Melting Glaciers, Ice Caps and Sea-level Rise
Fluctuations in surface melting are known to affect the speed of glaciers and ice sheets, while the contribution of glaciers and ice caps to global sea-level rise is uncertain at best. Much has been made of the “accelerating” loss of ice from the Greenland glaciers. Over the past decade, Arctic sea ice retreated substantially during the summer months, and some predicted that the ice loss could be irreversible, a tipping point that would boost global warming. A number of new papers in Nature, Geophysical Research Letters and Nature Geoscience, shed new light on these subjects, and the answers are not the ones usually heard in the media.
Surface melting is known to affect the speed of glacial ice as it flows downhill but its impact on the Greenland ice sheet in a warming climate remains contravention. Although some studies suggest that greater melting produces accelerated ice flow, others have identified a long-term decrease in Greenland’s flow despite increased melting. Appearing in the January 27, 2011, issue of the journal Nature, “Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage” takes a fresh look at glacial ice flow and what regulates it. Using satellite observations of ice velocity in Greenland, Aud Venke Sundal et al. provide empirical support for a recent theoretical prediction that subglacial drainage associated with high meltwater input can, in fact, reduce ice velocity. From the papers abstract:
Here we use satellite observations of ice motion recorded in a land-terminating sector of southwest Greenland to investigate the manner in which ice flow develops during years of markedly different melting. Although peak rates of ice speed-up are positively correlated with the degree of melting, mean summer flow rates are not, because glacier slowdown occurs, on average, when a critical run-off threshold of about 1.4 centimetres a day is exceeded. In contrast to the first half of summer, when flow is similar in all years, speed-up during the latter half is 62 ± 16 per cent less in warmer years. Consequently, in warmer years, the period of fast ice flow is three times shorter and, overall, summer ice flow is slower. This behaviour is at odds with that expected from basal lubrication alone. Instead, it mirrors that of mountain glaciers, where melt-induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient.
In other words, the rate at which glacial ice flows depends on more complicated mechanisms than previous, simplistic analyses have considered. Data from six major glaciers on the Greenland Ice Sheet show that, although the initial ice speedup was similar in all six years of the study, each glacier experienced a dramatic late summer slow-down during the warm years when more melt water was produced.
Local glaciers and ice caps in Greenland.
As is so often the case, the authors' conclusions point to deficiencies in computer models now in use: “Simulations of the Greenland ice-sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage; a simple model of basal lubrication alone misses key aspects of the ice sheet’s response to climate warming.” Once again, confusing model results with reality has led science astray—looks like the glaciers of Greenland will not be sliding ever faster into the sea.
Glaciers have gotten a reprieve, but what about the pack ice? Many would be climate change Cassandras have warned of an ice-free Arctic that will never recover—one of those pesky tipping points we hear so much about. It seems that there is good news regarding Arctic sea ice as well.
In “Recovery mechanisms of Arctic summer sea ice,” S. Tietsche and colleagues from the Max Planck Institute for Meteorology throw some cold water on the idea that the loss of Arctic ice could result in a calamitous climatic tipping point. Appearing in the January 26, 2011, issue of Geophysical Research Letters, here is how Tietsche et al. summarized their work and findings:
We examine the recovery of Arctic sea ice from prescribed ice-free summer conditions in simulations of 21st century climate in an atmosphere–ocean general circulation model. We find that ice extent recovers typically within two years. The excess oceanic heat that had built up during the ice-free summer is rapidly returned to the atmosphere during the following autumn and winter, and then leaves the Arctic partly through increased longwave emission at the top of the atmosphere and partly through reduced atmospheric heat advection from lower latitudes. Oceanic heat transport does not contribute significantly to the loss of the excess heat. Our results suggest that anomalous loss of Arctic sea ice during a single summer is reversible, as the ice–albedo feedback is alleviated by large-scale recovery mechanisms. Hence, hysteretic threshold behavior (or a “tipping point”) is unlikely to occur during the decline of Arctic summer sea-ice cover in the 21st century.
Though the loss of sea ice allows heat to build up in ocean surface waters during the summer, this analysis suggests that the heat is rapidly returned to the atmosphere the following autumn because of the lack of an insulating ice cover. As science has found in many previous cases, Earth's climate system is more stable and self correcting than the alarmists would have us believe. It seems that “hysteretic threshold behavior” is limited to climate change alarmists and hysterical news media reporters.
Pack-ice waxes and wains, but always returns..
