Testifying before the Select Committee on Energy Independence and Global Warming last year, I fumbled one question asked by the Honorable Chairman Edward J. Markey (D-MA): “Is it warming in the Petermann Glacier area?” I was unsure how the regionally relevant ocean temperatures had changed and how it impacts the melting glacier. A year late, we got the answer.
Floating ice shelf of Petermann Glacier on July 22, 2010 (NASA).
I was thinking of my former student Ms. Zweng. Three years earlier she had published a thorough analysis of ocean temperatures in Baffin Bay, that showed statistically significant warming by 0.11 +/- 0.06 degrees centigrade per decade for the 1916 through 2003 period (Zweng and Muenchow, 2006). But Baffin Bay is more than 800 miles away and it is not clear if those waters actually can make it to Petermann Fjord. I was also thinking of data in hand from only 80 miles away in Nares Strait whose waters definitely make it into Petermann, but I had not yet done the analyzes and thus did not know what the data would tell me. Now I do, and the peer-reviewed results (Muenchow et al., 2011) were published last week in Oceanography.
Time series of temperature (bottom) salinity (top) from the bottom of the ocean in Nares Strait between northern Greenland and Canada (from Muenchow et al, 2011). Trends are indicated for the 2003-06 and 2007-09 periods.
The data come from thermometers taking readings for years every 15 minutes. We placed the instruments on the bottom of the 300 meter deep ocean in 2003, recovered them in 2006, threw them back into the ocean in 2007 and found them again in 2009. We got data from three such instruments in 2003-06 and five in 2007-09 that all pretty much show the same thing: Bottom temperature change little during the 2003-06 period and about 0.06 +/- 0.02 degrees centigrade per year during the 2007-09 period of oberservations. Putting this together, we find a warming of 0.023 +/- 0.015 degrees centigrade per year. Next question would be, does this observed ocean warming in Nares Strait matter with regard to Petermann Glacier?
My current answer is a strong no. First, there is so much ocean heat already inside Petermann Fjord to melt away the entire floating section of the glacier (Johnson et al., 2011), that the extra ocean warming in recent years makes little difference. Second, the trends are from very short data sets that are dominated by physics unrelated to warming or could relate to a sequence of a few strong events that could either relate to man-made global warming or natural fluctuation at longer decadal cycles. This detection of signals in noise is a common problem in both engineering and geophysics, it is a required class for all our graduate students.
Very closely related is a paper entitled “Separating Signal and Noise in Atmospheric Temperature Changes: The Importance of Timescale” by Santer et al. (2011). Elegantly and comprehensively the authors expose and quantify the challenges one faces trying to extract the man-made warming signal from globally averaged near surface air temperature records sensed both from satellites and simulated in a number of numerical models. For this variable, the authors conclude convincingly, one needs records between 15-20 years long to extract a statistically significant man-made global warming signal from the much larger noise of natural variability.
So, if I had done my homework better last year, this should have been my answer to the question if it is warming in the Petermann Glacier area: “Yes, both the ocean and the atmosphere are warming in the Petermann region, but this may have little or no impact on the changing Petermann Glacier. Today we do not even know why Petermann Glacier has a floating ice shelf. Since we do not yet understand the physics of ice-ocean interactions, we can neither know nor predict what changes it has in store for us.”