The Arctic melts, but oceans and ozone hole may cool Antarctica

Icebergs calve from the Larsen Ice Shelf in Antarctica. Ted Scambos, NSIDC, CC BY-ND

Over recent decades, scientists have watched the Arctic and Antarctic polar regions respond in starkly different and perhaps surprising ways to the effects of increasing human influence on the Earth’s climate.

Whereas the Arctic has warmed far more than any other region and has steadily lost sea ice, Antarctica has cooled in most places and gained sea ice. Any hope of predicting the fate of these environmentally critical regions rests on understanding the elemental processes behind this polar climate conundrum.

In a paper published in the Philosophical Transactions of the Royal Society journal we lay out a framework for the response of polar environments to the effects of human-induced changes to the climate. The predominant factors are the ever-increasing concentration of greenhouses gases in the atmosphere and the ozone hole over Antarctica caused by an abundance of chlorofluorocarbons, which peaked at the turn of the century and are now slowly diminishing.

On account of its ability to absorb and transport enormous amounts of heat, the ocean also plays an outsized role in climate change and is an important factor in the explaining the asymmetric response of the north and south poles to the changing climate.

In our computer simulations of the ocean and climate, the excess heat from greenhouse gases is absorbed into the ocean around Antarctica and in the North Atlantic. The heat does not stay in place but instead is carried around by the moving ocean. In the Southern Ocean around Antarctica, currents draw heat towards the equator, away from the pole.

Meanwhile in the North Atlantic, ocean currents carry excess heat poleward towards the Arctic. So while Antarctica warms little, or even cools, the Arctic Ocean’s temperature increases to such an extent that heat is actually lost to the atmosphere over the Arctic.

GISS Surface Temperature Analysis 1979-2005 (-0.8°C to +0.8°C), showing asymmetrical distribution of warming and cooling between hemispheres. Marshall et al/Royal Society

This clearly reveals the differing responses to greenhouse gases in each region, with the Arctic warming more than twice as rapidly as the Antarctic. By mid-century, the Arctic may have warmed so much that summers could have no sea ice at all.

As for the ozone hole over Antarctica, scientists have only recently started to quantify the impact of ozone depletion and recovery on the surface climate. When we introduced an ozone hole into our model, the winds over the Southern Ocean grew faster. This intensification of winds initially triggers a rapid cooling of the sea surface and expansion of sea ice, but this is followed by a slow process of warming and sea ice contraction. We think this warming happens because the stronger winds eventually dredge up to the surface warm waters from the deep ocean.

Our calculations suggest that around Antarctica the ozone hole may have delayed warming due to greenhouse gases by several decades. It’s tempting to speculate that this is the period through which we now find ourselves passing. By mid-century, however, ozone hole-effects may instead be adding to the warming around Antarctica, but with diminished amplitude as the ozone hole heals.

When we bring our results together, an interesting picture emerges of the combined effect of greenhouse gases and the effects of the ozone hole on climate at the poles. It may be that the slight cooling currently measured around Antarctica today is a consequence of the cooling effects of the ozone hole. But as the century proceeds, both these human-induced effects on the climate may combine with the effect of warming waters around Antarctica with worrying but uncertain effects on Antarctic sea ice, glaciers and sea-level rise.

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