May 9, 2012

No sea level rise catastrophe?

Filed under: Climate History

As one of the central tenets of climate change catastrophe involves inundation by rapidly rising seas, we like to visit the issue from time to time here at World Climate Report. Interestingly, or perhaps some may prefer predictably, we usually are able to uncover plenty of science that indicates that the situation is not nearly so dire.

More evidence of this was published this week in Science magazine.

A paper by Twila Moon, Ian Joughin, Ben Smith, and Ian Howat titled “21st Century Evolution of Greenland Outlet Glacier Velocities” examined the flow characteristics from nearly 200 glaciers across Greenland for the period 2000-2010 as analyzed using synthetic aperture radar data collected from various satellites. Moon and colleagues assessed changes in the flow rate of each of the glaciers.

And what they found—much like what is found whenever the climate system is examined in detail rather than painted with a broad brush—was that the patterns of flow rate changes across Greenland were complex, both in space and time. Glaciers that were accelerating during a few years were found to be decelerating in others. Some accelerating glaciers were found in close proximity to other glaciers that were decelerating. The authors hypothesize that a variety of local factors are important in controlling the flow rate of individual glaciers including “fjord, glacier, and bed geometry,” “local climate” and “small-scale ocean water flow and terminus sea ice conditions.”

Moon et al. were able to make a few regional generalizations. Glaciers in the northwestern portion of Greenland typically showed accelerations thought the 2000-2010 period, while glaciers in southeastern Greenland showed a speed up from about 2000-2005 and then their flow rate remained fair steady from 2006-2010.

But overall, the speed up of glacier across Greenland was much lower than projections of their behavior under a warming climate have made them out to be. In fact, so much so, that Greenland’s glaciers are going to have to speed up even more than they have already just to reach the low end scenario of their contribution to sea level rise by the end of the century.

And just to show you that we are not making this stuff up, here is the complete concluding paragraph of the new Moon et al. study:

Finally, our observations have implications for recent work on sea level rise. Earlier research used a kinematic approach to estimate upper bounds of 0.8 to 2.0 m for 21st-century sea level rise. In Greenland, this work assumed ice-sheet–wide doubling of glacier speeds (low-end scenario) or an order of magnitude increase in speeds (high-end scenario) from 2000 to 2010. Our wide sampling of actual 2000 to 2010 changes shows that glacier acceleration across the ice sheet remains far below these estimates, suggesting that sea level rise associated with Greenland glacier dynamics remains well below the low-end scenario (9.3 cm by 2100) at present. Continued acceleration, however, may cause sea level rise to approach the low-end limit by this century’s end. Our sampling of a large population of glaciers, many of which have sustained considerable thinning and retreat, suggests little potential for the type of widespread extreme (i.e., order of magnitude) acceleration represented in the high-end scenario (46.7 cm by 2100). Our result is consistent with findings from recent numerical flow models.

Wow. The current rate of glacier flow is “far below” even low end projections of the increased flow rate between 2000 and 2010 and that there is “little potential” in reaching the high end scenario for the rate of flow. It is worth highlighting again in the low end scenario, Greenland’s glacier contributed 9.3 cm (3.7 in.) of total sea level rise by 2100 and in the high end scenario they were projected to contribute 46.7 cm (18.4 in).

That Greenland’s glaciers haven’t been cooperating with the oft-repeated (alarmist) notion that global warming will cause them simply to slide off into the sea has been becoming clearer and clearer over the past several years.

We have documented this progression has it has taken place through World Climate Report articles such as:

Slowdown in Greenland

Dead On Arrival: EPA/CCSP Sea Level Rise Report Already Outdated

Glacier Slowdown in Greenland: How Inconvenient

not to mention contributions we have made over at the Cato Institute website in the repeating Current Wisdom series:

The Iceman Goeth: Good News from Greenland and Antarctica

History to Repeat: Greenland’s Ice to Survive, United Nations to Continue Holiday Party

So, while some folks have been claiming that the IPCC was far too conservative in their projections of sea level rise contained in their Fourth Assessment Report—which spanned the range from 18 to 59 cm (7.1 to 23 in) with a central value of about 39 cm (15 in.), more and more science is showing that, as with most issue surrounding climate change—“modest” is the best description.

Reference:

Moon, T., I. Joughin, B. Smith, and I. Howat, 2012. 21st-century evolution of Greenland outlet glacier velocities. Science, 336, 576-578, doi:10.1126/science.1219985




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