Warming of The Atlantic Ocean

 Rapid and slower warming of The Atlantic Ocean have different effects  

      

For several years, climate scientists have shown that the Atlantic Meridional Overturning Circulation (AMOC) could come to a halt if the world warms too much. New research from Utrecht University shows that this picture is incomplete: The pace of warming also determines whether the AMOC stays stable. Scientists have found that the AMOC may be far more vulnerable to rapid warming than to temperature alone. When warming happens slowly, the ocean can adapt, but at faster rates similar to today’s, the massive Atlantic circulation could reach a tipping point at much lower temperatures. The fate of a crucial Atlantic Ocean circulation may depend not just on how hot the planet gets, but how quickly it gets there. Climate change could weaken the AMOC. The Atlantic’s massive ocean circulation may not have a fixed temperature breaking point, how fast the planet warms could determine whether it survives or collapses. The slower the warming, the more time the Atlantic Ocean has to adapt and the lower the near-term risk of an AMOC collapse. That has implications for how we think about climate policy: Much current policy, including the Paris Agreement, is aimed at limiting the eventual peak temperature. This is sometimes based on so-called overshoot pathways: temporary global warming past that limit, on the assumption that future technology could later bring the temperature back down.

Assumption is risky. A tipping point that induces an AMOC collapse could already be triggered during such a temporary overshoot. Once that happens, it can't simply be undone. It's comparable to coral reefs: Once they die off due to ocean warming, they don't come back, even if the temperature drops again later. If policy focuses only on the eventual warming peak, it may already be too late. The researchers therefore argue that emission reductions aimed at limiting the rate of global warming deserve their own place on the policy agenda. For years, climate scientists have warned that the Atlantic Meridional Overturning Circulation (AMOC) could eventually shut down if global temperatures rise too far. New findings from researchers suggest that temperature alone does not determine the fate of this major ocean circulation system. The speed at which the planet warms also appears to play a critical role in whether the AMOC remains stable. The Atlantic Meridional Overturning Circulation, or AMOC, is a vast network of ocean currents which carries warm water northward from the tropics. By moving heat around the planet, it strongly influences the global climate and helps maintain the relatively mild conditions found in Western Europe. Scientists have long viewed this Atlantic 'heat engine' as a system that could cross a tipping point. If that happened, the AMOC could transition from its current strong circulation to a much weaker state within decades. Possible triggers include growing amounts of melt water entering the ocean from polar regions as well as global warming itself.

AMOC is the system of ocean currents that transports warm water from the tropics northward. It plays a major role in redistributing heat across the planet and helps keep the climate in Western Europe relatively mild. This means that the system would shift from its present-day strong state to a much weaker state within decades. Until now, the AMOC was thought to tip and collapse around +4°C of warming. Researchers at the Institute for Marine and Atmospheric Research Utrecht now show that there's more to the story. "Our results show there is not necessarily a fixed temperature beyond which the AMOC inevitably collapses," says lead author René van Westen. "The stability of the circulation depends on how fast the climate is changing." AMOC strength for a slow (+0.5 ppm yr⁻¹, black) and fast (+2.5 ppm yr⁻¹, blue) increase in atmospheric CO2. The findings indicate that two worlds reaching the same eventual temperature could experience very different outcomes for the AMOC depending on how quickly the warming occurred.

To test the importance of warming speed, Van Westen and his colleagues ran two versions of a climate model. In both simulations, atmospheric CO2 increased gradually, but the rate of that increase differed substantially. In one simulation, CO2 concentrations increased slowly (0.5 ppm per year). In the second, they climbed much more rapidly (2.5 ppm per year), which is comparable to today's rate. The contrast produced dramatically different results. When warming occurred slowly, the AMOC remained stable well beyond +4°C and did not collapse even after warming reached +5°C. Under the faster warming scenario, however, the AMOC collapsed at around +2°C. "We deliberately looked at a scenario that is much slower than what we're experiencing today," explains co-author Reyk Börner. "That allowed us to isolate the effect of the warming rate alone, independent of how warm it eventually gets." The researchers say the difference comes down to the ocean's ability to respond to changing conditions. "Under slow warming, the entire ocean, from the surface down to its deepest layers, has time to gradually reorganize and adapt to the changing conditions," says co-author Henk Dijkstra, professor of Dynamical Oceanography. "Under faster warming, the ocean simply can't keep up." Slow climate change gives the ocean more time to adjust throughout its full depth. When temperatures rise more rapidly, those adjustments cannot happen quickly enough, leaving the circulation more vulnerable to instability.

