[1] We develop and use a new version of the Terrestrial Ecosystem Model (TEM) to study how rates of methane (CH 4 ) emissions and consumption in high-latitude soils of the Northern Hemisphere have changed over the past century in response to observed changes in the region's climate. We estimate that the net emissions of CH 4 (emissions minus consumption) from these soils have increased by an average 0.08 Tg CH 4 yr À1 during the twentieth century. Our estimate of the annual net emission rate at the end of the century for the region is 51 Tg CH 4 yr À1 . Russia, Canada, and Alaska are the major CH 4 regional sources to the atmosphere, responsible for 64%, 11%, and 7% of these net emissions, respectively. Our simulations indicate that large interannual variability in net CH 4 emissions occurred over the last century. Our analyses of the responses of net CH 4 emissions to the past climate change suggest that future global warming will increase net CH 4 emissions from the Pan-Arctic region. The higher net CH 4 emissions may increase atmospheric CH 4 concentrations to provide a major positive feedback to the climate system.

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.

The Massachusetts Institute of Technology (MIT) Integrated Global System Model is used to make probabilistic projections of climate change from 1861 to 2100. Since the model's first projections were published in 2003, substantial improvements have been made to the model, and improved estimates of the probability distributions of uncertain input parameters have become available. The new projections are considerably warmer than the 2003 projections; for example, the median surface warming in 2091-2100 is 5.18C compared to 2.48C in the earlier study. Many changes contribute to the stronger warming; among the more important ones are taking into account the cooling in the second half of the twentieth century due to volcanic eruptions for input parameter estimation and a more sophisticated method for projecting gross domestic product (GDP) growth, which eliminated many low-emission scenarios.However, if recently published data, suggesting stronger twentieth-century ocean warming, are used to determine the input climate parameters, the median projected warming at the end of the twenty-first century is only 4.18C. Nevertheless, all ensembles of the simulations discussed here produce a much smaller probability of warming less than 2.48C than implied by the lower bound of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) projected likely range for the A1FI scenario, which has forcing very similar to the median projection in this study. The probability distribution for the surface warming produced by this analysis is more symmetric than the distribution assumed by the IPCC because of a different feedback between the