Climate change played a key role in the “catastrophic” 2023 floods in the Himalayan state of Sikkim in India, a new study says.
The breach of one of the “largest, fastest-growing and most hazardous” glacial lakes in Sikkim, the South Lhonak lake, led to cascading floods that killed 55 people and washed away a 1,200 megawatt (MW) hydropower dam.
The event was identified as a glacial lake outburst flood (GLOF), which is a sudden release of water from a lake fed by glacial melt.
The research, published in Science, explores the many drivers of the GLOF, its extensive impacts and policy implications going forward.
“There are many, many factors that came together here,” the study’s lead author tells Carbon Brief, but the “main driver” was the destabilising effect caused by thawing permafrost.
The research also finds that the South Lhonak lake has been expanding for decades, due to meltwater from the glacier above, with its area growing 12-fold between 1975 and 2023.
The paper concludes that the GLOF highlights the “complex interactions” between climate change, glacier mass loss and human infrastructure in mountainous regions.
It also demonstrates the importance of “robust monitoring systems and proactive measures to minimise devastating consequences and enhance resilience”, the authors add.
Flood cascade
Sikkim is a small Himalayan state in north-east India, bordering China in the north, Bhutan in the east, Nepal in the west and the state of West Bengal in the south.
Part of the eastern Himalaya, Sikkim is host to more than 90 glaciers and Kanchenjunga, the world’s third-highest peak. Sikkim serves as the origin and upper river basin for the Teesta river, one of the largest tributaries of the Brahmaputra river system.
On the night of 3 October 2023, a ridge of frozen rock and other debris on the side of the South Lhonak glacier – called a “lateral moraine” – collapsed into the glacial lake. This set off a tsunami-like wave nearly 20 metres high that breached the front of the lake, sending 50m cubic metres of water – almost half the lake’s volume – downstream.
According to the study, the GLOF’s peak discharge “vastly exceeds” the magnitude of any meteorological flood in the region’s history, equivalent to a “rare” one-in-200-year event.
Dr Ashim Sattar, a glaciologist at the Indian Institute of Technology, Bhubaneswar and the lead author of the study, tells Carbon Brief the sheer scale of impact is not always evident in satellite images. He explains:
“Here, 270m cubic metres of sediment was eroded, enough to fill 108,000 Olympic swimming pools. The South Lhonak Lake itself is 2.2km long. Just walking around it will make you sweat.”
Two hours later, the GLOF and huge volumes of eroded sediment reached the village of Chungthang 35km away, destroying the 1,200MW Teesta-III hydropower project on impact and damaging four other dams downstream.
Exclusive visuals from completely damaged Chungthang Dam in Sikkim
Received from Pokhraj Rai Ji pic.twitter.com/x9gFxs1PC6
— Weatherman Shubham (@shubhamtorres09) October 12, 2023
As the GLOF travelled, it set off 45 secondary landslides, many of them deep-seated and up to 150 metres in depth, with impacts not just in Sikkim, but also in neighbouring West Bengal and Bangladesh.
In all, the flood cascade damaged 25,900 buildings, 31 major bridges and flooded 276km2 of agricultural land. The most heavily inundated zone was in Bangladesh 300km away, where intense cyclonic rainfall – initially attributed as a main GLOF driver – exacerbated flooding.
The figure below, taken from the study, shows before-and-after images and illustrations of the moraine collapse and the flood’s path from Sikkim to Bangladesh, where floodwaters finally discharged into the Brahmaputra river.
Dr Jakob Steiner, a geoscientist at the University of Graz and a member of the Himalayan University Consortium, who was not involved in the study, says the assessment captures the “cascading” impacts of GLOFs and their interaction with other complex, climatic factors in great detail. He tells Carbon Brief:
“Even if the glacial lake is relatively small, it can trigger other movements downstream and that can have far-reaching consequences, even for hydropower plants miles away from any lakes. So the message [of the study] is that you’re not safe anywhere and, hopefully, that’s a message that policymakers will get. Institutionally, however, we are not yet prepared to receive that kind of message.”
What caused the flood?
To study such a complex and multifaceted event, researchers combined satellite imagery, meteorological data, field observations and numerical modeling.
Study lead Sattar tells Carbon Brief that “capturing this entire process into one model is very tricky and complex”.
Throughout the paper, the authors emphasise the “multi-hazard” nature of the disaster, explaining that multiple short- and long-term changes in the climate and terrain converged to create the conditions needed for the event.
