Examining the influence of growing crops on tropical climate using a coupled crop-climate model

The growth, development and ultimately yield of many crops are strongly influenced by the environmental conditions during the growing season. In many regions, year-to-year variations in crop productivity are strongly related to fluctuations in climate. Therefore, the productivity of croplands at seasonal to longer timescales is vulnerable to variations in weather and climate, respectively. While the atmosphere has been shown to be sensitive to variations at the land surface, forecasts of the impact of seasonal climate variability and future climate change on crop productivity are commonly based on the output of crop models driven with weather data derived from atmospheric model simulations. Such procedures assume, therefore, that the growing crop has no influence on the climate. This study addresses this assumption by examining crops and climate as a coupled land-atmosphere system. The coupled crop-climate model developed by Osborne et al (2007) is used to examine the role of dynamic crop growth in the tropics on the magnitude of interannual climate variability.

Firstly, a coupled crop-climate simulation, forced with sea surface temperatures of the last 50 years, was integrated to examine the modelled relationship between crop productivity and the simulated climate variability. The relationships were shown to be consistent with those observed at equivalent spatial scales. To isolate the importance of the dynamic crop growth on climate a second parallel climate simulation was performed with the crop interannual variability removed. This was achieved by prescribing the median annual cycle of crop leaf area index, canopy height and rooting depth for each grid point every year.

A comparison of the interannual variability between climate simulations provided an indication of the fraction of variability due to variations at the land surface associated with the interannual variability in crop growth. The greatest impact was seen for surface fluxes and temperature. The role of dynamic crops within the soil moisture - precipitation feedback pathway was also analysed and shown to be important in parts of India and the Sahel; providing a positive feedback which enhanced the influence of soil moisture on determining the likelihood of subsequent precipitation events in the model.

As croplands occupy an ever increasing fraction of the Earth's land surface these results have implications for future seasonal forecasting and climate change crop impact assesments as surface climate is sensitive to dynamical crop growth and development.

References:

Osborne, T. M., D.M. Lawrence, A.J. Challinor, J.M. Slingo and T.R. Wheeler, 2007. Development and assessment of a coupled crop--climate model. Global Change Biology, 13, 169-183.