RealClimate: A couple of us (Eric and Mike) are co-authors on a paper coming out in Nature this week (Jan. 22, 09). We have already seen misleading interpretations of our results in the popular press and the blogosphere, and so we thought we would nip such speculation in the bud. The paper shows tha…
RealClimate: State of Antarctica: red or blue? .rll-youtube-player, [data-lazy-src]{display:none !important;} Skip to primary navigation Skip to main content Skip to primary sidebar Skip to footerRealClimateClimate science from climate scientists...MenuHome Start here Contributors Archives You are here: Home / Climate Science / Arctic and Antarctic / State of Antarctica: red or blue?State of Antarctica: red or blue? 21 Jan 2009 by eric Translations: () A couple of us (Eric and Mike) are co-authors on a paper coming out in Nature this week (Jan. 22, 09). We have already seen misleading interpretations of our results in the popular press and the blogosphere, and so we thought we would nip such speculation in the bud. The paper shows that Antarctica has been warming for the last 50 years, and that it has been warming especially in West Antarctica (see the figure). The results are based on a statistical blending of satellite data and temperature data from weather stations. The results don’t depend on the statistics alone. They are backed up by independent data from automatic weather stations, as shown in our paper as well as in updated work by Bromwich, Monaghan and others (see their AGU abstract, here), whose earlier work in JGR was taken as contradicting ours. There is also a paper in press in Climate Dynamics (Goosse et al.) that uses a GCM with data assimilation (and without the satellite data we use) and gets the same result. Furthermore, speculation that our results somehow simply reflect changes in the near-surface inversion is ruled out by completely independent results showing that significant warming in West Antarctica extends well into the troposphere. And finally, our results have already been validated by borehole thermometery — a completely independent method — at at least one site in West Antarctica (Barrett et al. report the same rate of warming as we do, but going back to 1930 rather than 1957; see the paper in press in GRL). Here are some important things the paper does NOT show: 1) Our results do not contradict earlier studies suggesting that some regions of Antarctica have cooled. Why? Because those studies were based on shorter records (20-30 years, not 50 years) and because the cooling is limited to the East Antarctic. Our results show this too, as is readily apparent by comparing our results for the full 50 years (1957-2006) with those for 1969-2000 (the dates used in various previous studies), below. 2) Our results do not necessarily contradict the generally-accepted interpretation of recent East Antarctic cooling put forth by David Thompson (Colorado State) and Susan Solomon (NOAA Aeronomy Lab). In an important paper in Science, they presented evidence that this cooling trend is linked to an increasing trend in the strength of the circumpolar westerlies, and that this can be traced to changes in the stratosphere, mostly due to photochemical ozone losses. Substantial ozone losses did not occur until the late 1970s, and it is only after this period that significant cooling begins in East Antarctica. 3) Our paper — by itself — does not address whether Antarctica’s recent warming is part of a longer term trend. There is separate evidence from ice cores that Antarctica has been warming for most of the 20th century, but this is complicated by the strong influence of El Niño events in West Antarctica. In our own published work to date (Schneider and Steig, PNAS), we find that the 1940s [edit for clarity: the 1935-1945 decade] were the warmest decade of the 20th century in West Antarctica, due to an exceptionally large warming of the tropical Pacific at that time. So what do our results show? Essentially, that the big picture of Antarctic climate change in the latter part of the 20th century has been largely overlooked. It is well known that it has been warming on the Antarctic Peninsula, probably for the last 100 years (measurements begin at the sub-Antarctic Island of Orcadas in 1901 and show a nearly monotonic warming trend). And yes, East Antarctica cooled over the 1980s and 1990s (though not, in our results, at a statistically significant rate). But West Antarctica, which no one really has paid much attention to (as far as temperature changes are concerned), has been warming rapidly for at least the last 50 years. Why West Antarctica is warming is just beginning to be explored, but in our paper we argue that it basically has to do enhanced meridional flow — there is more warm air reaching West Antarctica from farther north (that is, from warmer, lower latitudes). In the parlance of statistical climatology, the “zonal wave 3 pattern” has increased (see Raphael, GRL 2004). Something that goes along with this change in atmospheric circulation is reduced sea ice in the region (while