Estimating Earth’s climate sensitivity, the global average warming from doubling CO2 above pre-industrial levels, is an urgent task as it governs how much the world will warm from fossil-fuel emissions.
Recent work has essentially ruled out low values of climate sensitivity below about 2C, but how much do we know about high values of climate sensitivity?
There is an active debate about the likely upper bound of climate sensitivity, which informs the worst-case projections of warming over the coming decades.
One way to narrow this down is to study the major climate changes of Earth’s past seen in palaeoclimate “proxy data”. These records are scattered across the planet, found in everything from ice cores and ocean sediments to tree rings and coral reefs.
Combining proxy data with climate models helps scientists understand what past changes were caused by natural fluctuations in CO2 and what changes we can expect from human-caused CO2 increases in the future.
In our new study, published in Science Advances, we find that the Last Glacial Maximum – a period of extensive ice cover about 21,000 years ago – provides even stronger evidence for modern-day climate sensitivity than previously thought.
Our estimates suggest a central estimate of “equilibrium” climate sensitivity of 2.9C, with a very likely range of 2.1-4.1C.
This is around a 30% narrowing of the 2.0-5.0C range in the latest assessment from the Intergovernmental Panel on Climate Change (IPCC).
Our findings give us more confidence that very high climate sensitivity is unlikely. They also support the IPCC’s central estimate for climate sensitivity of around 3.0C and lower estimate of 2.0C, confirming that warming would still be severe if we do not reduce fossil-fuel emissions.
Progress in climate sensitivity
For many years, the estimated range for equilibrium climate sensitivity (ECS) remained stubbornly wide.
After the landmark Charney report established a “consensus” position from a study group convened by the US National Research Council back in 1979, many IPCC assessments over several decades were unable to narrow it further.
This is illustrated in the chart below, which shows the Charney report estimated range for ECS (grey bar) on the left, followed by each IPCC report in sequence, from the first in 1990 (“FAR”, light blue) through to the most recent AR6 range on the right (red). Central estimates are shown with a dot, where available. The coloured bars indicate the estimated likely range for ECS and very likely ranges are marked with whiskers.
Estimates of ECS published in successive IPCC assessments since the Charney report in 1979. Dots show central estimates. The coloured bars show the likely range and the very likely range is given by whiskers. Chart by Carbon Brief
Ahead of the IPCC’s sixth assessment report (AR6), a four-year project from an international team of scientists made a significant advance.
The resulting paper by Prof Steven Sherwood, Dr Mark Webb and colleagues, published in the Review of Geophysics in 2020, formally combined multiple lines of evidence from observed data to improve understanding of the climate “feedbacks” that can amplify or dampen surface warming. The paper played a key role in the narrower ECS range in AR6 (red bar in chart above) – particularly in raising the lower bound to 2.0C and setting the upper bound at 5.0C.
However, observed data from recent warming are not as useful for constraining the upper end of ECS estimates. This is because climate feedbacks – especially how warming affects cloud properties that either enhance or reduce their overall cooling effect – depend on spatial patterns of temperature change. Climate scientists have dubbed this phenomenon the “pattern effect”.
Pattern effects
Temperature patterns over recent decades differ substantially from what we expect in the long-term. This means that climate feedbacks are likely to change in the future too, making observed warming a relatively poor predictor of how high climate sensitivity could be.
This is where palaeoclimate data comes in. The Last Glacial Maximum (LGM) has been touted as the best evidence for or against high values of climate sensitivity. By estimating how much colder the Earth was when CO2 levels were so much lower during the LGM, scientists can predict how much warmer the Earth would be with higher CO2 levels decades from now.
However, doing so requires isolating how much of the LGM cooling came from lower CO2 and correcting for how the climate’s sensitivity to CO2 differs between the cold LGM and the warm modern-day climate. Prior to our study, no one had quantified the impact of temperature patterns on climate sensitivity in the palaeoclimate record.
Traditionally, the additional cooling effect of the vast LGM ice sheets, which covered much of Canada and northern Eurasia, has been estimated from the amount of sunlight they directly reflect. But the ice sheets also caused the nearby northern Pacific and Atlantic oceans to cool and produce more low clouds, amplifying global cooling by reflecting even more sunlight. This ocean cooling is illustrated by the dark blue shading in the left-hand map below.
By linking these adjacent cloud changes to the ice sheets, we found that relatively more of the LGM cooling was caused by the ice sheets and relatively less came from the lowered CO2.
Importantly, these amplifying cloud feedbacks are unique to the climate during the ice age and, therefore, do not apply to modern CO2-driven warming that will not have similarly large ice-sheet changes.
As a result, the amount of cooling from lower CO2 at the LGM is not consistent with very high values of modern-day climate sensitivity above around 4C.
