Hansen: 2025 could be the hottest year on record, despite La Niña

A worrying new theory has emerged at the forefront of climate science to explain anomalous and abrupt temperature spikes – and it’s predicting that 2025 could rank amongst the warmest years. Jed Thomas

After a record-shattering 2024, climate scientists are watching 2025 closely—not just because of the heat, but because this year will serve as a crucial test for our understanding of how fast the planet is warming.

James Hansen and Pushker Kharecha argue that 2025 will be an "acid test" for their theories on global warming acceleration. If global temperatures remain at or above 1.5°C of warming relative to pre-industrial levels—despite the fading El Niño—it will confirm that a fundamental shift in climate dynamics is occurring, driven by strong cloud feedback and the removal of aerosol cooling.

If, on the other hand, temperatures drop significantly, it would suggest that past El Niño-driven warming cycles still dominate, and that some of the recent temperature surge may have been temporary.

In short, this year will provide critical evidence about the future trajectory of global warming—whether it's accelerating due to human-driven changes in cloud cover, or whether natural variability still holds sway.

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A new (and contested) theory of anthropogenic climate change

Hansen and Kharecha identify three key factors that will determine whether 2025 will challenge 2024 for the hottest year on record:

1. The reduction of reflective clouds

Clouds play a huge role in regulating Earth’s temperature. Low-altitude clouds (such as marine stratocumulus) reflect sunlight back into space, cooling the planet. But satellite observations show that these clouds are disappearing, particularly in the mid-latitudes (30-50°N).

Why are these clouds vanishing?

• Warmer air holds more moisture, causing clouds to become thinner and dissipate more easily, allowing more sunlight to reach the surface.
• Fewer sulfate aerosols from ship emissions (due to recent pollution regulations) mean that clouds form less frequently over major ocean regions.
• Shifting wind and weather patterns caused by global warming may be reducing the persistence of low cloud cover.

The result? Less cloud cover = more heat absorption by the oceans, which keeps global temperatures high.

2. Warmer oceans strengthen non-reflective, heat-trapping clouds

While low clouds cool the planet, high-altitude cirrus clouds act like a heat-trapping blanket. They allow sunlight in but prevent heat from escaping, amplifying warming.

Why are cirrus clouds increasing?

• More water vapor in the upper atmosphere (caused by warming oceans) is fueling the formation of these heat-trapping clouds.
• Stronger atmospheric convection—rising warm air—is driving more high-altitude cloud formation, especially in the tropics.
• Disruptions to atmospheric circulation (linked to Arctic ice loss) are shifting cloud patterns in ways that favor warming rather than cooling.

Together, these cloud changes mean that the planet is losing a major cooling mechanism while strengthening a warming one—a dangerous feedback loop.

3. Reduction in aerosols has revealed hidden warming

For decades, human-made aerosol pollution (especially from burning sulfur-heavy fuels like ship diesel) artificially cooled the planet by scattering sunlight and increasing reflective cloud cover. But in recent years, new pollution regulations have dramatically reduced sulfate emissions, especially over the ocean.

Hansen et al. estimate that this aerosol reduction has contributed about 0.2°C of additional warming, meaning that a hidden cooling effect has been removed, allowing underlying greenhouse gas-driven warming to surge forward unmasked.

This means that even if the temporary warming from El Niño fades, the planet may still remain at record-high temperatures due to the aerosol effect no longer holding temperatures down.

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Temperatures in 2025 are a test for higher estimates of climate sensitivity

If 2025 remains near or above 2024’s temperature levels, it will be a major confirmation that:

• Cloud feedback is amplifying global warming more than most models expected.
• Climate sensitivity (how much the planet warms in response to CO₂) is at the high end of estimates—possibly 4.5-5°C per CO₂ doubling rather than the 3°C assumed in many models.
• Aerosol cooling was masking a significant portion of past warming, and its removal means we are now seeing the true force of greenhouse gas-driven climate change.

However, if 2025 cools significantly, it would suggest that past patterns still hold—meaning that natural variability (such as El Niño and La Niña) is still the dominant driver of short-term temperature fluctuations. While this wouldn't disprove Hansen et al.’s arguments entirely, it would challenge their assumption that cloud feedback and aerosol reductions are the primary drivers of the recent warming surge.

Faster warming threatens crossing tipping points sooner

Beyond setting records, the rapid warming of the past two years raises major concerns about tipping points, particularly the potential shutdown of the Atlantic Meridional Overturning Circulation (AMOC)—the system of ocean currents that distributes heat globally.

Hansen et al. warn that if accelerated warming continues, the melting of Arctic ice and Greenland glaciers could pour enough freshwater into the Atlantic to disrupt AMOC, potentially leading to:

• Extreme shifts in weather patterns, including prolonged droughts and stronger storms.
• Massive sea level rise, as Antarctic and Greenland ice sheets melt faster.
• Regional cooling in parts of Europe, even as global temperatures continue to rise.

This underscores why understanding what is driving current warming trends is urgent. If Hansen and Kharecha are right, the pace of warming is even faster than expected, meaning we have even less time to act to prevent catastrophic changes.

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Why 2025 is a crucial year for climate science

The coming months will provide a decisive test of whether global warming is accelerating due to cloud feedback and aerosol reductions—or whether natural variability still plays the dominant role in short-term temperature changes.

With January 2025 already the warmest January on record, if the rest of the year follows suit, it will strongly confirm that cloud feedback and human-driven factors are overriding past patterns.

If, instead, 2025 cools more than expected, it would suggest that El Niño’s influence was stronger than anticipated, and that we may still have some short-term cooling mechanisms at play.

Either way, this year is more than just another hot year—it’s a crucial experiment for climate science. If Hansen and Kharecha’s predictions are right, we could be entering a dangerous new phase of climate change where warming continues even without El Niño’s influence.

The question now is: will the world recognize the warning signs in time?

To read the full paper, click here. 

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