Scientists have left the causes of ice ages without a definite answer over the past decades, and recently a new study revealed that the oceans play a major role in the occurrence of these climatic phenomena.
This comes after finding evidence confirming that changes in the depths of the oceans have led to an increase in their ability to absorb carbon dioxide from the atmosphere, and thus reduce the average temperature of the Earth’s atmosphere. According to the researchers, explaining this climate phenomenon will improve our ability to predict future climate change.
The study, which was carried out by an international team of researchers under the supervision of Princeton University and the Max Planck Institute for Chemistry in Germany, was published recently in the scientific journal, Science.
Since its inception, the Earth has witnessed many great ice ages, the last of which occurred about 2.6 million years ago and is known as the Pleistocene. During these ages, small ice periods occur at the rate of one every about a hundred thousand years.
These frequent climatic fluctuations have been associated with the growth of massive ice sheets over continents and oceans in one period of time, and their shrinking in others. While the oscillations that occur in the Earth’s orbit around the sun and its rotation around itself, have a role in these fluctuations, they alone do not explain these large changes in climate.
In the seventies of the last century, scientists discovered an additional factor that could explain this phenomenon, as they found that the concentration of carbon dioxide in the atmosphere decreases during the ice ages by about 30%. But the reasons for the decline remained unknown.
Carbon gas pumps
In the new study, the researchers found, according to a Princeton University statement, evidence pointing to changes in the surface waters of the Antarctic Ocean during the ice ages that led to the storage of large amounts of coal gas in the deep oceans.
Researchers reconstruct detailed records of the chemical composition of organic matter trapped in fossils of floating algae (diatoms) that grew in surface waters, then died out and deposited on the surrounding sea floor, using sediment cores extracted from the Antarctic Ocean. Marine algae deposition is known to act as a pump of carbon dioxide gas in the deep ocean, a process known as a “biological pump”.
This pump works effectively in tropical, subtropical and temperate regions of the oceans, but its effectiveness is negligible near the poles, where carbon gas leaks from deep water into the atmosphere when it rises to the surface. In the Antarctic Ocean, the situation is even worse due to the strong easterly winds that encircle Antarctica, bringing deep waters saturated with coal gas to the surface.
Isotopes of nitrogen
The team of researchers measured the nitrogen isotope ratios of the rare organic materials confined within the shells of these fossils, which revealed the evolution of nitrogen concentrations in surface waters in the Antarctic Ocean over the past 150,000 years. It is a period that covers two ice ages and two warm periods.
These measurements made it possible to detect changes in surface water temperature, and to determine the timing of the deep ocean water flow decreases towards the surface that mark the beginning of the ice ages.
“The analysis of isotopes of nitrogen trapped in fossils reveals the concentration of surface nitrogen in the past,” says lead author Eileen Ie. .
Next Ice Age
According to the researchers, the results of this study will also enable to predict how the ocean will respond to global warming in the future. Researchers’ observations about the increase in deep water currents rising to the surface in the Antarctic Ocean during warm periods in the past indicate that the intensity of these currents will also be strengthened in light of global warming, which means that the next ice age will not be soon.
This rise is likely, according to the study authors, to accelerate the ocean’s absorption of heat from ongoing global warming, but more research is needed to understand how the Antarctic Ocean is changing and how this affects its ability to absorb carbon dioxide.