Published

12 months ago

June 23, 2025

Alice Rush

Volcanic eruptions pose a fundamental challenge for scientists and their climate models.

It is well known that explosive eruptions can cause sudden cooling at the Earth’s surface and that multiple eruptions shape climate variability over decades and centuries.

When sulphur dioxide is injected into the stratosphere during an eruption, it forms aerosols that block sunlight from reaching the Earth’s surface.

Unlike human influences on climate change, which occur slowly and can be accounted for in climate models under a range of socioeconomic scenarios, the sporadic nature of volcanic eruptions poses a challenge for climate projections.

Scientists cannot currently forecast the occurrence of volcanic eruptions – including when and where they will occur and how much sulphur they will emit.

How, then, to account for the climate impact of volcanic eruptions when projecting into the future?

In a recent study, published in Communications Earth & Environment, we show that volcanic eruptions make a substantial contribution to the uncertainty in projections of global temperatures.

Our findings suggest that, when sporadic volcanic eruptions are included in climate projections, breaching of the Paris Agreement’s 1.5C warming limit is slightly delayed – but we will also see more decades with rapid rates of warming and cooling.

Volcanic forcing in climate projections

Climate scientists refer to the influence volcanic eruptions have on the climate – largely through the release of sulphur dioxide gas into the atmosphere – as “volcanic forcing”.

Current climate models apply a constant volcanic forcing value when running future projections. This value is calculated based on the historical average of forcings from 1850 to the present day.

This is the case with the Coupled Model Intercomparison Project (CMIP), the international modelling effort that feeds into the influential assessment reports from the Intergovernmental Panel on Climate Change (IPCC).

However, this approach has significant limitations.

For starters, historically averaged forcing does not capture the episodic nature of eruptions.

Large-magnitude volcanic eruptions happen sporadically – sometimes clustering within decades and other times leaving century-long gaps between events.

Meanwhile, the reference period of 1850 to the present day has seen a relatively low frequency of large-magnitude eruptions that emitted more than 3 teragrams (Tg) of sulphur dioxide (SO2), when compared to multimillennial records.

Finally, volcanic forcing reconstructions used in earlier generations of CMIP climate models did not include small-to-moderate magnitude eruptions that emitted less than 3Tg of SO2.

This is because these eruptions went largely undetected before the satellite era began in 1980. Nonetheless, these smaller, but more frequent, eruptions contribute to 30-50% of long-term volcanic forcing.

Taking a new approach

Traditionally, climate scientists have recognised three main sources of uncertainty in climate projections: internal variability, model uncertainty and scenario uncertainty.

Here, “internal” variability refers to natural fluctuations that are generated within the climate system, such as by El Niño; model uncertainty refers to the differences in the results between multiple climate models; and scenario uncertainty refers to the different ways that the world could develop over the decades to come.

Our results show that volcanic eruptions should be specifically considered as a fourth significant source of uncertainty in climate projections.

To explore how climate projections change when accounting for volcanic forcing uncertainty, our study uses a probabilistic approach that builds on a 2017 methodology developed by Bethke et al.

To do this, we develop “stochastic forcing scenarios” – essentially, 1,000 different plausible timelines of volcanic activity extending to the end of the century.

These scenarios draw from past volcanic activity recorded in ice cores going back 11,500 years, along with satellite measurements and geological evidence. Each scenario represents different combinations of eruption magnitudes, location, timing and frequency.

(In mathematics, “stochastic” systems involve randomness or uncertainty of outcome, making them unpredictable. This is in contrast to “deterministic” systems, which are characterised by having outcomes that are completely predictable based on initial conditions and a set of rules or equations.)

We then simulate climate projections using both stochastic and historically-averaged volcanic forcing between 2015 and 2100, exploring temperature rise under three different emissions scenarios drawn from the Shared Socioeconomic Pathways (SSPs). These are a low-emission scenario (SSP1-1.9), an intermediate scenario that is in line with current climate policies(SSP2-4.5) and a very-high emissions scenario (SSP5-8.5).

For this step, we use a simple climate model, or “emulator”, called FaIR.

By simulating 1,000 different volcanic futures, we find that the climate uncertainty caused by future 21st century eruptions could exceed the internal variability of the climate system itself over the same period.

We also find that volcanic eruptions could account for more than one-third of total uncertainty in global temperature projections until the 2030s.

You can see these results in the plot below. It shows the contribution to the total uncertainty from the different sources. The colours represent volcanoes (orange), internal variability (dark blue), climate model response (yellow) and scenarios of future human emissions (green).