According to the researchers, the critical warming rate lies around 0.3°C/decade, a pace the world is already approaching. Van Westen compares it to driving a car: "If you're driving toward a wall, it makes sense to steer around it. To do that, you need to brake, otherwise you fly off the road. When it comes to global warming, the world is still pressing extra hard on the accelerator right now." The same research group has published on AMOC stability several times in recent years, each time from a slightly different angle. In 2024, the group showed that an increasing amount of melt water in the North Atlantic makes the AMOC more unstable. This mechanism had long been suspected, but this study was the first to demonstrate it in a modern, complex climate model. The results confirmed that there is a critical melt water threshold beyond which the AMOC becomes unstable. However, this threshold is unrealistically high, meaning that the present-day AMOC is unlikely to become unstable through this contribution alone. That study did not account for global warming or its pace. A later study explored several global warming scenarios. It concluded that the AMOC could reach a tipping point around 2060 under both an intermediate- and high-emission scenario. In those simulations, the tipping point occurred at approximately 2.5°C of global warming. The latest research helps explain why studies can produce different estimates for when the AMOC might reach a tipping point, as well as why temperature thresholds vary across climate models and emissions scenarios.

The AMOC does appear to have a critical threshold for melt water, but the researchers find no universal temperature threshold for its collapse. Instead, its stability depends partly on how quickly the planet warms. Faster warming leaves the AMOC more vulnerable, while slower warming gives the ocean more time to adjust and allows the circulation to remain stable under substantially higher levels of global warming. The findings suggest that slowing the pace of warming could reduce the near-term risk of an AMOC collapse by giving the Atlantic Ocean more time to adapt. The new findings suggest that the path taken toward a given temperature may matter alongside the temperature itself. The faster global warming occurs, the less time the Atlantic Ocean has to adjust, potentially increasing the vulnerability of one of the planet's most important circulation systems occurring around us.

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Titan greenhouse and antigreenhouse effects

 Titan's air warms the surface and its sky cools it : The greenhouse effect 

There are many parallels between the atmospheric thermal structure of the Saturnian satellite Titan and the terrestrial greenhouse effect; these parallels provide a comparison for theories of the heat balance of Earth. Titan's atmosphere has a greenhouse effect caused primarily by pressure-induced opacity of N2, CH4, and H2. H2 is a key absorber because it is primarily responsible for the absorption in the wave number 400 to 600 cm-1 "window" region of Titan's infrared spectrum. Planetary atmospheres can warm and cool at once, and Titan makes the split unusually clear. Saturn’s biggest moon has an atmosphere that heats its ground and a sky that chills it. The numbers on each side are large, so the tug-of-war between them is the whole point. Two opposing forces battle for control of Titan's frozen surface, with neither winning by much. What’s left after both effects fight it out is a surprisingly small margin.

Titan, the largest moon of Saturn, is the only satellite in the solar system with a significant atmosphere (1.5 bar). Titan’s atmosphere is composed primarily of N2, H2, and CH4 and like Earth, it has a greenhouse effect. Titan also has an anti-greenhouse effect due to an optically thick haze layer detected in its stratosphere preventing visible sunlight from reaching the moon’s surface. A total greenhouse warming effect of 10 K was previously estimated using 1D radiative-convective models, raising the surface temperature to about 95 K. The main goal of this stage is to evaluate the greenhouse and anti-greenhouse effects on Titan’s atmosphere. The concentration of CH4, also an important absorber, is set by the saturation vapor pressure and hence is dependent on temperature. In this respect there is a similarity between the role of H2 and CH4 on Titan and that of CO2 and H2O on Earth. Titan also has an antigreenhouse effect which results from the presence of a high-altitude haze layer that is absorbing at solar wavelengths but transparent in the thermal infrared. The antigreenhouse effect on Titan reduces the surface temperature by 9 K whereas the greenhouse effect increases it by 21 K. The net effect is that the surface temperature (94 K) is 12 K warmer than the effective temperature (82 K). If the haze layer were removed, the antigreenhouse effect would be greatly reduced, the greenhouse effect would become even stronger, and the surface temperature would rise by over 20 K.

