The greening of the McGill paleoclimate model. Part II: Simulation of Holocene millennial-scale natural climate changes

Various proxy data reveal that in many regions of the Northern Hemisphere (NH), the middle Holocene ( 6 kyr BP) was warmer than the early Holocene ( 8 kyr BP) as well as the later Holocene, up to the end of the pre-industrial period ( 1800 AD). This pattern of warming and then cooling in the NH represents the response of the climate system to changes in orbital forcing, vegetation cover and the Laurentide Ice Sheet ( LIS) during the Holocene. In an attempt to better understand these changes in the climate system, the McGill Paleoclimate Model (MPM) has been coupled to the dynamic global vegetation model known as VECODE ( see Part I of this two-part paper), and a number of sensitivity experiments have been performed with the "green'' MPM. The model results illustrate the following: ( 1) the orbital forcing together with the vegetation - albedo feedback result in the gradual cooling of global SAT from about 6 kyr BP to the end of the pre-industrial period; ( 2) the disappearance of the LIS over the period 8 - 6 kyr BP, associated with vegetation - albedo feedback, allows the global SAT to increase and reach its maximum at around 6 kyr BP; ( 3) the northern limit of the boreal forest moves northward during the period 8 - 6.4 kyr BP due to the LIS retreat; ( 4) during the period 6.4 - 0 kyr BP, the northern limit of the boreal forest moves southward about 120 km in response to the decreasing summer insolation in the NH; and ( 5) the desertification of northern Africa during the period 8 - 2.6 kyr BP is mainly explained by the decreasing summer monsoon precipitation.