Three recent studies on the West Antarctic Ice Sheet (WAIS) are making waves in the media, re-stoking fears of catastrophic sea-level rise, and putting a spring in the step of many a carbon-taxer.
Thomas Sumner summarizes two of the studies in a Science magazine commentary titled “No Stopping the Collapse of the West Antarctic Ice Sheet.” The studies, he writes, conclude that:
Thwaites Glacier, a keystone holding the massive West Antarctic Ice Sheet together, is starting to collapse. In the long run, they say, the entire ice sheet is doomed. Its meltwater would raise sea levels by more than 3 meters.
Specifically, Joughin et al., writing in Science, find that “in as few as 2 centuries Thwaites Glacier’s edge will recede past an underwater ridge now stalling its retreat. Their models suggest that the glacier will then cascade into rapid collapse.” Rignot et al., writing in Geophysical Research Letters (GRL), “describes recent radar mapping of West Antarctica’s glaciers and confirms that the 600-meter-deep ridge is the final obstacle before the bedrock underlying the glacier dips into a deep basin.”
In addition, McMillan et al., also writing in GRL, report that Antarctica as a whole is losing about 159 billion tons of ice per year. That’s an amount larger than previous estimates and translates to an overall sea-level rise contribution of 0.45 mm/year (1.7 inches per century).
The first two studies expressly conclude that the Thwaites and neighboring outlet glaciers have retreated to a point of no return and that, once gone, nothing can prevent the rest of the WAIS from flowing into the sea.
My initial reaction was: What’s really new here?
Conway et al. (1999), a study of the relentless retreat of the WAIS grounding line since the early-to-mid Holocene (i.e. 9,000 years ago or more), and Bindschadler (2006), a study of the inexorable melting of submarine glaciers in contact with warm ocean currents, both concluded that the WAIS is doomed.*
We can also infer as much from Dahl-Jensen et al. (2013), who found that in the last interglacial period, Greenland retained about 75% of its mass despite enduring temperatures 4°C-8°C warmer than the present for 6,000 years. Sea levels were 4-8 meters higher at the end of the last interglacial than at present, so much of that extra water must have come from Antarctica.
Thus, it is reasonable to assume that, anthropogenic global warming or no, the WAIS will someday be gone and sea levels will be several meters higher.
What’s new in the recent studies, apparently, is improved accuracy in estimating ice mass loss rates, and a shorter timetable than previously estimated for the demise of the WAIS. Joughin et al. expect the onset of rapid collapse to begin in 2 to 9 centuries.
There is no new insight for policy, though. If WAIS disintegration is unstoppable, regulating or taxing carbon is not going to save it. As Cato Institute scientist Chip Knappenberger points out, using EPA climate sensitivity assumptions, even if the U.S. shut down its entire economy tomorrow, that would avert less than 0.2°C of warming by 2100. That’s not enough to detectably slow WAIS ice loss rates much less stop the unstoppable.
Joughin et al. estimate that sea-level rise from Thwaites glacier retreat and thinning will be “moderate” over the course of the 21st century – less than 0.25 mm/year. That translates to 0.9 inches of additional sea-level rise. To be sure, the current rate of ice loss from Antarctica could increase during the decades to come. But at present, according to McMillan et al., all of Antarctica is contributing 1.7 inches per century to global sea level rise. That’s not a good reason to sell the beach house! For perspective, sea levels have risen about 8 inches since 1880.
The “crisis” spin on these studies is a trick of the imagination. People tend to imagine cities as immovable entities, stuck in harm’s way. But cities move continually in terms of where people build and live. If sea-level rise from Antarctica noticeably accelerates (whether in coming decades or after 2100), urban planners and real estate markets will alter development patterns accordingly.
The appropriate policy response remains adaptation. Indeed, as my colleague William Yeatman reminds us, although much of the Netherlands is at and below sea level, the Dutch built a flourishing society in the 16th and 17th centuries with pre-modern technology. How much better will sea defenses be in the century of Captain James Tiberius Kirk, or those of his great, great, great grandchildren?
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* Conway et al. 1999. Past and future grounding-line retreat of the West Antarctic Ice Sheet, Science: 286, 280–283.
This study estimates that the Ross Ice Shelf grounding line (the boundary of the ice sheet’s base on the sea floor) has retreated about 1,300 kilometers since the Last Glacial Maximum.
The researchers find that “most recession occurred in the middle to late Holocene in the absence of substantial sea level or climate forcing.” They conclude that grounding line retreat is mostly natural and will continue even in the absence of greenhouse forcing:
We suggest that modern grounding-line retreat is part of ongoing recession that has been under way since the early to mid-Holocene time. It is not a consequence of anthropogenic warming or recent sea level rise. In other words, the future of the WAIS may have been predetermined when grounding-line retreat was triggered in early Holocene time. Continued recession and perhaps even complete disintegration of the WAIS within the present interglacial period could well be inevitable.
Bindschadler, R. 2006. Hitting the Ice Sheets Where It Hurts. Science 311: 1720-1721
This study finds that water from the intermediate depths — the warmest water in polar oceans — is melting the submarine base of the glaciers, accelerating their flow towards the sea. Bindschadler is careful to point out “the absence of any indication of increasing sea surface temperature” in the polar oceans, and he notes that “warmth in the ocean arriving from lower latitudes would raise the temperature of this [comparatively warm] intermediate water a fraction of a degree, hardly enough to initiate a sudden glacier acceleration.” So why are glaciers accelerating?
According to Bindschadler, once the intermediate layer penetrates the moraine or sill (the barrier-like accumulation of boulders, gravel, and other debris deposited by the glacier as it retreats from its maximum extent), the water reaches the grounding line.
Figure explanation: Oceanic low blows. Schematic representing warm intermediate depth water breaching a submarine sill and striking in a water cavity beneath the ice shelf to access the grounding line of an outlet glacier.
“Increased pressure at these greater depths lowers the melting point of this ice, increasing the melting efficiency of the warmer water. Rapid melting results.” This explanation suggests a process that would occur with or without global warming. It also suggests a process that cannot be stopped. And that is what Bindschadler concludes:
Retreating glaciers lengthen the distance warmer water must travel from any sill to the grounding line, and eventually tidewater glaciers retreat to beds above sea level. This might limit the retreat in Greenland but will save neither West Antarctica, nor the equally large subglacial basin in East Antarctica where submarine beds extend to the center of the ice sheet.