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A 2007 Science study found that deep-sea warming began before atmospheric carbon dioxide rose and before tropical surface waters warmed near the start of the last deglaciation. The authors proposed that increasing austral-spring sunlight and sea-ice feedbacks helped drive that early warming. The result helps explain part of the transition out of the last ice age; it is not a complete, single-cause account of how the ice age ended.
What the scientists found
In their 2007 paper, Lowell Stott, Axel Timmermann and Robert Thunell reported that deep-sea temperatures rose by about 2°C between 19,000 and 17,000 years before present. In their reconstruction, this warming preceded both the rise in atmospheric CO2 and warming of the tropical surface ocean by about 1,000 years.
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That order matters: it suggests the initial deep-water warming did not result from the increase in atmospheric CO2. The authors instead identified increasing austral-spring insolation—solar energy reaching the Southern Hemisphere in spring—along with sea-ice albedo feedbacks as key factors in the early warming.
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How the study reconstructed the sequence
The team established a chronology from a marine sediment core in the western tropical Pacific. They used radiocarbon dating and analyzed benthic and planktonic foraminifera, microscopic organisms preserved in the sediment. Their stable-isotope and magnesium/calcium records provided evidence about past ocean conditions and allowed the researchers to compare changes in deep water with changes nearer the surface.
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The findings are therefore a reconstruction from one marine core, not a direct measurement of every ocean region or a global tally of how much and how quickly the entire ice age ended. The study’s title describes its central result: Southern Hemisphere and deep-sea warming led the deglacial CO2 rise and tropical warming.
How the proposed mechanism may have worked
The sequence proposed from the study and contemporaneous reporting is that stronger austral-spring insolation and sea-ice albedo feedbacks contributed to early Southern Hemisphere and deep-water warming. As sea ice melted, less sunlight was reflected, allowing the ocean to absorb more energy. The warming of deep water came before the atmospheric CO2 increase and tropical surface-ocean warming.
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Chemistry World reported that the warming apparently originated in the Antarctic Ocean, and that Antarctic sea-ice melting coincided with increased springtime solar radiation over Antarctica. That geographic origin is an interpretation reported at the time, not a direct conclusion that should be generalized beyond the study’s evidence.
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The report also described a further possibility: later ocean ventilation may have released CO2 from the ocean into the atmosphere, amplifying deglaciation. This is an expert interpretation of how the sequence could have developed, not proof that ventilation alone caused the ice age to end. Robert Mulvaney of the British Antarctic Survey summarized the proposed long-timescale relationship in a contemporaneous comment: “This paper confirms what we already suspected, that climate at the very long timescale is driven by temperature increases brought about by the orbit of the Earth around the Sun which warms up the deep ocean and that this in turn releases carbon dioxide into the atmosphere which amplifies the temperature increase.”
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What this result does—and does not—explain
The study offers evidence about the timing of deep-sea warming relative to atmospheric CO2 and tropical surface warming, and proposes a driver for the early warming. It does not establish that orbital forcing alone explains every regional change during deglaciation, nor does it provide a complete explanation for the overall duration or magnitude of the ice age’s end.
Dick Kroon, then Regius Professor of Geology at the University of Edinburgh, told Chemistry World that the work was important for understanding glacial-to-interglacial change, “but not necessarily for driving climate change in the near future.” The ancient sequence is evidence about long-term climate transitions, not a direct forecast of near-term climate.
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Read the original study and contemporaneous report
- Stott, Timmermann and Thunell’s 2007 paper in Science (abstract and publication record; DOI: 10.1126/science.1143791).
- Chemistry World’s 2007 report by Simon Hadlington, including contemporaneous expert comments.
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