February 16, 2011

Uncertainties Galore!

One word that comes up over and over in the global warming issue is “uncertainty”. The alarmists tend to minimize the discussion of uncertainties while the so-called skeptics seem to harp on how uncertain we are on so many fronts. Two articles have appeared in the literature during the past year highlighting amazing uncertainties dealing with ice loss from glaciers and water mass in the world’s oceans.

In our first feature article, a team of five scientists from France, Arizona, and British Columbia focused their attention on what has happened to glaciers in Alaska over the past 50 years. Berthier et al. begin their article noting that “Over the past 50 years, retreating glaciers and ice caps contributed 0.5 mm yr-1 to sea-level rise, and one third of this contribution is believed to come from ice masses bordering the Gulf of Alaska.” There is no doubt that melting has occurred in this area, there is no doubt that warming has occurred in the region, and there is no doubt that climate models predict warming in this area given the ongoing buildup of greenhouse gases. Images of ice breaking off the glaciers and falling into the sea have become standard in every presentation on global warming—watching calving tidewater glaciers in Alaska has become a major industry unto itself (Alaska cruise ships are more popular than ever). The dirty little secret is that glaciers have calved into the sea every summer whether the world is warming or cooling.

Nonetheless, there has been ice mass lost overall over the past five decades. Berthier et al. caution “However, these estimates of ice loss in Alaska are based on measurements of a limited number of glaciers that are extrapolated to constrain ice wastage in the many thousands of others. Uncertainties in these estimates arise, for example, from the complex pattern of decadal elevation changes at the scale of individual glaciers and mountain ranges.” Once again, we learn that uncertainties underlie any claims that massive melting is occurring in Alaska’s glaciers.

Berthier et al. used satellite-derived digital elevation models (think Google Earth on steroids) to more accurately determine the amount of mass loss in Alaska’s glaciers. As seen in Figure 1 below, the massive Columbian glacier (in the center) has experienced significant ice mass loss over the past five decades. However, the smaller Harvard glacier to the west has “thickened and advanced” according to the authors. That’s interesting—who would have thought that any major glacier in southern Alaska has actually thickened and advanced over the past five decades?


Figure 1. Map of surface elevation change in the Western Chugach Mountains between the 1950s and 2007 (from Berthier et al., 2010)

The bottom line of the work is expressed in the authors’ own words as they conclude “We find that between 1962 and 2006, Alaskan glaciers lost 41.9 ± 8.6 km3 yr-1 of water, and contributed 0.12 ± 0.02 mm yr-1 to sea-level rise, 34% less than estimated earlier. Reasons for our lower values include the higher spatial resolution of our glacier inventory as well as the reduction of ice thinning underneath debris and at the glacier margins, which were not resolved in earlier work. We suggest that estimates of mass loss from glaciers and ice caps in other mountain regions could be subject to similar revisions.” We are less than confident that the IPCC and others will be in a big hurry to suggest that estimates of mass loss from glaciers and ice caps will have to be reduced by 34%. Furthermore, had the authors suggested an increase in mass loss of 34%, they would have made front page news throughout the world.

A second article appeared recently with a title starting with “Uncertainty in ocean mass trends”—from the outset, this piece was never going to be a favorite of the global warming crowd. The article was published by two scientists from the private sector (a relatively rare happening in the peer-reviewed scientific literature), but the authors note that “This work was supported by the NASA GRACE Project and by the NASA Ocean Surface Topography Project.” They define “GRACE” stating “Since 2002 August, the Gravity Recovery and Climate Experiment (GRACE) satellite mission has produced time variable estimates of the Earth’s gravity field, which can be used to study changes in ocean mass.”

The Quinn and Ponte team state “Over the last century, the rate of sea level rise has been 1.7 ± 0.5 mm yr-1, based on tide gauge reconstructions. Recent estimates based on satellite altimetry are considerably higher at 3.1 ± 0.7 mm yr-1 from 1993 to 2003. Changes in mean sea level are primarily caused by changes in total ocean mass due to the exchange of water between the land and ocean and changes in mean ocean density (steric) due to changes in ocean temperature and salinity.” If you want to use GRACE data to study this issue, you must rely on GRACE data supplied from the GeoForschungsZentrum Potsdam, University of Texas Center for Space Research, and/or the Jet Propulsion Laboratory. The authors note “The three GRACE processing centres work with the same raw satellite data but process it using different orbit software, background models, GPS analysis, etc.” and the choice of dataset will impact the results of your work.

The bottom line is simple as the authors state in the abstract “We have examined trends in ocean mass calculated from 6 yr of GRACE data and found differences of up to 1 mm yr-1 between estimates derived from different GRACE processing centre solutions. In addition, variations in post-processing masking and filtering procedures required to convert the GRACE data into ocean mass lead to trend differences of up to 0.5 mm yr-1. Necessary external model adjustments add to these uncertainties, with reported postglacial rebound corrections differing by as much as 1 mm yr-1.” (note that post-glacial rebound processing improvements in GRACE recently led to a re-appraisal and substantial reduction of estimates of the amount of ice loss from Greenland and Antarctica). They conclude “The mean ocean mass trends we have calculated vary quite dramatically depending on which GRACE product is used, which adjustments are applied, and how the data is processed.”

These two articles underscore the many uncertainties that underlie key issues in the ongoing climate change debate.

References:

Berthier, E., E. Schiefer, G.K.C. Clarke, B. Menounos, and F. Rémy. 2010. Contribution of Alaskan glaciers to sea-level rise derived from satellite imagery. Nature GeoScience, 3, 92-95.

Quinn, K.J. and R.M. Ponte. 2010. Uncertainty in ocean mass trends from GRACE. Geophysical Journal International, 181, 762-768.




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