A recent study that is getting a lot of press suggests that the massive ice sheets of Antarctica are on average growing rather than shrinking, and thus, not contributing to sea level rise. (The authors of the study warn that this will reverse in the near future with global warming.) However, there is reason to believe that these conclusions are incorrect.
Antarctica is the sleeping giant of climate change. Human activity, mainly the release of greenhouse gasses from burning fossil fuels, has been changing the climate rather dramatically for the last few decades, and the consequences of this change are mostly negative. Failed agricultural systems have led to failed states and regional political instability. Dramatic changes in weather patterns, including droughts in Australia and California, a series of unprecedented tropical storms over the last several years, major flooding (if anyone from Texas is reading this, nice to know you have internet access in your tree), all have a global warming contribution, because weather is climate and climate is changed and changing. But sea level rise, while mostly a thing of the future rather than the present, may have the biggest effect of all, at least on land. As we warm the planet, the polar ice sheets will contribute much of their ice to the sea, and based on what we know of the past, direct measurements over the last 20 years or so, and from models of the medium term future, this could mean an increase in sea level of several meters. The best available science currently suggests that by the end of the century average sea levels could be about a meter higher than they are now. It would not be unreasonable to regard that as a conservative estimate.
I’d like to take a moment and point out an important aspect of the sea which people, especially those that don’t live on the sea, forget. The average altitude of the sea at a particular point along the shore is not the part you have to worry about. Well, that is important, but it is not the part that bites. Consider the cobra snake. A cape cobra can strike at a distance well over half its own body length. So if you are standing ten feet away from a fifteen foot long cobra, the snake might seem a safe distance away, but you are actually within its striking range. One could say that the sea has two overlapping but distinct distances at which it strikes. One is the normal storm range. If you raise the sea along a beach in Cape Cod by six inches, nothing interesting happens most days. But the dozen or so medium size storms that will occur over a year (especially in winter when the storms come in from the Atlantic) will convert that foot of elevation into several horizontal feet of beach erosion, in a very short amount of time. The second is what happens when more serious storms, like tropical cyclones or their extratropical spawn, come along. New York City was built and reinforced from the sea, over time, mainly when the Atlantic was about a foot lower than it is now. A couple of years ago, when Super Storm Sandy came along, the storm gathered up that extra foot of sea level and turned it into an extra large storm surge sufficient to flood the subway system in lower Manhattan. Long before the sea in that area rises another three feet, there will be the occasional storm surge that will be even more severe.
Since a large percentage of the world’s population, a large percentage of the world’s agricultural activity, and an even larger percentage (probably) of the world’s real estate value will become subject to flooding, sometimes severe, and eventually be replaced by the rising sea over the next century and beyond, sea level rise is a very important phenomenon.
You have probably already heard about the study, “Mass gains of the Antarctic ice sheet exceed losses” by H. Jay Zwally and others (see citation and abstract below), that came out a couple of weeks ago telling us that the contribution to sea level rise by the Antarctic is currently zero or negative. Or at least, that is how many press outlets are reporting the story.
There are two problems with this study that you need to know about. First, the study examines a data set that ends in 2008. The second problem is that there are indicators that the study is simply wrong, even though it likely has significant merits.
The last decade of research on Antarctica have shown, in many studies using a variety of techniques, that Antarctica is contributing to sea level rise. They have also shown that the rate of melting in Antarctic is probably increasing. Even more importantly, they have indicated that certain areas of Antarctic are current in a state of instability, suggesting that the rate of contribution of the southern continent’s ice mass to sea level rise may increase abruptly in the near future.
The fact that the study being reported uses older data could explain why it conflicts with everything else the science is telling us. Michael Mann, quoted in The Guardian, notes, “…the claims are based on seven-year-old data, and so cannot address the finding that Antarctic ice loss has accelerated in more recent years.” To this I’ll add that it is somewhat annoying that those reporting the story, including, oddly, the authors of the study, are using forms of the word “current” to describe the result. These results are old, out dated, and while potentially valuable, a data set ending in 2008, when speaking of a rapidly changing system, is not current.
Sou at HotWhopper has a nice graphic showing estimates of Antarctic ice melt before and after 2008, strongly indicating the problem with using a study from older date to understand current conditions.
Average global sea level is a measurable verifiable established fact, and the contribution of major ice sheets to this has been measured and found to be important. If the study is correct, and Antarctica was not contributing to sea level rise during that period prior to 2008, then something is terribly wrong. There is simply not enough wiggle room in the other sources of sea level rise to account for the missing volume of water. One could argue that a beautiful hypothesis (positive mass balance in Antarctic ice) has been killed by an ugly fact (actual observed sea level rise). But Zwally’s study does not present a mere hypothesis, but rather, is based on detailed observations incorporated into a set of carefully done calculations.
So, perhaps the observations are wrong. There may be two reasons the observations (and the calculations derived from them) are wrong. One is simply that the satellite data they use are inherently less accurate than needed. The measurements are of a very small change over time over a very large area. If the satellite method is just a little off, this could cause a problem. (By the way, the data end in 2008 because the instrumentation on the satellite stopped working then.) This study’s main contribution may, in the end, to be to point out a problem with the instrumentation prior to that time. This doesn’t seem that likely for the simple reason that the whole point of putting fancy instruments in a bird is to get super accurate information.
The second possible reason seems more likely. Part of the process of determining that Antarctica has a positive mass balance (more ice over time rather than less) involves assumptions (and some measurements) about the response of the bedrock underneath the very thick ice sheets. If that is wrong, then that is a problem.
Since the sea level has in fact been going up, and there is no easy way to account for that than a certain contribution to Antarctica, and all the other science shows an increasingly melting Antarctic, and the study uses older data, then I’m afraid I have bad news. Sea level is still going up, Antarctica is still contributing to it, and the amount of this contribution is still, as the science has been suggesting for several years no, only going to increase.
The following resources will be of interest to anyone following this story.
The original paper is here.
Zwally, H. Jay, 2; Li, Jun; Robbins, John W.; Saba, Jack L.; Yi, Donghui; Brenner, Anita C. 2015. Mass gains of the Antarctic ice sheet exceed losses. Journal of Glaciology, International Glaciological Society.
Mass changes of the Antarctic ice sheet impact sea-level rise as climate changes, but recent rates have been uncertain. Ice, Cloud and land Elevation Satellite (ICESat) data (2003–08) show mass gains from snow accumulation exceeded discharge losses by 82?±?25?Gt?a–1, reducing global sea-level rise by 0.23?mm?a–1. European Remote-sensing Satellite (ERS) data (1992–2001) give a similar gain of 112?±?61?Gt?a–1. Gains of 136?Gt?a–1 in East Antarctica (EA) and 72?Gt?a–1 in four drainage systems (WA2) in West Antarctic (WA) exceed losses of 97?Gt?a–1 from three coastal drainage systems (WA1) and 29?Gt?a–1 from the Antarctic Peninsula (AP). EA dynamic thickening of 147?Gt?a–1 is a continuing response to increased accumulation (>50%) since the early Holocene. Recent accumulation loss of 11?Gt?a–1 in EA indicates thickening is not from contemporaneous snowfall increases. Similarly, the WA2 gain is mainly (60?Gt?a–1) dynamic thickening. In WA1 and the AP, increased losses of 66?±?16?Gt?a–1 from increased dynamic thinning from accelerating glaciers are 50% offset by greater WA snowfall. The decadal increase in dynamic thinning in WA1 and the AP is approximately one-third of the long-term dynamic thickening in EA and WA2, which should buffer additional dynamic thinning for decades.