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Prof. Meissner’s research into ancient climates, where CO₂ levels rival or exceed today’s, also points to clouds as a major wildcard.

“Paleoclimate data shows warming near the poles far beyond what current models predict,” she says. “One explanation involves high-altitude clouds that most models don't simulate well due to upper-atmosphere limitations.”

In high-resolution tests, Prof. Meissner and Prof. Sherwood and their teams simulated such clouds under high CO₂ conditions.

“We found that these clouds not only formed but could contribute up to 7°C of additional warming at high latitudes – a staggering increase,” Prof. Meissner says.

Other atmospheric factors add further complexity. Air pollutants behave differently depending on whether they reflect sunlight or absorb heat, but their interactions with surrounding elements can also change their climate effects.

The (IPCC) is developing improved methods to help countries better track and report short-lived climate pollutants, to help refine future climate projections.

Complexity in the carbon cycle

Vegetation, soils, and microbes also interact in intricate ways and influence the amount of carbon stored on land. Likewise, life in the oceans, the chemistry of seawater, and ocean circulation influence the amount of carbon stored in oceans. Yet, says Prof. Meissner, models still treat these processes too simplistically to capture the full interaction with the climate, particularly the carbon cycle in oceans.

“The ocean holds 60 times more carbon than the atmosphere, and small shifts in ocean circulation or biological activity can reshape the carbon sink, with consequences for atmospheric CO₂.

“Unfortunately, our ocean biogeochemistry process understanding and models are still in their infancy and quite crude.”

Ice sheet dynamics are another relative weak spot. Processes such as the flow of ocean water beneath ice shelves, leading to melting and potential collapse, are too complex to simulate accurately in global models.

Tipping points and cascading risks

Tipping points  — critical thresholds where the climate system shifts suddenly and irreversibly — add further complexity to climate predictions.

History shows that the Earth’s climate has shifted abruptly before. In the Palaeocene-Eocene Thermal Maximum (PETM) around 56 million years ago, global temperatures rose by 5 to 8°C over a few thousand years, leading to mass extinctions in the deep ocean.

Carbon emissions during this period were estimated between 2,000 and 10,000 billion tonnes, but the annual release rate was likely below 1.1 billion tonnes, about one-tenth of today’s human-driven emissions.

“Comparing today’s climate change to events like the PETM puts our actions into perspective.

If such dramatic shifts unfolded under much slower carbon release, what might we expect today, when the brakes are off?", says Prof. Meissner.

Prof. Meissner says the events of the past show tipping points aren’t just theoretical risks, and that risk increases with global warming.

"Tipping points aren’t just about rising temperatures, but involve feedback loops and nonlinear reactions," she says.

“Current models rarely include these feedbacks because simulating them requires intensive computation and, occasionally, a deeper level of understanding of highly dynamic systems than our science currently allows.

“Tipping points can trigger others in a domino effect known as a tipping cascade. For instance, ice loss in West Antarctica or Greenland will lead to changes in and warming at high latitudes, which in turn could activate other tipping points, such as the thawing of permafrost or the degradation of boreal forests.

“So, crossing one tipping point can trigger more tipping points, and, at this point, we don’t know what the consequences will be. It is also possible that we don’t even know all the tipping points that exist in the climate system.

“Incorporating a better representation of clouds, feedbacks, and tipping cascades into our models is one of the biggest scientific hurdles we face,” she says. “It won’t be solved quickly, but we already know enough to act decisively.”