Chart: Annual mean contribution of uncertainties — Annual average contribution to the total uncertainty in global average surface temperature from volcanic eruptions, internal variability, climate model response and human emissions scenarios from 2020 to 2100. Credit: Amended from Chim et al. (2025).

What this means for the 1.5C threshold

Our simulations demonstrate that incorporating possible timelines of volcanic activity slightly reduces the probability of crossing the Paris Agreement’s aspirational 1.5C temperature limit in the near term.

We find that – depending on the emissions scenario – the probability of exceeding 1.5C decreases by 4-10%, compared to projections using constant volcanic forcing.

While this might sound encouraging, future volcanic activity does not provide any long-term mitigation of human-caused warming.

The eruption of Mount Tambora in 1815 offers a dramatic illustration of this point. While the event cooled global temperatures by an average of 0.8C, it led to a “year without a summer” and caused crop failures and widespread famine across Europe, North America and China.

Eruptions produce temporary cooling lasting just a few years. They do not alter the underlying warming trend driven by human emissions.

Our study finds that, taking into account a range of future volcanic activity, global warming will still exceed 1.5C within decades under all but the very lowest emissions scenarios.

A high level of volcanic activity over the 21st century would help offset just a small fraction of global warming – meaning that emission reduction remains essential for meeting long-term climate goals.

The charts below show the probability of scenarios exceeding 1.5C using stochastic volcanic forcing (solid lines) and constant volcanic forcing (dashed lines) under three emissions scenarios (top) and the difference in probability between the two forcing approaches (bottom).

Charts: Probability of exceeding 1.5C — The top chart (a) shows the probability of scenarios exceeding 1.5C using stochastic volcanic forcing (solid lines) and 1850-2014 mean historically-averaged forcing (dotted lines) for SSP1-1.9 (very low emissions), SSP2-4.5 (approximate current policies) and SSP5-8.5 (very high emissions) scenarios. The lower chart (b) shows the difference in probability in exceeding 1.5C between the simulations and historically-averaged forcing and stochastic volcanic forcing. Credit: Chim et al. (2025)

Decadal-scale temperature variability

Another important insight from our research is that extreme warm and cold decades become more likely once the variability of volcanic forcing is accounted for.

We find that the chance of a negative decadal trend – a decade where global surface temperature cools on average – increases by 10-18% under the intermediate emissions scenario.

We also find a corresponding increase in the probability of extremely warm decades, reflecting how volcanic forcing variability enhances the likelihood of both cooling and warming extremes.

This underscores how volcanic eruptions could introduce significant variability into the global temperature trends over decadal timescales.

Toward better climate projections

Understanding volcanic effects on the climate is essential for comprehensively assessing future risks to agriculture, infrastructure and energy systems.

Running thousands of volcanic scenarios with full-scale Earth system models is not practical as it requires too much computing power. On the other hand, current approaches have significant limitations, as described above.

However, there is a middle ground for future climate modelling efforts.

The next phase of future climate modelling experiments – the Scenario Model Intercomparison Project for CMIP7 – can use a more representative “average” volcanic forcing baseline that incorporates the effects of small eruptions often missed in historical records. This bias has now been addressed in the historical volcanic forcing dataset that will underpin the next generation of climate model simulations.

Additionally, modelling teams should run additional scenarios with high and low future volcanic activity to capture the range of volcanic uncertainty on climate projections.

While human-caused greenhouse gas emissions remain the dominant driver of climate change, properly accounting for volcanic uncertainty provides a more complete picture of possible climate futures and their implications for society.

The post Guest post: Investigating how volcanic eruptions can affect climate projections appeared first on Carbon Brief.

Guest post: Investigating how volcanic eruptions can affect climate projections

Climate Change

On the Historic Route From Selma to Montgomery, an AI Cloud Looms

Published

12 hours ago

June 10, 2026

Alice Rush

In this rural Alabama community, some residents can’t flush their toilets. Developers want to build a state-of-the-art data center next door.

By Lee Hedgepeth

HAYNEVILLE, Ala.—When Alabamians marched from Selma to Montgomery in 1965 to demand voting rights for African Americans, Highway 80 became their path toward freedom.

On the Historic Route From Selma to Montgomery, an AI Cloud Looms

Climate Change

Guest post: How a record-high ‘energy imbalance’ is driving global warming

Published

15 hours ago

June 10, 2026

Alice Rush

The planet is heating up more quickly than ever before.

For decades, greenhouse gas emissions caused by human activity have been building up in the atmosphere and trapping ever-higher levels of heat.

The resulting asymmetry between incoming solar energy and energy radiated back out into space – known as “Earth’s energy imbalance” – provides a direct measure of the extent to which humans are disrupting the Earth’s climate system.

