JPL bombshell: Polar ice sheet mass loss is speeding up, on pace for 1 foot sea level rise by 2050

The Greenland and Antarctic ice sheets are losing mass at an accelerating pace, according to a new NASA-funded satellite study. The findings of the study — the longest to date of changes in polar ice sheet mass — suggest these ice sheets are overtaking ice loss from Earth’s mountain glaciers and ice caps to become the dominant contributor to global sea level rise, much sooner than model forecasts have predicted.

The study, led by the U.S. Jet Propulsion Laboratory, was just published in Geophysical Research Letters here (subs. req’d).

It’s been clear for a while that the polar ice sheet mass loss is accelerating (see Large Antarctic glacier thinning 4 times faster than it was 10 years ago: “Nothing in the natural world is lost at an accelerating exponential rate like this glacier”).

But the new study is a bombshell because of its credibility and thoroughness — and because it provides perhaps the most credible estimate to date of the sea level rise we face in 2050 on our current emissions path, 1 foot.

The JPL news release runs through the calculation that leads to the 1-foot estimate:

The authors conclude that, if current ice sheet melting rates continue for the next four decades, their cumulative loss could raise sea level by 15 centimeters (5.9 inches) by 2050. When this is added to the predicted sea level contribution of 8 centimeters (3.1 inches) from glacial ice caps and 9 centimeters (3.5 inches) from ocean thermal expansion, total sea level rise could reach 32 centimeters (12.6 inches). While this provides one indication of the potential contribution ice sheets could make to sea level in the coming century, the authors caution that considerable uncertainties remain in estimating future ice loss acceleration.

It is always worthwhile to make clear that the projections are uncertain. On the other hand, one would have to say that the uncertainty is greater on the high side — since the rate of human-caused warming is itself projected to accelerate, and the poles are the place where the planet is heating up the most, much faster than expected (see “Deep ocean heat is rapidly melting Antarctic ice: Oceanographer at AGU: Western Antarctic Peninsula is seeing “the highest increase in temperatures of anywhere on Earth”).

“That ice sheets will dominate future sea level rise is not surprising — they hold a lot more ice mass than mountain glaciers,” said lead author Eric Rignot, jointly of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and the University of California, Irvine. “What is surprising is this increased contribution by the ice sheets is already happening. If present trends continue, sea level is likely to be significantly higher than levels projected by the United Nations Intergovernmental Panel on Climate Change in 2007. Our study helps reduce uncertainties in near-term projections of sea level rise.”

So if you are asked how much sea levels are likely to rise by midcentury on our current emissions path, I think a reasonable reply now is “about 1 foot — assuming the current rate of ice mass loss doesn’t accelerate further.”

UPDATE: This work is consistent with other recent sea level rise projections — Sea levels may rise 3 times faster than IPCC estimated, could hit 6 feet by 2100 [see figure]:

What makes this new study so credible is not just the groups involved — JPL, the Institute for Marine and Atmospheric Research in The Netherlands, and NCAR — and not just the 18-year dataset examined. Impressively, the authors were able to compare and reconcile two completely different approaches, the mass budget method (MBM) and the gravity method:

The study compared two independent measurement techniques. The first characterized the difference between two sets of data: interferometric synthetic aperture radar data from European, Canadian and Japanese satellites and radio echo soundings, which were used to measure ice exiting the ice sheets; and regional atmospheric climate model data from Utrecht University, The Netherlands, used to quantify ice being added to the ice sheets. The other technique used eight years of data from the NASA/German Aerospace Center’s Gravity Recovery and Climate Experiment (Grace) satellites, which track minute changes in Earth’s gravity field due to changes in Earth’s mass distribution, including ice movement.

The team reconciled the differences between techniques and found them to be in agreement, both for total amount and rate of mass loss, over their data sets’ eight-year overlapping period. This validated the data sets, establishing a consistent record of ice mass changes since 1992.

The team found that for each year over the 18-year study, the Greenland ice sheet lost mass faster than it did the year before, by an average of 21.9 gigatonnes a year. In Antarctica, the year-over-year speedup in ice mass lost averaged 14.5 gigatonnes.

“These are two totally independent techniques, so it is a major achievement that the results agree so well,” said co-author Isabella Velicogna, also jointly with JPL and UC Irvine. “It demonstrates the tremendous progress that’s being made in estimating how much ice the ice sheets are gaining and losing, and in analyzing Grace’s time-variable gravity data.”

You may have heard about that 2010 study (Wu et al.) suggesting that earlier estimates of ice mass loss using the GRACE data were too high. That study turns out to have had a number of issues, not the least of which is the short timeframe it examined: “mass losses between 2002 and 2008 in Greenland, Alaska/Yukon and West Antarctica.”

As the JPL-led study notes:

The excellent agreement of the GRACE and MBM records over the last 8 years validates the 18″year MBM record. The results also indicate that an observation period of 8 years is probably not sufficient for these methods to separate the long”term trend in ice sheet acceleration from temporal variations in SMB [surface mass balance], especially in Antarctica. When we use the extended time period 1992–2009, the significance of the trend improves considerably. The MBM record indicates an acceleration in mass loss of 21.9 ± 1 Gt/yr2 for Greenland and 14.5 ± 2 Gt/yr2 for Antarctica. The lower uncertainty reflects the reduced influence of temporal variations in SMB for the longer record. The uncertainty in acceleration is thus reduced to 5% for Greenland and 10% for Antarctica. When the mass changes from both ice sheets are combined together (Figure 2c), the data reveal an increase in ice sheet mass loss of 36.3 ± 2 Gt/yr2.

I emailed Rignot to ask him about Wu et al., since I heard he had some issues with it. He wrote me back:

Wu et al. employed a rather complex technique to infer mass losses all over the world combining GRACE and GPS. While the approach is promising, there is not enough GPS data near the ice sheets today to constrain their solution well. Their study was somewhat premature in terms of concluding about ice sheet mass balance. Their reconstruction of postglacial rebound in Greenland is at odds with what we know about the local glaciology or with prior reconstructions of the GIA [Glacial Isostatic Adjustment].

So Wu et al. used too short of a time frame to get a good picture of what’s happening in Antarctica and appears to have a flawed reconstruction of what’s happening in Greenland.

Finally, it bears remembering that even this study’s projections for 2050 and beyond assume a basically continuous process, whereas the West Antarctic ice sheet is fundamentally unstable because most of it is grounded far below sea level. The warmer it gets, the more unstable WAIS outlet glaciers will become. Since so much of the ice sheet is grounded underwater, rising sea levels may have the effect of lifting the sheets, allowing more-and increasingly warmer-water underneath it, leading to further bottom melting, more ice shelf disintegration, accelerated glacial flow, and further sea level rise, and so on and on, another vicious cycle. The combination of global warming and accelerating sea level rise from Greenland could be the trigger for catastrophic collapse in the WAIS (see, for instance, here).

The time to act is most definitely now, before we all but guarantee unimaginable levels of polar warming post-2050: