References
Beck, W. 1998. Warmer and wetter 6000 years ago? Science 279: 1003-1004.
Gagan, M.K., Ayliffe, L.K., Hopley, D., Cali, J.A., Mortimer, G.E., Chappell, J., McCulloch, M.T. and Head, M.J. 1998. Temperature and surface-ocean water balance of the mid-Holocene tropical western Pacific. Science 279: 1014-1017.
What was done
Beck (1998) discusses the work of Gagan et al. (1998), who used a double-tracer technique based on Sr/Ca and 18O/16O ratios in the skeletal remains of corals from the Great Barrier Reef, Australia, to infer climatic conditions for that region of the earth -- with implications for other regions as well -- at about 5350 years ago.
What was learned
Impressed with the use of coupled Sr/Ca and 18O/16O measurements to more accurately determine sea surface temperatures than has been possible in the past, Beck states that the new approach "promises to elucidate many important new clues about the dynamics of the coupled ocean-atmosphere-climate system for climate modelers to digest." Already, in fact, Gagan et al.'s work suggests that the tropical ocean surface about 5350 years ago was 1.2°C warmer than the mean that prevailed throughout the early 1990s, which accords well with terrestrial pollen and tree-line elevation records elsewhere in the tropical southwest Pacific for the entire period from 7000 to 4000 years ago. Their work also suggests that the higher tropical sea surface temperatures of that time likely enhanced evaporation from the tropical Pacific, and that the extra latent heat and moisture thereby exported to higher latitudes may have helped to maintain the equable climates known to have characterized the extratropics during this time period.
What it means
This new evidence adds credence to the plot of Earth's Climatic History: The Last 10,000 Years, which is featured in our Climate History (Overview) section in our Subject Index. In addition, when considered in conjunction with atmospheric CO2 data for The Last 10,000 Years, which is featured in our Carbon Dioxide (History) section of our Subject Index, it demonstrates, once again, that it is clearly possible to have warmer temperatures than those of the present with a lot less CO2 in the air than there is now. This decoupling of air temperatures from atmospheric CO2 concentration, i.e., the episodic warming and cooling of the globe during a period of relatively stable atmospheric CO2 concentration, which is evident over approximately 98% of the past 10,000 years (see the two items referenced above), strongly argues against attributing the warming of the last century or so to the concomitant rapid rise in the air's CO2 content.