This imbalance is growing and in 2025 its 10-year average reached a record high, indicating that global temperatures could increase at even higher rates in the future.

This is among the headline findings of the latest “indicators of global climate change” (IGCC) report, published in the journal Earth System Science Data, which tracks changes in the climate system on an annual basis.

The report, now in its fourth iteration, has been produced by dozens of scientists from around the world.

Its findings are designed to fill the gap between Intergovernmental Panel on Climate Change (IPCC) science reports, which are published every 5-7 years.

In this article, we unpack the IGCC report, which explores how human activity is driving a growing energy imbalance and why monitoring systems to track global climate are so crucial.

(For more on previous IGCC reports, see Carbon Brief’s coverage in 2023, 2024 and 2025.)

Greenhouse gas emissions remain at an all-time high

Global greenhouse gas emissions are continuing to increase, mostly as a result of the use of fossil fuels. However, deforestation, agriculture and industrial processes also play an important role.

Glossary

CO₂ equivalent: Greenhouse gases can be expressed in terms of carbon dioxide equivalent, or CO2e. For a given amount, different greenhouse gases trap different amounts of heat in the atmosphere, a quantity known as… Read More

Over the most recent decade (2015-24), emissions stood at the equivalent of 54.6bn tonnes of carbon dioxide equivalent (GtCO2e) per year. In 2024, the most recent year for which we have complete data, emissions reached 56.8GtCO2e.

As the chart below shows, these emissions have pushed up atmospheric levels of CO2, methane and nitrous oxide. In 2025, concentrations of these gases reached 425.6 parts per million (ppm), 1936.3 parts per billion (ppb) and 339.4ppb, respectively.

This represents a rise of 3.8%, 3.8% and 2.2%, respectively, since the 2019 levels reported in the IPCC’s sixth assessment report (AR6).

Atmospheric concentrations of CO2 (yellow), methane (blue) and nitrous oxide (green) over 2000-25. The grey-shaded region represents continuing changes since AR6. Note the different vertical scales for each gas. Credit: Forster et al. (2026)

At the same time, declines in emissions of aerosols such as sulphur dioxide, partly as a result of efforts to tackle air pollution, are increasing the Earth’s energy imbalance. This is because aerosols have a cooling effect on the Earth’s climate, counteracting warming from CO2 and other greenhouse gas emissions.

(Tackling sulphur dioxide, alongside other particulate emissions, remains critical because the immediate health and environmental damage they cause far outweighs their short-term cooling effect on the climate.)

The Earth’s energy imbalance is rising rapidly

The Earth’s energy imbalance has long been recognised as a key indicator of how the climate is being affected by human activities.

However, it is only in the last few decades that scientists have been able to record temperature changes deep enough in the ocean to accurately quantify it.

Earth’s energy imbalance measures how quickly excess heat is accumulating in every part of the Earth system, primarily in the ocean, but also in land, ice and atmosphere.

Through this accumulation of heat, the energy imbalance influences the rate of sea level rise and ice melt across the world, as well as increasing the frequency and intensity of extreme weather events, such as storms, floods and droughts.

Without human influence, the Earth’s energy imbalance would be close to zero.

But, as greenhouse gas emissions have built up in the atmosphere, the imbalance has been growing since the 1970s. Recent increases to Earth’s energy imbalance have outpaced those projections made by climate models — indicating the planet could see more warming than expected in the future.

As the right-hand chart below shows, the imbalance is now at a record high, having more than doubled over the past two decades.

It has increased by around 40% since 2019, from an average 0.79 watts per square metre (Wm2) over 2006-18, according to IPCC AR6, to 1.12Wm2 over 2013-25.

The left-hand chart shows how heat is accumulating in the ocean (blues), ice (grey), land (orange) and atmosphere (purple).

Left: Observed changes in the Earth heat inventory for the period 1971-2020. Right: Estimates of the Earth energy imbalance for successive overlapping 20-year periods and the most recent decade (right). Shaded regions indicate the very likely range (90-100 % probability), while the stars show the CERES (NASA Clouds and the Earth’s Radiant Energy System) estimates for comparison. Credit: Forster et al. (2026)

Global temperature rise

The excess heat building up in the climate system from the energy imbalance is pushing up global temperatures at a record rate of 0.27C per decade.

We estimate that human-induced warming – the amount of observed global surface

temperature increase attributable to both the direct and indirect effects of human activities – reached 1.37C in 2025. This has risen from 1.0C in 2017, as reported in IPCC AR6.

While natural variability in the climate system – such as El Niño or La Niña events – can also influence temperatures year-to-year, the upward temperature trend we are seeing is being driven by the persistent imbalance in energy.

