炭素循環の欠陥が地球を氷期に導く可能性（Carbon Cycle Flaw Can Plunge Earth into an Ice Age）

2025-09-25 カリフォルニア大学リバーサイド校（UCR）

＜関連情報＞

• https://news.ucr.edu/articles/2025/09/25/carbon-cycle-flaw-can-plunge-earth-ice-age
• https://www.science.org/doi/10.1126/science.adh7730

地球の気候の地質学的調節における不安定性 Instability in the geological regulation of Earth’s climate

Dominik Hülse and Andy Ridgwell
Science  Published:25 Sep 2025
DOI:https://doi.org/10.1126/science.adh7730

Editor’s summary

The stabilization of Earth’s climate by negative feedbacks between atmospheric carbon dioxide and silicate rocks can potentially be overridden by faster processes causing rapid burial of large amounts of organic carbon. Hülse and Ridgwell report model results showing how high rates of organic carbon burial caused by elevated levels of atmospheric carbon dioxide, climate-sensitive phosphorous mineral weathering, and phosphate regeneration from marine sediments can turn warming events into ice ages. The juxtaposition of these conditions might help to explain the origins of past “snowball” climates such as those seen in the Neoproterozoic. —Jesse Smith

Structured Abstract

INTRODUCTION

How climate is regulated on geologic timescales (hundreds of thousands of years) is one of the foremost questions in natural science, with implications spanning the evolution and persistence of life on our planet and others to the long-term consequences of anthropogenic carbon (C) emissions. The prevailing view is that regulation occurs as a result of the climate-sensitive response of carbon dioxide (CO₂) removal by the weathering of silicate rocks on land. This negative, stabilizing feedback on CO₂ acts as a planetary climate “thermostat.” However, the episodic occurrence of extreme cooling and “snowball Earth” events during the Precambrian (before 539 million years ago) hints at times in the past when effective regulation breaks down, whereas the occurrence of prominent intervals of widespread organic carbon burial throughout Earth’s history points to the action of an additional planetary CO₂ removal mechanism and, potentially, a second thermostat.

RATIONALE

Owing to the lack of suitable coupled global carbon-cycle and climate models, the nature of the interaction between weathering and organic carbon burial, as well as the factors that determine which one dominates, remains unknown. The first thermostat requires that the global balance between weathering and carbonate burial, and their relationship to atmospheric CO₂ and surface temperature, is represented in the model, whereas organic carbon burial depends on spatial patterns of ocean circulation plus oxygen and nutrient cycling. The computational challenge is simulating all these processes on the ~100-thousand-year timescale relevant to Earth’s thermostats. To resolve this, we took an efficient ocean circulation–based global carbon-cycle model, which already included the silicate weathering thermostat and added the key organic geological processes—the release of CO₂ from kerogen weathering on land and its removal through organic carbon burial in marine sediments. We also accounted for the primary sources and sinks of phosphorus (P), which regulate the availability of nutrients to marine plankton and, hence, organic carbon production and burial.

RESULTS

We found that the silicate weathering thermostat can be “outcompeted” by the more responsive organic carbon burial thermostat, which then dominates long-term climate regulation. In response to an initial CO₂ release and global warming perturbation in the model, weathering rates and nutrient supply to the ocean increase. Dissolved oxygen (O₂) availability in the marine environment declines through the reduced gas solubility and higher rates of organic matter decay prevailing in a warmer and more productive ocean. As seafloor anoxia spreads, P is recycled more efficiently back to the ocean rather than buried alongside C in marine sediments, fueling yet higher productivity and lower O₂. CO₂ declines more rapidly than P, and eutrophic conditions persist even as atmospheric CO₂ falls below its starting value. Within a few hundred thousand years, global surface climate becomes colder than at the start of the experiment. The magnitude of this “overcooling” depends on the background state of C and P cycling, and for atmospheric O₂ levels that are 60% lower than present-day levels, excess cooling can surpass 6°C, exceeding the global temperature difference between today and the Last Glacial Maximum.

CONCLUSION

Fast feedbacks involving organic matter are not only essential for Earth system recovery from perturbation but also create unexpected climate instability. Overcooling is most strongly expressed at intermediate oxygenation states of the ocean and atmosphere in our model and provides a causal link between major transitions in oxygenation during the Precambrian and the occurrence of extreme cooling snowball Earth events. The existence of critical instability in geological climate regulation is a consequence of the presence of microbial life; this has implications for the coevolution of life and environment as well as long-term habitability on other planets. A fresh look is needed into the operation of Earth’s thermostats.

Sequence of events induced by massive CO₂ release.
The path traces the time evolution (dark through light colors) of the modeled change in global mean surface air temperature (y axis) versus ocean phosphate (x axis) inventory. Different states of rising and falling global temperature (°C) and P inventory (as % change) are highlighted, alongside age [thousand years (kyr)]. The black and red arrows in the insets represent C and P fluxes, respectively. Pco₂, partial pressure of CO₂ in the atmosphere.

Abstract

Negative feedback between climate and atmospheric carbon dioxide (CO₂), mediated by the weathering of silicate minerals on land, is thought to provide the primary regulation of Earth’s climate on geological timescales. By contrast, we found that faster feedbacks involving organic matter are not only critical to Earth system recovery but can also create unexpected instability. Our Earth system model experiments show how sedimentary organic carbon burial, amplified by redox-sensitive phosphorus regeneration, can outweigh silicate weathering and paradoxically drive climate overcooling in response to massive CO₂ release. This instability depends on the initial balance between silicate weathering and organic carbon burial in addition to the state of global phosphorus cycling. It is most strongly expressed at intermediate ocean redox states, which may help us understand the timing of past ice ages.