sea ice in Antarctica has been increasing on average, there have been significant declines off the West Antarctic coast for the last 25 years, and probably longer). And in fact this is self reinforcing (less sea ice, warmer water, rising air, lower pressure, enhanced storminess). The obvious question, of course, is whether those changes in circulation are themselves simply “natural variability” or whether they are forced — that is, resulting from changes in greenhouse gases. There will no doubt be a flurry of papers that follow ours, to address that very question. A recent paper in Nature Geosciences by Gillet et al. examined trends in temperatures in the both Antarctic and the Arctic, and concluded that “temperature changes in both … regions can be attributed to human activity.” Unfortunately our results weren’t available in time to be made use of in that paper. But we suspect it will be straightforward to do an update of that work that does incorporate our results, and we look forward to seeing that happen. Postscript Some comment is warranted on whether our results have bearing on the various model projections of future climate change. As we discuss in the paper, fully-coupled ocean-atmosphere models don’t tend to agree with one another very well in the Antarctic. They all show an overall warming trend, but they differ significantly in the spatial structure. As nicely summarized in a paper by Connolley and Bracegirdle in GRL, the models also vary greatly in their sea ice distributions, and this is clearly related to the temperature distributions. These differences aren’t necessarily because there is anything wrong with the model physics (though schemes for handling sea ice do vary quite a bit model to model, and certainly are better in some models than in others), but rather because small differences in the wind fields between models results in quite large differences in the sea ice and air temperature patterns. That means that a sensible projection of future Antarctic temperature change — at anything smaller than the continental scale — can only be based on looking at the mean and variation of ensemble runs, and/or the averages of many models. As it happens, the average of the 19 models in AR4 is similar to our results — showing significant warming in West Antarctica over the last several decades (see Connolley and Bracegirdle’s Figure 1). AddThis Sharing ButtonsShare to FacebookFacebookShare to TwitterTwitterShare to EmailEmailShare to Copy LinkCopy LinkShare to MoreAddThis4Filed Under: Arctic and Antarctic, Climate modelling, Climate Science, Instrumental Record Reader Interactions128 Responses to "State of Antarctica: red or blue?" Comments navigation 1 2 3 Next » Tom G says 21 Jan 2009 at 1:28 PM Would I be wrong to think that the West Antarctic ice shelves are under greater stress than previously thought? [Response: West Antarctic is a lot colder to begin with than the Peninsula, so we’re not talking about anything catastrophic in the near term. Still, surface melting has increasingly been observed on the West Antarctic ice shelves (see here). If current trends continue (and that is definitely an “if”), then of course we will see more of that. But it is not really significant yet — it is not yet influencing the stability of the ice shelves outside the Peninsula region–eric] Jim Bouldin says 21 Jan 2009 at 1:33 PM Eric I just spent about 10 minutes searching every possible way and can’t find the article. Can you give the ref? [Response: Here. It’s also on the cover. – gavin] Andy Gunther says 21 Jan 2009 at 2:00 PM Any chance the authors can provide a larger version of their figure, properly referenced, that could be used in digital presentations by those of us who talk to the public about climate change? [Response: I will put this up on my University of Washington web site early next week and let you know.–eric] Richard Pauli says 21 Jan 2009 at 2:09 PM So the Wilkins Ice Shelf is supposedly still held together by a shrinking strip of ice, now about 500 meters wide at its narrowest point. News reports say a break-up expected at any time – 2 weeks to 2 months. (article 1-19-09 http://uk.reuters.com/article/environmentNews/idUKTRE50I4G520090119 ) Why aren’t there bets on on when this will happen? Catastrophic events have long associated with gambling – and this is one of the rare sudden AGW events. Who is running the pool? I say Feb 14th just for the poetic factor. James Staples says 21 Jan 2009 at 2:15 PM Gentlepeople, well done on nipping any controversy in the bud – as usual; though I’m left wondering if the warming trend isn’t related to a subject that i’d like to see Real Climate Address more often; The possible shut-down of The North Atlantic Conveyor – as extreme warming of the Southern Oceans, along with the plunging of Europe into a new Ice Age would be the result of this, as I’m sure you all know. I’ve also been wondering, and will now do a little investigating of the issue, whether or not the Level of Oxygen in our Atmosphere has been declining measurably; as it seems to