Accounting for differences between the LGM and modern climates allows us to lower the upper end of climate sensitivity estimates, representing a major shift in how palaeoclimate data is used to inform our future.
Combining palaeoclimate data with climate models
Quantifying the climate differences mentioned above for the LGM requires estimating temperature patterns and climate feedbacks from 21,000 years ago.
An obvious challenge is that our instrumental temperature records span only centuries and our satellite observations of clouds span only decades. Fortunately, there have been two major advances in recent years that allowed us to quantify the pattern effects from the distant past.
First, palaeoclimate data assimilation is a recently developed method that combines climate model simulations with proxy data, producing globally complete reconstructions of surface temperatures during the LGM. However, there are still major uncertainties in this method. Various groups have recently reconstructed the ice age, finding somewhat different temperature patterns. To assess this uncertainty, we use four available reconstructions.
Second, we use these reconstructed surface temperatures in global atmosphere models to estimate climate feedbacks during the LGM and compare them to feedbacks expected from CO2 doubling. Because different models produce different climate feedbacks – even when given the same reconstructed surface temperatures – we assess this feedback uncertainty using five different atmosphere models.
Despite the uncertainties, a consistent story emerged. Climate feedbacks strongly amplified LGM temperature changes, much more so than climate feedbacks under modern-day warming from CO2 alone. We traced these differences primarily to cloud feedbacks in the North Pacific and North Atlantic Oceans where the patterns of temperature change strongly differ between the LGM – due to the presence of large ice sheets – and future warming from CO2.
At first glance, the extra amplifying feedbacks of the LGM could appear to suggest that climate sensitivity is higher than we thought. A recent study led by Dr James Hansen, former director of the NASA Goddard Institute for Space Studies, reported that some of the same reconstructions used here are consistent with a modern-day climate sensitivity of around 4.8C.
However, that interpretation is reversed when we recognise that those amplifying feedbacks are unique to the ice age climate and do not apply to the modern-day climate.
The result is that modern-day climate sensitivity, as estimated from the LGM, is actually lower than has been reported by studies such as Hansen’s that have not accounted for the unique effect of ice sheets on climate feedbacks.
Our findings also challenge a common assumption that warmer climates are more sensitive than colder climates. While that assumption appears likely to be true for climates warmer than today, it appears to be false for climates colder than today when the effect of ice sheets on temperature patterns and feedbacks is included.
Implications for future warming
The LGM, after accounting for how temperature patterns impact climate feedbacks, ends up being an even stronger constraint on modern-day climate sensitivity – especially the upper bound.
To the best of our knowledge, this is the first time that considering pattern effects has helped constrain climate sensitivity instead of adding to its uncertainty for modern-day climate.
Given the importance of combining multiple lines of evidence, we use the community framework from the Sherwood, Webb et al (2020) study, updated to include our estimate of how climate feedbacks differ between the LGM and modern-day warming.
As the chart below shows, our revised best estimate of climate sensitivity (blue) becomes 2.9C with a very likely range of 2.1-4.1C, a substantial narrowing of uncertainty by reducing the upper bound.
In contrast, the IPCC AR6 very likely range was 2.0-5.0C with a best estimate of 3C (red), while the Sherwood, Webb et al (2020) study reported 2.3-4.7C and central estimate of 3.1C (yellow).
Modern-day ECS estimates from recent assessments, including the Sherwood, Webb et al (2020 study (yellow), IPCC AR6 (red) and this study (blue). Dots show central estimates. The coloured bars show the likely range and the very likely range is given by whiskers. Chart by Carbon Brief based on data provided by V Cooper and K Armour
While the lower bound and central estimates are mostly unchanged, our findings reduce the upper bound by approximately 1C.
Based on the modelled relationship between climate sensitivity and future warming, lowering the upper bound on climate sensitivity from 5C to 4.1C translates to a reduction of 0.4-0.8C in the upper end of global warming estimates by 2100 (the range reflects uncertainty in future emissions). In other words, this is a substantial reduction in the likelihood of extremely high warming.
Our results for the LGM show that temperature patterns and their effects on climate feedbacks must be accounted for when using past climates to estimate modern-day climate sensitivity.
Spatial reconstructions of past climates are a necessity and there are opportunities for further progress in constraining climate sensitivity by analysing temperature patterns and feedbacks in other past climates.
The post Guest post: Ice-age analysis suggests worst-case global warming is less likely appeared first on Carbon Brief.
Guest post: Ice-age analysis suggests worst-case global warming is less likely
Drought and heatwaves occurring together – known as “compound” events – have “surged” across the world since the early 2000s, a new study shows.
Compound drought and heat events (CDHEs) can have devastating effects, creating the ideal conditions for intense wildfires, such as Australia’s “Black Summer” of 2019-20 where bushfires burned 24m hectares and killed 33 people.
The research, published in Science Advances, finds that the increase in CDHEs is predominantly being driven by events that start with a heatwave.
