March 23, 2010 – Original Source: Climate Progress
A new study has lowered the carbon pollution threshold or “tipping point” for collapse of the Greenland ice sheet to 400 to 560 ppm. We’re currently at about 390 parts per million atmospheric concentrations of CO2, rising about 2 ppm a year (and yes, total collapse would take a while).
Another new study documents the unexpectedly fast spread of ice loss into northwest Greenland (animation below). And the Director of the International Polar Year [IPY] Program Office, Dr. David Carlson, told the Senate last year:
“A clear consensus has emerged during IPY that the Greenland Ice sheet will disappear as a consequence of this current global warming.” Carlson added that a “very plausible outcome” was “a meter or more of sea level rise in this century from Greenland alone.”
A 2009 study by Velicogna of JPL using the GRACE (Gravity Recovery and Climate Experiment) satellite to “determine the ice mass-loss for the Greenland and Antarctic Ice Sheets during the period between April 2002 and February 2009″ concluded:
We find that during this time period the mass loss of the ice sheets is not a constant, but accelerating with time, i.e., that the GRACE observations are better represented by a quadratic trend than by a linear one, implying that the ice sheets contribution to sea level becomes larger with time. In Greenland, the mass loss increased from 137 Gt/yr in 2002–2003 to 286 Gt/yr in 2007–2009, i.e., an acceleration of −30 ± 11 Gt/yr2 in 2002–2009.
The figure above is from the study via Skeptical Science with the caption, “Ice mass changes for the Greenland ice sheet estimated from GRACE satellite measurements. Unfiltered data are blue crosses. Data filtered for the seasonal dependence are shown as red crosses. The best-fitting quadratic trend is shown as a green line.”
Also in 2009, the University of Alaska reported on a study in the journal Hydrological Processes (subs. req’d) that found “The Greenland ice sheet is melting faster” — and contributing to sea level rise more — than expected.
This research is consistent with data presented at the annual meeting of the American Geophysical Union in December 2008 (see “Two trillion tons of land ice lost since 2003, rate of Greenland summer ice loss triples 2007 record“). This staggering ice loss is all the more worrisome because it was not predicted by the IPCC’s climate models. As Penn State climatologist Richard Alley said in March 2006, the ice sheets appear to be shrinking “100 years ahead of schedule.” In 2001, the IPCC thought that neither Greenland nor Antarctica would lose significant mass by 2100. They both already are.
And Nick Sundt just blogged on yet another new GRL study, “Spread of ice mass loss into northwest Greenland observed by GRACE and GPS” (subs. req’d). The AGU press release reports :
“When we look at the monthly values from GRACE, the ice mass loss has been very dramatic along the northwest coast of Greenland,” says CU-Boulder physics professor and study co-author John Wahr….
“This is a phenomenon that was undocumented before this study,” Wahr says. “Our speculation is that some of the big glaciers in this region are sliding downhill faster and dumping more ice in the ocean.”
Especially worrisome for North America is that a 2009 study in Geophysical Research Letters (subs. req’d, UCAR summary here) finds that sustained high rates of Greenland ice loss could lead to staggering increases in coastal sea level rise. As reported:
If Greenland’s ice melts at moderate to high rates, ocean circulation by 2100 may shift and cause sea levels off the northeast coast of North America to rise by about 12 to 20 inches (about 30 to 50 centimeters) more than in other coastal areas. The research builds on recent reports that have found that sea level rise associated with global warming could adversely affect North America, and its findings suggest that the situation is more threatening than previously believed.
“If the Greenland melt continues to accelerate, we could see significant impacts this century on the northeast U.S. coast from the resulting sea level rise,” says NCAR scientist Aixue Hu, the lead author. “Major northeastern cities are directly in the path of the greatest rise.”
All that is needed for the 20 inches of extra sea level rise is if Greenland’s melt rate continues at its current rate through 2050. And that is on top of new, much high projections for overall SLR (see “Sea levels may rise 3 times faster than IPCC estimated, could hit 6 feet by 2100“)
Another 2009 study in Nature using “high-resolution ICESat (Ice, Cloud and land Elevation Satellite) laser altimetry” found “Dynamic thinning of Greenland and Antarctic ice-sheet ocean margins is more sensitive, pervasive, enduring and important than previously realized.” The study noted that dynamic thinning is “ice loss as a result of accelerated flow,” which is something the IPCC basically ignored in its 2007 sea level rise projections (see here).
