The Little Medieval Warm Period in the Cariaco Basin

Working with a sediment core extracted in 1990 from the northeastern slope of the Cariaco Basin (10°45.98'N, 64°46.20'W), Black et al. (2007) derived an 800-year Mg/Ca history of the planktic foraminifer Globigerina bulloides, which they correlated with spring (March-May) sea surface temperatures (SSTs) measured between AD 1870 and 1990, which ultimately allowed them to reconstruct an 800-year SST history of the region.

A plot of the authors' findings is reproduced in the figure below; and as may readily be seen, it reveals dramatic 20th-century warming, the prior Little Ice Age, and (at the beginning of the plot) what they describe as "the end of the Medieval Warm Period." What stands out most boldly of all, however, is the remarkable rise and fall of the region's SST that occurred between the Medieval Warm Period and the Little Ice Age, which is sometimes referred to as the Little MWP.

Figure 1. The 800-year Mg/Ca-derived SST history of the northeastern slope of the Cariaco Basin. Adapted from Black et al. (2007).

Compared to its "bigger brother," the Little MWP gets much less respect. Nevertheless, its prowess is indicated by the fact that Black et al. write that "on average, twentieth-century temperatures are not the warmest in the entire record," which finding contradicts the IPCC-based claim that 20th-century warming was unprecedented over the past millennium or more.