- Why Scientists Track Atmospheric Carbon With Giant Towers - October 7, 2025
- The Link Between Melting Ice and Rising Sea Levels Explained - October 6, 2025
- How Ocean Currents Help Regulate Global Climate - October 5, 2025
Why Ocean Water Expands When It Gets Warmer
Think of the ocean like a massive swimming pool that’s slowly being heated. When water gets warmer, it naturally expands in volume, and about half of the measured global sea level rise on Earth is from warming waters and thermal expansion. More than 90 percent of the heat trapped by greenhouse gases is absorbed by the oceans, causing ocean temperatures to rise and water to expand, which contributes to an increase in global sea level. It’s similar to how a mercury thermometer works – as the liquid heats up, it takes up more space.
In 2024, those contributions flipped dramatically, with two-thirds of sea level rise coming from thermal expansion. Global sea level rose faster than expected in 2024, mostly because of ocean water expanding as it warms, with last year’s rate of rise being 0.23 inches (0.59 centimeters) per year, compared to the expected rate of 0.17 inches (0.43 centimeters) per year.
The Greenland Ice Sheet Is Shrinking at Record Speed
Last year, Greenland’s ice sheet lost 55 gigatons of water, which is equivalent to 2.5 million litres per second the entire year, night and day. The ice sheet has lost over a trillion tonnes of its total mass since the 1980s, with melting rates six times higher in the last decade, and an average of 30 million tonnes of ice is now being lost every hour. To put this in perspective, this amounts to 150 million litres per minute, 9 billion litres per hour, and 216 billion litres per day – or three Olympic-sized swimming pools of fresh water being added to the ocean every second.
The Greenland Ice Sheet has experienced net-annual mass loss for 27 years running, for every year since 1998. However, the ice sheet lost 55 ± 35 Gt of mass in 2024, the lowest annual ice loss since 2013, which occurred due to above-average snowfall and below-average melting.
Antarctic Ice Loss Is Accelerating Faster Than Expected
Antarctic ice loss nearly quadrupled from 51 billion tons per year between 1992 and 2001 to 199 billion tons per year from 2012-2016. Antarctica is losing ice mass at an average rate of about 135 billion tons per year, and Greenland is losing about 266 billion tons per year. What makes Antarctica particularly concerning is the massive scale – the Totten Glacier alone contains as much ice as the entire West Antarctic Ice Sheet and has the potential to contribute 11.5 feet (3.5 meters) of sea level rise.
Not only is Antarctic ice melting more quickly than new snow can replace it, but the rate of loss due to melting and calving is increasing, with each year the ice sheet shrinking more rapidly, and by 2020, the speed of ice loss had multiplied sixfold over thirty years. Glaciers along the Wilkes Land coast in East Antarctica doubled their rate of melting between 2009 and 2018, with satellite observations recording a drop of about 9 feet in the height of the ice surface.
Mountain Glaciers Are Disappearing at Alarming Rates
The decadal average loss from glaciers quintupled over the past few decades, from the equivalent of 6.7 inches of liquid water in the 1980s, to 18 inches in the 1990s, to 20 inches in the 2000s, to 33 inches for 2010-2018. Mountain glaciers might seem small compared to massive ice sheets, but they’re disappearing much faster. Today, the Arctic is warming twice as fast as anywhere on earth, and the sea ice there is declining by more than 10% every 10 years.
These glaciers act like natural water towers for communities downstream. Glaciers act as reservoirs of water that persist through summer, providing continual melt that contributes water to the ecosystem throughout dry months, creating perennial stream habitat and a water source for plants and animals. When they’re gone, entire ecosystems lose their primary water source.
Sea Level Rise Has Doubled Its Speed in Recent Decades
Global mean sea level has risen about 8–9 inches (21–24 centimeters) since 1880, mostly due to a combination of melt water from glaciers and ice sheets and thermal expansion of seawater as it warms. In 2023, global mean sea level was 102.0 millimeters (4.02 inches) above 1993 levels, making it the highest annual average in the satellite record. Since the satellite record of ocean height began in 1993, the rate of annual sea level rise has more than doubled, with global sea level rising by 4 inches (10 centimeters) since 1993.
What’s particularly troubling is the acceleration. As one NASA researcher noted, “The rise we saw in 2024 was higher than we expected. Every year is a little bit different, but what’s clear is that the ocean continues to rise, and the rate of rise is getting faster and faster”.