This brings us to the subject of sea-level rise (SLR) and the impact that melting glaciers might have on it. While pack ice does not affect sea-level, since it floats upon ocean water, glaciers and the Antarctic ice cap are supported by solid ground. When glaciers melt they add to the total amount of water in Earth's oceans. But Greenland and Antarctica are not the only places on Earth that have glaciers—around the world there are numerous mountain glaciers and many of them are melting.
In “Regionally differentiated contribution of mountain glaciers and ice caps to future sea-level rise,” Valentina Radić and Regine Hock present an analysis of the possible contribution to sea-level rise from mountain glaciers. Published online on January 9, 2011, by Nature Geoscience, the authors claim that though mountain glaciers and ice caps include only a minor fraction of all ice bound water on Earth—1% compared with the Antarctic and Greenland ice sheets >99%—their retreat has dominated the eustatic sea-level contribution in the past century.
Local mean sea level (LMSL) is defined as the height of the sea with respect to a specific landmark, averaged over a period of time to compensate for the effects of tides, storm surges, etc. Eustatic sea-level, on the other hand, varies in response to changes in the volume of ocean water and the size of ocean basins. This is global change as opposed to uplift and subsidence caused by local geologic change. Sweden and the west coast of the US are rising while Thailand and the Netherlands are sinking independent of any change in eustatic sea-level.
The contribution of glaciers and ice caps to global sea-level rise is uncertain: they are incompletely counted and the calculation is challenging. Radić and Hock's new estimate suggests a contribution of about 12 cm by 2100. To arrive at this new estimate they calculated mass balance explicitly for each of the roughly 122,000 glaciers and ice caps that are listed in the extended format of the World Glacier Inventory (WGI-XF). They then applied the projections from ten different global climate models to a macroscopic glacier-response model that explicitly considers the shrinking area of glaciers and ice caps in the future. These data are captured in the graphs shown below.
Contribution of glacier melting to sea-level rise.
The authors summarize the study's results:
According to our multi-model mean, sea-level rise from glacier wastage by 2100 will amount to 0.124±0.037 m, with the largest contribution from glaciers in Arctic Canada, Alaska and Antarctica. Total glacier volume will be reduced by 21±6%, but some regions are projected to lose up to 75% of their present ice volume. Ice losses on such a scale may have substantial impacts on regional hydrology and water availability.
According to Radić and Hock, the largest contributions to sea-level rise originate in Arctic Canada, Alaska and the Antarctic Peninsula—where the gaps in glacier data are most severe and the differences between the various climate models are largest. Uncertainties in the extrapolated glacier response for these regions are therefore high. Indeed, the ongoing flap over whether Himalayan glaciers are melting shows how uncertain the data are.
The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4) projects twenty-first-century global sea-level rise due to wastage of mountain glaciers and ice caps to range between 0.07 and 0.17 m, depending on different initial ice volume estimates and emission scenarios. This places the new estimate almost exactly in the middle of that range. Wild estimates of 10, even 20 foot increases in ocean levels by 2100 are not supported by science, even the flawed science of IPCC supporters.
In summary, Radić and Hock predict that sea-level rise due to melting mountain glaciers will amount to ~12.5 cm (~5 inches) by 2100. If the IPCC is correct and this corresponds roughly to one-third of total predicted sea-level rise, total increase will be around 37.5 cm (~15 inches). Compare that to last century's global mean sea level (GMSL) rise of 10 to 20 cm (4 to 8 inches) and things no longer seem so abnormal or unprecedented. And of course, the predicted rise depends on significant melting of both the Greenland and Antarctic ice-sheets, something that is not at all assured.
This is not going to happen by the end of the century.
As discovered in the first paper of this review, glaciers speed up and slow down naturally. Under some global warming scenarios, a warming climate leads to greater snowfall in the Arctic, Antarctic and high mountains, which could actually increase glacial ice mass. Certainly, the models used by Radić and Hock would not have reflected the findings by Sundal et al. Regardless, this amount of SLR is significantly less than the 88 cm used by the European impact study I reported on recently. That study found the impact of global warming, including sea-level rise, to be minor.
The evidence continues to mount. Even if global warming progresses as the IPCC predicts—something that I have grave doubts about—the impact on the planet will not be catastrophic. Glaciers are not racing to the sea, Arctic pack-ice waxes and wains but will not permanently disappear, and sea-level rise will be moderate at worst. The stories of an impending climate change apocalypse can be seen for what they are: attempts to garner research funding, attract TV viewers or promote hidden political agendas. Anthropogenic global warming, if it exists, will have even less impact than predicted by the climate change alarmists—and that is not much at all.
Be safe, enjoy the interglacial and stay skeptical.