A Linkage Between the LIA and Gulf Stream?

Michael Mann & Gavin Schmidt

The precise factors underlying the so-called “Little Ice Age” (LIA) have been intensely debated within the scientific community. One key metric in this debate is the spatial pattern of cooling which may provide a ‘fingerprint’ of the underlying climate change, whether that was externally forced (from solar or volcanic activity) or was part of an intrinsic mode of variability.

Surface temperatures in parts of Europe appear to have have averaged nearly 1°C below the 20th century mean during multidecadal intervals of the late 16th and late 17th century (and with even more extreme coolness for individual years), though most reconstructions indicate less than 0.5°C cooling relative to 20th century mean conditions for the Northern Hemisphere as a whole. There is much less data during these time intervals for the Southern Hemisphere, and that severely limits what conclusions can be drawn there. Just what combination of factors could explain this pattern of observations has remained somewhat enigmatic. A new ingredient in this debate comes with a recent paper in Nature by Lund et al.

Their study uses oxygen isotopes records from foraminifera (small zooplankton) contained within a set of long sediment cores taken off the coast of Florida to infer past changes in currents through the Florida Straits, which represents part of the greater “Gulf Stream” current system. This method is based on the observation that the isotope values are proportional to the water density, and as with the geostrophic calculations that are the basis of ocean transport estimates today, changes in isotope gradients across the strait should tell us something about the strength of the currents. They conclude that the Gulf Stream (at least, the portion of it which travels through the Florida Straits) may have been weaker by about 10% during the broad period (AD 1200-1850) commonly associated with the LIA (see figure, and note the time goes backward). The time resolution of the records doesn’t really allow for a much more precise chronology.

Such changes would have little if any impact on global mean temperatures, and only a modest impact on Northern Hemisphere mean temperatures, though they would have a more substantial impact on North Atlantic Sea Surface Temperatures (SSTs) – possibly up to a degree in some locations. This could help to explain the evidence for greater cooling during that period during the LIA in the North Atlantic and surrounding regions This possibility was touched upon in one news article about the study that appeared in The New Scientist.

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