The work of two Earth and Life Institute researchers has led to a better understanding of glaciation and deglaciation. We now know how variations in earth’s orbit influence the passage from a glacial period to an interglacial period.
Over the last two million years, earth’s climate has been characterised by two alternating periods: glacial and interglacial. ‘During the former, the ocean level was up to 120 metres lower than the current level, which means you could have walked across the English Channel’, explains Earth and Life Institute Researcher and Professor Michel Crucifix. ‘And North America was covered by a dome of ice 3,000 metres high. This ice melted during interglacial periods, when average temperatures approached those of our own time.’
Influence of solar energy
The passage from a glacial period to an interglacial period is closely tied to variations in earth’s orbit around the sun, so-called insolation cycles. More specifically, earth’s position relative to the sun influences the distribution of solar energy on the earth: the closer the sun, the greater the solar energy and the temperatures and the more glaciers melt. ‘However, when we compare the two periods, we see that the cause-and-effect relationship is not systematic. Indeed, insolation cycles recur every 20,000 years whereas glacial/interglacial cycles recur only every 100,000 years. So a peak in solar energy doesn’t always cause glacier melt and temperature rise.’
Zooming in on deglaciation
To find out why, Prof. Crucifix, Earth and Life Institute Research Assistant Takahito Mitsui, and colleagues at the University of Cambridge and University College London focused on deglaciation. ‘Our data showed that deglaciation is quite abrupt: the Last Glacial Maximum was 20,000 years ago and we know that 10,000 years ago the earth’s climate was similar to today’s. So deglaciation took no more than 10,000 years, which is very rapid.’
How does ice that formed over tens of thousands of years start to melt ‘overnight’? ‘We hypothesised that for glaciers to begin melting, two conditions must be met: solar energy must be sufficient and the system must be ‘ripe’ for deglaciation. And these two conditions are linked: the more time elapsed since the last deglaciation, the less the solar energy level must be raised to melt the glaciers.’ Which explains why only certain insolation cycles initiate deglaciation. The team proved the hypothesis and published it in the journal Nature: they first precisely identified the deglaciation periods, then statistically confirmed the robustness of their model.
Causes? Basins and CO2
Next, the researchers tried to explain why the passage from a glacial period to an interglacial period could be so abrupt. ‘There are two main hypotheses. First, we know that when ice accumulates, it distorts the lithosphere by forming large basins in which it builds up. Beginning in the 1980s, we saw something that made the ice more vulnerable: when a rise in solar energy melts surface ice, the resulting water infiltrates the basin and accelerates melting. The system is thus more prone to melting when these basins have had time to form.’
‘Second, when we enter a glaciation period the concentration of CO2 in the atmosphere diminishes. Yet the French scientist Didier Paillard suggested that when ocean level falls owing to glaciation, abyssal circulation – deep ocean currents – in the Southern Ocean can change. And the amount of CO2 that had been accumulated in the ocean up to that point is thus released into the atmosphere by the change in abyssal circulation. This abrupt rise in concentration could trigger the start of deglaciation. So it’s probable that these two phenomena act together.’
A glance at Michel Crucifix's bio
1998 Bachelor’s Degree in Physics, University of Namur
2002 Doctorate in Physical Sciences, UCL
2002 – 2006 Researcher, Met Office Hadley Centre (UK)
Since 2006 Research Associate, then Senior Research Fellow, FNRS; Associate Professor, then Professor, UCL
2015 Elected to the Belgian Royal Academy of Sciences