The Impacts of Agricultural Land Use and Management on Regional Climate Systems: Interactions with the Asian Summer Monsoon

Increasingly intensive crop management and land use change has substantially altered the South Asian land surface¹. Regional areas classified as semi-arid and arid are now intensively irrigated, while forested domains have been cleared for extensive crop rotations. Such changes serve to alter the surface energy balance and fluxes of heat and moisture with the overlying atmosphere^2,3,4,5. This can potentially modulate the Asian Summer monsoon, and thus affect annual rainfall for nearly 60% of South Asian agricultural holdings^2,6,7,8. These land surface changes are an important regional climate forcing alongside anthropogenic greenhouse gas (GHG) and aerosol emissions.

This paper builds upon previous work^3,9,10 to quantify the independent and combined regional impacts of a) land conversion from natural vegetation to croplands, b) intensive management relating to both large-scale irrigation and cropping patterns, and c) increasing GHG forcing on South Asian climate dynamics. To explore these interactions, we present novel global climate model experiments with the NASA GISS ModelE2.1 that vary over modern agricultural land cover and management configurations. We perform two categories of experiments to evaluate 1) climatological differences resulting from present-day (centered on the year 2000) land cover and management, iterating over land conversions, crop calendars and rotations, and irrigation, and 2) time-varying climate responses from the separate and combined impacts of irrigation and historical GHG trends.

Prior results suggest that time-varying irrigation can weaken components of the monsoon system. Specifically, irrigation re-partitions the surface energy balance and increases the evaporative fraction, which reduces surface temperatures and the land-sea thermal contrast critical to monsoon establishment (Figure 1). The latter effect is compounded by GHG-induced rising sea surface temperatures. However, it is important to consider the impacts of irrigation alongside land cover conversions and agricultural management (i.e. crop timing and rotations), which may incite different climate responses. Figure 2 compares the climatological responses resulting from land cover change versus those of land cover change plus irrigation. Both differences are taken with respect to a natural vegetation experiment (i.e. no crop cover and irrigation). The two experiments exhibit opposing temperature responses of similar magnitude in the northwestern (intensively cultivated) areas of the domain (Figure 2a,b). The experiments also show similar patterns of net radiation change (Figure 2c,d), though these are driven by different radiative components. Under land cover change only conditions, western increases in net radiation were driven largely by increased incident shortwave radiation, while eastern reductions reflect albedo changes from the natural vegetation. In the irrigated scenario, increased net radiation is associated with enhanced cloud cover and moisture availability, which serves to increase downward longwave radiation but limits incident shortwave. Though irrigation leads to a weakened monsoon system, we also identify resulting circulation changes that enhance moisture convergence over northwestern areas, thereby contributing to increased summertime precipitation (Figure 2e,f). Precipitation reductions do however occur towards the east in all experiments, indicating that land cover and management changes can impact the spatial distribution of South Asian rainfall.

We present additional simulated results testing scenarios of land conversion, land management with respect to cropping calendars, and irrigation to further elucidate the above responses. Particular emphasis is placed on the interactions between these land surface modifications and background climate conditions, including soil moisture-atmosphere coupling and evaporative regime. Furthermore, we illustrate how such land surface modifications impact regional synoptic-scale circulation features, and the transport of moisture and energy through the atmospheric column.

In general, we find that the impacts of the combined land surface and historical forcings are highly dependent upon a-priori and background climate conditions and the implementation details of each forcing (e.g. vegetation characteristics, distributions, and initial soil moisture). However, irrigation emerges as a robust and consequential regional forcing, consistent with previous study.¹⁰ These findings have implications for monsoon variability and, furthermore, agricultural suitability and management. We suggest future work to: better quantify model sensitivities to the managed land surface; better represent major features of time-varying agricultural land management; and understand the South Asian regional interactions between climate change, particularly rising Indo-Pacific sea surface temperatures, and agricultural land management.

References:

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