Tag: Global Science Report

Carbon Tax Follies

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

There seems to be a noticeable murmur around town about a carbon tax—a tax on the amount of carbon dioxide that is released upon generating a unit of energy. Since fossil fuels—coal, oil, natural gas—are both the source of over 75% of our energy production and emitters of carbon dioxide when producing that energy, a carbon tax insures that the price of everything goes up.

There is one and only one justification for a carbon tax—an attempt to influence the future course of the earth’s climate (or, as some people prefer, to mitigate anthropogenic climate change) by trying to force down the emissions of the most abundant human-generated greenhouse gas.

But of all the things that a carbon tax will do (raise prices, increase bureaucracy, elect Tea Partiers, etc), mitigating anthropogenic climate change in any meaningful manner is not one of them.

The annual carbon dioxide emissions from the U.S., currently about 5,500 million metric tons per year, only contributes roughly 0.003°C/per year of warming pressure on global temperatures (see here for a handy way of making that calculation). So the best that a carbon tax could ever hope to achieve, climatically, would be to prevent this amount of warming each year by completely eliminating all carbon dioxide emissions from the U.S.

If we went to zero emissions tomorrow,  the carbon tax would prevent about 0.26°C of global temperature rise by the year 2100. According to the latest projections from the Intergovernmental Panel on Climate Change (IPCC), the projected temperature rise by the end of the century ranges from about 1.1 to 6.4°C, with a business-as-usual rise of around 3°C (put me down for 1.6° until then, unless nature is being a blatant liar).  The “mitigated” rise is proportional to the expected temperature rise. A carbon tax enacted today that is immediately and completely successful at eliminating all U.S. CO2 emission would lower rise in temperature expected by the end of the century around 10%.  This amount is small, of little consequence, and in fact will be difficult to detect.

It is also not going to happen.  We only have the capacity to produce about 30% of our electricity from non-carbon emitting fuel sources (primarily nuclear and hydroelectric). So it will take time, and probably a lot of time (many decades) before our energy needs could possibly be met without emitting CO2 into the atmosphere.  And of course, as time ticks by before eliminating or at least appreciably reducing  our emissions, the amount of global warming saved by such action declines (and become less and less consequential), as does the justification for the carbon tax.

I am just in the early stage of this analysis, so the numbers above are a bit rough (but conservative). In the future I hope to produce a menu of emissions reductions/climate savings options—but one without prices.  That way the policymakers will see what they are going to be getting for whatever price they decide to assign. So too will the general public. And what they will all see is that whatever level of carbon tax they decide upon,  they will get a lot of climate nothing  for a lot of financial something.

The best thing would be for policymakers to just leave well enough alone, for on their own, carbon dioxide emissions in the U.S. have been declining for more than a decade (and in fact are pushing levels of the early 1990s, http://www.eia.gov/environment/emissions/carbon/). And even if such a reduction doesn’t result in any scientifically detectable climate impacts, at least it hasn’t cost us anything.

Arguing over Sandy

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

On Monday of this week, three prominent and influential scientists published an opinion piece in Politico arguing that anthropogenic global warming was responsible for making the destruction from “super storm” Sandy significantly worse than it otherwise would have been. They added that if we don’t “cut industrial carbon pollution,” we are going to get more of the same, and then some.

On the same day, I argued here at Cato@Liberty that it could be reasoned that anthropogenic global warming lessened the impact of Sandy.

The scientific truth about the situation is that it is impossible to know who is right—the uncertainties are just too large. But I think it is fair to say that no matter what direction the influence of anthropogenic global warming was on Sandy, in net it was quite small.

One difference between my piece and the Politico article co-signed by Dr. Robert Corell (“senior policy fellow for the American Meteorological Society and former Chair of the United States Global Change Research Program”), Dr. Jeff Masters (“the founder and Director of Meteorology for Weather Underground and a former NOAA Hurricane Hunter”) and Dr. Kevin Trenberth (“Distinguished Senior Scientist in the Climate Analysis Section at the National Center for Atmospheric Research”) was that I tried to stick to a scientifically defensible argument.  In their piece, they juiced up their case with some—how should I say this?—rather dubious facts.

The worst of these was claiming that “[o]n the stretch of the Atlantic Coast that spans from Norfolk to Boston, sea levels have been rising four times faster than the global average.” They implied that anthropogenic global warming was the reason why.

This is simply untrue.

