed while the stratosphere has cooled. Increasing greenhouse gases explain both changes—more heat is retained below and less reaches the stratosphere. A stronger Sun would tend to warm the atmosphere more broadly, so it does not match this fingerprint.
The ocean
The ocean has absorbed most of the excess heat in the climate system. Ocean heat content has increased over recent decades even when the surface record temporarily slows. Internal variability can move heat between the ocean and atmosphere, creating a few cooler surface years without removing the underlying energy gain. Ocean warming, thermal expansion and related sea-level rise provide an independent energy-accounting check on surface-temperature records.
Land and ice
Glaciers and ice sheets are losing mass in a warming climate, and spring snow cover and Arctic sea ice have declined over the long term. Permafrost is thawing in many areas. These responses are not controlled by a single thermometer and therefore strengthen the attribution when they occur alongside atmospheric and ocean changes.
Biological and seasonal indicators
Earlier spring timing, shifts in species ranges and changes in growing seasons can reflect warming, although each ecological record has additional local influences. Scientists use these indicators as part of a network of evidence rather than as a standalone proof.
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What climate-model experiments reveal
Attribution studies run ensembles of simulations under different forcing assumptions. In a “natural-only” experiment, the model receives estimated solar and volcanic changes but no human emissions. Those simulations can reproduce some short-term fluctuations, including cooling after major eruptions, yet they do not reproduce the observed long-term global warming pattern.
When greenhouse-gas emissions, human-made aerosols, land-use changes and natural forcings are included, the simulated trend and many regional details agree with observations. The comparison is not a claim that every year or location is predicted exactly. Weather remains chaotic, and models have known limitations. The relevant result is that the observed combination of warming, ocean heat uptake, atmospheric structure and regional patterns is very unlikely under natural forcing alone and is expected when human forcing is included.
How natural variability affects the recent record
Internal variability is generated within the climate system. Ocean-atmosphere patterns can shift heat between the upper ocean, deeper water and the atmosphere, while changes in circulation redistribute heat between regions. These processes can make a decade warmer or cooler than the long-term trend would suggest.
Short records can mislead
A trend calculated from only a few years is highly sensitive to the starting and ending points. A strong El Niño event can temporarily raise global surface temperature; a volcanic eruption can lower it for a year or two. Choosing a start year after a warm event and an end year after a cool event can make a real long-term trend appear flat.
Temporary slowdowns are not a reversal
During a period when the atmosphere warms slowly, the climate system can still gain energy. Heat may be entering the ocean, and natural variability can offset some surface warming. Scientists therefore examine ocean heat, sea level, ice and atmospheric measurements together instead of using a short surface-temperature interval as a test of the cause.
Regional weather is not the global mean
Natural variability can dominate a particular region for years or decades. A cold winter, a wet season or a local drought does not by itself confirm or refute global human-caused warming. Attribution of a specific event requires a separate analysis of that event’s probability, background climate and other drivers. Confidence is generally higher for the long-term global mean than for every local variable.
Why “the climate has always changed” is not a complete explanation
Past natural changes had identifiable causes, such as changes in Earth’s orbit, volcanic activity or ocean circulation. For the modern trend, scientists can measure the relevant forcings and test their expected responses. The question is not whether nature can change climate; it is whether the natural changes observed during the industrial era are large enough and have the right pattern.
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A practical way to evaluate a claim about the cause of warming
- Define the claim’s scope. Is it about global average temperature, a region, one season or a single event? A global attribution result should not be applied automatically to a local observation.
- Check the time period. Multi-decadal records are more informative about forced trends than a handful of years. Ask whether the chosen interval begins or ends with a known El Niño, volcanic eruption or other unusual event.
- Ask which driver has been measured. A proposed explanation should have an observed change in the relevant forcing, not merely a correlation with temperature.
- Look for a fingerprint. Does the explanation predict the observed vertical profile, regional pattern, ocean response and ice changes? Matching the global average alone is not enough.
- Compare alternative simulations. The strongest attribution studies test natural-only and human-plus-natural scenarios against the same observations.
- Read the uncertainty statement. “Likely” ranges describe assessed probability, not a lack of knowledge about the direction of the result. A range can be broad while still showing that one explanation is much more consistent than another.
What remains uncertain—and what does not
Scientists continue to refine estimates of climate sensitivity, regional precipitation changes, cloud responses, ocean circulation and the timing of individual extremes. Historical observations are sparser in some oceans and earlier decades, and internal variability limits how precisely a short record can be partitioned among causes.
Those uncertainties do not put the basic attribution in doubt. The evidence that human activities have warmed the atmosphere, ocean and land is assessed as unequivocal by the IPCC. Uncertainty affects the exact size of the contribution, the regional details and the projection of future change—not whether human influence is the dominant explanation for the observed long-term global warming.
The bottom line
Natural variability explains the wiggles: annual records, regional swings and some multi-year or decadal pauses. Human activities explain the rising baseline. Greenhouse-gas measurements, the physical fingerprint of warming, changes in the ocean and cryosphere, and model experiments all point to the same conclusion. No single hot year, cold season or short-term slowdown can overturn that multi-line evidence.
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