Should it read .75 Fahrenheit or .4 Celsius? Thanks

Robert

Dr. Monaghan’s response:

Not having access to the photos/data of Antarctica, I’m not certain over what time frame the originator of the comment is referring (i.e., ‘this year is the coldest in decades’, ‘this decade is the coldest in decades’, etc…). So, I’ll make a more general statement about Antarctic near-surface temperatures over the past several decades.

Near-surface temperatures have not changed much over most of continental Antarctica over the past 50 years. If one evaluates the trends over the past five decades, there has been a very, very slight warming (averaged over the continent) that is not statistically different from a trend of zero (meaning that there is so much year-to-year variability that the noise in the data prevents any trend from being considered real). If one evaluates the trends only over the past 20-30 years, there is actually a slight cooling (averaged over the continent), but it too is not statistically different from a trend of zero. So, based on the past 20-30 years, some people point out that there has been recent cooling over Antarctica. However, it might be more accurate to say that there hasn’t been much change over most of the continent. In any case, compared to the considerable warming that has been widespread over the rest of the globe during the past 50 years, the lack of warming over most of Antarctica has been a subject of much interest.

As you might expect, the picture is more complicated than generalizing that Antarctica is warming or cooling. This isn’t surprising, since Antarctica is quite large — about 1-1/2 times the size of the United States. Just as there has not been uniform climate change over the U.S., there has not been uniform climate change over Antarctica. For example, despite little change overall, the Antarctic Peninsula — a California-sized thumb that juts out into the Southern Ocean toward the tip of South America - has been one of the most rapidly warming regions on earth (and the positive temperature trends there are statistically different from zero for the most part). The breakup of several large ice shelves on the Peninsula has coincided with this warming and is thought to be unprecedented in recent history (the Larsen B ice shelf existed for at least 10,000 years prior to its breakup). In turn, the land-based glaciers on the Antarctic Peninsula that were buttressed by these former ice shelves have now sped up, in some cases by a factor of 8 (and, unlike the ice shelves, these contribute to sea level rise). Therefore, though the Antarctic Peninsula only comprises about 4-5% of Antarctica (and there has not been much change over the other 95% of the continent), the changes occurring there seem to be disproportionately important, and perhaps foreshadow the types of (rapid) changes that might occur further south on the coastal margins of the Antarctic mainland if the region of warming becomes larger.

In addition to near-surface atmospheric temperatures, there is another twist when considering Antarctic climate change: the ocean. Regional ocean warming (we still don’t know the cause) is driving the acceleration of the large glaciers in West Antarctica (Pine Island and Thwaites glaciers) that are in turn the largest contributors to contemporary sea level rise coming from Antarctica. The West Antarctic Ice Sheet (WAIS) is a marine-based ice sheet (meaning its base sits on the ocean floor, below sea level, instead of resting on land above sea level). So, this exposure of the lower portion of WAIS to ocean waters and the potential consequences if ocean temperatures continue to warm is currently a major concern of climate scientists (more so than the prospect of atmospheric warming, for now). In summary, the changes to the Antarctic ice sheet (which affect sea level rise - the main consequence of climate change in Antarctica) are driven from the ocean in addition to the atmosphere, so both must be considered.

Finally, its important to note that the cooling (or lack of warming)

in recent decades over continental Antarctica has mainly been

attributed to a strengthening of the belt of westerly winds that

encircles Antarctica (called the “Southern Hemisphere Annular Mode”).

In turn, it appears that a combination of natural climatic

variability, stratospheric ozone depletion (the ‘ozone hole’), and

greenhouse gas increases are the cause of these strengthening winds.

The natural variability and the ozone depletion are the two

‘wildcards’ in this equation (since we know that greenhouse gases will

continue to increase). If the ozone hole becomes smaller as projected

(via the limits on CFC emissions put in place by the Montreal

Protocol), several recent studies suggest that the strengthening of

westerlies will taper off. In this case, our understanding of how the

Southern Hemisphere Annular Mode affects Antarctic temperatures

suggests that temperatures over most of Antarctica may begin to

increase.

The contribution to cooling near the surface of Antarctica due to the stratospheric ozone hole is complex, but it can be explained roughly as follows: the stratospheric ozone layer, situated about 15 km above the surface, absorbs ultraviolet radiation from the sun, which in turn keeps that layer warmer than it would be in the absence of ozone. So, when the ozone hole forms in the austral springtime, the temperatures are colder than normal in the stratosphere over Antarctica, which in turn strengthens the gradient of temperature between Antarctica and equatorial regions of earth. A stronger temperature (energy) gradient between the equator and Antarctica causes the belt of westerly winds that encircles Antarctica to strengthen. In turn, this stronger ‘tube’ of westerly winds that encircles Antarctica and extends from the stratosphere all the way down to the surface counteracts the direction of the prevailing winds near the surface of the Antarctic ice sheet, causing them to weaken. This then causes changes to the near surface temperature inversion over Antarctica. The temperature inversion over Antarctica is a layer of very, very cold temperatures near the surface of the ice sheet caused by the constant radiative cooling of the ice. Its strength is modulated by the near-surface winds. If they are strong, they mix warmer air aloft down to the surface and weaken the inversion. If they are weak, the warmer air aloft is not mixed down toward the surface and the inversion remains strong. So, when the near-surface winds become weaker due to the chain of events set off by the ozone hole, it has the effect of cooling the surface by allowing the cold temperature inversion to strengthen. It is important to note, however (as noted in the previous paragraph), that ozone is only one factor affecting the overall change in temperature near the surface. Warming due to greenhouse gases, and natural variability of the climate — which is very large in Antarctica — also play a role.