While it is true that the long-term (~20th century) rate of sea level rise along that stretch has been about twice the global average over the same period, it is scientifically well-established that the regional enhanced rate of sea level rise is due to ongoing geologic processes resulting from end of the last ice age. When these non-anthropogenic processes are properly subtracted out of the tide gauge record of sea level observations, the rate of sea level rise that is left over is virtually the same as the global rate of rise, not four times faster.

The same conclusion is reached if you limit your comparison to the period of satellite observations of sea level, which began about 20 years ago.  The figure below shows a map of the satellite-measured trends in sea level from 1993 through mid-2012.  The global average rate of rise is 0.12 inches per year, which is represented by a sort of greenish yellow color.  Turning your attention to the Northeast coast of the United States (you might have to squint a bit), you see that the color there is also sort of greenish yellow—in other words, right about the global average.  Places where the sea level is rising four times faster than the global average are colored a light pink; while there are a few such places, none of them are anywhere near the stretch of coast between Norfolk and Boston.

Figure 1. Spatial distribution of the rate of sea level rise across the globe as measured by satellite altimeters (Source: University of Colorado Sea Level Group, modified to reflect English units).

It is somewhat telling when prominent climate scientists have to resort to incorporating incorrect (and readily debunked) science to try to bolster their case for climate alarm—an alarm that was raised to try to scare us into accepting regulations on greenhouse gas emissions.

New Evidence that Plants Are Slowing the Growth of Greenhouse Gases

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

Scientists have known for decades that, as global carbon dioxide levels increase, so too does the standing biomass of the world’s plants. Carbon dioxide is a strong plant fertilizer.

As plants grow better, they also increasingly act as carbon sinks as they convert atmospheric carbon dioxide, with a little help from water and sunshine, into carbohydrates stored as biomass. Some of that carbon is returned to the air annually through decomposition, but other portions are are stored for longer periods in the soil, downed logs, houses, etc.  This plant-based carbon sink helps to offset the growth of global carbon dioxide emissions from human activities (primarily from the burning of fossil fuels). Together, the terrestrial carbon sink, along with the oceanic carbon sink, annually takes up more than half of the anthropogenic CO2 emissions—and remarkably, as global CO2 emissions have increased, so too has the global CO2 sink.

But now comes new evidence that plants may be helping to combat global warming through another mechanism as well, slowing the build-up of the atmospheric concentration of methane (a greenhouse gas some 25 times more effective than CO2 on a molecule-for-molecule bases at adding pressure for the world to warm).

As shown in the fugure below the jump, the growth rate of the atmospheric concentration of methane (CH4)—which is projected by the IPCC to be rising rapidly—began slowing down in the early 1990s and even topped out for a few years in the mid-2000s. Since about 2007, the atmospheric concentration of CH4 has been rising again, but only at about half that of the pre-1990 rate.

Figure 1. Atmospheric methane concentration (source: National Oceanic and Atmospheric Administration).
This behavior is not understood by climate scientists. It contravenes alarmist scenarios of runaway global warming fueled by a positive methane feedback (the scenario for which is that warming leads to thawing of the arctic permafrost, which releases methane, which leads to more warming, and so on).

A team of scientists from Lund University and Stockholm University set out to investigate recent claims that some plants release methane and are therefore a source of global methane emissions. They set up instruments to measure methane exchange on a collection of individual branches of four different tree species in a 100-year-old forest in central Sweden. A set of control experiments was also conducted in a laboratory setting. They just published their findings in the journal Geophysical Research Letters. Much to their surprise, the researchers found that the trees (both in the field and in the lab) were taking up methane rather than releasing it. They suggest that the presence of a “bacteria with the ability to consume [methane] would be a possible explanation for [the observed behavior].”

That’s not the only good news.

The researchers then executed the extremely risky (and oft ill-advised) maneuver of scaling up from a few tree branches in central Sweden to the level of the global forest canopy. Their research “indicates that the canopy might play an equally important role [in CH4 uptake] as the soil in the global context.” In other words, their results show that trees are playing a large (and hitherto unknown) role as a sink in the global methane cycle.

The culprit?  Increasing atmospheric carbon dioxide.

