Tag Archives: NASA Study

NASA Study of Antarctic Ice Melt Misunderstood

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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.

  • Goldberg, Suzanne. 2014. Western Antarctic ice sheet collapse has already begun, scientists warn. (The Guardian)
  • Lewis, Renee. 2014. West Antarctic ice melt is now ‘unstoppable,” NASA report says. (Al Jazeera report)
  • Lewis, Renee. 2015 Experts dispute NASA study showing Antarctic ice gain. (Al Jazeera report)
  • Plait, Phil. 2015. Is Antarctica Gaining or Losing Ice? Hit: Losing. (Slate)
  • Sea Level Rise Research Group. 2015 global mean sea level time series. (data site)
  • Sinclair, Peter. 2015. Keeping it simple on sea level rise. (Blog post)
  • Sou. 2015. Antarctic ice – growing or shrinking? NASA vs Princeton and Leeds etc. (Hot Whopper)

    • The original paper is here.

    Citation:

    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.

    Abstract:

    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.

    What is the role of the deep ocean in global warming? Climate science deniers get this wrong.

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    What is the role of the ocean’s abyss in global warming?1

    I’ve already posted on a study published in Nature Climate change that shows that the amount of extra global warming related heat in the Southern Oceans is greater than previously thought. There is another paper in the same journal by Llovel et al, “Deep-ocean contribution to sea level and energy budget not detectable over the past decade.” This paper verifies previous research that the oceans absorb a lot of the excess heat, but looks specifically at the ocean below 2,000 meters, which the paper referrs to in places as “deep” but that we should probably call “abyssal.”1 The paper concludes that the abyss is not warming. This is bad news, because if it was warming the total effects of global warming on the surface would be potentially less, or at least, stretched out over a longer period of time. But, it is not unexpected news. We already suspected that the abyssal ocean does not absorb much of the surface heat, while the shallower ocean absorbs quite a bit.

    Research done prior to 2012 (e.g. Hansen et al 2011) parceled out the energy imbalance the Earth experiences from anthropogenic global warming. The extra heat caused by AGW from 2004 to 2010 was divided among the upper ocean (71%), the deeep ocean (5%), with the rest going various other places (only 4% over land). The new paper suggests that the abyssal ocean takes up closer to zero heat.

    There are three complexities you need to be aware of to interpret this finding. First is the complexity in the climate system, second is the complexity of the research itself, and third is the relatively straight forward statistical problem of assigning meaning to specific numbers. That third one is important for journalists and regular people to pay attention to, because the climate science denial community is already exploiting it to misrepresent this study.

    This is a complex and difficult problem

    We know that the vast majority of the extra heat resulting from global warming ends up in the ocean, and also, we know there is a lot of interaction between the ocean and the atmosphere, with heat that might otherwise add to the atmosphere seemingly entering the ocean on a regular basis, with some of it occasionally coming out in large quantitates during El Nino events. This relationship is expected to change over time as the ocean warms, as the transfer of heat between ocean and atmosphere depends in part on the relative difference between them. At some point it is likely that the degree to which the ocean takes up net heat will decrease if the ocean warms up beyond a certain point.

    Over the medium and long term this matters a lot. Because of the ocean (and polar ice and a few other things) the effect of increasing greenhouse gasses is not instantaneous. If the Earth was a simple rock with no water, but a Nitrogen atmosphere with, say, 250ppm of CO2, the greenhouse effects of the CO2 would ensure that the atmosphere was at least a little warm. If we doubled the CO2 the atmosphere would warm further, and it would do so very quickly. A new equilibrium would be reached in a geological instant (a few years?). But with the ocean, that change is much slower slower (decades, perhaps many decades), because the ocean buffers the atmospheric change.

    When heat goes into the ocean, it then moves around in the ocean because it disperses across the aqueous medium, and because water is always moving in currents or mixing. An El Nino is a change in the movement of water that has been warmed with contact with the surface, so that warm water that has been building up at depth over time changes its movement pattern and moves closer to the surface (and to a different horizontal location) where heat is released. That is one (especially large and important) example of the complex dynamic of atmosphere, ocean, and heat. Currents that move through the upper ocean then dive down to depth may move some of the surface heat to the deeper waters, especially where the currents have dived not just from cooling water (hot water would tend to go up, cold water would tend to go down) but because it is driven in “conveyor” systems which may run counter to expectations of where water should go when considering only local conditions, and especially, if the water is dropping because of an increase in salinity. Again, this is an example of the complexity of the system.

