Kazi Farzan Ahmed

Future changes of the terrestrial ecosystem based on a dynamic vegetation model driven with RCP8.5 climate projections from 19 GCMs

Future changes of terrestrial ecosystems due to changes in atmospheric CO2 concentration and climate are subject to a large degree of uncertainty, especially for vegetation in the Tropics. Here, we evaluate the natural vegetation response to projected future changes using an improved version of a dynamic vegetation model (CLM-CN-DV) driven with climate change projections from 19 global climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The simulated equilibrium vegetation distribution under historical climate (1981–2000) has been compared with that under the projected future climate (2081–2100) scenario for Representative Concentration Pathway 8.5 (RCP8.5) to qualitatively assess how natural potential vegetation might change in the future. With one outlier excluded, the ensemble average of vegetation changes corresponding to climates of 18 GCMs shows a poleward shift of forests in northern Eurasia and North America, which is consistent with findings from previous studies. It also shows a general “upgrade” of vegetation type in the Tropics and most of the temperate zones, in the form of deciduous trees and shrubs taking over C3 grass in Europe and broadleaf deciduous trees taking over C4 grasses in Central Africa and the Amazon. LAI and NPP are projected to increase in the high latitudes, southeastern Asia, southeastern North America, and Central Africa. This results from CO2 fertilization, enhanced water use efficiency, and in the extra-tropics warming. However, both LAI and NPP are projected to decrease in the Amazon due to drought. The competing impacts of climate change and CO2 fertilization lead to large uncertainties in the projection of future vegetation changes in the Tropics.

Potential impact of climate change on cereal crop yield in West Africa

Resilience of crops to climate change is extremely critical for global food security in coming decades. Decrease in productivity of certain crops as a consequence of changing climate has already been observed. In West Africa, a region extremely vulnerable to climate change, various studies predicted significant reduction in productivity of the major crops because of future warming and shift in precipitation patterns. However, most studies either follow statistical approaches or involve only specific sites. Here, using a process-based crop model at a regional scale, we project the future changes in cereal crop yields as a result of climate change for West African countries in the absence of agricultural intensification for climate adaptation. Without adaptation, the long-term mean of crop yield is projected to decrease in most of the countries (despite some projected increase of precipitation) by the middle of the century, while the inter-annual variability of yield increases significantly. This increase of yield variability is attributed to an increase of inter-annual variability of growing season temperature and/or precipitation in future climate scenarios. The lower mean yield and larger year-to-year variation together make the regional food security extremely volatile. For a comprehensive understanding of climate change impact on crop yield, the distribution of temperature and precipitation over specific growth stages, in addition to growing season averages, should be accounted for. Although uncertainties are rife in calibrating and running a process-based crop model at regional scale, the present study offers insight into potential vulnerabilities of the agricultural system in specific countries or West Africa as a whole because of regional climate change.

Projecting regional climate and cropland changes using a linked biogeophysical-socioeconomic modeling framework: 1. Model description and an equilibrium application over West Africa

Agricultural land use alters regional climate through modifying the surface mass, energy, and momentum fluxes; climate influences agricultural land use through their impact on crop yields. These interactions are not well understood and have not been adequately considered in climate projections. This study tackles the critical linkages within the coupled natural-human system of West Africa in a changing climate based on an equilibrium application of a modeling framework that asynchronously couples models of regional climate, crop yield, multimarket agricultural economics, and cropland expansion. Using this regional modeling framework driven with two global climate models, we assess the contributions of land use change (LUC) and greenhouse gas (GHGs) concentration changes to regional climate changes, and assess the contribution of climate change and socioeconomic factors to agricultural land use changes. For future cropland expansion in West Africa, our results suggest that socioeconomic development would be the dominant driver in the east (where current cropland coverage is already high) and climate changes would be the primary driver in the west (where future yield drop is severe). For future climate, it is found that agricultural expansion would cause a dry signal in the west and a wet signal in the east downwind, with an east-west contrast similar to the GHG-induced changes. Over a substantial portion of West Africa, the strength of the LUC-induced climate signals is comparable to the GHG-induced changes. Uncertainties originating from the driving global models are small; human decision making related to land use and international trade is a major source of uncertainty.

Projecting regional climate and cropland changes using a linked biogeophysical‐socioeconomic modeling framework: 2. Transient dynamics

Understanding climate-cropland interactions and their impact on future projections in West Africa motivated the recent development of a modeling framework that asynchronously couples four models for regional climate, crop growth, socioeconomics, and cropland allocation. This modeling framework can be applied to a future time slice using an equilibrium approach or to a continuous projection using a transient approach. This paper compares the differences between these two approaches, examines the transient dynamics of the system, and evaluates its impact on future projections. During the course of projection up to midcentury, food demand is projected to increase monotonically, while the projected crop yield shows a high degree of temporal dynamics due to strong climate variability. Such temporal dynamics are not accounted for by the equilibrium approach. As a result, the transient approach projects a generally faster future expansion of cropland, with the largest differences over Benin, Burkina Faso, Ghana, Senegal, and Togo. Despite the relative large differences between the two approaches in projecting land cover changes associated with cropland expansion, the projected future climate changes are fairly similar. While the additional cropland expansion in the transient approach favors a wet signal, both the transient and equilibrium approaches project a future decrease of rainfall in the western part of West Africa and an increase in the eastern part. For quantifying climate changes, the equilibrium application of the modeling framework is likely to be sufficient; for assessing climate impact on the agriculture sector and devising mitigation and adaptation strategies, transient dynamics is important. This article is protected by copyright. All rights reserved.