Greenhouse Warming: Is the Mid-Holocene a Good Analogue?

Abstract

The mid-Holocene period (from approximately 9000 to 6000 years beforepresent) is often suggested as an analogue for enhanced greenhousewarming. The changes in net radiative forcing at the top of atmosphereare very different; increases in greenhouse gases producing a smallannual mean warming of little seasonal or latitudinal variation,whereas during the Holocene the annual mean did not change but therewere large seasonal and latitudinal variations. Two climate modelexperiments, one in which CO2 amounts are doubled and the other inwhich the value of the earth's orbital parameters are altered tothose appropriate to 9000 years before present (BP), are compared.Any similarity in the simulated response is found to be limited tothe northern continents and, even there, the mechanisms producingthe changes differ between the two experiments. Assuming that thegross behavior of the model is realistic, the Holocene is not a goodanalogue for a "greenhouse" warming. Furthermore, as the mechanismsoperating in the two experiments are different, a model which producesa realistic simulation for the mid-Holocene and present climate neednot necessarily produce a reliable simulation of greenhouse warming.However, a comparison of simulated climates for the mid-Holoceneand that reconstructed from paleoclimatic data may help to constrainthe existing range of subgridscale parametrizations used in climatemodels.