Abstract: Land use in the U.S. is driven by multiple forces operating at the global level such as income and population growth, yield and productivity improvement, trade policy, climate change, and changing diets. Future land use has implications for biodiversity, run-off, carbon storage, ecosystem values, agriculture and the broader economy. We investigate those forces in the U.S. and their implications from a multi-sector, multi-system dynamics (MSD) perspective focused on understanding dynamics and resilience in complex interdependent systems.

Historical trends show slightly increased grassland and natural forest areas and decreases in cropland. We project these trends will be intensified under higher pressures for agricultural land or reduced under lower pressures, with no evidence of tipping points toward larger agricultural land abandonment or deforestation. However, U.S. sectoral output and trade, fertilizer use, N2O and CH4 emissions from agriculture activities, and CO2 emissions from land-use change are substantially impacted under several land-use forcing scenarios.

Summary: Previous studies on the impacts of climate change on agriculture have the following shortcomings: a) most focus only on a few major crops (maize, wheat, rice or soybeans); b) site-level and global gridded crop models (GGCMs) provide very different impacts of climate effects on crops; c) effects of climate change on livestock are well documented, but rarely quantified; d) there are several elements, causal relations and feedbacks among biophysical, environmental and socioeconomic aspects usually not taken into account in these studies. The goal of this paper is to investigate at the global level how alternative assumptions about these four aspects may affect agricultural markets, food supply, consumer well-being and environmental metrics.

To that end, this study simulates changes in crop yield and livestock productivity in a large-scale socio-economic model of the global economy with detailed representation of the agriculture sector, the MIT EPPA-Agriculture model. The economic model considers many complex socio-economic relationships and feedbacks, such as changes in management and land-use allocation, shifts in demand for food as prices and incomes change, and changing patterns of global trade. The climate shocks considered were median agricultural productivity changes taken from several site-level crop models revised by IPPC and several GGCMs.

The researchers find global welfare impacts several times larger when climate impacts all crops and all livestock. At the regional level, food budget impacts are 10% to 25% in many developing countries, which may challenge food security. Most of the results are due to the role of land area expansion as a major source of adaptation. Climate impacts from site-level crop models revised by the IPCC generate most challenging socio-economic outcomes, while median climate impacts from GGCMs on yield were positive for major crops. However, due to the wide range of impacts from these two types of models, caution is warranted in comparing those median effects.

The study’s conclusions indicate that the agricultural research community should expand efforts to estimate climate impacts on many more crops and livestock. Also, careful comparison of the GGCMs and traditional site-level models are needed to understand their major differences and implications for agricultural systems and food markets.

Abstract: The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO₂ emissions and as such is a critical parameter for calculating the remaining carbon budget. The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) was established to gain a better understanding of the potential magnitude and sign of ZEC, in addition to the processes that underlie this metric. A total of 18 Earth system models of both full and intermediate complexity participated in ZECMIP. All models conducted an experiment where atmospheric CO₂ concentration increases exponentially until 1000 PgC has been emitted. Thereafter emissions are set to zero and models are configured to allow free evolution of atmospheric CO₂ concentration. Many models conducted additional second-priority simulations with different cumulative emission totals and an alternative idealized emissions pathway with a gradual transition to zero emissions. The inter-model range of ZEC 50 years after emissions cease for the 1000 PgC experiment is −0.36 to 0.29 ^∘C, with a model ensemble mean of −0.07 ^∘C, median of −0.05 ^∘C, and standard deviation of 0.19 ^∘C. Models exhibit a wide variety of behaviours after emissions cease, with some models continuing to warm for decades to millennia and others cooling substantially. Analysis shows that both the carbon uptake by the ocean and the terrestrial biosphere are important for counteracting the warming effect from the reduction in ocean heat uptake in the decades after emissions cease. This warming effect is difficult to constrain due to high uncertainty in the efficacy of ocean heat uptake. Overall, the most likely value of ZEC on multi-decadal timescales is close to zero, consistent with previous model experiments and simple theory.