Scientists have challenged the conclusions of a new study suggesting that the planet has already exceeded the 1.5C warming threshold set under the Paris Agreement.
Climate change is typically measured as the average global temperature increase relative to a “pre-industrial baseline”. The UN Intergovernmental Panel on Climate Change (IPCC), for example, uses the average temperature over 1850-1900 as their historical baseline. The planet has already warmed by around 1.2C compared to this period.
The new study, published in Nature Climate Change, uses proxy data from sea sponges in the Caribbean Sea to create a record of ocean temperatures from 1700 to the present day. This data suggests that warming started 40 years before the IPCC’s pre-industrial baseline period began.
Based on this new record, the authors say “warming is 0.5C higher than IPCC estimates”.
This means that “the global warming clock for emission reductions to minimise the risk of dangerous climate change has been brought forward by at least a decade”, the lead author told a press briefing.
However, many experts have warned that the framing of the study is misleading, arguing that the finding has no bearing on the Paris Agreement 1.5C limit, because it specifically “describes temperature rise relative to the late 19th century”.
Prof Richard Betts, head of climate impacts research at the UK Met Office Hadley Centre, who was not involved in the study, tells Carbon Brief that, crucially, the study “does not mean that impacts of climate change will occur earlier than expected”.
Other experts raised doubts that the 0.5C warming in the 1800s is human-caused, while many cautioned that proxy data from a single location cannot be used to make assumptions about the entire planet.
The University of Oxford’s Prof Yadvinder Malhi, who was also not involved in the study, cautions that “the way these findings have been communicated is flawed, and has the potential to add unnecessary confusion to public debate on climate change”.
Shifting baselines
Humans have been releasing greenhouse gases into the atmosphere for centuries, causing global temperatures to rise.
In IPCC reports – considered the most authoritative summaries on climate science – scientists use a combination of land surface air temperatures and sea surface temperatures to assess changes in global mean surface temperatures (GMST).
The UN body reports global warming against a “pre-industrial baseline” of 1850-1900. It describes this baseline as “a pragmatic choice based upon data availability considerations” – in part because much of the observed climate data they use is only available from 1850.
For example, the Met Office’s HadSST4 dataset – one of the three datasets used in IPCC estimates of sea surface temperatures – goes back as far as 1850.
The IPCC also recognises that “both anthropogenic and natural changes to the climate occurred” before the 1850-1900 baseline. For example, in its 2021 report on climate science, the IPCC estimates that between 1750 and 1850-1900, GMST increased by around 0.1C. Of this, human activity was responsible for 0.0-0.2C, it says.
Nonetheless, researchers have typically followed suit in using the 1850-1900 average as their “pre-industrial baseline” to measure global warming.
In 2015, countries agreed under the Paris Agreement to hold the increase in the global average temperature to well below 2C above pre-industrial levels and pursue efforts to limit warming to 1.5C. “Pre-industrial” was not clearly defined in the agreement, but it has generally been taken to mean the average temperature over 1850-1900.
However, some scientists argue that the “pre-industrial baseline” period should begin before 1850.
The new study uses proxy data taken from sea sponges from the Caribbean sea, to present a timeseries of regional ocean temperatures from 1700 to the present day. Scientists collected sclerosponges from the ocean mixed layer – a region of ocean where heat is exchanged between the atmosphere and the ocean interior.
Between 1700-90 and 1840-60, the proxy data shows ocean warming of around 0.9C, according to the study. In the intervening time, there was some cooling, largely caused by volcanic eruptions, the authors say.
The plot below shows the proxy data (blue) from the year 1770, alongside the HadSST4 observed temperature record (purple), which begins in 1850, relative to a 1961-90 reference period. The authors have applied a 0.9C “offset” to their proxy data to account for pre-industrial temperature increase.
By comparing their proxy data against existing records of global temperature changes, the authors find “strong empirical evidence that the Caribbean ocean mixed layer has warmed proportionately to the average global increase in sea surface temperature, over the last ~50 years”.
The authors assume that the 0.9C offset “can be applied to land-air as well as the ocean mixed layer anomalies”, therefore concluding that GMST increased by 0.9C between 1700-1860 and 1961-90.
Meanwhile, global ocean temperatures measured using HadSST4 show only 0.4C of warming relative to the IPCC’s 1850-1900 pre-industrial period.
As such, the authors suggest that human-caused warming to date is actually 0.5C higher than IPCC estimates.
Dr Malcolm McCulloch – an emeritus professor at the University of Western Australia and lead author on the study – told a press briefing that, according to his study, the 1.5C Paris temperature threshold has already been crossed in around 2010-12.
He continued:
“It means that now, temperatures are at least 1.7C above the pre-industrial level. It also means that the 2C target will be passed in late 2020 unless there are major reductions in emissions…
“The big picture is that the global warming clock for emission reductions to minimise the risk of dangerous climate change has been brought forward by at least a decade”.