The global area affected by such “heatwave-led” compound events has more than doubled between 1980-2001 and 2002-23, the study says.
The rapid increase in these events over the last 23 years cannot be explained solely by global warming, the authors note.
Since the late 1990s, feedbacks between the land and the atmosphere have become stronger, making heatwaves more likely to trigger drought conditions, they explain.
One of the study authors tells Carbon Brief that societies must pay greater attention to compound events, which can “cause severe impacts on ecosystems, agriculture and society”.
Compound events
CDHEs are extreme weather events where drought and heatwave conditions occur simultaneously – or shortly after each other – in the same region.
These events are often triggered by large-scale weather patterns, such as “blocking” highs, which can produce “prolonged” hot and dry conditions, according to the study.
Prof Sang-Wook Yeh is one of the study authors and a professor at the Ewha Womans University in South Korea. He tells Carbon Brief:
“When heatwaves and droughts occur together, the two hazards reinforce each other through land-atmosphere interactions. This amplifies surface heating and soil moisture deficits, making compound events more intense and damaging than single hazards.”
CDHEs can begin with either a heatwave or a drought.
The sequence of these extremes is important, the study says, as they have different drivers and impacts.
For example, in a CDHE where the heatwave was the precursor, increased direct sunshine causes more moisture loss from soils and plants, leading to a drought.
Conversely, in an event where the drought was the precursor, the lack of soil moisture means that less of the sun’s energy goes into evaporation and more goes into warming the Earth’s surface. This produces favourable conditions for heatwaves.
The study shows that the majority of CDHEs globally start out as a drought.
In recent years, there has been increasing focus on these events due to the devastating impact they have on agriculture, ecosystems and public health.
In Russia in the summer of 2010, a compound drought-heatwave event – and the associated wildfires – caused the death of nearly 55,000 people, the study notes.
The record-breaking Pacific north-west “heat dome” in 2021 triggered extreme drought conditions that caused “significant declines” in wheat yields, as well as in barley, canola and fruit production in British Columbia and Alberta, Canada, says the study.
Increasing events
To assess how CDHEs are changing, the researchers use daily reanalysis data to identify droughts and heatwaves events. (Reanalysis data combines past observations with climate models to create a historical climate record.) Then, using an algorithm, they analyse how these events overlap in both time and space.
The study covers the period from 1980 to 2023 and the world’s land surface, excluding polar regions where CDHEs are rare.
The research finds that the area of land affected by CDHEs has “increased substantially” since the early 2000s.
Heatwave-led events have been the main contributor to this increase, the study says, with their spatial extent rising 110% between 1980-2001 and 2002-23, compared to a 59% increase for drought-led events.
The map below shows the global distribution of CDHEs over 1980-2023. The charts show the percentage of the land surface affected by a heatwave-led CDHE (red) or a drought-led CDHE (yellow) in a given year (left) and relative increase in each CDHE type (right).
The study finds that CDHEs have occurred most frequently in northern South America, the southern US, eastern Europe, central Africa and south Asia.
Threshold passed
The authors explain that the increase in heatwave-led CDHEs is related to rising global temperatures, but that this does not tell the whole story.
In the earlier 22-year period of 1980-2001, the study finds that the spatial extent of heatwave-led CDHEs rises by 1.6% per 1C of global temperature rise. For the more-recent period of 2022-23, this increases “nearly eightfold” to 13.1%.
The change suggests that the rapid increase in the heatwave-led CDHEs occurred after the global average temperature “surpasse[d] a certain temperature threshold”, the paper says.
This threshold is an absolute global average temperature of 14.3C, the authors estimate (based on an 11-year average), which the world passed around the year 2000.
Investigating the recent surge in heatwave-leading CDHEs further, the researchers find a “regime shift” in land-atmosphere dynamics “toward a persistently intensified state after the late 1990s”.
In other words, the way that drier soils drive higher surface temperatures, and vice versa, is becoming stronger, resulting in more heatwave-led compound events.
Daily data
The research has some advantages over other previous studies, Yeh says. For instance, the new work uses daily estimations of CDHEs, compared to monthly data used in past research. This is “important for capturing the detailed occurrence” of these events, says Yeh.
He adds that another advantage of their study is that it distinguishes the sequence of droughts and heatwaves, which allows them to “better understand the differences” in the characteristics of CDHEs.
Dr Meryem Tanarhte is a climate scientist at the University Hassan II in Morocco, and Dr Ruth Cerezo Mota is a climatologist and a researcher at the National Autonomous University of Mexico. Both scientists, who were not involved in the study, agree that the daily estimations give a clearer picture of how CDHEs are changing.
Cerezo-Mota adds that another major contribution of the study is its global focus. She tells Carbon Brief that in some regions, such as Mexico and Africa, there is a lack of studies on CDHEs:
“Not because the events do not occur, but perhaps because [these regions] do not have all the data or the expertise to do so.”
