Abstract
Abstract. Reconstructions of the mid-Holocene climate, 6000 years before present, suggest that spring temperatures were higher at high northern latitudes compared to the pre-industrial period. A positive feedback between expansion of forest and climate presumably contributed to this warming. In the presence of snow, forests have a lower albedo than grass land. Therefore, the expansion of forest likely favoured a warming in spring, counteracting the lower insolation at the mid-Holocene. We investigate the sensitivity of the vegetation-atmosphere interaction under mid-Holocene orbital forcing with respect to the strength of the forest-albedo feedback by using a comprehensive coupled atmosphere-vegetation model (ECHAM5/JSBACH). We perform two sets of model simulations: a first set of simulations with a relatively weak reduction of albedo of snow by forest; and a second set of simulations with a relatively strong reduction of the albedo of snow by forest. We show that the parameterisation of the albedo of snow leads to uncertainties in the temperature signal. Compared to the set with weak snow masking, the simulations with strong snow masking reveal a spring warming that is three times higher, by 0.34 °C north of 60° N. This warming is related to a forest expansion of only 13%.
Highlights
Most temperature reconstructions indicate that during the mid-Holocene, temperatures were on average higher compared to the pre-industrial period at high northern latitudes
We investigate the sensitivity of the vegetation-atmosphere interaction under mid-Holocene orbital forcing with respect to the strength of the forest-albedo feedback by using a comprehensive coupled atmosphere-vegetation model (ECHAM5/Jena Scheme for Biosphere-Atmosphere Coupling in Hamburg (JSBACH))
We investigate how much the parameterisation of the albedo of snow-covered forest influences the pure contribution of vegetation-atmosphere interaction to the midHolocene spring warming
Summary
Most temperature reconstructions indicate that during the mid-Holocene (about 6000 years before present), temperatures were on average higher compared to the pre-industrial period at high northern latitudes. Cannot be explained by the insolation signal, as the high northern latitudes received less insolation during midHolocene spring compared to the pre-industrial period This leads to the assumption that some feedbacks between the land surface and atmosphere may have caused warmer springs during the mid-Holocene. We assume that the spread of albedo values may lead to a spread of temperature signals in simulations of mid-Holocene climate To test this assumption, we investigate how much the parameterisation of the albedo of snow-covered forest influences the pure contribution of vegetation-atmosphere interaction to the midHolocene spring warming. We analyse two different sets of simulations: (a) simulations with a relatively weak reduction of albedo of snow by forest, and (b) simulations with a relatively strong reduction of the albedo of snow by forest
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