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Abstract. We used the General Ensemble biogeochemical Modeling System (GEMS) to simulate responses of natural and managed ecosystems to changes in land use and land cover, management, and climate for a forest/savanna transitional zone in central Ghana. Model results show that deforestation for crop production during the 20th century resulted in a substantial reduction in ecosystem carbon (C) stock from 135.4 Mg C ha −1 in 1900 to 77.0 Mg C ha −1 in 2000, and in soil organic C stock within the top 20 cm of soil from 26.6 Mg C ha −1 to 21.2 Mg C ha −1 . If no land use change takes place from 2000 through 2100, low and high climate change scenarios (increase in temperature and decrease in precipitation over time) will result in losses of soil organic C stock by 16% and 20%, respectively. A low nitrogen (N) fertilization rate is the principal constraint on current crop production. An increase in N fertilization under the low climate change scenario would lead to an increase in the average crop yield by 21% with 30 kg N ha −1 and by 42% with 60 kg N ha −1 (varying with crop species), accordingly, the average soil C stock would decrease by 2% and increase by 17%, in all cropping systems by 2100. The results suggest that a reasonable N fertilization rate is critical to achieve food security and agricultural sustainability in the study area through the 21st century. Adaptation strategies for climate change in this study area require national plans to support policies and practices that provide adequate N fertilizers to sustain soil C and crop yields and to consider high temperature tolerant crop species if these temperature projections are exceeded.

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Many savannas in West Africa have been converted to croplands and are among the world's regions most vulnerable to climate change due to deteriorating soil quality. We focused on the savanna-derived cropland in northern Ghana to simulate its sensitivity to projected climate change and nitrogen fertilization scenarios. Here we show that progressive warming-drying stress over the twenty-first century will enhance soil carbon emissions from all kinds of lands of which the natural ecosystems will be more vulnerable to variation in climate variables, particularly in annual precipitation. The carbon emissions from all croplands, however, could be mitigated by applying nitrogen fertilizer at 30-60 kg N ha −1 year −1 . The uncertainties of soil organic carbon budgets and crop yields depend mainly on the nitrogen fertilization rate during the first 40 years and then are dominated by climate drying stress. The replenishment of soil nutrients, especially of nitrogen through fertilization, could be one of the priority options for policy makers and farm managers as they evaluate mitigation and adaptation strategies of cropping systems and management practices to sustain agriculture and ensure food security under a changing climate.

The possibility of future climate change in Ghana has received much attention due to repeated droughts and floods over the last decades. The savanna zone which is described as the food basket of Ghana is highly susceptible to climate change impact. Scenarios from 20-year time slices of the near future – 2046-2065 – and the far future – 2081-2100 – climate change meant to help guide policy remain a challenge. Empirical downscaling performed at the local-scale of Wa District in the savanna zone of Ghana under the IPCC A2 SRES emissions scenario showed evidence of probable climate change with mean annual temperatures expected to increase over an estimated range of 1.5°C to 2.3°C in the near future, with number of cool nights becoming less frequent, especially during the Harmattan1 period. The dry season is expected to be warmer than the wet season, with high inter-annual variations projected in both maximum (Tmax) and minimum (Tmin) temperatures. Given an average of 1 day of Tmax > 40°C per month in the control period of 1961-2000, the number of hot days is expected to increase to 12 by 2046-2065. An increase in total rainfall is projected with possible shifts in distribution toward the end of the year, with a slight increase in rainfall during the dry season and an increase of rainfall at the onset and toward the end of the wet season. However, a decrease in June rainfall is projected in the wet season. The objective of this paper is to improve the understanding of future climate as a guide to local level medium-term development plans of effective adaptation options for Wa district in the savanna zone of Ghana.