Volume 7, Number 35: 1 September 2004
In 1986, a network of automatic weather stations was established throughout the McMurdo Dry Valleys (MDV) of Antarctica. Sixteen years later, Doran et al. (2002) reported that the data they collected revealed that seasonally-averaged surface air temperature there had decreased at a rate of 0.7°C per decade from 1986 to 1999, that summer (Dec-Feb) temperatures had declined at a rate of 1.2°C per decade, and that autumn (Mar-May) temperatures had dropped at a rate of 2.0°C per decade.
Flash forward to the present and the paper of Bertler et al. (2004), who propose that this cooling was caused by "a change of the atmospheric circulation," in support of which concept they present a detailed mechanism and proffer much data. Whether or not they are correct on this point remains to be seen. What is clearly debatable, however, is their ancillary contention that the MDV cooling "does not reflect a regional cooling" and is of short duration, i.e., a fluke or anomaly that is "superimposed on [a] longer-term warming trend."
In support of this claim, Bertler et al. present two additional instrumental temperature records: (1) autumn at nearby Marble Point from 1980-1999 and (2) annual and autumn at the slightly more distant Scott Base from 1958-2000. The former of these records reveals a cooling of 2.65°C per decade for the period of time (1986-1999) covered by the study of Doran et al. and a cooling of 2.54°C per decade for the extended period of 1980-1999, providing no evidence whatsoever for Bertler et al.'s claim, as these cooling trends are even greater than those derived by Doran et al. The latter of the records, however, does provide some support for their position, for although the Scott Base annual and autumn records depict coolings of 0.45 and 1.82°C per decade, respectively, from 1986 to 1999, they depict slight warming trends of 0.29 and 0.14°C per decade over the entire 1958-2000 period. Last of all, Bertler et al. present a proxy temperature history of nearby Victoria Lower Glacier derived from δ18O measurements of snow, which yields a summer cooling of 0.32°C per decade for 1986-2000 but a summer warming of 0.49°C per decade for the period 1969-2000. Including the record of Doran et al., there are thus four temperature histories that depict cooling from 1986 to 1999 and two that depict warming when the initial year of the temperature series is set at either 1958 or 1969, which collection of results is not all that definitive. And, of course, these data apply to but a very small portion of East Antarctica.
Further - and, we might add, decisive - insight into the difference of opinion between the two groups is provided by Doran et al., who in addition to their analysis of McMurdo Dry Valleys temperatures conducted a "spatial analysis of Antarctic meteorological data," which yielded a net cooling over the entire Antarctic continent between 1966 and 2000. Considering Antarctica without the Antarctic Peninsula, for example, Doran et al. found annual cooling to prevail over 65.9% of the continent, summer cooling to prevail over 76.3% of the continent, and autumn cooling to prevail over 99.7% of the continent. Clearly, therefore, over the period of time (1958-2000) for which instrumental temperature data were available at the times of the studies of both Bertler et al. and Doran et al., there was considerably more cooling occurring than warming over the bulk of the time period studied.
So why did Bertler et al. couch their statements about the findings of Doran et al. in language that suggests they are flawed? Was it because of the ultimate conclusion of Doran et al., i.e., that "continental Antarctic cooling, especially the seasonality of cooling, poses challenges to models of climate and ecosystem change"? It is a question well worth contemplating.
| Sherwood, Keith and Craig Idso |
References
Bertler, N.A.N., Barrett, P.J., Mayewski, P.A., Fogt, R.L., Kreutz, K.J. and Shulmeister, J. 2004. El Niño suppresses Antarctic warming. Geophysical Research Letters 31: 10.1029/2004GL020749.
Doran, P.T., Priscu, J.C., Lyons, W.B., Walsh, J.E., Fountain, A.G., McKnight, D.M., Moorhead, D.L., Virginia, R.A., Wall, D.H., Clow, G.D., Fritsen, C.H., McKay, C.P. and Parsons, A.N. 2002. Antarctic climate cooling and terrestrial ecosystem response. Nature 415: 517-520.