Reference
Goni, M.A., Woodworth, M.P., Aceves, H.L., Thunell, R.C., Tappa, E., Black, D., Muller-Karger, F., Astor, Y. and Varela, R. 2004. Generation, transport, and preservation of the alkenone-based U37K' sea surface temperature index in the water column and sediments of the Cariaco Basin (Venezuela). Global Biogeochemical Cycles 18: 10.1029/2003GB002132.
What was done
Based on the degree of unsaturation of certain long-chain alkenones synthesized by haptophyte algae contained in a sediment core retrieved from the eastern sub-basin of the Cariaco Basin (20°30'N, 64°40'W) on the continental shelf off the Venezuelan central coast, the authors reconstructed a history of sea surface temperatures for that location covering the past 6000 years.
What was learned
Goni et al. report that the highest alkenone-derived sea surface temperatures "were measured during the Medieval Warm Period (MWP)," which they identified as occurring between AD 800 and 1400. From the graph of their results reconstructed below, it is further evident that peak MWP temperatures were approximately 0.35°C warmer than peak Current Warm Period (CWP) temperatures, and that they were fully 0.95°C warmer than the mean temperature of the last decade of the 20th century.
What it means
The results of this research are striking in the clarity of their depiction of the MWP, CWP and intervening Little Ice Age (LIA) in a tropical setting. Consequently, they do much to substantiate the global nature of these multi-centennial climatic excursions. They are also significant in that they indicate (1) the LIA was the coldest period of the last six 6000 years, (2) the MWP was the warmest period of the same time interval, and (3) the CWP - especially most recently (in this part of the world at least) - has a long way to go before it can even be said to approach the warmth of the MWP.