Abstract
The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the climate system, however its sensitivity to the terrestrial biosphere has been largely overlooked. Here the HadCM3 coupled climate model is run for millennial timescales to investigate the feedbacks between vegetation and the AMOC at increasing CO2. The impact of agricultural conversion (termed land-use change; LUC) and the role of the simulated ‘background’ vegetation (termed land cover change; LCC) are investigated. LUC cools climate in regions of high crop fraction due to increased albedo. LCC is shown to evolve at higher CO2, with a northward migration of the tree line in the Northern Hemisphere and dieback of the Amazon. This generally acts to enhance the impact of climate change primarily due to albedo changes. Density in the Greenland-Iceland-Norwegian (GIN) Seas is crucial in driving the AMOC. Increasing CO2 decreases regional sea surface density, reducing convection and weakening the AMOC. The inclusion of LCC is shown to be responsible for a significant proportion of this weakening; reflecting the amplification effect it has on climate change. This acts to decrease the surface density in the GIN Seas. At elevated CO2 (1400 ppm) the inclusion of dynamic vegetation is shown to drive a reduction in AMOC strength from 6 to 20%. Despite the cooling effect of LUC, the impact on the AMOC is shown to be small reflecting minimal impact it has on GIN Sea density. These results indicate the importance of including dynamic vegetation in future AMOC studies using HadCM3, but LUC may be insignificant. In the context of other climate models however, the importance of vegetation is likely to be overshadowed by other systemic model biases.
Highlights
The Atlantic Meridional Overturning Circulation (AMOC) has a significant impact on regional and global climate
This study aims to answer this question in the context of increasing concentrations of C O2 using millennial simulations of the Hadley Centre model version 3 (HadCM3) coupled climate model
This study has used the HadCM3 coupled climate model run for millennial timescales to investigate the sensitivity of the AMOC to vegetation at four equilibrium CO2 concentrations; 350, 700, 1050 and 1400 ppm (1x, 2x, 3x and 4x respectively)
Summary
The Atlantic Meridional Overturning Circulation (AMOC) has a significant impact on regional and global climate. There remain a number of research areas that have been largely excluded from past AMOC studies One such example is the sensitivity of the AMOC to the terrestrial biosphere, such as the distribution of vegetation used in climate models. Vegetation alters the land surface structure and impacts biogeophysical processes including albedo, moisture fluxes and leaf-area index (LAI). This influences the surface energy balance, the partitioning of latent and sensible heat and shortwave radiation reaching the surface (Bala et al 2007; Boisier et al 2012; Brovkin et al 2009; Pielke et al 2002).
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