B. Govindasamy

Land use changes and northern hemisphere cooling

Recent reconstructions of Northern Hemisphere mean temperatures over the past millennium show a long-term cooling of about 0.25K between 1000 and 1900 AD, prior to the 20th centurys warming. In this paper, we present the results of equilibrium climate model simulations that indicate that the land-use change occurring over this period may largely explain this observed cooling, although other factors also could have played a significant role. The simulated annual mean cooling due to land-use change is 0.25K globally and 0.37 K for the Northern Hemisphere, suggesting that the cooling of the prior centuries could have been largely the result of anthropogenic interference in the climate system.

Geoengineering Earth's radiation balance to mitigate climate change from a quadrupling of CO2

It has been suggested that climate change induced by anthropogenic CO2 could be counteracted with geoengineering schemes designed to diminish the solar radiation incident on Earth’s surface. Though the spatial and temporal pattern of radiative forcing from greenhouse gases differs from that of sunlight, it was shown in a recent study that these schemes would largely mitigate regional or seasonal climate change for a doubling of the atmospheric CO2 content. Here, we examine the ability of reduced solar luminosity to cancel the effects of quadrupling of CO2 content. In agreement with our previous study, geoengineering schemes could markedly diminish regional and seasonal climate change. However, there are some residual climate changes: in the geoengineered 4CO2 climate, a significant decrease in surface temperature and net water flux occurs in the tropics; warming in the high latitudes is not completely compensated; the cooling effect of greenhouse gases in the stratosphere persists and sea ice is not fully restored. However, these residual climate changes are much smaller than the change from quadrupling of CO2 without reducing solar input. Caution should be exercised in interpretation because these results are from a single model with a number of simplifying assumptions. There are also many technical, environmental and political reasons not to implement geoengineering schemes. D 2003 Elsevier Science B.V. All rights reserved.

Increase of carbon cycle feedback with climate sensitivity: results from a coupled climate and carbon cycle model

Coupled climate and carbon cycle modelling studies have shown that the feedback between global warming and the carbon cycle, in particular the terrestrial carbon cycle, could accelerate climate change and result in greater warming. In this paper we investigate the sensitivity of this feedback for year 2100 global warming in the range of 0 to 8 K. Differing climate sensitivities to increased CO2content are imposed on the carbon cycle models for the same emissions. Emissions from the SRES A2 scenario are used. We use a fully coupled climate and carbon cycle model, the INtegrated Climate and CArbon model (INCCA), the NCAR/DOE Parallel Climate Model coupled to the IBIS terrestrial biosphere model and a modified OCMIP ocean biogeochemistry model. In our integrated model, for scenarios with year 2100 global warming increasing from 0 to 8 K, land uptake decreases from 47% to 29% of total CO2emissions. Due to competing effects, ocean uptake (16%) shows almost no change at all. Atmospheric CO2 concentration increases are 48% higher in the run with 8 K global climate warming than in the case with no warming. Our results indicate that carbon cycle amplification of climate warming will be greater if there is higher climate sensitivity to increased atmospheric CO2 content; the carbon cycle feedback factor increases from 1.13 to 1.48 when global warming increases from 3.2 to 8 K.

The Equilibration of Short Baroclinic Waves

Abstract The life cycles of short baroclinic waves are investigated with the intention of completing a simple classification of nonlinear equilibration scenarios. Short waves become important in moist environments as latent heating reduces the scale of maximum baroclinic instability. Long-wave life cycles (wavenumber 6) were previously found to depend on details of the low-level momentum fluxes established during the earliest stages of development. These fluxes also serve as a focal point for the present study. For a realistic, zonally symmetric jet on the sphere, the normal-mode life cycle of a short wave (wavenumber 8) under both dry and moist conditions is described. Latent heating intensifies the low pressure system and frontal zones but does not alter the broader details of the life cycle. The normal modes have predominantly equatorward momentum fluxes, in contrast to the mainly poleward momentum fluxes of long waves. The short waves are more meridionally confined. The equatorward momentum fluxes dir...