However, she notes that the reanalysis data used by the study does have limitations with how it represents rainfall in some parts of the world.
Compound impacts
The study notes that if CDHEs continue to intensify – particularly events where heatwaves are the precursors – they could drive declining crop productivity, increased wildfire frequency and severe public health crises.
These impacts could be “much more rapid and severe as global warming continues”, Yeh tells Carbon Brief.
Tanarhte notes that these events can be forecasted up to 10 days ahead in many regions. Furthermore, she says, the strongest impacts can be prevented “through preparedness and adaptation”, including through “water management for agriculture, heatwave mitigation measures and wildfire mitigation”.
The study recommends reassessing current risk management strategies for these compound events. It also suggests incorporating the sequences of drought and heatwaves into compound event analysis frameworks “to enhance climate risk management”.
Cerezo-Mota says that it is clear that the world needs to be prepared for the increased occurrence of these events. She tells Carbon Brief:
“These [risk assessments and strategies] need to be carried out at the local level to understand the complexities of each region.”
The post Heatwaves driving recent ‘surge’ in compound drought and heat extremes appeared first on Carbon Brief.
Heatwaves driving recent ‘surge’ in compound drought and heat extremes
Greenhouse Gases
DeBriefed 6 March 2026: Iran energy crisis | China climate plan | Bristol’s ‘pioneering’ wind turbine
Welcome to Carbon Brief’s DeBriefed.
An essential guide to the week’s key developments relating to climate change.
This week
Energy crisis
ENERGY SPIKE: US-Israeli attacks on Iran and subsequent counterattacks across the Middle East have sent energy prices “soaring”, according to Reuters. The newswire reported that the region “accounts for just under a third of global oil production and almost a fifth of gas”. The Guardian noted that shipping traffic through the strait of Hormuz, which normally ferries 20% of the world’s oil, “all but ground to a halt”. The Financial Times reported that attacks by Iran on Middle East energy facilities – notably in Qatar – triggered the “biggest rise in gas prices since Russia’s full-scale invasion of Ukraine”.
‘RISK’ AND ‘BENEFITS’: Bloomberg reported on increases in diesel prices in Europe and the US, speculating that rising fuel costs could be “a risk for president Donald Trump”. US gas producers are “poised to benefit from the big disruption in global supply”, according to CNBC. Indian government sources told the Economic Times that Russia is prepared to “fulfil India’s energy demands”. China Daily quoted experts who said “China’s energy security remains fundamentally unshaken”, thanks to “emergency stockpiles and a wide array of import channels”.
‘ESSENTIAL’ RENEWABLES: Energy analysts said governments should cut their fossil-fuel reliance by investing in renewables, “rather than just seeking non-Gulf oil and gas suppliers”, reported Climate Home News. This message was echoed by UK business secretary Peter Kyle, who said “doubling down on renewables” was “essential” amid “regional instability”, according to the Daily Telegraph.
China’s climate plan
PEAK COAL?: China has set out its next “five-year plan” at the annual “two sessions” meeting of the National People’s Congress, including its climate strategy out to 2030, according to the Hong Kong-based South China Morning Post. The plan called for China to cut its carbon emissions per unit of gross domestic product (GDP) by 17% from 2026 to 2030, which “may allow for continued increase in emissions given the rate of GDP growth”, reported Reuters. The newswire added that the plan also had targets to reach peak coal in the next five years and replace 30m tonnes per year of coal with renewables.
ACTIVE YET PRUDENT: Bloomberg described the new plan as “cautious”, stating that it “frustrat[es] hopes for tighter policy that would drive the nation to peak carbon emissions well before president Xi Jinping’s 2030 deadline”. Carbon Brief has just published an in-depth analysis of the plan. China Daily reported that the strategy “highlights measures to promote the climate targets of peaking carbon dioxide emissions before 2030”, which China said it would work towards “actively yet prudently”.
Around the world
- EU RULES: The European Commission has proposed new “made in Europe” rules to support domestic low-carbon industries, “against fierce competition from China”, reported Agence France-Presse. Carbon Brief examined what it means for climate efforts.
- RECORD HEAT: The US National Oceanic and Atmospheric Administration has said there is a 50-60% chance that the El Niño weather pattern could return this year, amplifying the effect of global warming and potentially driving temperatures to “record highs”, according to Euronews.
- FLAGSHIP FUND: The African Development Bank’s “flagship clean energy fund” plans to more than double its financing to $2.5bn for African renewables over the next two years, reported the Associated Press.
- NO WITHDRAWAL: Vanuatu has defied US efforts to force the Pacific-island nation to drop a UN draft resolution calling on the world to implement a landmark International Court of Justice (ICJ) ruling on climate, according to the