Saving our Skins: Chapter 23. Back to the Beginning

Atmospheric Reflections from a Lauder Stargazer

The continuing story of “Saving our Skins”. The final chapter. Antarctica again.
Updated September 5, 2021.

My story started in Antarctica, with the crash of flight NZ901 and the discovery of the ozone hole soon after. It ends there too, with ozone recovery, climate change and its effect on ice, and some interactions there between climate change and ozone depletion.

The Antarctic ice sheet is the largest single mass of ice on the planet. In some places, it’s 3 km thick. It contains 30 million cubic kilometres of ice, which accounts for 90 percent of the fresh water on Earth. If all that ice were to melt, sea level would rise by 60 metres. The ice locked up there holds the key to our long-term future because a sea level rise of that magnitude would inundate most of the world’s cities.

There’s a tipping point beyond which we are consigned to that fate, but the time scale for that is enormous. A complete ice-melt would take thousands of years. Of more immediate concern is what happens in the next hundred years or so. While the situation is quite clear in the Arctic, where ice-melt is increasing rapidly, and an ice-free Arctic Sea is already on the horizon, it is more contentious in Antarctica.

And the merchants of doubt remain alive and well today. For several years they sowed doubt arguing that climate change couldn’t be occurring because global temperatures plateaued after a peak associated with an El Nino event in the early part of the 21st century. But as the rate of change has again increased, they’ve now had to shelve that one. A favourite now is to ask why isn’t ice decreasing in all parts of Antarctica?

That’s a good question. In some parts of Antarctica, ice is currently accumulating, rather than eroding. This may be a temporary respite. The reasons for the increase in ice cover in parts of Antarctica are a topic of heated debate that has been seized upon by climate deniers. The cause of  this local increase in ice-thickness is not yet fully understood. But, despite the increased air temperatures there, temperatures are still well below zero most of the time, so we don’t yet expect that to be a direct source of ice-melt in the region.

The processes are complex, involving connections between the Arctic, the atmosphere and the oceans. Several lines of research are beginning to shed light on the paradox.

Estimates of the amount of Antarctic ice are constantly being improved, as higher resolution measurements of the underlying topography become available, and as we learn more about the rate of rebound of the underlying terrain as glaciers are eroded. These improvements will lead to better estimates of long-term changes. During the first decade of this century, there was a period of increased precipitation that may have contributed to ice increases, and this may recur in the future because as the temperature of air increases, its capacity to hold water also increases, and episodes of heavier precipitation are expected. That higher water capacity also contributes to the higher probability of destructive storms, because the condensation of water vapour into liquid water (rain drops) releases energy. That energy is released as heat, which contributes to up-draughts and subsequent storm winds, including tropical cyclones.

Because most of the ice in the Arctic is floating on water, its melting doesn’t contribute to sea level rise. But the melting of freshwater glaciers changes climate by changing the patterns of circulation in the ocean. Freshwater has a lower density than saltwater. That’s why you feel so much more buoyant swimming in the sea. When the glacial ice-melt flows to the sea, the less dense freshwater floats on top of the saltwater, rapidly cooling the surface. This is especially important in regions where ocean surface waters are normally warmed by large scale currents like the Gulf Stream.

Without the Gulf Stream, the temperature of London would be 2°C colder in winter. But it has already slowed significantly in recent years, and that might be a contributor to the severe winters currently being experienced in parts of Europe (while others are unusually warm). The Gulf Stream continues its northern march until the surface temperatures become cooler than the underlying deeper water. As it becomes cooler it becomes denser and eventually descends to the ocean floor where it begins its long, slow deep-water journey southward to Antarctica. With the influx of freshwater melt from glaciers, the northern extent of its march is curtailed.

This planetary-scale pattern of oceanic circulation is called the “thermohaline” circulation because it involves heat (thermo) and salinity (haline) effects. Because of changes in those effects caused by increasing greenhouse gases, the temperature of the deep water that eventually collides with Antarctica is also increasing. These warmer waters are already eroding the base of the ice sheet and so contributing to the ice melt being observed in Antarctica.

Ozone depletion in Antarctica may also have temporarily masked the effects of climate change in the region. In addition to its well-known capacity to absorb UV radiation, ozone also absorbs at visible and infra-red wavelengths. All these absorptions contribute to effects on climate. Although its UV absorption is important for health, its heating effect below the peak of the ozone layer near 20 km is small because only a very small fraction of incoming UV from the Sun penetrates that far, and there’s even less outgoing energy from the Earth at those wavelengths. In those lower regions of the atmosphere it’s the absorptions at longer wavelengths that are most important.

During the Antarctic ozone hole, which has recurred every spring since the late 1970s and has been comparable in size each spring since the 1990s, lower ozone in the 15-25 km range of altitudes means that less energy is absorbed, so there’s less heating at those altitudes. The cooler atmosphere causes changes in wind patterns that can lead to changes in surface winds and precipitation patterns as far away as Australia.

But the normal pattern of temperature change from increasing greenhouse gases is also to decrease stratospheric temperatures because the heat emitted from the surface is absorbed lower down in the atmosphere. It’s a chicken-and-egg situation. Is the reduced temperature in the stratosphere entirely a consequence of these radiative effects of ozone depletion, or is it amplified by climate change? Changes in climate attributable to ozone changes in Antarctica may have mitigated against those due to increasing greenhouse gases. If that’s the case, over the years ahead as ozone slowly recovers, we should expect accelerated warming and ice-melt.

Whatever the reason, the net result is that ice cover is currently decreasing, and that rate of decrease is accelerating. And the physics tells us that eventually, any localised increases in ice cover will eventually be overcome by the inexorable warming. It’s just a matter of time.

It won’t be the end of the world. According to James Lovelock’s Gaia hypothesis, the planet will look after itself. But it will be a very different world. Climate change and sea-level rise, if left unchecked, will affect the survivability and habitability of parts of the developing world, and will affect the profitability of others in the developed world. Low-lying countries and countries with long coastlines relative to their area, will be most affected. These include The Pacific Islands and New Zealand.

We’ve probably already averted a disaster from ozone depletion. We fixed it with a momentous international agreement: the Montreal Protocol. The threat we face from climate change will be harder to solve. Not least because it is the developing world of dark-skinned people- rather than the developed world of white skinned people – that’s unfairly most at risk. The poor will bear the brunt of a problem caused by rich countries. The economic advantages of solutions are still being denied, but we now have the technology to move away from fossil fuels. And through the Montreal Protocol, we also have a political framework to attack the problem. All we need now is the will to act. That will be strengthened by reduced political interference from vested interests, and by the continued fading-away of climate-change-denying curmudgeons.

I made the graphic below for inclusion in the 2006 UNEP Assessment to show how the ozone problem is entwined with climate change.

Sadly, my version never saw the light of day and was replaced in the final report by a much more complicated version that looks sexier, though I like it less.

The wheels of progress will continue to turn and as the threats from ozone depletion in the air above Antarctica recede, the threats from climate change warming the ocean and melting the ice below advance. While we now understand the first (mostly), there’s still much to be learnt of the other.

Though closely linked to Ozone Depletion, Climate Change is another story. A story that’s yet to play out.

[1]. There is an apparent positive trend at tropical Saint Denis, but because the time series available there is short, the uncertainty in determining the trend is large. In this case, the error bars encompass zero, so the trend is not statistically significant.

[2]. Shortly after posting this chapter, the first evidence of ozone hole recovery affecting climate was published in Nature.

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We’re just about done now. Just the appendix to go, and that’ll be all for “Saving our Skins”.

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