It is well understood that human-caused climate change is causing sea levels to rise around the world.
Since 1901, global sea levels have risen by at least 20cm – accelerating from around 1mm a year for much of the 20th century to 4mm a year over 2006-18.
Sea level rise has significant environmental and social consequences, including coastal erosion, damage to buildings and transport infrastructure, loss of livelihoods and ecosystems.
The Intergovernmental Panel on Climate Change (IPCC) has said it is “virtually certain” that sea level will continue to rise during the current century and beyond.
But what is less clear is exactly how quickly sea levels could climb over the coming decades.
This is largely due to challenges in calculating the rate at which land ice in Antarctica – the world’s largest store of frozen freshwater – could melt.
In this article, we unpack some of the reasons why projecting the speed and scale of future sea level rise is difficult.
Drivers of sea level rise
There are three principal components of sea level rise.
First, as the ocean warms, water expands. This process is known as thermal expansion, a comparatively straightforward physical process.
Second, more water gets added to the oceans when the ice contained in glaciers and ice sheets on land melts and flows into the sea.
Third, changes in rainfall and evaporation – as well as the extraction of groundwater for drinking and irrigation, drainage of wetlands and construction of reservoirs – affect how much water is stored on land.
In its sixth assessment cycle (AR6), the IPCC noted that thermal expansion and melting land ice contributed almost equally to sea level rise over the past century. Changes in land water storage, on the other hand, played a minor role.
However, the balance between these three drivers is shifting.
The IPCC projects that the contribution of melting land ice – already the largest contributor to sea level rise – will increase over the coming decade as the world continues to warm.
The lion’s share of the Earth’s remaining land ice – 88% – is in Antarctica, with Greenland accounting for almost all of the rest. (Mountain glaciers in the Himalaya, Alps and other regions collectively account for less than 1% of total land ice.)
However, it is difficult to project exactly how much Antarctic ice will make its way into the sea between now and 2100.
As a result, IPCC projections cover a large range of outcomes for future sea level rise.
In AR6, the IPCC said sea levels would “likely” be between 44-76cm higher by 2100 than the 1995-2014 average under a medium-emissions scenario. However, it noted that sea level rise above this range could not be ruled out due to “deep uncertainty linked to ice sheet processes”.
The chart below illustrates the wide range of sea level rise projected by the IPCC under different warming scenarios (coloured lines) as well as a possible – but unlikely – worst-case scenario (dotted line).
The shaded areas represent the “likely range” of sea level rise under each warming scenario, calculated by analysing processes that are already well understood. The worst-case scenario dotted line represents a future where various poorly understood processes combine to lead to a very rapid increase in sea levels.
The graph shows that sea level rise increases with warming – and would climb most sharply under the “low-likelihood, high-impact” pathway.
Retreat of glacier grounding lines
In Antarctica, the melting of ice on the surface of glaciers is limited. In many locations, warmer temperatures are leading to increases in snowfall and greater snow accumulation, which means the surface of the ice is continuously gaining mass.
Most of Antarctica’s contribution to global sea level rise is, therefore, not linked to ice melt at the surface. Instead, it occurs when giant glaciers push from land into the sea, propelled downhill by gravity and their own immense weight.
These huge masses of ice first grind downhill across the land and then along the seafloor. Eventually, they detach from the bedrock and start to float.
These floating ice shelves then largely melt from below, as warm ocean water intrudes into cavities on its underside. This is known as “basal melting”.
The boundary between grounded and floating ice is known as the “grounding line”.
In many regions of Antarctica, grounding lines typically sit at the high point of the bedrock, with the ice sheet deepening inland. This is illustrated in the graphic below.
When a grounding line is at a high point of the bedrock, it acts as a block which limits the area of ice exposed to basal melting.
However, if the grounding line retreats further inland, warm water could “spill” over the high point in the bedrock and carve out large cavities below the ice. This could dramatically accelerate the retreat of grounding lines further inland across Antarctica.
There is evidence to suggest that the retreat of grounding lines might cause a runaway effect, in which each successive retreat causes the ice behind the line to detach from the land even more quickly.
Recent climate modelling suggests that many grounding lines are not yet in runaway retreat – but some regions of Antarctica are close enough to thresholds that tiny increases in basal melting push model runs toward very different outcomes.
Whether – and to what extent – grounding lines might retreat will depend on a wide range of factors, including the exact shape of the bedrock beneath the ice. However, the bedrock on the coast of Antarctica has not yet been precisely mapped in many places.
Ice shelves
Once Antarctic ice detaches from the seabed, it floats on the ocean surface. These floating ice shelves slow the flow of ice from land towards the sea, acting as a brake as they wedge between headlands and little hills on the seafloor.
If these ice shelves break apart, the flow of glaciers towards the sea can accelerate.
