Overview - Polar Ice-Caps and Sea-Level Rise

Roughly 3 percent of the water on planet earth exists as ice. Of this, 90 percent is in the ice sheets of Antarctica and Greenland, the remainder being tied up in glaciers and floating sea ice (which does not contribute to changes in sea-level). If the grounded portion of this ice (that is, the portion that is directly supported by underlying land mass or in ice shelves that are attached to one) were discharged into the world’s oceans sea-level would rise by over 80 meters (260 feet). This would wreak havoc on coastal populations and ecosystems worldwide (floating sea-ice already contributes its mass displacement to sea-level so its melting would not alter it). This raises important questions about the extent to which human-caused global warming could be contributing to ice-cap melting.

Contrary to popular belief ice-cap melting is not actually the issue, discharge is. When ice sheets break up large portions of them slide into (are discharged into) the surrounding oceans as ice icebergs. This water content drives sea-level change regardless of whether it is frozen or not (Archimedes’ principle—which is why existing sea-ice is not a factor in sea-level rise). As the earth warms, ice-caps and ice shelves may become unstable and break up, discharging their ice into the oceans long before they actually melt. Many factors contribute to this process including ones that are not easily observed. A warming climate leads to both air and ocean temperature changes that may drive, or inhibit ice-cap discharge. Below freezing increasing air temperatures lead to increases in snowfall that build ice sheets. As this water content ultimately comes from the ocean, this results in a net decrease in sea level.

Increasing ocean temperatures contribute to ice discharge in at least two ways. There is a direct contribution to melting and breakup of the ice shelves surrounding the ice caps. This accelerates the flow of the glaciers that drain the world’s ice sheets. Second, as ocean currents warm they feed warmer groundwater to the land masses underlying these glaciers. This increases melting underneath them making the bedrock they rest on more slippery—a process known as basal lubrication that speeds up glacier-borne ice discharge even further.

If warming induced increases in ice-cap discharge rates exceed snowfall driven decreases there will be a net loss of the world’s ice reserve, and a corresponding rise in global sea-level. Global warming skeptics regularly cite recent increases in Antarctic and Greenland snowfall as proof that sea-level rise should not concern us. The most illiterate of these go so far as to claim that increasing snowfall in these regions disproves global warming—a fact that is easily refuted by reading any high school physics or chemistry textbook. But these criticisms are irrelevant unless they are accompanied by comparisons with the coastal ice discharge rates around both continents, which are almost never provided in skeptic forums. On a few occasions skeptic commentators have even gone so far as to deliberately obscure such comparisons with partial datasets and even bad mathematics to give the appearance of net sea-level decreases (see for instance “Scare of the Century” by Jason Lee Steorts, National Review, June 2006).

Current observations suggest that neither the Antarctic, nor the Greenland ice sheets will break up before the end of the 21st century. But these are based on observed discharge rates under current snowfall, temperature, and basal lubrication conditions. There is no guarantee that any of these factors will change gradually under the most likely climate change scenarios for the coming century. A non-linear change in any of them could result in a sudden ice sheet breakup that may appear with little or no warning (the world was taken by surprise in 2002 when the Larsen B ice shelf on the Antarctic Peninsula broke up in less than 35 days. This in turn has significantly sped up the flow of glaciers in that region). Indeed, recent studies of oceanic currents and basal lubrication rates in West Antarctica and Greenland suggest that this is a very real possibility.

Even a partial breakup of Antarctic or Greenland ice sheets could have catastrophic consequences. It has been estimated that a breakup of the West Antarctic Ice Sheet alone (which has been showing some signs of instability in recent years) would contribute over 16 feet to global sea-level and possible as much as 50 feet. The sea-level rise resulting from even a partial breakup of the Greenland ice sheet would likely contribute to destabilizing the West Antarctic Ice Sheet. The net effect of this alone could be as much as 20 feet of net sea-level rise. This would be enough to wreak havoc on coastal communities worldwide.

Nor is the loss of sea-ice without its impacts. The bulk of the world’s sea-ice is in the Arctic, where 20th century temperature rise has been most noticeable. Satellite and other datasets indicate that since the early 70’s Arctic sea-ice has been decreasing by an average of around 3 percent per decade. The September Arctic Ice Sheet size for the 1979-2004 period has decreased by 7.7 percent per decade reaching a record low in 2002 that was 4 percent lower than at any other time since 1978. These losses are well simulated by state-of-the-art climate models, but only when human generated greenhouse gases and land use impacts are included. They are not being compensated for by increases in sea-ice elsewhere (most notably, the Antarctic). Though these losses will not contribute to changes in sea-level they could have serious impacts on Arctic ecosystems. Arctic and Antarctic ice sheets provide crucial habitat and food sources for krill, which in turn support much of the food chain in these regions. Loss of this habitat could destabilize the ecology of the entire region. Losses during spring and summer have significantly shortened hunting seasons for polar bear populations. In the Hudson Bay region where the annual sea-ice season has been shortened by over 3 weeks since the 50’s, polar bear populations are known to be having 15 percent fewer cubs today than during that period.

Global climate impacts could be severe as well. Arctic sea-ice also provides significant thermal insulation to the underlying ocean though its high reflectivity (or albedo) and the insulation provided by its snow cover (contrary to popular belief, snow is a relatively good thermal insulator compared to water and ice), so its loss creates a positive feedback that accelerates further melting. This in turn will result in changes to the salinity of Arctic seas that will directly impact the thermohaline circulation of the world’s oceans. This process drives global oceanic currents—the so-called ocean “conveyor belt”—and will likely have far-reaching effects for communities and ecosystems around the globe. Even without significant acceleration, continuation of current loss rates will result in the vanishing of the entire Arctic Sea-Ice Sheet by 2060.

There are many today who would have us believe that none of this should concern us. Strangely enough, virtually all are affiliated with industry and Far-Right special interests, most of whom stand to profit heavily from the elimination of caps on greenhouse gas emissions, pollution controls, and conservation measures. To those who are less self-centered and more attuned to climate science and our childrens’ future, the time to wake up and pay attention to these matters is now.