However, Sattar tells Carbon Brief that the “main driver” of the GLOF was the long-term impact of rising temperatures on permafrost – the perennially frozen ground that makes up much of the mountain’s slope.
According to the authors, decades of rising temperatures have led to permafrost thaw, which caused “extensive, rapid deformation” of the slope for years preceding the collapse. The paper estimates that permafrost warming has reached a depth of 100 metres below the surface of the soil.
The study also identifies the expansion of the South Lhonak lake as an important driver. The authors find that the South Lhonak glacier, which sits above the lake, has been melting for decades. Meltwater from the glacier flows directly into the lake, which has been gradually filling up.
The charts below show the annual mass balance of the glacier (left) – where a negative number indicates a shrinking glacier – and the increasing area of the lake (right) between 1951 and 2023.
The research finds that the lake has been expanding by 0.32km2 per year over 1975-2023. It notes there has been a “doubling” in the rate of expansion over the past two decades.
The authors suggest that rising temperatures are responsible for the glacier losing mass, as the annual average temperature in the region has been increasing by 0.08C per decade since the 1950s.
The long-term permafrost thaw and growth of the lake means that, by October 2023, the region was in a state of “increased sensitivity” to a multi-hazard cascade, the paper says.
The authors say the final “trigger” was the intense rainfall that hit Sikkim on 3-4 October. Though the rainfall was “typical” for the region and season, the authors say that it “saturated the soil and increased the vulnerability of slopes to failure”.
Dr Stephan Harrison – a researcher from the University of Exeter – tells Carbon Brief that the study is “very significant” and is “written by some of the leading scientists in the field”.
Dr Miriam Jackson is the programme coordinator for the cryosphere initiative at the International Centre for Integrated Mountain Development, and was not involved in the study. She echoes Harrison’s praise, but warns about the “lack of good data” in the region for these sorts of studies. She says:
“We desperately need more data on the status of glaciers and glacial lakes, more meteorology measurements at high elevation and more data on the status of frozen ground in the Hindu Kush Himalaya.”
Harrison and Jackson gave conflicting answers about whether GLOFs are increasing or decreasing globally. However, both pointed to the lack of data on GLOFs, noting that datasets are incomplete or unavailable in many regions and emphasised the need to get better records before definitive answers can be drawn.
Hydropower rush
The Sikkim GLOF event joins a chain of recent disasters in high-mountain Asia that have destroyed hydropower plants. Given the sheer “physical magnitude” of these events and their impacts, the study highlights “potential limits to adaptation” in the Himalaya, warning that “even the most diligent and comprehensive suite of disaster risk reduction strategies [is] unlikely to entirely prevent” loss and damage.
The study draws attention to a “surge” of hydropower development in the Himalayan region near glacial lakes, which it attributes to a rising demand for “stable and renewable energy”.
With more than 650 projects planned or under construction in high-mountain Asia, it warns that many dams are “moving closer to these hazard-prone areas” and this could “exacerbate” GLOF impacts. The Teesta basin, for instance, hosts the highest density of hydropower projects in the Himalayan region, with 47 dams planned, including the reconstruction of the Teesta-III project.
While dams themselves are “susceptible” to a wide array of high-mountain hazards, they also increase the exposure of communities, workers and infrastructure investments to a “greater likelihood” of GLOFs in the future, according to the paper.
Comprehensive risk assessments, stringent building standards, regulating land use and regional cooperation among river-sharing countries are among the measures suggested by the study to reduce GLOF risks.
Sattar says governments “can make a start” by developing “basin-scale” early-warning systems. However, he cautions that structural measures such as draining glacial lakes “are easy to say, but difficult to do” in harsh terrain.
Meanwhile, geoscientist Steiner says it is critical that the key role played by infrastructure development in damage caused by GLOFs is not downplayed – noting that a failure to do so risks “absolv[ing] local institutions of their responsibility”. He concludes:
“As scientists, we find it important to show that climate change is involved, but we have to be aware that the science we create is very, very political… [A] big part of the disaster is not climate change; it’s institutional failures, it’s infrastructural failures.
“If nobody takes the responsibility and everyone just says: ‘it’s my neighbour and not me’, then we are truly in deep shit. Maybe we already are.”
The post ‘Catastrophic’ 2023 lake outburst in India driven by glacial melt and permafrost thaw appeared first on Carbon Brief.
‘Catastrophic’ 2023 lake outburst in India driven by glacial melt and permafrost thaw
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