    If we add a lot of CO2 to the atmosphere, the atmosphere will warm up, but because of the complexities cited above, it is hard to say how much or how long it will take. The ocean serves to slow the process down. In fact, it is quite possible that if the ocean would be so kind as to absorb a certain amount of this heat permanently, maybe global warming would be somewhat reduced. The ocean is potentially a way of stretching out the effects of global warming. But this effect is likely reduced if the abyssal ocean is not in the game.

    Complexities in the research

    Meanwhile the measurement of heat in the ocean has been very sparse. Over the last decade more measurements have been taken using new technology, but even that is not as good as we would like to understand what is going on at depth. So, when it comes to understanding heat in the ocean, we may sometimes feel like we are at sea. The two papers in this week’s Nature Climate Change are much more important as studies that calibrate or refine the process of measuring ocean heat dynamics under global warming than they are studies that change our view of global warming. Neither paper concludes anything unexpected, both provide important refinements to key numbers, exploiting the last decade of improved data collection.

    One of the complexities is in the details of the Llovel et. al study as compared to the handful of previous related studies. One of the key numbers is the energy imbalance where the ocean absorbs extra AGW produced heat. Energy imbalance is measured in terms of Watts per m–2. The present study yields a value of 0.72. A previous study reported 0.54. Other estimates have varied in this range. Llovel et al point out, however, that these differences may be due to differences in the ocean depth considered in each study and the time periods covered. At least one earlier study measured energy imbalance for the top 1,800 meters, while Llovel et al look at the top 2,000 meters, and all the studies cover somewhat different time periods.

    So, we have changing quality of data, a data set that is growing incrementally over time, studies that look at slightly different time and space parameters. And, on top of this, we have the increasingly advanced methods of figuring this all out. Both of the Nature Climate Change studies used a combination of direct measurements of temperature at various depths, a measurement of the altitude of the top of the ocean (sea level) from highly accurate satellite instruments, and measures of the mass of the water in the ocean, from the GRAIL gravity research project. If the mass of the ocean stays the same (same number of water molecules) but the surface rises, that is from heat, and that allows an estimate of energy imbalance. If the ocean goes up more than it should from heat expansion, the extra may be from glacial melting. And that is the simple version.

    Statistical reasoning

    The statistical part of this is not really so complex. Well, it is, but the part I want to point out is not. Llovel et al concluded “Accounting for additional possible systematic uncertainties, the ocean below 2,000 m contributes ?0.13 ± 0.72 mm yr?1 to global sea-level rise and ?0.08 ± 0.43 W m?2 to Earth’s energy balance.” Sea level rise is close to 3 mm a year, so the abyss is decreasing sea level rise by close to 4%. And, the abyss is in negative energy balance, while the upper ocean is in positive energy balance.

    But look at the numbers. –0.13 plus or minus 0.72. There is actually no way to say that the abyssal ocean is contributing negatively to sea level rise. Zero (or small positive numbers) are well within the range of statistical probability. For energy imbalance, –0.08 plus or minus 0.43. Again, zero and small positive numbers are well within the statistical range for this value.

    But for some reason we see various individuals, including sadly at least one climate scientist (Judith Curry: “Evidence of deep ocean cooling?“), but mostly anti-science climate trolls, crowing that the “deep ocean” is cooling therefore we are not experiencing global warming. However, the truth is that the total amount of heat that is going into the ocean, instead of the atmosphere or other places, was thought to be large, is still known to be large, and in fact is larger than we were originally thinking (from these papers and several others that have come out recently). And, the contribution of the abyss ocean to both sea level rise and energy imbalance is statistically nil. It might be negative, it might be positive, but it is tiny either way. The deep ocean, on the other hand, is in strong positive energy balance.

    1The terminology used in this discussion and some of the research papers (or reporting thereof) is a bit confused. Climate scientists have repeatedly said that heat is going into the “deep ocean” and this paper seems to say it is not. But it is. It is a matter of terminology. This is a source of confusion sometimes exploited by climate science denialists. A good way to define these terms is as follows:

    Shallow ocean = 0-700 meters
    Deep ocean = 700-2,000 meters
    Abyss = > 2,000 meters

    Hansen, J., Mki. Sato, P. Kharecha, and K. von Schuckmann, 2011: Earth’s energy imbalance and implications. Atmos. Chem. Phys., 11, 13421–13449, doi:10.5194/acp–11–13421–2011.

    Llovel, W. J. K. Willis, F. W. Landerer, I. Fukumori. 2014. Deep-ocean contribution to sea level and energy budget not detectable over the past decade. Nature Climate Change, 5 October.