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Friday, January 21, 2011

Jason Box: Record setting 2010 Greenland temperatures and long term trends

Record setting 2010 Greenland temperatures and long term trends

by Jason Box, blog, January 20th, 2011

Year 2010 surface air temperature observations around west and south Greenland are unprecedented in the instrumental record. Year 2010 and year 2003 temperatures dwarf high yearly averages occurring in the 1920s and 1930s.

Warming and Cooling
Fig. 1. 170 years of annually resolved whole Greenland ice sheet averaged surface air temperature from a reconstruction driven by a statistical fusion of long term meteorological station data with calibrated regional climate data assimilation model output (Box et al. 2009). A pink circle denotes the record setting year 2010 value.The thick gray line is a 31-year, two-tailed, Gaussian-weighted, smoothing of the annual values. As the "boxcar" gets within 15 years of the beginning and end of the series, the "tail" that runs into the end of the series is cut off and the weighting shifts accordingly.
Over the full 170-year record (1840-2010) of the reconstruction, the ice sheet average surface air temperature increased 1.26 °C. The warming rate was 0.74 °C/century. The recent 17-year Greenland ice sheet warming rate is 30% smaller in magnitude than a 17-year period in the 1920s. The intervening 63-year period (1932 to 1992) was cooling at -0.19 °C/decade. This  cooling can be attributed to a cooling phase of the Atlantic Multidecadal Oscillation (AMO) (e.g., Schesinger et al. 1994; Trenberth et al. 2006). Cold episodes in 1983-84 and 1991-92, caused primarily by major volcanic eruptions (see Box, 2002) enhance this cooling trend. West Greenland is a locus of sulfate aerosol-induced cooling (see Box et al. 2009). Another contributor to the 1932 to 1992 cooling is “Global Dimming,” that is, cooling at the surface induced by increases in sulfate aerosols. Liepert et al (2002) estimated that there was globally a reduction of about 4% in solar radiation reaching the ground between 1961 and 1990. The Wikipedia Global Dimming article is worth the read. The recent (post-1994) warming, is attributable to: (1) a growing absence of sulfate cooling because there has not been a major volcanic eruption since 1991; (2) recent warming phase of AMO; (3) a possible reversal of the global dimming trend; and (4) ongoing and intensifying anthropogenic global warming (AWG) [elephant in the room] owing to a dominance of enhanced greenhouse effect despite other anthropogenic cooling factors such as aerosols and contrails (IPCC, 2007). The primary factor responsible for the warming trend is very likely to be AWG (IPCC, 2007).

Fig. 2. Three long term Greenland meteorological station records, illustrating the long term time series of yearly-average temperatures. Triangles denote record setting values coinciding in 2010. Also interesting to note is the strong 1983-1984 El Chichon volcanic cooling (see Box 2002).

Refuting Denial
It is scientific to question if year 2010 record setting temperatures are real or due to some spurious aspect of the measurements. Former television meteorologist Anthony Watts, for one, expended quite a lot of effort to discredit apparent record setting 2010 temperatures in Nuuk, Greenland. However, Watts seems in error, as one would not expect the same pattern at other locations and in independent periods of time (Fig. 2), if the Nuuk 2010 temperatures are spurious. Rather, record high temperatures are evident at other Greenland stations in the same months, for example, in May, August, September, November, December 2010. Watts implicates the fact that the Nuuk measurements are near an airport to discredit the anomalous year 2010 values. Heat spewing from airplanes seems a valid concern and incidentally Aasiaat measurements are also from the grounds of an airport. However, the Prince Christian Sound (a.k.a. Prins Christian Sund) data are not obtained from near any airport (J. Cappelen, DMI, personal communication).

We are fortunate to have continuous temperature records from Greenland’s capital Nuuk beginning in 1866 in addition to century-plus records from other locations in Greenland (Box 2002; Vinther et al. 2006; Cappelen 2010; Box et al. 2009), providing instrumental climate records rivaling many of the longest records on Earth. I have used these data record and others available from the Danish Meteorological Institute and NASA to reconstruct Greenland ice sheet average surface air temperatures (see Box et al. 2009). I update the Box et al. (2009) reconstruction and make further analysis in this blog entry. This work is in preparation for my 7th consecutive annual Greenland entry for the Bulletin of the American Meteorological Society’s “State of the Climate” report published each June.


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