Background
The authors note that "increasing freshwater inputs may slow North Atlantic Deep Water (NADW) formation, a major driver of [the] Atlantic meridional overturning circulation (MOC)," and that "a slowing or cessation of [the] MOC in response to global warming could lead to relative cooling in some regions and amplified warming in others," perhaps the most significant of which phenomena is a postulated failure of the Gulf Stream that is often claimed to have the potential to dramatically cool much of Europe. The question they thus consider within the context of their research is: "How may changes in arctic and subarctic river discharge affect [the Atlantic] MOC?"

What was done
In a study designed to provide some perspective on the issue, McClelland et al. analyzed discharge records of 16 Eurasian and 56 North American rivers over the period 1964-2000. Of these rivers, all Eurasian ones and 14 of the North American ones flow directly into the Arctic Ocean, while the other 42 North American rivers flow into the Hudson, James and Ungava Bays (HJUBs).

What was learned
The five researchers determined that "discharge to the Arctic Ocean increased by 5.6 km³/yr/yr during 1964-2000, the net result of a large increase from Eurasia moderated by a small decrease from North America," but that "discharge to Hudson/James/Ungava Bays decreased by 2.5 km³/yr/yr during 1964-2000."

What it means
McClelland et al. say they "expect decreasing river discharge to Hudson, James, and Ungava Bays and increasing river discharge to the Arctic Ocean to have opposing effects on NADW formation," which leads us to ask: How significant is the net result for the maintenance of the Atlantic MOC?

The researchers go on to say that "the observed changes in river discharge over 1964-2000 amount to an increase of about 0.007 Sv to the Arctic Ocean and a decrease of about 0.003 Sv to HJUBs by the end of the record," and that "these values are relatively small compared to the ~0.1 Sv [increase] that lead[s] to abrupt reductions in NADW formation in a variety of models (Clark et al., 2002; Rahmstorf, 2002)."

McClelland et al. are right on the mark in this assessment. In fact, the net increase in freshwater discharge to the Arctic Ocean that is revealed by their analysis to have occurred between 1964 and 2000 amounts to only about 4% of the "tipping point" value that is predicted by some climate models to lead to an abrupt Atlantic MOC reduction. Hence, there is little cause for alarm in their findings. In addition, it has recently been noted by Wunsch that the models that predict decreases in, or even a cessation of, NADW formation and the Atlantic MOC are still too crude to be given much credence. In fact, he reports that depending on how the mixing coefficients are modified, fresh water additions can actually increase the North Atlantic mass circulation (Nilsson et al., 2003).

In conclusion, the totality of these several observations suggests that all of the hype surrounding the subject of a Gulf Stream shutdown due to a warming-induced increase in freshwater input to the Arctic Ocean is without a sound basis in either observation or theory.

References
Clark, P.U., Pisias, N.G., Stocker, T.F. and Weaver, A.J. 2002. The role of the thermohaline circulation in abrupt climate change. Nature 415: 863-869.

Nilsson, J., Brostrom, G. and Walin, G. 2003. The thermohaline circulation and vertical mixing: does weaker density stratification give stronger overturning? Journal of Physical Oceanography 33: 2781-2795.

Rahmstorf, S. 2002. Ocean circulation and climate during the past 120,000 years. Nature 419: 207-214.

Wunsch, C. 2006. Abrupt climate change: An alternative view. Quaternary Research 65: 191-203.