The Physics Behind Ice Sheet Collapse
Ice sheets don’t melt uniformly – they can reach critical tipping points that trigger rapid collapse. One process involves moulins – large vertical shafts in the ice that allow surface meltwater to penetrate to the base of the ice sheet, while another involves vast ice shelves off Antarctica that buttress the grounded continental ice sheet. Glaciers don’t melt in a linear, predictable way; sometimes they melt slowly for decades before reaching a critical tipping point that signals a new, more chaotic phase of rapid melting.
Ice sheets are so massive that they exert significant gravitational pull, drawing surrounding ocean water toward them, and as an ice sheet melts and loses mass, its gravitational pull weakens, allowing water to move away, which means that sea level near the ice sheet may actually decrease in response to melting while waters rise elsewhere.
How Different Types of Ice Affect Sea Levels Differently
Sea ice forms and melts strictly in the ocean while glaciers are formed on land, and when glaciers melt, because that water is stored on land, the runoff significantly increases the amount of water in the ocean, contributing to global sea level rise. Sea ice, on the other hand, is often compared to ice cubes in a glass of water: when it melts, it does not directly change the level of water in the glass.
However, sea ice loss creates other problems. The Arctic is warming twice as fast as anywhere on earth, and the sea ice there is declining by more than 10% every 10 years, and as this ice melts, darker patches of ocean start to emerge, eliminating the effect that previously cooled the poles, creating warmer air temperatures and disrupting normal patterns of ocean circulation.
The Greenland Tipping Point Scientists Are Watching
Researchers found that when approximately 230 gigatons of ice is lost in a single year, corresponding to 60% of the surface mass balance compared to pre-industrial times, this is the pivotal tipping point that could initiate complete loss of the Greenland ice sheet, which corresponds to a global mean temperature increase of 3.4°C. If all the ice on Greenland melted, it would raise global sea levels by 20 feet.
As the century continues, loss of ice from surface melting is likely to outweigh that from the ice sheet margins extending into a warming ocean, and ice loss would then be further exacerbated by surface albedo feedbacks, whereby less ice means there is reduced “white” surface to reflect incoming solar radiation, meaning it is instead absorbed by the comparatively “dark” land and ocean, warming the ambient environment and causing more melting.
West Antarctica’s Dangerous Instability
There are fears that as the West Antarctic Ice Sheet melts, it could contribute over 10 feet (3 meters) to rising sea levels within the next two hundred years, and a worst case climate scenario could see these changes occurring much more quickly. The West Antarctic Ice Sheet is the frontier of dramatic ice loss in Antarctica and one of the most rapidly changing ice sheets on earth, with undisputed ice loss across West Antarctica that is increasing and accelerating, though exactly how much and how quickly this ice will flow, calve and melt into the ocean is one of the areas of greatest uncertainty in climate science today.
The Thwaites Glacier in Antarctica is disintegrating more quickly than anticipated and is nicknamed the ‘doomsday glacier’ because sea levels could rise more than three metres without it and its supporting ice shelves. The concern isn’t just about Thwaites itself – it acts like a cork in a bottle, holding back much larger ice masses behind it.
Recent Surprising Changes in 2024
Melting on the Antarctic Ice Sheet for the 2024 to 2025 season began with above average melt extents in all regions, but melt extent dropped to nearly zero or below average from February 1 to March 15, with net accumulation of snow far above average for the year ending in February 2025, helping reduce the ice sheet’s net contribution to sea level rise for this period. Total net accumulation of snow has been far above average for the March 2024 to February 2025 period, by nearly 200 billion tons.
Meanwhile, a strong and persistent melt event occurred in southern Greenland in mid-August 2024, preceded by extensive rain in the same area, with snow cover following the rain events covering the lower elevation bare ice areas along the western side, brightening the surface again.
What Complete Ice Sheet Loss Would Mean
If all glaciers and ice sheets melted, global sea level would rise by more than 195 feet (60 meters). There is still some uncertainty about the full volume of glaciers and ice caps on Earth, but if all of them were to melt, global sea level would rise approximately 70 meters (approximately 230 feet), flooding every coastal city on the planet. If all 2,900,000 cubic kilometres of the Greenland ice sheet were to melt, it would increase global sea levels by ~7.4 m (24 ft).
The combination of seawater’s thermal expansion and the melting of mountain glaciers is predicted to lead to an increase in global sea level of 0.28–1.01 metres (11–39.8 inches) by 2100. However, these projections could be conservative if ice sheet collapse accelerates.
The connection between melting ice and rising seas isn’t just about physics – it’s about the future of our coastal civilizations. Every fraction of a degree of warming, every melting glacier, and every expansion of warming ocean water adds up to reshape the world map. Understanding this relationship helps us grasp why climate scientists speak with such urgency about tipping points and why coastal communities worldwide are already adapting to a reality that seemed like science fiction just decades ago.