me to be of an issue of Great Concern, now that we’ve cut down so much our Rainforests and are in danger of destroying the ability of Chloroplankton to form the Carbonate shells necessary to their existence, through Oceanic Acidulation. Even if you, at Real Climate, are too busy to address these issues; I’ll share what I can find out in a Technorati Blog posting, and – if it seems urgent enough – I’ll send you guys an E-mail as well. Love Ya! ike solem says 21 Jan 2009 at 2:28 PM “Our paper — by itself — does not address whether Antarctica’s recent warming is part of a longer term trend.” Isn’t that because it ignores the ocean component? For example, see this article from Reuters, Feb 2008:http://www.alertnet.org/thenews/newsdesk/SYD100332.htm The paper is here, I believe. Satellite altimetry data show a strong increase in sea level in various parts of the Southern Ocean over the 1990s. In this paper, we examine the causes of the observed sea level rise in the region south of Australia, using 13 years of repeat hydrographic data from the WOCE SR3 sections, and the SURVOSTRAL XBT and surface salinity data. The hydrographic data show a poleward shift in the position of the Subtropical and the Subantarctic Fronts over the period. In the Antarctic Zone, the Antarctic Surface Water has become warmer and fresher, and the Winter Water tongue has become warmer, fresher, thinner and shallower. Increased freshening south of the Polar Front is linked to increased precipitation over the 1990s. Temperature changes over the upper 500 m account for only part of the altimetric sea level rise. The CTD sections show that the deeper layers are also warmer and slightly saltier and the observed sea level can be explained by steric expansion over the upper 2000 m. ENSO variability impacts on the northern part of the section, and a simple Sverdrup transport model shows how large-scale changes in the wind forcing, related to the Southern Annular Mode, may contribute to the deeper warming to the south. Also, isn’t Antarctic sea-ice thickness data incredibly poor? Unlike the Arctic, there is no long-term submarine record of ice thickness – but with the warming Southern Ocean, it seems likely that that has been going on as well. Not that it isn’t a good paper that shows the temperature trends, but it’s important (as the postscript admits) that future predictions will depend heavily on getting the ocean component right. It is encouraging that the coupled ocean-atmosphere GCMs give similar results to the data analysis, though. Chris Colose says 21 Jan 2009 at 2:32 PM Tom G (#1) It’s hard to make a lot of sense out of “previously thought” because the “old thought” in general was that ice sheets were fairly stagnant on decadal timescales. So it’s true that things are happening that are more interesting than what was “previously thought,” but that isn’t very meaningful. Over the last decade, there has really been a paradigm shift in the understanding and coverage/modeling of the polar areas, and how dynamic they are. I recently attended a seminar at UChicago where Michael Oppenhemier gave a talk on ice sheets and sea level rise. A recurring statement was that we have essentially no skill at modeling dynamic processes of accelerated glacier flow, and thus there is low to no predictive ability for either the collapse of ice shelfs or the effects of accelerated flow following ice shelf collapse. David B. Benson says 21 Jan 2009 at 2:44 PM Eric — Thank you. Most useful. John Gribbin says 21 Jan 2009 at 2:45 PM A beautifully clear contribution. Thanks. Sekerob says 21 Jan 2009 at 2:58 PM How does the current state per e.g. the uni Bremen map http://www.iup.uni-bremen.de:8084/ compare to the papers observations. Ross Sea seams to be in a horrible state and big gaps all around the coast lines. Area per CT was a few days ago over a 5 day period above 79-00 mean: –2008- 79-00mn Anomaly date, all in km square 4003000 3716000 287000 jan11 3917000 3629000 288000 jan 12 3916000 3552000 364000 jan 13 3696000 3466000 230000 jan 14 3624000 3376000 248000 jan 15 Is the same as for the Arctic, volumetric estimation of the total Sea Ice done, if that at all is of relevance when no longer land fast? Surely the Jeff Id’s of this world will be following this thread as he’s come up with these wonderful “play down” equations of global sea ice, which presently is about 600,000 km2 behind 79-00 mean. Already saw an amazing statement of “the current state is right on the 30 year mean” by S. Goddard at WotSuUp, which I interpret as 1979-2008 mean. Ostrich policy of the guys that know the glass was half filled with now vaporised red wine, but when looking from the side the red haze of the former content used, to have the ignorami believe it ain’t so. Joseph O'Sullivan says 21 Jan 2009 at 3:03 PM “We have already seen misleading interpretations of our results in the popular press and the blogosphere” I saw the NY Times article first ‘Warming in Antarctica Now Looks Certain’ http://www.nytimes.com/2009/01/22/science/earth/22climate.html?partner=rss&emc=rss then I went to RC to see if there was a post up. Does the NYT article do a fair job covering the paper? [Response: Yes, it’s a pretty good article. Kenneth Chang tends to get these things pretty much right in my experience.