Sea-Level Rise: Still Inches, Not Feet

In recent days, several stories have hit the presses regarding climate change and, more specifically, the future threat from rising sea levels. The EPA has just released a report on the potential impacts of sea level rise along the mid-Atlantic coast, while NASA’s Jim Hansen, apparently worrying that the economy is going to dominate the Obama administration’s attention, has cited rapidly rising sea levels as the primary basis for his warning that we only have four years to save the world.

Hansen is one of the popular alarmist voices of global warming, and his main weapon of fear is the threat of catastrophic sea level rise—upwards of 20 feet by century’s end—if we don’t immediately take drastic action to curtail the emission of greenhouse gases. However, a level-headed examination that combines past and present observations with model projections of the future shows the aforementioned 20 feet should really be more like 20 inches. And there is a world of difference between the two.

Hansen’s 20-foot threat has found champions among alarm ringers such as Al Gore, who made hay with it in An Inconvenient Truth. But in recent years, this over-the-top scenario has been pretty well beaten down in the scientific literature (e.g., Pfeffer et al., 2008; Joughin et al., 2008) and even the bloggers at Real Climate have stated that “We stress that no-one (and we mean no-one) has published an informed estimate of more than 2 meters of sea level rise by 2100.”

The Intergovernmental Panel on Climate Change (IPCC) in its Fourth Scientific Assessment (AR4) put the bounds of future sea level rise (by the year 2100) at somewhere between 7 and 23 inches. The IPCC actually left itself an additional bit of wiggle room by stating that it does not include the dynamic effects of ice loss from Greenland or Antarctica—that is, possible processes which act to change the velocity of the flow rate of glaciers—but goes on to speculate that such changes may add additional (6-ish) inches to the total rise by century’s end. Not all that alarming.

Since the publication of the IPCC’s AR4, investigations into the controlling factors of glacial flow across Greenland and Antarctica have picked up speed (much like some of the glaciers there), and for the most part the results that have been published to date have not lent a great deal of support to a large contribution from such processes to future sea level rise. Two research teams (Joughin et al., 2008; van de Wal et al., 2008) published results in which they concluded that melt water draining from the surface to the bottom portions of the glacier did not substantially increase long-term flow rate by lubricating the flow channel (contrary to Gore’s favorite hypothesis). Other researchers did find, however, that a thinning of the terminal portions of glaciers, whether through surface melting (Howat et al., 2008) or through basal melting from increased ocean temperatures (Holland et al., 2008) led to a reduction in the resistance to flow and thus an increase in the flow rate. So this is a possible mechanism for future increases in flow rate.

The latest and greatest, though, was just published last week. A research team led by Faezeh Nick of the U.K.’s Durham University produced one of the first numerical flow models for Greenland glaciers and used it to test various hypothesis for (marine-terminating) glacial retreat there (Nick et al., 2009). They determined that Greenland’s outlet glaciers were very sensitive to processes which acted on their calving termini—either surface melting from rising air temperatures or basal melting from rising ocean temperature. Their biggest finding, however, was that although the glaciers are fast to respond to rising temperatures, they rapidly reach a new equilibrium—in other words, the rapid rates of retreat are not sustained. Nick and his colleagues conclude:

From our numerical modelling, we conclude that Greenland tidewater outlet glaciers are highly sensitive to changes in their terminus boundary conditions and dynamically adjust extremely rapidly, providing an explanation for their almost synchronous behaviour to short-term fluctuations in climate. This implies that discharge changes near the glacier terminus reflect short-term dynamical adjustments, and do not provide a reliable measure for the longer-term mass balance of an ice sheet. We predict that longer-term rates of mass loss, at least for Helheim Glacier, may be less marked than observed in recent years. [emphasis added]

The authors also speculate that “extreme mass loss cannot be maintained in the long-term, and that the recent rates of mass loss through increased outlet discharge should not be extrapolated to the future” and suggest that “in the long term, non-dynamical processes, such as direct surface melt under a warming climate, may dominate the future mass loss of the Greenland ice sheet.”

These non-dynamical processes are the types of things already covered by the IPCC estimates. Therefore, the findings in Nick et al. (2009) do not suggest that sea level rise is going to proceed quickly, in an out-of-control fashion, nor that the IPCC projections are gross underestimates.

In fact, if anything, the high end of the IPCC range seems like it is losing support as global temperature trends (on which the IPCC sea level scenarios are strongly dependent) are slowing and proceeding at a rate beneath IPCC projections. (See my recent post on global temperatures). Simply put, a slower temperature rise equals a slower sea level rise.

So, the next time you come across someone pitching a catastrophic sea level rise, of upwards of 20 feet by 2100, realize that such a number is intended only to scare you into taking some action—soon to be prescribed by the same source. The best science tells you otherwise, and that the feet should be inches.

References:

Howat, I. M., 2008. Synchronous retreat and acceleration of southeast Greenland outlet glaciers 2000-2006; Ice dynamics and coupling to climate. Journal of Glaciology, 54, 646–660.

Intergovernmental Panel on Climate Change, 2007. Climate Change 2007: The Physical Science Basis. Solomon, S., et al. (eds.), Cambridge University Press, Cambridge, U.K., 996 pp.

Joughin, I., et al., 2008. Seasonal speedup along the western flank of the Greenland Ice Sheet. Science, 320, 781–783.

Nick, F. M., et al., 2009. Large-scale changes in Greenland outlet glacier dynamics triggered at the terminus. Nature Geoscience, DOI:10.1038, published on-line January 11, 2009.

Pfeffer, W. T., et al., 2008. Kinematic constraints on glacier contributions to 21st-century sea-level rise. Science, 321, 1340–1343.

van de Wal, R. S. W., et al., 2008. Large and rapid melt-induced velocity changes in the ablation zone of the Greenland ice sheet. Science, 321, 111–113.