In 1991, Christopher McKay, James Pollack and Régis Courtin published a model of Titan’s surface heat balance in Science. Nearly everything below comes from that model, so treat the exact figures as one careful study’s estimate, not fixed constants. When a probe finally landed on Titan years later, its direct surface reading came in almost exactly at the 94 K value used in the model. Titan has the only thick atmosphere of any moon in the solar system. It’s mostly nitrogen, with a few percent methane and a trace of hydrogen. The greenhouse there works differently from Earth’s familiar case: the 1991 paper says it is caused primarily by pressure-induced opacity involving nitrogen, methane and hydrogen, dominated by collision-induced absorption from N2-N2, CH4-N2 and H2-N2 pairs. In the 1991 accounting, that warming is large. The greenhouse effect increases the surface temperature by 21 K, about 38 °F. But it doesn’t act on its own. High above the surface, Titan is wrapped in a thick orange haze, the smog that makes the moon look like a featureless ball in most images. This haze does something unusual. It absorbs incoming sunlight before it can reach the ground while remaining relatively transparent in the thermal infrared. This combination cools the surface rather than warming it.

Titan also has an antigreenhouse effect that results from the presence of a high-altitude haze layer that is absorbing at solar wavelengths but transparent in the thermal infrared. In plain terms, it’s the mirror image of a greenhouse: instead of trapping heat below, it blocks warmth on the way in. In the model, this antigreenhouse effect reduces the surface temperature by 9 K, about 16 °F. The haze is not a thin veil. NASA’s Cassini imaging team described how “this thick, orange-colored haze absorbs visible sunlight, allowing only perhaps 10% of the light to reach the surface.” That’s why the same team noted that “despite the fact that Titan has a thicker atmosphere than Earth, the thick global haze causes the greenhouse effect there to be somewhat weaker than it is on Earth.” More atmosphere doesn’t automatically mean more warming when part of that atmosphere is busy blocking the sun. Add 21 K of warming, subtract 9 K of cooling, and you’re left with 12 K of net warming. In the model’s words, “the net effect is that the surface temperature (94 K) is 12 K warmer than the effective temperature (82 K).” That 12 K works out to about 22 °F, the figure the whole balance comes down to.

Each effect moves the surface by tens of degrees, but they partly cancel, leaving it only a little above the model’s 82 K effective temperature. According to the same model, strip the haze away and the surface temperature would rise by 20 K. That’s a what-if, not something anyone has measured, but it shows how strongly the haze holds the surface temperature down. On 14 January 2005, the Huygens probe from the Cassini-Huygens mission dropped through Titan’s atmosphere and landed on the surface. Its instruments measured a surface temperature of 93.65 K and a surface pressure of 1,467 hPa, about one and a half times Earth’s. That 93.65 K sits almost exactly on the 94 K surface temperature used in the 1991 paper. It was not a blind prediction: the paper already described Titan’s surface temperature as near 94 K based on pre-Huygens observations. What Huygens did was confirm that earlier estimate directly at the surface. A separate check from orbit also agreed,  Cassini’s infrared spectrometer found a surface brightness temperature of 93.7 K near the landing site.

None of this makes the 21 K and 9 K splits themselves directly measured. Those remain the model’s internal bookkeeping, its way of dividing the temperature balance into two competing causes. Huygens confirmed the surface temperature, not the model’s exact division of that temperature into greenhouse and antigreenhouse effects. Titan refuses to be understood as a single number. Its climate is a subtraction problem, a warming term and a cooling term that only make sense together, and the tidy 94K at the surface hides a much noisier fight underneath. Any world’s temperature is a balance, and Titan is unusually good at showing the two sides: large effects partly canceling, leaving a surface about 22 °F warmer than its effective temperature noted in different observations.

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