–eric] Gary Plyler says 21 Jan 2009 at 3:15 PM This is very interesting. Over 31 years, 1969 to 2000, East Antarctica cooled by 0.1 degree-C per decade. Over 50 years, 1957 to 2006, East Antarctica warmed by 0.1 degree-C per decade. So, must have warmed by an average of 0.2 degrees-C per decade during the 19 years, (some during the 12 years 1957 to 1969 and some during the 6 years 2000 to 2006 or all 0.4 degrees-C total during one of those periods). What atmospheric hypothesis can account for such behavior? Gavin, you are the computer modeler that knows all the scientific laws, theories, and hypotheses that have been mathematically entered and weighted in the GCMs. Which hypothesis or hypotheses can cause this and what scientific method can prove, modify, or disprove any of the individual hypotheses? Hank Roberts says 21 Jan 2009 at 3:53 PM Gary Plyler, you’re being too precise in claiming you can isolate 12 year and 6 year trends. Did you read this part of the article above? “… Substantial ozone losses did not occur until the late 1970s, and it is only after this period that significant cooling begins in East Antarctica.” t_p_hamilton says 21 Jan 2009 at 4:09 PM Richard Pauli Said in comment #4: “So the Wilkins Ice Shelf is supposedly still held together by a shrinking strip of ice, now about 500 meters wide at its narrowest point. News reports say a break-up expected at any time – 2 weeks to 2 months. (article 1-19-09 http://uk.reuters.com/article/environmentNews/idUKTRE50I4G520090119 ) Why aren’t there bets on on when this will happen? Catastrophic events have long associated with gambling – and this is one of the rare sudden AGW events. Who is running the pool? I say Feb 14th just for the poetic factor.” I thought guys were supposed to prefer breakups before Valentine’s Day for financial reasons. Chip Knappenberger says 21 Jan 2009 at 4:15 PM Guys, A couple of questions: 1) Wouldn’t it have been more instructive to have shown in Steig et al. (2009) Figure 3b the period, from say, 1969 to present, or 1979 to present, so that we could see the overall behavior in recent decades rather than from a period that ended in 2000 for the sake of comparison to an earlier study? I understand that it is important to compare your new results with previous results to show consistency, but that could have been stated, or perhaps moved into the Supplementary Material. Since, as seen in Monaghan et al. (2008), and as Chapman and Walsh (2007) drew attention to, “Trends computed using these analyses show considerable sensitivity to start and end dates with starting dates before 1965 producing overall warming and starting dates from 1966 to 1982 producing net cooling rates over the region” it would have been interesting to see if such was the case in your analysis. The results of such an analysis would help get a firmer handle on whether or not the predominance of warming in Antarctica took place from the late-1950s to the mid-1960s, or whether it occurred fairly consistently through time. A better understanding of this would help make comparisons to model expectations perhaps a bit clearer. 2) Your spatial reconstruction procedure does not seem to be pinned down by actual observations. For instance, in your Figure 3a, the observed trend at the South Pole station is given as -0.1ºC/dec while the reconstruction appears to show a positive trend there. [Our figure 3a also shows that the observed trend is +0.1C/decade at Vostok, and the reconstruction shows no significant trend at either location.] Is this an isolated instance, or are their other observed records with show signs of trends opposite than the reconstruction produces? Your map in Figure 3a (and the cover of Nature) gives the casual reader the perception that all of Antarctica has exhibited a warming trend from 1957-2006, when in fact, it seems that at least one location, direct, local observations indicate otherwise. I understand that your procedure tried to intelligently fill in data where there was none, but in the places that actually do have data, isn’t it best to use that data? It would have been instructive to superimpose an appropriately colored dot at the location of each long-term occupied weather station representing the observed trend (in Figure 3) rather than just the two values included in 3a and 3b. 3) Why do you need reconstructions of the period of record when you have full satellite coverage? Am I correct to assume that the trend from the satellite data and the trend for the reconstructed temperatures are the same during the satellite data period? And that therefore the slightly higher trend that you all report (as compared with other efforts) comes about from cooler conditions in the pre-satellite era than has been previously determined from the direct observatio… truncated (26,877 more characters in archive)