In the authors’ own words (with my emphasis):

Two recent studies give alternative explanations to the slow-down in the growth rate of atmospheric methane in the last decades. One of them indicates that it is due to a stabilization of fossil-fuel emissions (Aydin et al., 2011) whereas the other explains it by a decrease in microbial methane sources in the northern hemisphere (Kai et al., 2011). Our results offer a third explanation: that an increasing amount of CH4 has been taken up by vegetation during the last decades as a consequence of increased greenness (Myneni et al., 1997), NPP [net primary production] (Nemani et al., 2003) and GPP [gross primary production] (Chen et al., 2006) as observed by satellite remote sensing.

This is still highly a highly speculative result and one that will require a heck of a lot more study and independent confirmation. But it is a novel finding and goes to show that there is still a lot of interesting research ongoing in the field of climate (change), and that most definitely the science is not “settled.”


Reference:

Sundqvist, E., et al., 2012. Atmospheric methane removal by boreal plants. Geophysical Research Letters, 39, L21806, doi:10.1029/2012GL053592

Did Global Warming Reduce the Impacts of Sandy?

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

The press has been quick to jump on the idea that post-tropical cyclone Sandy (it was not a hurricane at landfall) was worsened by anthropogenic global warming and that “superstorms” are here to stay.

But I must ask the impertinent question: could anthropogenic global warming actually have lessened the impacts of Sandy?

There are basically three pro-global warming talking points involving Sandy: 1) global warming has caused sea levels to rise, thus making the storm surge larger, 2) global warming has led to higher sea surface temperatures and thus stronger hurricanes, and 3) global warming is making extratropical circulation features more conducive to intense and slower moving storm systems.

There is precious little evidence to definitively support any of these points when applied to Sandy, and, in fact, there exists a body of evidence pointing to the opposite conclusion—that anthropogenic global warming may have actually acted to mitigate the intensity of Sandy. Perhaps what lies closest to our current best understanding is that anthropogenic global warming made little contribution one way or the other.

Let’s start with sea level rise.  Water levels at New York City’s Battery Park location have been measured and recorded since 1856. The full record shows an overall (relatively steady) rise of about 0.11 inches per year, for a total rise between 1856 and now of just a bit more than 17 inches. How much of this has to do with anthropogenic global warming? Maybe a third, or about 6 inches. Of the rest, about half was caused by a subsidence of the land (geological processes related to the end of the last ice age, see Engelhart et al., 2009 for example), and the remainder to a warming up from the naturally occurring cold period which ended in the mid-19th century. So of the total 17.34 feet of water (above the station datum) recorded at The Battery tide gauge during the height of Sandy, about 0.5 feet of that could probably be linked to anthropogenic global warming.  This is not nothing, but the overwhelming majority of the damage done by the storm surge would have happened anyway. For comparison, the influence of the full moon that night was about as large as the influence of anthropogenic global warming.

As to anthropogenic global warming’s impact on the path, frequency, and intensity of hurricanes, there is a mixed bag of potential outcomes which may be detectable far in the future (towards the end of the century)  if anthropogenic greenhouse gas emissions continue to rise. The current science suggests that the frequency of hurricanes could decrease, the intensity may increase slightly, and the preferred path may be displaced out to sea (Wang et al., 2010). The net effect on the U.S. is anyone’s guess at this point (but 2 of the 3 argue for fewer hurricane impacts in the U.S.).  But what virtually everyone does agree upon is that any influence of anthropogenic global warming on hurricane characteristics is not detectable in today’s climate (see for example, Knutson et al., 2010). So that talking point is basically off the table.

Which brings us to the third global-warming-made-Sandy-worse talking point—the influence of anthropogenic global warming on the extratropical circulation characteristics.

This is where the rubber really meets the road when it comes to Sandy’s behavior.  Without the northward, and ultimately westward pull from the upper atmospheric jet stream, Sandy would have progressed harmlessly eastward, away from the Northeast coast, and out to sea.  But that is not what happened. Instead, a fairly deep trough (southward excursion) of the jet stream was coincidentally passing through the eastern U.S. just as hurricane Sandy was progressing up (but offshore) the U.S. Eastern Seaboard. This trough had the effect of attracting Sandy, and drawing it northwestward, pumping energy into it, and changing its character from a hurricane to an extra/post tropical storm system (also known as a Nor’easter in this part of the country).  In October, this type of behavior is not particularly unusual. The preferred tropical cyclone track maps provided by the National Hurricane Center (Figure 1) indicate a general tendency for tropical cyclones in October to curve back into the northeastern U.S.—just like Sandy did.

