So what did Peterson et al. do to cause such a flurry of concern and lead certain folks, who should know oh-so-much-better, to go so far astray in their intimations about the future?  Very simply, they plotted annual values of the combined discharge of the six largest Eurasian Arctic rivers (Yenisey, Lena, Ob', Pechora, Kolyma and Severnaya Dvina) - which drain about two-thirds of the Eurasian Arctic landmass - against the globe's mean annual surface air temperature (SAT), after which they ran a simple linear regression through the data and determined that the combined discharge of the six rivers seems to rise by about 212 km³/year in response to a 1°C increase in mean global air temperature.  Then they calculated that for the high-end global warming predicted by the Intergovernmental Panel on Climate Change (IPCC) to occur by 2100, i.e., a temperature increase of 5.8°C, the warming-induced increase in freshwater discharge from the six rivers could rise by as much as 1260 km³/year (we calculate 5.8°C x 212 km³/year/°C = 1230 km³/year), which represents a 70% increase over the mean discharge rate of the last several years.

The link between this conclusion and the postulated shutting down of the thermohaline circulation of the world's oceans resides in the hypothesis that the delivery of such a large addition of freshwater to the North Atlantic Ocean (augmented by presumed increased freshwater discharges from other Arctic rivers and meltwater from Greenland) may slow - or even stop - that location's production of new deep water, which latter phenomenon constitutes one of the main driving forces of the great oceanic "conveyor belt" that redistributes heat around the world and brings considerable warmth to Europe [see North Atlantic Deep Water in our Subject Index].  Although we, too, are inclined to give a certain amount of credence to this concept, we are not inclined to accept the tremendous extrapolation that Peterson et al. make in extending their Arctic freshwater discharge vs. global surface air temperature (SAT) relationship to the great length that is implied by the IPCC's predicted high-end warming of 5.8°C over the remainder of the current century.

Consider, for example, that "over the period of the discharge record, global SAT increased by [only] 0.4°C," according to Peterson et al.  Do you think it's reasonable to extend the relationship they derived across that small range of temperature variability fourteen and a half times beyond the range of the independent variable used to derive it?  We surely don't, nor should any other rational person.

Consider also the Eurasian river discharge anomaly vs. global SAT plot of Peterson et al. (their Figure 4), which we have replotted in the figure below.  Enclosing their data with simple straight-line upper and lower bounds, it can be seen that the upper bound of the data does not change over the entire range of global SAT variability, suggesting the very real possibility that the upper bound corresponds to a maximum Eurasian river discharge rate that cannot be exceeded in the real world under its current geographic and climatic configuration.  The lower bound, on the other hand, rises so rapidly with increasing global SAT that the two bounds intersect less than two-tenths of a degree above the warmest of Peterson et al.'s 63 data points, suggesting that 0.2°C beyond the temperature of their warmest data point is all the further any relationship derived from their data may validly be extrapolated.

Annual Eurasian Arctic river discharge anomaly vs. annual global surface air temperature (SAT) over the period 1936 to 1999.  Adapted from Peterson et al. (2002).

In light of these observations, which are so plain as to be almost impossible to not understand, there would appear to be absolutely no reason for making the type of attention-grabbing statement contained in the headline of Science magazine's News of the Week report on the article of Peterson et al., especially when it is known that statements of this type serve as welcome fodder for climate alarmists who typically blow them even further out of proportion, to where they ultimately bear almost no relationship to what is really known.  As Stokstad (2002) correctly notes, "many experts caution that too little is known to make any solid predictions about such effects [as those discussed in the Peterson et al. article]."  In fact, he reports, one of those experts - oceanographer Knut Aagaard of the University of Washington in Seattle - says he would be very careful about considering any predictions about "the influence of the runoff and changes in the overturning of the North Atlantic" as being "anything more than very loose speculation."

We wholeheartedly agree.

References
Peterson, B.J., Holmes, R.M., McClelland, J.W., Vorosmarty, C.J., Lammers, R.B., Shiklomanov, A.I., Shiklomanov, I.A. and Rahmstorf, S.  2002.  Increasing river discharge to the Arctic Ocean.  Science 298: 2171-2173.

Stokstad, E.  2002.  River flow could derail crucial ocean current.  Science 298: 2110.