"Systematic reviews are a well established technique in other science domains, notably medical research (synthesizing results from experimental or clinical trials), but not widely adopted in environmental science and climate-change impact research," Jerry Knox of Cranfield University told environmentalresearchweb. "[Our findings] highlight crops and countries where the evidence base is robust and conversely others where there are significant knowledge gaps."

Knox and colleagues from Cranfield and the University of Reading found that wheat yields across Africa are, on average, likely to decrease by 17%, with maize seeing a 5% drop, sorghum experiencing a 15% decrease and a 10% cut for millet. South Asia, meanwhile, looks set for a 16% fall in maize production and 11% less sorghum. Rice yields appeared to be unaffected by future climate change while there was not enough data to analyse the outlook for cassava, sugarcane and yams. The eight crops examined account for more than 80% of total crop production in Africa and South Asia.

The study also highlighted the magnitude of some of the uncertainties surrounding the impacts of climate change on mean yields. "It is important to remember that farmers don't grow crops under 'average' conditions – it's always the extreme years and climate uncertainty that cause the biggest problems for crop production," said Knox.

The analysis revealed a growing evidence base for climate change and maize yield in Kenya, Zimbabwe and Cameroon but a lack of studies for Nigeria, Tanzania and Malawi, despite the importance of maize in these nations. Similarly, more data was available for crops in India, Nepal, Pakistan and Sri Lanka than the South-East Asian nations of Bangladesh and Bhutan.

"The research findings will be extremely useful in highlighting key crops and regions where there are still major gaps in knowledge," said Knox. "For example, sugar cane is one of the most important commodity crops in the world, yet detailed knowledge of climate change impacts on both rainfed and irrigated production in Africa is sparse."

Knox said that for these under-examined crops and regions, new studies can be focussed to help build research capacity and develop appropriate strategies for climate adaptation.

"The systematic review approach could also be applied to other sectors and regions of the world where concerns regarding climate change or other stresses are growing, and where there is a need for unbiased robust evidence to inform decision making," he added.

Knox and colleagues screened 1144 studies, such as biophysical-based crop modelling research and statistical analyses, for inclusion in the review. A total of 52 studies were suitable for further analysis.

"Policy professionals in government departments work in fast-paced environments where it is usually impossible to assess the latest body of evidence within the time frame available," said Knox. "The way evidence is currently mediated means they sometimes have to rely on single studies, well placed experts or traditional unsystematic studies. There is thus a gap in the provision of a systematic and unbiased assessment of, in this case, development evidence."

The UK Department for International Development (DFID) has generated a systematic review programme to support the creation of new policies. "Our systematic review will therefore inform DFID policy and practice options, including resource allocation, for agricultural systems in Africa and South Asia under a changing climate," said Knox.

Writing in Environmental Research Letters (ERL), the researchers say that climate change will clearly threaten farming livelihoods of the rural poor in Africa and South Asia, particularly where soils and climate are already marginal for production and where limited access to agricultural knowledge and technology will hamper their ability to adapt. "Some autonomous adaptations, such as shifting planting dates, modifying crop rotations or the uptake of pre-existing crop varieties will help offset some negative impacts of climate change," they wrote. "However, it is reported that the greatest benefits in food-insecure regions are likely to arise from more expensive adaptation measures including the development of new crop varieties and uptake of new technologies including, for example, the expansion of irrigation infrastructure."

Most of the crops considered in the study are grown for local food or subsistence, under rainfed conditions. Commodity crops such as sugarcane grown for export tend to be irrigated, which should help buffer the impacts of rainfall and temperature changes. But there are signs that even irrigated commodity cropping in Africa will be at risk, wrote the researchers: "In many African countries, commodity cropping is a major employer in rural communities, so climate change could affect local populations involved in both subsistence and commercial crop production, albeit in differing ways."

The team reported the study in Environmental Research Letters (ERL).