Figure 1. Prevailing tropical cyclone tracks for the month of October (source: National Hurricane Center, http://www.nhc.noaa.gov/climo/)

In fact, since the beginning of the 20th century, there have been about a dozen or so tropical cyclones that have made landfall in the U.S. north of Cape Hatteras which had a westerly component to their trajectory either immediately before or just after they came ashore. This includes historically damaging storms such as the 1903 New Jersey hurricane, the 1938 Long Island Express hurricane, and 1972’s Hurricane Agnes which is still the flood of record in many parts of the Northeast.

The last one was tropical storm Danielle, over twenty years ago. This is the longest interval in the record (since 1900) between westward-component storms north of Hatteras.  So much for the influence of global warming!

So, given this fairly typical behavior—why would anyone even consider that anthropogenic global warming played a role in Sandy?

For two reasons: 1) any bad weather these days is immediately linked to global warming by someone with an agenda, and 2) there was a paper published last spring (Francis and Vavrus, 2012) in which the authors concluded that the decline of Arctic sea ice (tied to anthropogenic global warming) was causing the Arctic to warm up faster than the lower latitudes, reducing the natural north-south temperature gradient which is where the jet stream (and extratropical storms) gain energy.  According to Francis and Vavrus, a less energetic jet stream contracts and becomes more meandering, with relatively deeper troughs and higher ridges which produce slower moving storm systems and more extreme weather.

Since Sandy was strengthened and pulled ashore by a deep trough/ridge system in the jet stream, folks are quick to assume that the Francis and Vavrus mechanism tying in anthropogenic global warming must be involved.

Not so fast!

This is like claiming to have made a new discovery that, when flipping a coin, heads are now more likely to occur than tails.  And wouldn’t you know, the next time the coin is flipped, it came up heads—to which you proclaim, “See, I told you so.” And since heads are associated with a bad outcome, the press flock to your explanation.  But what is completely overlooked, is that other researchers have examined every coin flip for the past 60 years and found that heads and tails occur with equal likelihood. So the current heads outcome is simply part of the natural 50-50 occurrence of heads or tails.

In this case, the other researchers are a pair of atmospheric scientists from Cornell University which have examined the forward speed of all nor’easters along the East Coast from 1951 through 2006 (Bernhardt and DeGaetano, 2012). And what they found, in their own words, was “There was no clear trend in [nor’easter forward] speed during the time period, although considerable season-to-season variability was present.” In other words, while there is a lot of storm-to-storm and season-to-season variability, there is no overall trend towards slower moving nor’easters (Figure 2)—so much for the Francis and Vavrus hypothesis.

Figure 2. Average speed of East Coast winter storms (nor’easters), from 1951-2006 (source: Bernhardt and DeGaetano, 2012).

And, there has been a lot of other research on changes in the patterns and characteristics of the Northern Hemisphere jet stream during the period of anthropogenic global warming which did not find that same thing that Francis and Vavrus found (we detailed many of these findings in our March 8, 2012 Current Wisdom).  At least one of those papers suggested that the methodology employed by Francis and Vavrus “can generate false, or mask actual, variability patterns including trends” (Strong and Davis, 2007). Others concluded that global warming contracted, the jet stream, flattened it over the eastern U.S., and sped it up a bit—characteristics, which, along with a decreased temperature gradient, if applied to Sandy, would have combined to produce a less intense post tropical storm system than if global warming had not been occurring.

So rather than anthropogenic global warming making Sandy worse, it could have actually lessened its intensity and impacts.

The truth is, is that it is impossible to know how, or even if, global warming played any role at all in the lifecycle of Sandy. The science is all over the map, and the signal-to-noise ratio is so low that no matter what is occurring its impact in any direction is undetectable.

But it is sexier and has much more press appeal to proclaim that the destruction wrought by “superstorm” Sandy is the product of our unrestrained fossil fuel consumption, rather than the equally plausible opposite—that anthropogenic climate changes may have combined to lessen Sandy’s intensity.


References:

Bernhardt, J.E., and A.T. DeGaetano, 2012. Meteoro­logical factors affecting the speed of movement and related impacts of extratropical cyclones along the U.S. east coast. Natural Hazards, 61, 1463-1472, doi:10.1007/s11069-011-0078-0

Engelhart, S.E., et al., 2009. Spatial variability of late Holocene and 20th century sea-level rise along the Atlantic coast of the Unites States. Geology, 37, 1115-1118.

