When and why does our planet go from a warm, interglacial period to an ice age? What triggers the abrupt changes typical of the end of these periods? They’re associated with a threshold in the energy the Earth receives from the Sun, our researchers have shown. When will our interglacial period end? What about cumulation with the effects of global warming? Qiuzhen Yin, an FNRS research associate at the UCLouvain Earth and Life Institute, provides answers.
Why is it important to study the phenomena associated with the end of interglacial periods?
We’re now in a warm period called an interglacial. This interglacial started about 12,000 years ago and it’s extremely important to know when it will end naturally. To do this, we studied the interglacials of the last million years and their endings. The end of all these interglacials is characterised by abrupt changes. We were able to show that these changes are associated with a threshold in the energy that the Earth receives from the Sun.
Why does solar irradiance vary over time? Is it cyclical? If so, why?
Solar irradiance varies over long periods of time due to the long-term variation of elements of the Earth’s orbit and its rotational axis. These elements characterise the shape of the Earth’s orbit, the position of the seasons in that orbit, and the tilt of the Earth’s rotation axis. The variations of these elements are quasi-periodic and the average periods are 100,000 years (for the shape of the Earth’s orbit), 41,000 years (for the inclination of the Earth’s rotation axis), 23,000 and 19,000 years (for the position of the seasons). All these variations lead to more or less similar variations in solar irradiance.
When will the next solar irradiance threshold be reached and thereby put an end to our interglacial period?
Analysis of the last 11 interglacials has shown that when the northern hemisphere’s summer irradiance decreases to the threshold, a strong and abrupt cooling occurs in the northern hemisphere due to an abrupt weakening of the circulation in the Atlantic Ocean. Our analysis shows that the threshold’s value is not exactly the same for all past interglacials but lies within a narrow range. The next threshold will occur in 50,000 years. If this threshold is also the end of our interglacial, it should end within 50,000 years. This shows that our interglacial is exceptional in its length, especially when compared to the length of previous interglacials, which is on the order of 10,000 to 20,000 years. We emphasise that this length has nothing to do with the possible impact of human activities on climate but is entirely due to variations in astronomical elements and the resulting solar irradiance. In our study, the concentration of CO2 in the air is fixed at its typical interglacial level, 280 ppmv, compared to the current 415 ppmv.
What could be the cumulative effects on our planet of reduced solar irradiance and global warming?
Global warming’s impact on the threshold related to the end of the interglacial is an extremely difficult question, because it depends not only on astronomical variation but also on the non-linear response of the climate system to global warming. An earlier study by UCLouvain’s André Berger and Marie-France Loutre (Berger and Loutre, 2002, Science) showed that our interglacial would last about 50,000 years in response to astronomical variation alone. This study also showed that a CO2 concentration of around 750 ppmv would lead to the total disappearance of the Greenland ice sheet over the next 10,000 years, which would be rebuilt over the following tens of thousands of years under the influence of a natural CO2 concentration (280 ppmv).
Your results have been published in the journal Science. What new avenues for research do they create?
To study the possible impact of global warming on the length of our interglacial, it’s imperative to start simulations that take into account the behaviour not only of the atmosphere and the ocean (which we’ve done in our Science article), but also of the ice caps and the carbon cycle (including the impact of vegetation and soil) and their interactions with climate. For this, it’s important to develop climate models that integrate the interactions of the climate system’s components and that are able to correctly simulate past glacial-interglacial cycles. It’s therefore essential to distinguish between the impact of natural phenomena and the impact of human activities on the climate.