The image below on the left shows a present-day ice shelf that is pinned in place by bedrock, which slows the flow of the ice into the sea.
The image on the right shows a future scenario in which ocean water continues to intrude under the ice, accelerating basal melting on the underside of the floating ice until it completely detaches from the “pinning point” that had previously held it in place.
In this scenario, the bedrock is no longer acting as a break on glaciers pushing to the sea and the ice shelf starts flowing into the sea more quickly and begins breaking up. Ice masses inland then begin to push more rapidly towards the sea.
This dynamic was directly observed during the collapse of the Larsen-B ice shelf on the Antarctic Peninsula in 2002, which led to accelerated glacial ice flow and is believed to have contributed to a dramatic glacial retreat two decades later.
However, the factors affecting the stability of the floating ice shelves around Antarctica’s coast are complex. The strength of ice shelves depends on their thickness, how and where they are pinned to the seafloor, how cracks grow, as well as air and sea temperatures and levels of snow and rainfall. For example, meltwater at the surface can lever cracks further apart, in a process known as hydrofracturing.
A 2024 review of the stability of ice shelves found big gaps in scientific understanding of these processes. There is currently no scientific consensus on how rapidly various ice shelves might collapse – the pace is likely to vary greatly from one ice shelf to the next.
Ice-cliff collapse
If, and when, ice shelves collapse and drift away from the coast, they will expose the towering ice cliffs that loom behind them directly to the sea. These ice cliffs can be more than 100 metres tall.
This exposure could potentially lead to those cliffs to become structurally unstable and collapse in a runaway process – further accelerating the advance of the glaciers pushing towards the sea.
The images below illustrate how such a collapse might unfold. In the top image, a floating ice shelf buttresses the ice masses behind it. In the middle image, the ice shelf has largely broken apart and melted into the sea. In the bottom image, the ice shelf has completely disappeared, leaving a steep wall of ice towering over the sea. At this point, the exposed cliffs might collapse and crash into the water below.
Researchers are still debating whether or not this “marine ice cliff instability” is likely to happen this century.
Modelling ocean dynamics
The speed at which grounding lines retreat, ice shelves collapse and ice cliffs cascade into the sea partially depends on complex ocean dynamics.
The temperature and speed of water intrusion underneath the ice depends on multiple factors, including ocean currents, winds, sea ice, underwater ridges and eddies. These factors vary from one location to the next and can vary by season and by year.
Once water reaches a given cavity, the ways in which turbulent flows and fresh meltwater plumes meet the ice can significantly affect melt levels – further complicating the picture.
In other words, predicting future melt depends on models that integrate macro-level ocean circulation with local-level turbulence. This remains a major modelling challenge that, despite ongoing progress, is unlikely to be conclusively resolved any time soon.
Planning for future sea level rise
Scientists agree that human-caused climate change is causing sea levels to rise and that the oceans will continue to rise during the current century and far beyond.
However, the combination of the complexity of modelling ice-ocean interactions and the threat of potential runaway processes means that, for the foreseeable future, there is considerable uncertainty about the magnitude of future sea level rise.
(While this article focuses on Antarctica, it is worth noting that Greenland’s contribution to future sea level rise is also highly uncertain.)
To complicate matters further, the ocean does not rise like water in a bathtub, creeping up equally on all sides. Instead the Earth’s surface is highly dynamic.
For example, during the last ice age, the immense mass of the glaciers that covered much of northern Europe pressed the Earth’s surface downwards. Even though most of that ice disappeared millennia ago, much of Scandinavia is still rebounding today, causing the land to rise gradually.
In contrast, the city of Jakarta in Indonesia is sinking at a rapid pace of 10cm per year due to sprawling urbanisation and extraction of groundwater for household and industrial uses. That rate may increase or decrease over the coming decades, depending on urban planning and water management decisions.
This mix of natural and human-driven factors means that, even if researchers could perfectly predict average global sea level rise, calculating how much the sea will rise in any given location will remain challenging.
Another key unknown is around future levels of human-caused greenhouse gas emissions which drive climate change.
The scientific community is working to better understand the dynamics driving sea level rise and improve predictions, including through Antarctic sea bed mapping, field observations and improved models. Those advances in knowledge will not erase uncertainty, but they could reduce the range of possible outcomes.
Nevertheless, while that range may narrow, it will not completely disappear.
Plans drawn up by policymakers and engineers to prepare society for future sea level rise should never be based on a single point estimate.
Instead, they should take into account a range of possible “likely” outcomes – and include contingency plans for less likely, but entirely possible, scenarios in which the oceans rise far faster than currently expected.
The post Guest post: The challenges in projecting future global sea levels appeared first on Carbon Brief.
Guest post: The challenges in projecting future global sea levels
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