Francis, J., and S. Vavrus, 2012. Evidence linking arctic amplification to extreme weather in mid-latitudes. Geophysical Research Letters, 39, L06801, doi:10.1029/2012GL051000.

Knutson, T. R., et al., 2010. Tropical cyclones and climate change. Nature Geoscience, 3, 157-163, doi: 10.1038/ngeo779

Strong, C., and R. Davis, 2007. Winter jet stream trends over the Northern Hemisphere. Quarterly Journal of the Royal Meteorological Society, 133, 2109-2115, doi:10.1002/qj.171

Wang, C., et al., 2011: Impact of the Atlantic warm pool on United States landfalling hur­ricanes. Geophysical Research Letters, 38, L19702, doi:10.1029/2011GL049265.

Global Science Report and The Current Wisdom Now on Cato@Liberty

Fans of Cato@Liberty may have noticed two new features from the Center for the Study of Science.  These are a weekly Global Science Report and a monthly Current Wisdom

While the Wisdom has been a monthly feature that can be found under my publications, World Science Report is new and is modelled after my original blog, Global Climate Report, which is the Web’s longest running climate change blog.  Our first release was September 11, 1995. The enormous archive at http://www.worldclimatereport.com is cross-referenced by subject and date, and can provide valuable information on virtually any climate question.  We also reserved the right to write in a humorous fashion.

As the Center adds new affiliates, you will see much more in the new World Science Report than mere climate.

Continuing to Lower the Sea Level Rise Contribution from Antarctica

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

The good news keeps coming in about sea level rise—or more precisely, Antarctica’s (minimal) contribution to it. Last time, we reviewed recent scientific findings indicating Antarctica was on the verge of gaining ice mass (and thus acting to draw down global sea level) as a slightly warmer Southern Ocean results in increasing snow accumulation which acts to offset ice loss from its peripheral (marine-terminating) glaciers.  Without a contribution from Antarctica, alarming visions of a large and rapid sea level rise this century—upwards of a meter  (and by some reckoning up to 6 meters)—are pretty much out the door.  Sans Antarctica, we are looking at a foot to foot-and-a-half of rise, give or take a few inches. Such an amount will undoubtedly require some adjustment and adaptation, but will not involve a wrenching transformation of society. Most of us probably wouldn’t even notice. Consider that, due to a combination of geology and oceanic warming, this same amount (or more) has been experienced in many East Coast locations in the last 100 years.

The good science news may be one reason why global warming has been so absent in the election debates. In response,  last week, the Union of Concerned Scientists helped a collection of local government officials and scientists from Florida pen an open letter to the candidates imploring them to address the issue of sea level rise during their third and final debate (held in Boca Raton).  They didn’t.

It is a good thing that they left the issue alone, for in this week’s Nature magazine comes more evidence that Antarctica is perhaps not going to be the great sea level rise contributor that other research as made it out to be (e.g. Velicogna et al., 2009; Rignot et al., 2011).

Matt King, from Newcastle University, and colleagues set out to refine the Antarctic ice mass change calculations that have been performed using data collected by the Gravity Recovery and Climate Experiment (GRACE) satellite. GRACE determines how the mass is changing underneath the satellite by measuring temporal variations in the pull of gravity.  If the strength of the local gravitational attraction increases over time, then it is inferred that the local mass must be increasing (and vice versa).  This is a handy tool for assessing trends in dynamic ice/snow mass in places like Greenland and Antarctica.

But, variations in the ice/snow burden are not the only thing that can change the gravitational pull observed by the GRACE satellite. The ground underlying the ice and snow may be changing as well. And, in fact, it is. The ground in many places around the world is still adjusting to the burdening and subsequent unburdening from the coming and going of the massive amount of snow and ice from the last ice age (and its termination). This process is known as glacial isostatic adjustment (GIA).

The problem is that while we understand that GIA is taking place, we really don’t precisely know the details, like where, when, and how fast—especially over sparsely monitored and studied places like Antarctica.

Two  years ago, a study was published that showed that the GIA model used in most GRACE-based studies was in error, and that when it was corrected, the rate of calculated ice mass loss from across Antarctica declined by some 40 percent  (from ~150 gigatons/yr to ~87 Gt/yr). Since it takes about 374 Gt of melted ice to produce 1 millimeter of global sea level rise, these findings indicated that Antarctica was contributing to sea level rise at a rate of about one-quarter of a millimeter per year (or about 1 hundredth of an inch per year). We detailed that finding, by Xiaoping Wu and colleagues, in a Cato Current Wisdom article in October of 2010.

