Cold-blooded sea creatures seeking refuge from warming ocean waters may find themselves at increasing risk of deadly cold shocks due to changes in ocean currents, new research warns.
Climate change is pushing species to higher latitudes in an attempt to stay within their range of comfortable temperatures, but this migration can come with unforeseen consequences.
The new study, published in Nature, documents a mass mortality event in March 2021 that saw at least 260 dead sea creatures wash up on the shores of South Africa.
Using satellite data, ocean observations and data on the movements of bull sharks, the researchers link the event to a sudden influx of cold water coming up from the deeper ocean.
They also show that such events have been increasing in frequency over the past three decades and forecast that this trend may continue into the future as the world continues to warm.
One of the study authors tells Carbon Brief that “we predict this is going to become a more regular phenomenon and could impact a lot of different species”.
Marine migration
As the Earth warms, many species that are able to do so are migrating to higher latitudes, allowing them to maintain their place within their “thermal niche” – the set of temperatures at which they can comfortably survive.
Nowhere is this effect more pronounced than in the global oceans, where there are fewer barriers to migration than there are on land. On average, highly mobile marine species have been moving polewards by nearly 60km per decade since the 1950s, according to the latest report on climate impacts from the Intergovernmental Panel on Climate Change (IPCC).
But this migration comes with its own risks.
These shifting ranges due to climate change can introduce species to new, unfamiliar stressors – such as shipping lanes or fisheries, says Dr Natalie Posdaljian, a bioacoustician at Scripps Institution of Oceanography in La Jolla, California, who was not involved in the study.
One of these risks is what the researchers describe as a temperature “bait and switch” – where creatures seeking warmer waters can instead be trapped by a sudden cold event. Posdaljian tells Carbon Brief that the new study is the first time that she’s seen evidence of this hazard.
Mass mortality
On 2 March 2021, dead sea creatures started washing up on the south-eastern shores of South Africa between Port Elizabeth and East London. In all, more than 250 individual organisms and 82 separate species were found, including large, migratory species such as manta rays and bull sharks.
In deducing what had happened, the team of researchers examined the temperature data in the days leading up to the event. Using satellite and other observational data, they found that the temperature of the surrounding ocean had dropped by up to 9.2C in less than 24 hours.
The cold event persisted for seven days and had “severe physiological consequences” for the marine organisms there, including hypothermia, malfunction and death, the paper says.
Similar cold shocks have previously occurred in south-eastern South Africa, dating back to at least 1989 and affecting a wide array of creatures, according to the study. But this instance was “probably the biggest cold-water shock [mass mortality event]” ever recorded, Dr Ryan Daly, a marine scientist at the Oceanographic Research Institute in Durban, South Africa, tells Carbon Brief. Daly is one of the authors of the new study.
The influx of cold water was due to a process called “upwelling”, which carries cold, nutrient-rich water from the ocean depths to its surface.
The study identifies three factors that make rapid upwelling events likely to happen: strong currents interacting with the continental shelf, strong winds blowing from the east to the west and meanders in the current. Such winds occur predominantly during the southern hemisphere’s summer, between October and April. They often act as a harbinger of temperature drops occurring in the coming 0-72 hours, the study notes.
All three of these factors are characteristic of both the south-eastern coast of South Africa and the eastern coast of Australia, where strong currents known as the Agulhas and the East Australian Current, respectively, run up against the continental shelf.
‘Trapped’
Dr Camrin Braun, an ocean ecologist at the Woods Hole Oceanographic Institution in Massachusetts, finds it surprising that even large, migratory species such as rays and bull sharks were killed by the cold snap. Braun, who was not involved in the new research, tells Carbon Brief that these animals “can move really far and really fast”.
Daly says that this surprised the research team as well. But it’s possible, he says, that the onset of the cold temperatures was quick enough and large enough that the animals got “trapped” instead of being able to escape.
To underscore this, the researchers use data on bull shark movements and ocean temperatures from tags attached to sharks before, during and after the event.
They find that the sharks consistently demonstrate “attempted avoidance” of lower temperatures – moving closer to the surface while swimming through upwelling areas and only travelling at deeper depths once they reach warmer waters.