However, many scientists are concerned about this framing of the study.
Warming limits
Dr Friederike Otto, who was not involved in the study, is a senior lecturer in climate science at Imperial College London’s Grantham Institute. She says the paper “does not tell us anything about whether we have exceeded the 1.5C temperature limit set in the Paris Agreement”.
She continues:
“That limit was established as the threshold of unacceptably dangerous warming and describes temperature rise relative to the late 19th century. If this study has indeed identified warming from before the mid-1800s, that doesn’t mean the planet is any closer to breaking the 1.5C limit as it is widely understood.”
(The IPCC best estimate – in all but the highest emission scenario – is that global warming will pass 1.5C in the first half of the 2030s.)
Mahli adds:
“Our models of climate warming impacts are based on warming relative to 1850-1900 and moving the baseline definition of pre-industrial does not make these expected impacts worse…
“It is the date of the reference period that matters rather than whether it is labelled pre-industrial or not. The period 1850-1900 is a period of relatively reliable global data when industrial era human-caused climate change was likely negligible.”
Dr Andrew King is a senior lecturer in climate science at the University of Melbourne and was not involved in the study. He tells Carbon Brief that the findings of the study do not have any implications for the Paris Agreement warming limits, because these were “written in 2015 with a view to limiting further global warming from that point onwards”.
He adds:
“While the lack of clarity on what pre-industrial means was problematic, it doesn’t really affect that goal or any of the analyses on climate impacts at global warming levels that have been performed.”
King also tells Carbon Brief that the authors have not demonstrated that pre-1850s warming is due to human activity.
Malhi agrees that “this early industrial-era warming, if real, is almost certainly not human-caused”. He notes that human-caused emissions over 1750-1900 account for only 2.5% of total emissions to date, and says they are “unlikely to have caused substantial warming compared to the 1.4C of warming caused by the remaining 97.5% of cumulative emissions”.
Dr Duo Chan, a lecturer in climate sciences at the University of Southampton, also advises “caution” when interpreting the results, noting that “this new warming estimate does not align” with historical estimates of the different factors that affect the climate.
He notes that, according to Berkeley Earth temperature estimates, the land warmed by around 0.05C per decade over 1850-1900. The new proxy data from the sponges suggests that the ocean warmed almost twice as quickly as the land over this time – a “puzzling observation given the ocean warms more slowly than land”, he says.
Dr Zeke Hausfather, Carbon Brief’s contributing science writer, adds that the study authors are “conflating ocean mixed layer temperature with sea surface temperature in a way that is confusing”. He adds that “their reconstruction also seems a bit at odds with other palaeoclimate reconstructions – such as PAGES2k – that do not see large differences in pre-1900 temperatures”.
The sclerosponge record
Coralline sclerosponges are an ancient type of calcifying sea sponge which can live for hundreds of years. As they grow, chemicals called strontium and calcium build up in their skeletons. The ratio of strontium to calcium in their skeletons is higher during warm periods and lower during cool periods.
Scientists collected live specimens of sclerosponge from the Caribbean sea and analysed the ratios of strontium to calcium in their skeletons to reproduce a timeseries of ocean temperatures in the region from the year 1700 to the present day.
Dr Amos Winter is a professor of Earth and environmental systems at Indiana State University and author on the study. He told the press briefing that there is no such thing as a “perfect proxy”, but said the sclerosponge record is “as good as possible – the holy grail of reconstruction”.
He explained that the Caribbean is “the ideal location to measure global trends”. According to the paper, the region is “ideally positioned” to have a “minimal” impact from the Atlantic Meridional Overturning Circulation, while “still registering the broader effects” of the El Niño-Southern Oscillation climate phenomenon.
He adds that the sclerosponge temperature reconstruction is “very robust” when compared to other assessments of temperature trends.
Dr Gavin Schmidt, director of the NASA Goddard Institute for Space Studies, says that the new data is a “useful addition to the database” of palaeoclimate proxies. However, he adds:
“Estimates of the global mean temperatures before 1850 require multiple proxies from as wide a regional variation as possible, thus claims that records from a single record can confidently define the global mean warming since the pre-industrial are probably overreaching.”
Prof Gabi Hegerl, a professor of climate system science at the University of Edinburgh, says that the paper presents a “nice new record” of ocean temperatures, but says that “the interpretation in terms of global warming goals overstretches it”.
She warns that “a single location cannot substitute global data, as climate varies across the globe, which is why the only way to measure global temperature is to get data from across the globe”.
Similarly, Hausfather calls the finding “interesting”, but says it “should be combined with other proxy records in a larger synthesis before it will change our prevailing views here”.
The post Scientists challenge ‘flawed communication’ of study claiming 1.5C warming breach appeared first on Carbon Brief.
Scientists challenge ‘flawed communication’ of study claiming 1.5C warming breach
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 ha