Traditionally, the reigning consensus in climate science depicts Earth’s climate regulation as predominantly controlled by the gradual weathering of silicate rocks, such as granite. This geological process acts as a planetary thermostat: atmospheric carbon dioxide (CO₂) dissolves into rainwater, which falls on exposed rocks and chemically reacts to slowly break down minerals, sequestering the carbon by eventually depositing it on the ocean floor in the form of carbonate minerals. This slow but dependable cycle has been credited with keeping Earth’s climate relatively stable over geological timescales, mitigating drastic temperature swings through balancing CO₂ levels.

However, geological records paint a more complicated picture, especially when examining past “Snowball Earth” episodes during which the planet became almost entirely encased in ice. These extreme glaciations are not adequately explained by a mere steady-state cooling process. Therefore, the UC Riverside team pursued inquiry into the missing dynamics that could instigate such extreme climatic transitions, seeking to integrate additional biogeochemical feedbacks into climate models.

The key addition to these models involves marine carbon burial processes that hinge on nutrient fluxes, particularly phosphorus. When atmospheric CO₂ rises and drives global temperatures upward, enhanced weathering not only liberates carbon but also washes increased quantities of phosphorus into the world’s oceans. This nutrient enrichment stimulates the proliferation of marine phytoplankton, microscopic algae which photosynthesize and absorb CO₂, channeling more carbon into biological forms suspended in the ocean’s upper layers.

As phytoplankton flourish, they eventually die and sink, transporting organic carbon to the seafloor – a process termed the biological pump. This mechanism acts as a carbon sink, contributing to long-term carbon sequestration. Yet, as the ocean responds to warmer surface conditions and altered biological productivity, oxygen levels within marine depths decline—a state known as ocean deoxygenation. This phenomenon fundamentally alters nutrient cycling by promoting phosphorus recycling within oxygen-poor environments, effectively halting its burial and amplifying nutrient availability in surface waters.

This phosphorus feedback instigates a nonlinear, self-reinforcing cycle: more nutrients fuel more plankton growth, which after death exacerbates oxygen depletion, leading to more efficient phosphorus recycling, perpetuating the cycle. Such feedback departs from traditional notions of smooth regulatory mechanisms, introducing the possibility of climate overshoot where cooling trends surpass initial equilibria, resulting in climate states far colder than previously predicted by simpler models.

Computer simulations incorporating this refined biogeochemical interplay illustrate how these feedbacks could precipitate pronounced cooling phases following periods of warming, potentially ushering in glacial periods of significant intensity. This dynamic contrasts sharply with the gentler, stabilizing controls previously assumed, painting a vivid picture of Earth’s climate system as finely balanced yet inherently prone to sharp swings under specific conditions.

Andy Ridgwell, a geologist and lead author of this study, likens this phenomenon to a thermostat that overshoots its target temperature. Conventional thermostats maintain room temperature by cooling or heating air until a set point is reached, then turning off. However, if the thermostat is misaligned or situated away from the environmental source—like an air conditioner—its control becomes erratic and overshoots, causing the room to become colder than desired. Similarly, Earth’s climate system regulates temperature on immense timescales, but feedbacks can cause disproportionate responses that overshoot equilibrium, triggering extreme climatic events.

The study also highlights the role of Earth’s atmospheric oxygen levels in modulating this feedback loop. Geological epochs characterized by lower atmospheric oxygen, such as during the Proterozoic, rendered the climate thermostat even more erratic, fostering more profound and longer-lasting ice ages. In contrast, the modern atmosphere’s relatively higher oxygen concentration acts to dampen these nutrient feedbacks, making climate oscillations milder and somewhat more predictable.

This insight is crucial because, while humanity’s rapid increase in atmospheric CO₂ contributes to short-term warming, the model indicates that in the geological timescale, subsequent cooling overshoots remain possible. Nevertheless, the severity of these future ice ages is expected to be less dramatic than past events due to the moderating influence of current oxygen levels, effectively moving the thermostat closer to the air conditioning unit in Ridgwell’s analogy.

Despite the long-term eventual cooling prospects illuminated by this research, Ridgwell cautions that the timeframes involved are far beyond human lifespans. The onset of future ice ages—whether sooner or later by tens or hundreds of thousands of years—is largely inconsequential when juxtaposed with pressing climate challenges faced today. Current policies and scientific efforts must prioritize mitigating warming and its immediate impacts, as natural cooling processes will neither occur rapidly nor reliably enough to offer reprieve within this century or the next.

This study therefore reframes our understanding of Earth’s climate regulation by revealing a complex interplay of geochemical and biological processes capable of destabilizing the climate system in profound ways. Integrating nutrient-driven carbon burial feedbacks into existing models not only explains ancient climatic extremes but also sharpens predictions for future Earth system behavior, underscoring the intricate balance of forces shaping planetary climate across eons.

In summary, as the scientific community expands knowledge of Earth’s long-term carbon cycle and climate regulation, it becomes evident that the planet’s thermostat is seldom static or linear. This newfound appreciation for the interlinked biochemical cycles provides a more nuanced framework to interpret past climate events and anticipate future trajectories, emphasizing the delicate interplay between geological processes, ocean biology, atmospheric chemistry, and climate dynamics.

