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Late Holocene Variability of Florida Current Surface Density
Reference
Lund, D.C. and Curry, W.B.  2004.  Late Holocene variability in Florida Current surface density: Patterns and possible causes.  Paleoceanography 19: 10.1029/2004PA001008.

What was done
The authors note that "while the Florida Current-Gulf Stream system is arguably one of the most studied features in modern oceanography, almost nothing is known about its behavior on centennial to millennial timescales."  To rectify this situation, Lund and Curry thus analyzed planktonic foraminiferal δ18O time series obtained from three well-dated sediment cores retrieved from the seabed near the Florida Keys (24.4N, 83.3W) that cover the past 5200 years.

What was learned
In the words of Lund and Curry, isotopic data from the three cores "indicate the surface Florida Current was denser (colder, saltier or both) during the Little Ice Age than either the Medieval Warm Period or today," and that "when considered with other published results (Keigwin, 1996; deMenocal et al., 2000), it is possible that the entire subtropical gyre of the North Atlantic cooled during the Little Ice Age ... perhaps consistent with the simulated effects of reduced solar irradiance (Rind and Overpeck, 1993; Shindell et al., 2001)."  In addition, they report that "the coherence and phasing of atmospheric 14C production and Florida Current δ18O during the Late Holocene implies that solar variability may influence Florida Current surface density at frequencies between 1/300 and 1/100 years."

What it means
Once again, we have a situation where both centennial- and millennial-scale climatic variability is explained by similar-scale variability in solar activity, much as Bond et al. (2001) found for ice-rafting variability in the subpolar North Atlantic, which suggests there is no need to invoke the historical increase in the air's CO2 content to explain the increase in temperature that represents earth's transition from the Little Ice Age to the Modern Warm Period.

References
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G.  2001.  Persistent solar influence on North Atlantic climate during the Holocene.  Science 294: 2130-2136.

deMenocal, P., Ortiz, J., Guilderson, T. and Sarnthein, M.  2000.  Coherent high- and low-latitude variability during the Holocene warm period.  Science 288: 2198-2202.

Keigwin, L.  1996.  The Little Ice Age and Medieval Warm Period in the Sargasso Sea.  Science 274: 1504-1508.

Rind, D. and Overpeck, J.  1993.  Hypothesized causes of decade- to century-scale climate variability: Climate model results.  Quaternary Science Reviews 12: 357-374.

Shindell, D.T., Schmidt, G.A., Mann, M.E., Rind, D. and Waple, A.  2001.  Solar forcing of regional climate during the Maunder Minimum.  Science 294: 2149-2152.

Reviewed 9 November 2005