Climate Change: It’s the Variability, Stupid

November 19, 2014

Studies evaluating the impact of climate change have mostly focused on the effects of mean changes in climate. This approach may severely underestimate the vulnerability of human society to anthropogenic-driven climate change. This is because the biological and agricultural sectors are also affected by changes in climate variability and extreme events. A recent article by Thornton et al. (2014) reviews our current understanding on the topic and highlights significant gaps in the research. Expected changes due to climate variability, in terms of frequency, intensity, and extent of extreme events are discussed. Depending on the region of interest, an increased variance in the probability distribution of temperatures (ie. hotter summers and colder winters or very hot summers with mild winters) is observed in global climate modeling simulations of future climates. The downstream impacts of these changes on ecosystems and biological systems can be substantial. For example, modern studies show a positive relation between the annual gross domestic product of a country and their agricultural yield, which in turn is coupled to the variability in rainfall in the region. However, the direct effects of climate variability on ecosystems, biological systems and the economy have been hard to quantify because a single extreme event cannot be attributed to climate change. The authors discuss the qualitative effects of climate variability on biological systems and food security.

Changing climates impact biological systems by changing the timing of maturity (flowering), timing and duration of growing seasons, risk of crop failures, crop yields, and yield quality. Studies have shown that interannual yield variability can be explained by variability in rainfall (Hlavinka et al., 2009; Rowhani et al., 2011). While several aspects of this relation are poorly understood, ignoring climate variability and only using climate means in impact models to project future changes can significantly underestimate the impact of changing climate (Rowhani et al., 2011). For example, an increase in the intensification of droughts can lead to lower crop yields which can lead the local farmer to switch to growing more drought-tolerant crops with a shorter growing season. This may have drastic downstream effects on the quality and quantity of feed used to maintain livestock populations. For example, in Kenya it is projected that ~1.8 million extra cattle could be lost due to increased frequency of droughts in 2030 because of greater water and heat stresses and drop in feed levels (Erickson et al., 2012). These effects however vary from region to region and an improved understanding on a local level is essential for future planning. Globally, the negative effects of climate change, such as decreased freshwater availability in parts of the tropics and sub-tropics are expected to outweigh the benefits of overall increases in rainfall in a warmer world (primarily in the high-latitudes). Similarly climate variability can also affect food access, transport and storage. For example, a qualitative modeling study showed that a drought in North America in 2030, with a magnitude similar to a historical drought in 1988 will have a significant effect (temporarily at least) on world market prices for wheat. This will have harmful consequences in several developing countries including Nigeria which imports most of its wheat and would have to raise the average domestic price of wheat above 50% of its baseline 2030 price. These sorts of global effects of localized extreme events have been very poorly studied, but have potential to cause the biggest harm.

The authors then discuss the concept of human vulnerability, defined in the context of food security as the product of impacts of climate change on biological systems (food availability) and social and economic impacts (affecting food security, utilization and access) (Ericksen, 2008). A simple analysis is performed to evaluate a potential link between rainfall variability and food security. The total amount of kilocalories produced by different regions around the world by livestock and crops is calculated using information from the year 2000. Rainfall variability is calculated by estimating the coefficient of variation of annual rainfall for the globe. The proportion of underweight children under the age of 5 is used as a proxy for food security in the region. While this proxy is rather crude, it is simply meant to be indicative as the age group vulnerable to direct effects of changes in food security. Several key inferences are made based on this analysis. It is shown that while 78% of the world population lives in developing countries, only 40% of the calories are produced in the region. Conversely, temperate regions account for 60% of produced calories with only 22% of the population. Interestingly, the relation between the food security proxy and rainfall variability is not completely straightforward. In developed countries, food insecurity increases with rainfall variability. In developing countries, food insecurity increases up to rainfall variability value of 30% and then falls slightly. A possible explanation for this trend is that in the region with higher coefficient of variability, food is brought in from other countries either via imports or food aids. Critically, small changes in climate variability in these regions can cause large changes in food security. This problem is expected to get worse in the future due to substantial growth of the population in developing regions.

The authors discuss human and societal responses to this vulnerability and review strategies to stabilize food security in different regions. Potential strategies include changes in crop varieties to tolerate heat and drought stress, increasing crop and livestock system efficiency by using more efficient soil and nutrition management, water harvesting and retention, improved ecosystem management, and using methods for managing risk such as index-based insurance and risk transfer products (Barnett  et al., 2008).  Finally, gaps in our existing knowledge are discussed and areas of further research that need greater focus are discussed. Briefly, these gaps can be filled by (1) improving our understanding of the effects of climate variability; (2) improving impact models such as crop and livestock models; (3) improving monitoring at a local scale, and developing effective adaptation strategies such as early-warning systems and risk-insurance schemes at a regional scale; (4) enhancing aspects of food security, especially in developing countries where an estimated 86% of the 9.5 billion population will reside by 2030; and (5) improving communication between scientists, social scientists, and policy-makers for effective use of all available information to develop informed adaptation strategies.


Philip K. Thornton. Climate variability and vulnerability to climate change: a review.  Global Change Biology (Impact Factor: 8.22). 03/2014; DOI: 10.1111/gcb.12581