The team also observe one shark taking up residence in a sheltered bay during one upwelling event to escape the cold waters. The researchers write that these actions “probably represent behavioural strategies to avoid/survive intense temperature declines”.
On its own, the shark-movement data is “kind of limited” and does not “make a very convincing case”, Braun says. But combining it with other data “really up[s] the ante on the importance” of the research, he adds.
Climate patterns
The researchers also look at several decades’ worth of sea surface temperature and wind data to understand whether these upwelling events are changing in frequency or intensity.
They identify clear increasing trends in the proportion of winds that favour upwelling events across three sites in South Africa. (Previous research has shown a similar increase in such winds in south-eastern Australia.)
Then, for the three South African sites and three Australian sites, they compare temperature data from three locations: “inshore”, defined as between 0-15km from the shore, “midshelf”, which is 15-30km from the shore and “offshore” – located within the warm “core” of the current. The inshore and midshelf locations fall within the upwelling zone, but the offshore ones do not.
If, as they hypothesised, upwelling events were becoming more frequent, the number of cold events inshore would increase over time, while the number of such events offshore would stay the same. Similarly, an upwards trend in the intensity of cold snaps would be revealed in the inshore and midshelf, but not the offshore, data.
The chart below shows that the proportion of upwelling-favourable winds (top left) at three sites in South Africa has been steadily increasing since the “upwelling season” – the period of upwelling-favourable winds stretching from October to April – of 1988-89.
The other three charts show increasing trends in the number of cold events (top right), the average intensity of cold events (bottom left) and the average rate of onset of such cold events (bottom right) for a single site, Port Alfred, over the same period. All three characteristics increase over time for the inshore (blue) and midshelf (pink) locations, but not the offshore (green) one, supporting the idea that the cold snaps are linked to upwelling.
These increases persist over long enough time periods, the authors argue, to be clear evidence of long-term trends, rather than natural variation. Furthermore, the study points to previous research – dating back more than 30 years – that shows evidence of climate change increasing upwelling intensity due in part to increasingly strong winds driven by the land warming faster than the ocean.
This trend analysis is one of the most valuable contributions of the new study, Posdaljian says. She tells Carbon Brief:
“It’s often hard to be able to have that kind of concrete evidence about how something could be increasing in intensity or frequency over time.”
The idea that climate change could lead to an increase in cold snaps may seem counterintuitive. But those increased temperatures “mean more energy in the climate [system] too”, Daly says. He explains:
“This wind-driven upwelling, linked to climate change, is essentially an extreme event – just like we might have more flooding and stronger cyclones and hurricanes.
“If you think about equivalent on land, that might be fires being fuelled by more intense wind. It takes an existing natural phenomenon and basically supercharges that to become [more] intense.”
He adds:
“Going forward, we predict that this is going to become a more regular phenomenon and could impact a lot of different species.”
The researchers “did a really good job of creating this foundational understanding” of how such cold events could hit marine ecosystems in future, Posdaljian says.
Looking ahead, she adds that she would like to see more work focusing on projecting future trends in cold snaps and perhaps even being able to predict them. She tells Carbon Brief:
“A lot of these animals are not just dealing with one stressor from climate change…We can’t necessarily mitigate these [extreme events], but what we can do is maybe reduce the other stressors that we can control.”
The post Climate change ‘bait and switch’ threatens sharks and rays appeared first on Carbon Brief.
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 Guardian.
98
The number of nations that submitted their national reports on tackling nature loss to the UN on time – just half of the 196 countries that are part of the UN biodiversity treaty – according to analysis by Carbon Brief.
Latest climate research
- Sea levels are already “much higher than assumed” in most assessments of the threat posed by sea-level rise, due to “inadequate” modelling assumptions | Nature
- Accelerating human-caused global warming could see the Paris Agreement’s 1.5C limit crossed before 2030 | Geophysical Research Letters covered by Carbon Brief
- Future “super El Niño events” could “significantly lower” solar power generation due to a reduction in solar irradiance in key regions, such as California and east China | Communications Earth & Environment
(For more, see Carbon Brief’s in-depth daily summaries of the top climate news stories on Monday,