Now along comes the new study Matt King et al. (2012) that further refines the local GIA over Antarctica. Here is how they did it:

Here we applied a new GIA model (W12a) to GRACE data to estimate the ice-mass balance for 26 independent Antarctic drainage basins from August 2002 to December 2010. The W12a model comprises a glaciologically self-consistent ice history constrained to fit data that delimit past ice extent and elevation, and an Earth viscosity model chosen such that GIA predictions from W12a best fit a suite of relative sea-level records around Antarctica. The advance of W12a on previous models applied to GRACE data is illustrated by the misfit to GPS uplift rates being halved. Our use of W12a addresses the dominant GRACE-related error in previous Antarctic analyses.

With this new model in hand, they were able to produce a new estimate of the rate of ice mass change over Antarctica from 2002 through 2010. That estimate is a loss of only 69 Gt/yr (+/- 18Gt/yr). And further, they found no statistically significant change in this rate when averaged over the whole continent—in contrast to other prominent studies (e.g. Rignot et al., 2011) which claimed a significant acceleration was taking place.

So King and colleagues’ latest refinement puts the Antarctic contribution to global sea level rise at a rate of about one-fifth of a millimeter per year (or in English units, 0.71 inches per century).

Without a significantly large acceleration—and recall the King et al. found none—this is something that we can all live with for along time to come.


References:

King, M., et al., 2012. Lower satellite-gravimetry estimates of Antarctic sea-level contribution. Nature, doi:10.1038/nature, http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11621.html

Rignot, E., et al., 2011. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters, 38, L05503, http://www.agu.org/pubs/crossref/2011/2011GL046583.shtml

Velicogna, I., 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, 36, L19503, http://www.agu.org/pubs/crossref/2009/2009GL040222.shtml

Is the Long-Awaited Snowfall Increase in Antarctica Now Underway?

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

Whenever the topic of rising seas comes up, we point out that Antarctica is expected to gain mass through enhanced snowfall in a warmer climate, and therefore its contribution to global sea level rise should be negative—that is, the water locked up in the added snowfall there will act to reduce the level of the globe’s seas. The models used by the Intergovernmental  Panel on Climate Change (IPCC) in their 2007 Fourth Assessment Report project the sea level reduction from this mechanism by the end of the 21st century to amount to somewhere between 2 cm and 14 cm (roughly 1 to 6 inches). While this is not a lot, the main point is that Antarctica is not expected to be a contributor to rising seas as the climate warms. Without a large contribution from Antarctica, we will not approach alarmist projections of a meter-plus of sea level rise by century’s end.

Up to now, though, Antarctica has not exactly been with the program.

Instead of gaining mass through increased snowfall, there have been indications that Antarctica is losing ice (contributing to sea level rise) as ice discharge from its coastal glaciers exceeds gains from snow increases (which have been hard to find).  One has to wonder whether Antarctica, contrary to expectations, will continue to lose mass and become an important contributor sea level rise, or whether the projected increases in snowfall have just not yet reached a magnitude sufficient to offset the loss from glacial discharge.

Things are starting to change down there.

The research that has gotten the most attention on the subject of Antarctic mass balance has been based on observations made by the Gravity Recovery And Climate Experiment (GRACE) satellite.  This orbiter senses changes in gravity (i.e., mass) which can be caused by increasing snow and ice loads over the continent.  One key piece of information which must be factored into the calculations of ice mass change is the change in the underlying geologic formations, which are still rebounding from enormous amounts of ice lost after the end of the last ice age.  This geologic motion, known as the glacial isostatic adjustment (GIA), is largely modeled rather than directly observed. Our level of knowledge (or lack thereof) of the true GIA adds a sizable amount of uncertainty to GRACE-based estimates of the ice mass changes over time in Antarctica (and Greenland, the northern hemisphere’s cheap imitation of Antarctica).

In a widely cited finding, Velicogna (2009) reported that Antarctica was losing ice at a rate of about 104 gigatons per year (Gt/yr) during the period 2002–2006, increasing to a loss rate of 246 Gt/yr during 2006–2009 (about 374 Gt of ice are equivalent to 1 mm of sea level).  Rignot et al. (2011) also found an acceleration of ice loss there, increasing from a loss of about 209 Gt/yr (in 2003-2007) to about 265 Gt/yr from 2007 to 2010.  However, Wu et al. (2010) argued that the GIA model used in these previous studies is incorrect, and that when a more accurate GIA model is incorporated in the GRACE-based ice mass change calculations, Antarctica was only losing about 87 Gt/yr during the period 2002–2008.

