For decades, Earth’s climate system has been understood as a largely self-regulating mechanism, governed by slow but reliable geological processes that prevent temperatures from drifting too far in either direction. Central to this understanding is the carbon cycle, particularly the role of rock weathering in removing carbon dioxide from the atmosphere. However, new research from the University of California, Riverside challenges this long-standing view by revealing a missing component in Earth’s carbon recycling system—one that can drive the planet’s climate beyond simple balance and into extreme states, including global-scale ice ages.
At the heart of traditional climate theory lies the concept of chemical weathering. Rainwater absorbs carbon dioxide from the atmosphere to form weak carbonic acid, which falls onto land and slowly breaks down exposed rocks. Silicate rocks such as granite are especially important in this process. As these rocks weather, they release minerals that bind with carbon, allowing carbon dioxide to be transported by rivers into the oceans. Over geological time scales, this mechanism acts as a thermostat for the planet. When Earth warms, weathering accelerates, removing more carbon dioxide and cooling the climate. When the planet cools, weathering slows, allowing carbon dioxide to accumulate and temperatures to rise again.
This negative feedback loop has long been viewed as a stabilizing force that keeps Earth’s climate within habitable limits. According to this model, the planet responds gradually and proportionally to changes in atmospheric carbon dioxide, avoiding runaway heating or cooling. Yet geological records paint a far more dramatic picture of Earth’s past. Evidence from ancient sediments indicates that some of the earliest ice ages were extraordinarily severe, with ice and snow extending across nearly the entire globe. These so-called “Snowball Earth” events cannot be adequately explained by a climate system that merely fine-tunes itself.
This discrepancy led researchers to search for an additional mechanism capable of pushing Earth’s climate beyond gentle equilibrium. Their investigation revealed a powerful feedback involving ocean chemistry, nutrient cycling, and microscopic marine life. This newly identified process fills a critical gap in the understanding of how carbon is buried in the ocean and how this burial can dramatically alter global temperatures.
As atmospheric carbon dioxide levels rise and the planet warms, increased rainfall carries larger quantities of nutrients—particularly phosphorus—from land into the oceans. Phosphorus is a key nutrient for plankton, microscopic organisms that form the foundation of marine food webs. Through photosynthesis, plankton absorb carbon dioxide from the atmosphere and convert it into organic matter. When these organisms die, they sink to the ocean floor, transporting the carbon they captured into deep-sea sediments.
Under certain conditions, this process can become self-reinforcing. Warmer temperatures and higher nutrient availability stimulate greater plankton growth, which in turn removes more carbon dioxide from the atmosphere. However, the decay of large amounts of plankton consumes oxygen in the ocean. As oxygen levels decline, the chemistry of marine sediments changes. Instead of phosphorus being permanently buried, it is more likely to be released back into the water.
This recycled phosphorus fuels additional plankton blooms, whose decay further depletes oxygen levels. The result is a feedback loop in which nutrients and carbon cycle rapidly between the ocean and sediments, driving increasingly efficient carbon burial. As enormous quantities of carbon are locked away on the seafloor, atmospheric carbon dioxide levels fall sharply, leading to global cooling.
Unlike the traditional rock weathering feedback, this ocean-based mechanism does not stop neatly once temperatures return to their original state. Instead, it can overshoot, pushing the climate into extreme cold. Computer simulations conducted by the research team showed that this feedback is powerful enough to trigger an ice age, even in the absence of other major forcing events.
Andy Ridgwell, a geologist at UC Riverside and co-author of the study published in Science, explains the process using a household analogy. In a typical air-conditioned home, a thermostat regulates temperature by switching the system on and off as needed. If the thermostat is placed far from the air conditioner, however, the system may overreact—cooling the room too much before shutting off. Similarly, Earth’s climate control system is not broken, but under certain conditions, it may respond unevenly and excessively.
The severity of Earth’s earliest ice ages can be partly explained by differences in atmospheric composition. During ancient periods, oxygen levels were significantly lower than they are today. Low oxygen conditions enhance the nutrient feedback in the oceans, making it easier for phosphorus to be recycled rather than buried. This amplifies plankton growth and carbon burial, increasing the likelihood of extreme cooling events. In contrast, modern Earth’s higher oxygen levels act as a stabilizing influence, weakening this feedback and reducing the risk of runaway cooling.
The findings have important implications for understanding Earth’s future climate. Human activities continue to release large amounts of carbon dioxide into the atmosphere, driving rapid warming. According to the researchers’ model, the planet will eventually respond by increasing carbon removal processes, leading to a cooling rebound. However, this rebound is expected to occur over tens to hundreds of thousands of years and is unlikely to be as extreme as ancient ice ages due to modern oxygen-rich conditions.
Ridgwell emphasizes that this long-term cooling offers little comfort for present-day society. While Earth’s climate system may eventually counterbalance human-driven warming, the timescale is far too slow to mitigate the impacts unfolding now. Rising sea levels, extreme weather events, and ecosystem disruptions will continue to affect human populations for generations to come.
Ultimately, the study underscores the complexity of Earth’s climate system and the dangers of assuming it always behaves in a smooth and predictable manner. Feedbacks involving oceans, nutrients, and life itself can push the planet into dramatic extremes under the right conditions. While Earth has mechanisms that can eventually restore balance, they operate on geological timescales that are irrelevant to human lifespans.
The lesson is clear: climate action remains essential. Relying on Earth’s natural processes to correct human-induced warming is neither practical nor safe. Understanding these deep-time feedbacks enriches scientific knowledge and sharpens awareness of how delicate—and occasionally volatile—the planet’s climate system truly is.
Source: University of California - Riverside
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