Subject of Research: Instability in Earth’s geological climate regulation through carbon cycle feedbacks
Article Title: Instability in the geological regulation of Earth’s climate
News Publication Date: 25-Sep-2025
Web References: 10.1126/science.adh7730
Image Credits: Andy Ridgwell/UCR
Keywords: Climate change, Anthropogenic climate change, Climate change effects, Earth sciences, Climate sensitivity, Climate stability, Climate systems, Earth climate, Global temperature, Ice ages, Carbon cycle, Carbon flux, Biogeochemical cycles

The research team meticulously analyzed data from 188 long-term permanent forest plots distributed throughout the Amazon rainforest. This extensive ground-based inventory was conducted under the auspices of the RAINFOR (Amazon Forest Inventory Network) initiative, which has been tracking individual tree metrics such as diameter at breast height (DBH), species identity, growth rates, and mortality over multiple decades. The investigators’ ability to revisit and measure each tree multiple times allowed the capture of nuanced changes in forest structure, providing unprecedented insights into the growth patterns and demographic shifts within this globally significant biome.

Integral to the study’s impact is the revelation that not only the largest canopy giants but also smaller tree cohorts have exhibited significant growth over the past three decades. This suggests a pervasive biological response enhanced by CO2 fertilization, a phenomenon whereby elevated atmospheric carbon dioxide concentrations enhance photosynthetic capacity and water-use efficiency in plants, leading to accelerated biomass accumulation. While climate change and habitat fragmentation pose severe threats to rainforest integrity, these findings show that in relatively intact forest areas, carbon fertilization is currently fostering a net increase in tree size.

Professor Beatriz Marimon, who spearheaded data collection efforts in the southern Amazon, highlights the paradox this presents: “While there is constant discourse on the destruction facing the Amazon due to deforestation and climate extremes, our data reveal an ecosystem actively thriving in many undisturbed areas. The upward trend in tree size, including among massive, centuries-old specimens, is reassuring evidence of the forest’s resilience amid ongoing environmental pressures.”

Of particular significance is the role of these large trees as carbon sinks. According to Dr. Adriane Esquivel-Muelbert, formerly affiliated with the University of Birmingham and currently at the University of Cambridge, “Large trees disproportionately contribute to carbon sequestration, making their growth vital to the global carbon cycle. This research confirms that despite climate volatility, CO2 enrichment continues to stimulate growth and maintain the Amazon’s critical function in mitigating anthropogenic climate change.” Her comment underscores how large tropical trees operate as keystones in maintaining atmospheric carbon balance.

Contrastingly, Dr. Rebecca Banbury Morgan from the University of Bristol draws attention to the irreplaceability of mature forests compared to reforested or secondary-growth areas. “Old-growth tropical forests represent centuries of accumulated biomass and biodiversity. Merely planting new trees cannot replicate these complex carbon reservoirs or ecosystem services. Our study therefore amplifies the urgent need to halt deforestation and preserve mature forest structure to sustain these carbon sinks.”

The study also contributes a new dimension to understanding forest dynamics under climate change by revealing alterations in the forest’s size structure. The largest trees have increasingly dominated competition for light and nutrients, altering ecological interactions within the canopy and potentially affecting species composition. Professor Tim Baker of the University of Leeds, a joint senior author, notes, “The Amazon is not static—it’s actively changing. This size increase across all tree classes indicates fundamental shifts in forest composition and carbon storage capacity rarely captured before.”

This robust response, however, is not without caveats. The researchers caution that the synergistic effects of ongoing deforestation and climate pressures remain a formidable threat to the sustainability of these large tree populations. Professor Oliver Phillips from the University of Leeds, an expert in tropical ecology, stresses the importance of connectivity within the Amazonian ecosystem to preserve seed dispersal networks and genetic flow essential for the survival of these majestic giants. “Connectivity is mission-critical; fragmentation fragments these lifelines, imperiling the future health and resilience of the forest.”

Scientific consensus recognizes the abundance of mature trees as crucial to maintaining global climatic stability. The Amazon, often described as “the lungs of the planet,” sequesters billions of tons of carbon annually. By combining comprehensive field measurements with atmospheric science, this study provides compelling evidence that the heightened levels of anthropogenic CO2, while responsible for climate challenges, concurrently reinforce the carbon uptake capability of intact Amazonian forests.

This nuanced revelation adds a critical layer to the broader climate discourse, highlighting the vital role of conserving intact tropical ecosystems not only for biodiversity but as a frontline defense against climate change. The findings anticipate crucial discussions at the upcoming COP30 climate conference in Brazil, where emissions reduction commitments and conservation strategies will be at the forefront.

Incorporating state-of-the-art techniques, the RAINFOR network protocol classified trees by species and repeatedly measured individual growth metrics, providing statistically robust evidence of these trends rather than relying solely on remote sensing or modeling. This field-based experimental approach affirms the continued scientific value of hands-on ecological monitoring in capturing long-term ecological processes in a changing world.

Despite the positive news about tree growth and carbon uptake, researchers unanimously emphasize that such benefits hinge on the maintenance of large, connected forest landscapes. Initiatives to curb deforestation and promote sustainable land use in the Amazon Basin remain paramount, as unchecked forest loss would negate the natural gains observed and exacerbate climate instability worldwide.

This study, therefore, embodies a beacon of hope amid environmental concerns. It illustrates how the Amazon rainforest, often perceived as vulnerable and faltering, exhibits an intrinsic capacity for resilience and adaptation, buoyed by scientific evidence of thriving tree growth in intact regions under rising CO2. However, the balance is delicately poised and contingent on robust conservation efforts and international cooperation to safeguard this irreplaceable global resource.

Subject of Research: Not applicable

Article Title: Increasing tree size across Amazonia

News Publication Date: 25-Sep-2025

Image Credits: Pauline Kindler (Scientists in Colombia measuring a giant Ceiba tree), Adriane Esquivel Muelbert (View of the rainforest canopy)

Keywords: Climate change, Forests, Rainforests, Tropical forests, Trees, Atmospheric carbon dioxide, Anthropogenic carbon dioxide, Carbon emissions