Helal Ahammad

Climate change effects on agriculture: Economic responses to biophysical shocks

Significance Plausible estimates of climate change impacts on agriculture require integrated use of climate, crop, and economic models. We investigate the contribution of economic models to uncertainty in this impact chain. In the nine economic models included, the direction of management intensity, area, consumption, and international trade responses to harmonized crop yield shocks from climate change are similar. However, the magnitudes differ significantly. The differences depend on model structure, in particular the specification of endogenous yield effects, land use change, and propensity to trade. These results highlight where future research on modeling climate change impacts on agriculture should focus. Agricultural production is sensitive to weather and thus directly affected by climate change. Plausible estimates of these climate change impacts require combined use of climate, crop, and economic models. Results from previous studies vary substantially due to differences in models, scenarios, and data. This paper is part of a collective effort to systematically integrate these three types of models. We focus on the economic component of the assessment, investigating how nine global economic models of agriculture represent endogenous responses to seven standardized climate change scenarios produced by two climate and five crop models. These responses include adjustments in yields, area, consumption, and international trade. We apply biophysical shocks derived from the Intergovernmental Panel on Climate Change’s representative concentration pathway with end-of-century radiative forcing of 8.5 W/m2. The mean biophysical yield effect with no incremental CO2 fertilization is a 17% reduction globally by 2050 relative to a scenario with unchanging climate. Endogenous economic responses reduce yield loss to 11%, increase area of major crops by 11%, and reduce consumption by 3%. Agricultural production, cropland area, trade, and prices show the greatest degree of variability in response to climate change, and consumption the lowest. The sources of these differences include model structure and specification; in particular, model assumptions about ease of land use conversion, intensification, and trade. This study identifies where models disagree on the relative responses to climate shocks and highlights research activities needed to improve the representation of agricultural adaptation responses to climate change.

Alternative ways of measuring and decomposing equivalent variation

Abstract Two different concepts of ‘equivalent variation’ have been used to measure the welfare effects of policy changes. One applies to uncompensated changes in which utility can vary, the other to compensated changes in which it is held constant. Harberger [J. Economic Lit. 9 (1971) 785–797] decomposed infinitesimally small changes in the uncompensated measure into components that measure the alleviation, or exacerbation, of existing distortions. We extend his methodology to obtain an exact decomposition of the compensated measure for finite changes, and show that no corresponding decomposition of the uncompensated measure is generally possible for finite changes, without introducing a residual or a scaling factor.

A CGE Approach to Measuring the Sectoral Contribution to an Economy: An Application to Western Australian Agriculture

Typically, sectoral contribution to an economy is estimated by measures such as shares in gross domestic/regional product, employment and exports. According to these measures, agricultures contribution declines as the economy grows. However, the indirect contribution of the sector is likely to rise due to the expansion of agriculture–based processing activities in the process of economic growth. Input–output multiplier analysis is often used to capture such flow–on contributions of the sector. This paper proposes to use a computable general equilibrium framework to estimate the direct as well as indirect contributions of agricultural growth in an economy. It also develops a methodology to distinguish the broader effects of agricultural output growth by source — input growth and productivity growth. The proposed methodologies are illustrated by applying these to analyze the contribution of agriculture to the economy of Western Australia, an agriculture and resources based state of Australia. The results for Western Australia suggest that the indirect benefits of agriculture exceed its direct contributions and that the contributions made by enhancing productivity in agriculture are indeed substantial. Importantly, the broader contribution of the agriculture sector depends critically on the specific source of the sectoral growth — input or productivity growth.