A world in warming, under most circumstances, tipping toward deep cooling does seem counterintuitive; yet, Earth’s geological record is replete with such episodes when rising greenhouse gases were followed by abrupt climatic reversals. Today, scientists increasingly look to the oceans for an explanation, where the quiet work of microscopic organisms regulates the exchange of carbon between the atmosphere, water, and rocks. Long timescales are involved, but their cumulative effects can be dramatic. How ancient climate shifts unfolded has become crucial to understanding future risk, as modern warming alters marine ecosystems that once helped nudge the planet into an ice age.
How ancient oceans pulled carbon from the air and cooled the planet
Over the years, evidence has sharpened for a dramatic climate transition during the Ordovician period through various synthesis works, like the review in the Annual Review of Earth and Planetary Sciences on the causes and consequences of Ordovician cooling. About 445 million yrs ago, Earth moved from a warm greenhouse state to widespread glaciation without any obvious decline in atmospheric carbon dioxide at the outset. Geological proxies suggest that enhanced carbon removal and not reduced volcanic emissions played a decisive role. This cooled concurrently with major changes in marine life, indicating a biological driver located within the ocean carbon cycle.The Ordovician seas were dominated by planktonic organisms that fixed carbon through photosynthesis. As these organisms proliferated, more organic carbon sank to the seafloor, reducing the amount returned to the atmosphere. As this biological pump developed, it essentially helped to energise chemical weathering on land by lowering atmospheric carbon dioxide, and more carbon was pulled down through the reactions of rocks. The result was a tightly coupled system where marine biology and geochemistry were interlaced in a feedback capable of reshaping global climate, a sobering example of how life itself can become a planetary force.
How microscopic marine organisms influence global temperatures
Phytoplankton, though perhaps invisible to the naked eye, represent one of the most vital elements in marine food chains and are considered capable of exerting strong controls over climate. While they are growing, these organisms absorb carbon dioxide from the atmosphere as they are a sink for it, which eventually gets stored in both ocean waters and sediments. If environmental conditions are right for big blooms, the efficiency of this transfer will be higher, thus the cooling effects will be stronger. During the Ordovician, evolutionary changes likely improved plankton productivity, enabling larger volumes of carbon to be locked away for long periods.Modern climate research shows that plankton respond sensitively to temperature, nutrient supply, and ocean circulation. Warming can initially stimulate growth in some regions, but sustained heat alters species composition and nutrient mixing. If future oceans experience changes in the same order of magnitude as in the geological past, the balance between the transport of carbon to depth and the transport to the atmosphere may become unpredictable. The Ordovician example suggests that biological responses do not always stabilise climate, and under specific thresholds, they may accelerate transitions toward cooler states.
When oceans changed composition and ice began to spread
The Ordovician cooling was not a story of carbon removal alone. Changes in ocean chemistry and the distribution of oxygen supplemented this path toward the climate outcome. Increased burial of organic carbon raised atmospheric oxygen levels, which in turn intensified wildfires on land and changed terrestrial vegetation. Such changes further amplified weathering rates and drew down additional carbon dioxide. Cascading effects linked marine productivity with atmospheric composition and surface processes, generating conditions favourable for the formation of ice sheets.The cooling oceans also changed the circulation patterns that led to the production of cold, dense waters at high latitudes. As the polar regions became colder, ice sheets expanded rapidly and reflected more of the sunlight back to space, thus global cooling was strengthened. This albedo feedback is what keeps the planet at a glacial state, at least until higher carbon dioxide levels can be returned by volcanic emissions and reduced weathering. The sequence underscores how tightly coupled Earth systems are, in which changes in ocean life can flow through to the atmosphere and cryosphere.
What ancient ice ages reveal about today’s warming oceans
Drawing lessons from the Ordovician does not mean that global warming today will soon pitch the planet into an ice age. Timescales are very different, and human-driven emissions are unfolding much faster than ancient biological feedbacks. Yet history does show that the Earth’s climate can respond nonlinearly when important thresholds are crossed. Small marine organisms, through the power of cumulative action, have in the past changed atmospheric chemistry enough to alter global conditions.Today’s oceans are undergoing rapid change, including warming, acidification, and deoxygenation-all drivers that impact plankton behaviour. If these changes alter the efficiency of the biological carbon pump, then long-term climate trajectories could shift in unexpected directions. The Ordovician record illustrates that gradual trends alone do not ensure climate stability and that the tiny forms of life can, when conditions are right, serve to help guide the planet toward radically different states.Also Read | A hidden Arctic world: Methane mounds and life found 3.6 km below the Greenland Sea