We now expect global temperatures to exceed the Paris Agreement limit of 1.5C above pre-industrial levels around the year 2030.

This is significant because 1.5C has been identified as the critical dividing line between manageable climate risks and catastrophic, potentially irreversible damage to global ecosystems and human societies.

Heat accumulating throughout the Earth system

While heat is accumulating throughout the Earth system, it is not being distributed evenly around the globe.

Since the 1970s, around 90% of this heat has been taken up by the ocean, affecting marine ecosystems, ocean circulation patterns, sea level rise and climate extremes.

For example, the number of marine heatwave days – periods of unusually high sea surface temperatures – has more than tripled globally since the early 1990s. The year 2025 alone saw 65 days of marine heatwaves – meaning they occurred, on average, more than one day a week.

Meanwhile, the cryosphere – the portion of the Earth made up of frozen water, including glaciers, ice sheets and permafrost – is experiencing widespread ice loss and thawing in response to the growing energy imbalance. This affects ecosystems, sea level rise and infrastructure in polar and high-latitude regions.

Rapid warming has also resulted in record extreme temperatures over land, with average maximum temperatures for any single day over 2016-25 around 1.92C above pre-industrial levels). This is an increase of almost half a degree compared to the previous decade (2006-15).

Sea level rise and the energy imbalance

Sea level rise provides one of the clearest long-term signals of a changing planet.

It is closely linked to Earth’s energy imbalance. As heat accumulates in the ocean, water expands, raising sea levels. Meanwhile, a warming land and atmosphere means addition of water to the oceans through melting of glaciers and ice sheets, also adding to sea level rise.

Over the long-term, sea levels have been rising, on average, at a rate of around 1.8mm per year since 1901, totalling a record 23cm in 2025. This is increasing the risk of coastal flooding, erosion and habitat loss in many low-lying areas around the world.

This rise can be seen in the left-hand chart below, which shows observed global sea level changes from tide gauges (grey and blue dashed lines) and satellites (red dashed lines) since 1901. The solid lines indicate the average across multiple datasets.

Sea level rise is accelerating consistent with the observed increase in Earth’s energy imbalance. Over 2006-25, sea levels have risen at a rate of 3.67mm per year – more than double the rate of 1.69mm per year seen over 1976-95.

This increasing rate is shown in the right-hand figure below, which shows four successive overlapping 20-year periods and the most-recent decade.

(Last year’s transition from El Niño to weak La Niña conditions affected global rainfall patterns and led to a small and temporary fall in global average sea level in 2025. This explains the slight decrease in rate of sea level rise for the most recent decade, which is affected more than the 20-year period 2006-25.)

Left: Global average sea level rise over 1901-2025, relative to a 1995-2014 baseline. Individual timeseries are shown with dashed lines, while the black solid line shows the average (from tide gauges and satellites) used in AR6 and the solid red line shows the 1993-2025 average from satellites. Right: Global mean sea-level rates (in mm per year) for four successive overlapping 20-year periods and the most-recent decade. The shading indicates the very likely range. Credit: Forster et al. (2026)

The bigger picture

Despite greenhouse gas emissions not increasing as rapidly as in the 2000s, this year’s IGCC findings continue to show how far and how fast the climate is changing due to human activity.

A significant increase in decarbonisation efforts in the second half of this decade is required to slow down the rate of human-caused warming and limit the escalation of climate risks and impacts.

These findings, like many others produced by scientists across the globe, rely on international expertise, partnership and the maintenance and availability of global climate datasets and the global observing programmes that underpin them.

This year’s edition of IGCC used more than 40 global datasets produced by research teams around the world, including the NASA satellite record of the Earth’s energy imbalance and the ARGO deep ocean float network.

However, a number of long-term monitoring programmes could be threatened by funding decisions made by governments around the world, most notably the Trump administration in the US.

Local meteorological data and weather balloon measurement programmes in many countries have declined in recent years, especially in Africa, the west Pacific and South America. This reduces scientists’ ability to monitor and understand key indicators of climate change.

This is not just an issue for climate science. Many of these observations are key to weather forecasts and systems that provide early warning for extreme weather. For example, media reports have suggested that recent reductions in weather balloon measurements in Alaska led to a lack of warnings for a recent winter storm.

The continuity and integrity of the climate observations that scientists use to understand how the climate is changing depends on effective and sustained coordination by international organisations, such as the Global Climate Observing System, the World Meteorological Organization and World Climate Research Programme.

Without this data and its coordination, future assessments will be much more difficult at a time when urgent climate action is needed.