Support for the GRACE-based calculations comes from the general agreement between the GRACE numbers and those calculated from studies of changes in the grounding lines of coastal glaciers and the ice flow across those grounding lines in association with the other aspects of the mass balance.  This method is known as the Input-minus-Output Method (IOM).  The IOM estimates of the average ice loss from Antarctica over the past several decades (1992–2007) lie somewhere around 136 Gt/yr, in rough agreement with the GRACE-based estimates.  However, the IOM is also subject to a lot of uncertainty. An attempt by Zwally and Giovinetto (2011) to reduce the uncertainty and increase the accuracy resulted in an IOM-based estimate of a loss of only 13 Gt/yr over the same 18-yr period and led the researchers to conclude that:

Although recent reports of large and increasing rates of mass loss with time from GRACE-based studies cite agreement with IOM results, our evaluation does not support that conclusion.

It seems that as the calculations and derivations are improved, the amount of ice mass that Antarctica is supposedly losing gets less and less.

Or perhaps it isn’t losing any mass.

Using a set of observations from a series of satellites that have been in orbit since 1992 and that measure changes in the height of the surface of the ice (ICESat), NASA’s Jay Zwally and colleagues (2012) report that Antarctica is gaining mass. Zwally recently presented his findings to a workshop of the Ice-Sheet Mass Balance and Sea Level expert group of the Scientific Committee on Antarctic Research and the International Arctic Science Committee. According to his abstract, Zwally reported that “During 2003 to 2008, the mass gain of the Antarctic ice sheet from snow accumulation exceeded the mass loss from ice discharge by 49 Gt/yr (2.5% of input), as derived from ICESat laser measurements of elevation change.”

Zwally further added, “A slow increase in snowfall with climate warming, consistent with model predictions, may be offsetting increased dynamic losses.”

So the “global warming, leading to increased snowfall, leading to a drawdown of global sea level” mechanism may be operating after all.

A paper to soon appear in Geophysical Research Letters give us another enticing look at recent snowfall changes in Antarctica.  In “Snowfall driven mass change on the East Antarctic ice sheet,” Carmen Boening and colleagues from NASA’s Jet Propulsion Laboratory report that extreme precipitation (snowfall) events in recent years (beginning in 2009) have led to a dramatic gain in the ice mass in the coastal portions of East Antarctica amounting to about 350 Gt in total (Figure 1).

Figure 1. Timeseries of snow accumulation in coastal East Antarctica (shaded region in inset).
(Source: Boening et al., 2012)
Boening et al. reported that the increase in ice mass in East Antarctica has not completely offset the loss of ice mass during the same time in West Antarctica, but as this comparison is made using GRACE data, it is hard to know just how accurate it is.

Also note that a few years with a lot of snowfall does not mean that a change in the long-term snowfall rate has occurred.  Nevertheless, the situation bears careful watching.

Putting everything together, we conclude that many of the claims that Antarctica is rapidly losing ice and increasingly contributing to a rise in global sea levels must now be, at the very least, tempered, if not overturned entirely. Time will certainly tell. And time will also tell just how much we need to worry about future sea level rise. Currently, the answer seems to be “not overly much.”


References:

Boening, C. et al., 2012. Snowfall-drive mass change on the East Antarctic ice sheet. Geophysical Research Letters, in press, DOI:10.1029/2012GL053316.

Rignot, E., et al., 2011. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters, L05503, DOI:10.1029/2011GL046583

Velicogna, I., 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, 36, L19503, DOI: 10.1029/2009GL040222.

Wu, X., et al., 2010. Simultaneous estimation of global present-day water transport and glacial isostatic adjustment. Nature Geoscience, 3, DOI: 10.1038/NGEO938.

Zwally, H.J., and M.B. Giovinetto, 2011. Overview and assessment of Antarctic ice-sheet mass balance estimates: 1992-2009. Surveys in Geophysics, 32, 351-376, DOI: 10.1007/s10712-011-9123-5.

Zwally, H.J., et al., 2012. Mass gains of the Antarctic ice sheet exceed losses. Presentation to the SCAR ISMAA Workshop, July 14, 2012, Portland Oregon.

Pages