How Northern Ice May Have Reached the Southern Ocean
AOver the past few million years, Earth's climate has swung repeatedly between long, cold ice ages and shorter, warmer periods like the one we are living in today. Scientists broadly agree that these swings were set in motion by slow, natural changes in Earth's orbit around the Sun, which alter how much sunlight reaches different parts of the planet in different seasons. Yet orbital changes are only part of the story. Another key ingredient was carbon dioxide: during ice ages, the atmosphere contained considerably less of this greenhouse gas, because more carbon was stored in the oceans.
BMuch of this storage is thought to have been controlled by the Southern Ocean, the vast body of water that surrounds Antarctica. In this region, deep water rises towards the surface and comes into contact with the atmosphere, so changes in the way the ocean circulates there can help decide whether carbon escapes into the air or remains locked away in the depths. For this reason, research into ice-age climate has traditionally concentrated on the Southern Ocean itself. Only recently have scientists begun to ask seriously whether distant parts of the ocean might also play a role.
CThis raises an awkward question. The great ice sheets that expanded during ice ages lay mainly in the Northern Hemisphere, covering large parts of North America and northern Europe, and their growth was initially driven by the changes in Earth's orbit. The Southern Ocean, however, lies at the opposite end of the planet, separated from those ice sheets by the entire length of the Atlantic. How, then, could ice building up in the far north affect the amount of carbon absorbed by waters thousands of kilometres away around Antarctica? Until this question is answered, the explanation of how ice ages deepen remains incomplete.
DA study led by Madison Shankle of the University of St Andrews, published in the Journal of Climate, offers a possible answer. The team proposes a chain of events that links the two hemispheres through the ocean. As ice sheets spread across the north, they cool the nearby waters of the North Atlantic. This colder water then travels southwards until it reaches the Southern Ocean. If the cooling is strong enough, it can change the circulation around Antarctica in a way that allows more carbon dioxide to be stored below the surface. With less of the gas in the atmosphere, the planet cools further, and this extra cooling may in turn make the ice age more severe.
ETo test the idea, the researchers turned to computer simulations of ocean circulation, which allow scientists to change one factor at a time and watch how the rest of the system reacts. Rather than cooling the whole planet, they cooled only the most northerly latitudes, imitating the effect of a large ice sheet, and then observed how the Southern Ocean responded. The result was striking. Cooling in the north alone produced 74 per cent of the fall in carbon dioxide that occurred in a separate simulation in which the entire planet was cooled. In other words, a change that began in one hemisphere was able to achieve most of the effect of a global one, with the ocean itself carrying the influence from north to south.
FIt is worth being clear about what this finding does and does not show. Shankle herself describes the work as providing a plausible mechanism that connects northern ice sheets with changes in circulation in the south, and as demonstrating that connection in model simulations. A computer model, however sophisticated, is a simplified version of the real ocean, and a mechanism that works in a simulation still has to be checked against evidence from the past. Such evidence may come from sediments on the sea floor, which can preserve chemical traces of how ancient oceans behaved. Even so, the study is valuable, because it turns a vague possibility into a specific proposal that other scientists can test.
GNor is the study merely a matter of historical curiosity. The authors point out that understanding how the ocean took up carbon during past ice ages helps scientists to understand the natural carbon cycle, and to predict how the oceans may respond to the rising levels of carbon dioxide in the atmosphere today. The ocean already absorbs a large share of the carbon dioxide released by human activity, so knowing what controls its ability to store carbon is of practical importance. Some of the ways in which the ocean stores carbon are physical: colder water, for example, can hold more dissolved gas than warmer water. Others depend on how water moves between the surface and the deep ocean, which is the kind of circulation change at the heart of the new study. If a change in one region can alter such processes across the globe, then predicting the ocean's future behaviour will require models that treat the planet as a single connected system. Perhaps the broadest lesson of the research is that the climate system is deeply interconnected. A change in the North Atlantic, it seems, can be felt as far away as Antarctica, and that is worth remembering whether we are studying the distant past or preparing for the future.