Back in 2004, the Arctic Climate Impact Assessment, warned “Models indicate that warming over Greenland is likely to be of a magnitude that would eventually lead to a virtually complete melting of the Greenland ice sheet, with a resulting sea-level rise of about seven meters (23 feet).” And that was before we had better modeling of the high emissions path we are now on coupled with carbon-cycle feedbacks.
It is implausible that Greenland could survive sustained exposure to the warming we face if we stay anywhere near our current emissions path — see “M.I.T. doubles its 2095 warming projection to 10°F — with 866 ppm and Arctic warming of 20°F.”
Indeed, as John Cook of Skeptical Science notes, the last time global temperatures were just 1 to 2°C higher than today (with polar temps ~3–5 °C warmer) was 125,000 years ago. A December 2009 Nature study of that time, “Probabilistic assessment of sea level during the last interglacial stage” (subs. req’d), concluded:
We find a 95% probability that global sea level peaked at least 6.6 m higher than today during the last interglacial; it is likely (67% probability) to have exceeded 8.0 m but is unlikely (33% probability) to have exceeded 9.4 m…. The results highlight the long-term vulnerability of ice sheets to even relatively low levels of sustained global warming.
That is, sea levels were probably more than 26 feet (!) higher when it was as warm as most models suggest it will be by mid-century if we stay near our current emissions path. Greenland and Antarctic ice sheets thus appear to be very sensitive to sustained warming.
Moreover, it seems likely that there is a “point of no return” or threshold beyond which collapse cannot be stopped because of dynamic feedbacks. As a Geophysical Research Letters paper led by Kaser (subs. req’d) noted, the warming-driven melting is
reinforced by feedbacks among which the most important are probably the balance-altitude feedback (net melting lowers the glacier surface to warmer altitudes, increasing net loss) and the albedo feedback (more darker ice exposed at the surface promotes further melting).
So it is no surprise scientists are trying to figure out what that threshold might be. The new study that attempts to do so is “The effect of more realistic forcings and boundary conditions on the modelled geometry and sensitivity of the Greenland ice-sheet,” in The Cryosphere, “An Interactive Open Access Journal of the European Geosciences Union” Cook has a nice summary:
This paper uses updated data on bedrock topography and ice thickness to produce more accurate modelling results of Greenland ice sheet behaviour. They model how the Greenland ice sheet will respond to three different scenarios with atmospheric CO2 held at 400 ppm, 560 ppm and 1120 ppm. The simulations are run over a 400 year period.Although not completely collapsed, the 400 ppm ice-sheet loses ice mass in the north of the island, with a total reduction in ice volume ranging between 20 to 41%. Note – due to the large inertia of the Greenland ice sheet, this mass loss doesn’t happen at the moment CO2 levels reach 400 ppm but over a period of centuries. Under a 560 ppm climate, the Greenland ice sheet loses between 52 to 87% of its ice volume. If CO2 reaches 1120 ppm, there is almost complete elimination of the Greenland ice sheet with loss between 85 to 92%. The important result from this paper is that there is a critical threshold where the Greenland ice sheet becomes unstable somewhere between 400 and 560 ppm.
This is a large uncertainty range and one imagines there will be much research in the next few years to reduce the uncertainty. However, the 400 to 560 ppm range is put into perspective when you look at the projected CO2 levels for the various IPCC scenarios. The business as usual scenario has CO2 levels reaching 1000 ppm by 2100. Even the most optimistic scenario tops 500 ppm by 2100.
Figure 3: Atmospheric CO2 concentrations as observed at Mauna Loa from 1958 to 2008 (black dashed line) and projected under the 6 IPCC emission scenarios (solid coloured lines). (IPCC Data Distribution Centre)
Of course, Figure 3 displays projected scenarios. What has been happening in the real world? Observed CO2 emissions in recent years have actually been tracking close to or above the worst case scenario.
Figure 4: Observed global CO2 emissions from fossil fuel burning and cement production compared with IPCC emissions scenarios. The coloured area covers all scenarios used to project climate change by the IPCC (Copenhagen Diagnosis).
Satellite measurements, paleoclimate data and ice sheet modelling all paint a consistent picture. Global warming is destabilising the Greenland ice sheet which is highly sensitive to sustained warmer temperatures. Our current trajectory with CO2 emissions will likely cause at least several metres sea level rise from the Greenland ice sheet over the next few centuries. Of course, we shouldn’t forget that this estimate doesn’t include Antarctica – the Antarctic ice sheet is also losing ice at an accelerating rate.
The time for action was quite some time ago, but now is still better than later!