A global biofuels program will lead to intense pressures on land supply and can increase greenhouse gas emissions from land-use changes. Using linked economic and terrestrial biogeochemistry models, we examined direct and indirect effects of possible land-use changes from an expanded global cellulosic bioenergy program on greenhouse gas emissions over the 21st century. Our model predicts that indirect land use will be responsible for substantially more carbon loss (up to twice as much) than direct land use; however, because of predicted increases in fertilizer use, nitrous oxide emissions will be more important than carbon losses themselves in terms of warming potential. A global greenhouse gas emissions policy that protects forests and encourages best practices for nitrogen fertilizer use can dramatically reduce emissions associated with biofuels production.

The impact of carbon-nitrogen dynamics in terrestrial ecosystems on the interaction between the carbon cycle and climate is studied using an earth system model of intermediate complexity, the MIT Integrated Global Systems Model (IGSM). Numerical simulations were carried out with two versions of the IGSM's Terrestrial Ecosystems Model, one with and one without carbon-nitrogen dynamics.Simulations show that consideration of carbon-nitrogen interactions not only limits the effect of CO 2 fertilization but also changes the sign of the feedback between the climate and terrestrial carbon cycle. In the absence of carbon-nitrogen interactions, surface warming significantly reduces carbon sequestration in both vegetation and soil by increasing respiration and decomposition (a positive feedback). If plant carbon uptake, however, is assumed to be nitrogen limited, an increase in decomposition leads to an increase in nitrogen availability stimulating plant growth. The resulting increase in carbon uptake by vegetation exceeds carbon loss from the soil, leading to enhanced carbon sequestration (a negative feedback). Under very strong surface warming, however, terrestrial ecosystems become a carbon source whether or not carbonnitrogen interactions are considered.Overall, for small or moderate increases in surface temperatures, consideration of carbon-nitrogen interactions result in a larger increase in atmospheric CO 2 concentration in the simulations with prescribed carbon emissions. This suggests that models that ignore terrestrial carbon-nitrogen dynamics will underestimate reductions in carbon emissions required to achieve atmospheric CO 2 stabilization at a given level. At the same time, compensation between climate-related changes in the terrestrial and oceanic carbon uptakes significantly reduces uncertainty in projected CO 2 concentration.

Cloud cover and relative humidity in the Tropics are strongly influenced by organized atmospheric convection, which occurs across a range of spatial and temporal scales. One mode of organization that is found in idealized numerical modelling simulations is self‐aggregation, a spontaneous transition from randomly distributed convection to organized convection despite homogeneous boundary conditions. We explore the influence of domain geometry on the mechanisms, growth rates and length‐scales of self‐aggregation of tropical convection. We simulate radiative–convective equilibrium with the System for Atmospheric Modeling (SAM), in a non‐rotating, highly elongated three‐dimensional (3D) channel domain of length >104 km, with interactive radiation and surface fluxes and fixed sea‐surface temperature varying from 280–310 K. Convection self‐aggregates into multiple moist and dry bands across this full range of temperatures. As convection aggregates, we find a decrease in upper tropospheric cloud fraction but an increase in lower tropospheric cloud fraction; this sensitivity of clouds to aggregation agrees with observations in the upper troposphere but not in the lower troposphere. An advantage of the channel geometry is that a separation distance between convectively active regions can be defined; we present a theory for this distance based on boundary layer. We find that surface fluxes and radiative heating act as positive feedback mechanisms, favouring self‐aggregation, but advection of moist static energy acts as a negative feedback, opposing self‐aggregation, for nearly all temperatures and times. Early in the process of self‐aggregation, surface fluxes are a positive feedback at all temperatures, shortwave radiation is a strong positive feedback at low surface temperatures but weakens at higher temperatures and longwave radiation is a negative feedback at low temperatures but becomes a positive feedback for temperatures greater than 295–300 K. Clouds contribute strongly to the radiative feedback, especially at low temperatures.