Simulation of the climate, ocean, vegetation and terrestrial carbon cycle in the holocene

• Main Author: Wang, Yi, 1969-
• Format: Thesis
• Language: English
• Published: McGill University 2005
• Subjects: Biogeochemistry; Environmental Sciences; Southern Ocean; Taylor Dome; Ice Sheet; North Atlantic; Sea ice
• Online Access: http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=86064

In this thesis, the "green" McGill Paleoclimate Model (MPM) is developed by interactively coupling the five-component physical MPM with a Dynamic Global Vegetation Model (DGVM) known as VECODE (VEgetation COntinuous DEscription model). Three applications to the pre-industrial Holocene climate, ocean, vegetation and terrestrial carbon cycle dynamics are presented, after a new land surface scheme is introduced. In these applications, orbital (Milankovitch) forcing and prescribed atmospheric CO2, starting from eight thousand years before present (8 kyr BP), are applied. In addition, a prescribed retreat of the Laurentide Ice Sheet (LIS) from 8 to 6 kyr BP is introduced. [Note: All acronyms used in this thesis are given in Appendix A.] The first application, in which the atmospheric CO 2 is fixed at 280 ppmv, shows that the vegetation-albedo feedback together with the retreating LIS allows the global annual mean surface air temperature to increase starting from 8 kyr BP and reach a maximum at around 6 kyr BP. The decreasing Northern Hemisphere summer insolation (orbital forcing) together with the vegetation-albedo feedback can explain the gradual cooling during the past 6 kyr. The southward shift of the boreal forest treeline from 6 to 0 kyr BP and the desertification of northern Africa from 8 to 2 kyr BP are also simulated, in good agreement with paleoclimatic reconstructions. In the second application, the reconstructed (Taylor Dome) atmospheric CO2 is used as a variable radiative forcing, and an inverse method is introduced to investigate the global carbon cycle dynamics. The model results indicate that the retreating LIS, in association with the vegetation-albedo and vegetation-precipitation (biogeophysical) feedbacks, causes the terrestrial carbon store to reach its maximum at around 6 kyr BP. Based on the inverse method, it is inferred that the first 10 ppmv atmospheric CO 2 increase from 8 to 6 kyr BP comes from the ocean carbon pool, which includes sedimentation processes. However, the land carbon release of about 68 PgC (95 PgC without CO2 fertilization) from 6 to 0 kyr BP can only contribute about 5 to 7 ppmv increase in atmospheric CO2; additional carbon sources are needed from the ocean. The simulated desertification results in a 70-PgC decrease in total carbon in the Sahara desert. This decrease is partially compensated by a 40-PgC increase in total carbon in the Southern Hemisphere. Finally, in the third application, the total volume of meltwater/freshwater from the retreating LIS is estimated, and four discharge scenarios are proposed to investigate the impact of this freshwater on the Holocene ocean, climate and terrestrial carbon cycle. During each freshwater perturbation, the simulated maximum Atlantic meridional overturning circulation (MOC) intensity is reduced, by amounts of up to 8 Sv. However, it rebounds to a higher level than the original state, within 10 to 20 years after the termination of the freshwater input. During the time of a weakened MOC, the SST is reduced in the high-latitude North Atlantic and increased in the Southern Ocean due to decreased northward oceanic heat transport. Only a large freshwater perturbation (>0.1 Sv) has a significant impact on the Holocene climate and terrestrial carbon cycle; it results in an enhanced cooling of about 1°C in the Northern Hemisphere (caused by the appearance of the North Atlantic sea ice) and notable drops in the global net primary productivity (2 PgC/yr) and total land carbon storage (40 PgC).