The mechanism keeping them elevated involves something climate science has struggled to pin down, the presence of low clouds over the ocean.
A second set of simulations stripped out everything except the atmosphere and fed it the warmed ocean surface as a fixed input.
The warming pattern at the surface appears to drive the feedback, the thinned clouds and retreating ice.
Atmosphere-only runs confirmed that the surface pattern alone explains the extra ocean heat uptake.
Low cloud responses to a warming ocean vary between models.
Most sea level projections run through 2100, maybe 2150, tracking how much ice melts and how much water expands.
The implicit assumption is that once greenhouse gases stabilize, the rise eventually levels off. In response, the ocean adjusts to a new steady state.
However, new climate modeling shows that timeline is off by centuries. Even after emissions drop, seas will keep climbing well past 2300.
The mechanism keeping them elevated involves something climate science has struggled to pin down, the presence of low clouds over the ocean.
Heat that’s locked in
Oceans both warm slowly and cool slowly. Right now, they’re holding decades of heat that humanity poured into the air through burning fossil fuels.
That stored energy doesn’t vanish the day emissions cease. Instead, it keeps expanding the water around it.
A team led by climate scientist Jong-Seong Kug at Seoul National University (SNU) ran the numbers on this.
Even under aggressive emission cuts, sea levels look set to rise about 17 inches and stay elevated for hundreds of years.
Thermal expansion alone, with no help from melting ice, does most of that work. Deep ocean layers keep absorbing surface heat, the volume of water creeps upward, and coastlines pay the cost.
Centuries-forward model
To watch this play out, the team built a climate model in which ocean, atmosphere, ice cover, and land all respond to each other.
They ran it forward for hundreds of years, past where typical projections stop to understand the future better.
A second set of simulations stripped out everything except the atmosphere and fed it the warmed ocean surface as a fixed input.
That trick let the researchers isolate what the surface temperature pattern alone could do to the air above it.
Patterns in warming
Sea surface temperatures don’t rise evenly. Some regions soak up heat faster than others, and the resulting map of warm and cool patches has a structure.
That structure appears to drive the climate response long after greenhouse gases stop climbing.
In the simulations, the warmest patches lined up with regions where flat decks of low cloud sit over the sea, and had edges of polar ice.
As those patches heated, clouds and ice changed. Those changes fed back into the warming itself, amplifying the original push from below.
Thinned-out clouds
Low clouds are the gray, flat layers that sit over much of the ocean and bounce sunlight back into space. They exist as some of the planet’s most overlooked air conditioners.
Brighter, denser decks mean more sunlight goes right back out instead of warming the water below.
In the warmed simulations, those decks thin and the reflector grows patchier. More sunlight slips through and hits the water, which absorbs it.
The same warming that started the problem dims the very cooling system that should have pushed back.
Researchers have known for years that low cloud responses are among the biggest uncertainties in climate projections. What was less clear was their tight grip on long-term sea level rise.
Retreating sea ice
Something similar plays out at higher latitudes. Sea ice reflects most of the sunlight that hits it.
When it retreats and exposes darker open water, that water absorbs the heat instead of bouncing it back.
Climate scientists have understood this ice-reflectivity feedback for decades. What this paper adds is the joint picture of both clouds and ice in concert together.
They reacting to the same warming pattern, letting more sunlight reach the ocean each year.
A lingering rise
Until now, the centuries-long elevation in sea level was usually explained as the ocean slowly adjusting to already added surface heat.
What the model runs contribute is something different. The warming pattern at the surface appears to drive the feedback, the thinned clouds and retreating ice.
In turn, that feedback keeps pulling additional sunlight into the ocean. Even as the original push from greenhouse gases fades, the expansion sustains itself.
Atmosphere-only runs confirmed that the surface pattern alone explains the extra ocean heat uptake.
Missing from forecasts
Most climate models capture some of this, but not all of it. Low cloud responses to a warming ocean vary between models.
Moreover, warming gets distributed across the surface differently as well. Those errors compound into very different long-term outcomes.
Coastal planners using current projections may be underestimating how long the sea will keep climbing after emissions reach zero.
The runs suggest centuries of continued rise. Not from new heat being added, but because the heat already trapped keeps finding ways in.
Kug and his colleagues have nailed down a specific mechanism, surface pattern driving feedback driving sustained heat uptake.
That gives the field of research a concrete target to model better and observe more carefully.
This generates new questions about how the ocean equilibrates after warming, and how it will affect the future.
The study is published in Nature Communications.
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