In a recent study to come out of China, Liu et al. (2014) write “food security under the changing climate is a great challenge for the world,” noting it has been stated by Porter et al. (2014) in the IPCC’s Fifth Assessment Report that “the negative impact of global climate warming on crop yield is more common than the positive impact according to the data from the past fifty years.”
That’s not true. Crop yields continue to rise, to the consternation of many, at the exact same rate that they have been rising at since the end of World War II. Even more telling, Liu et al. report studies based on historical data for the past several centuries suggest just the opposite, i.e. that “climate warming is good for crop harvests while climate cooling is bad for crop harvests in the world’s main crop production areas such as Europe (Braudel, 1992; Parker and Smith, 1997; Holopainen and Helama, 2009; Zhang et al., 2011) and China (Zhang, 1996; Ge, 2010; Su et al., 2014) in the temperate region.” They conclude “the current lengths of studies used to evaluate climate impacts on agriculture are too short to detect long-term trends.”
In making their case, the five Chinese scientists employed proxy data-based climate reconstructions that indicate that the Sui dynasty (581–618 AD) and Tang dynasty (618–907 AD) had warm climates comparable with the present, citing in this regard the study of Ge et al. (2003) that shows a strong periodicity in China temperatures. They additionally note that within this primarily warm climate regime, there were imbedded temperature variations—with cooling segments of inter-annual, multiple-decade and century-scale magnitude—which enabled them to assess crop yield responses to both heating and cooling from information provided about food availability in numerous historical documents that have been brought together in several historical compilations that deal with various aspects of China’s past, including Wang (1955), Wei et al. (1973), Li (1974), Liu (1975), Ouyang et al. (1975), Sima (1975), Dong (1985), Wang et al. (1985) and Song (2008). What did they thereby discover?
As indicated in the figure below, “on the one hand,” as Liu et al. describe it, “a declining trend of crop yield co-occurred with the climate cooling trend from 601 to 900 AD” (Figure 1a), while on the other hand, they indicate that crop yields were positively correlated with 30-year periods of warming (Figure 1b).
Figure 1. (a) Reconstructed Central East China temperatures (relative to the mean of 1951–1980) and 30-year regional mean crop yields over the period 601–900 AD, (b) the correlation between crop yield and temperature. Adapted from Liu et al. (2014).
Based on these real-world observations recorded in the historical documents they scrutinized, Liu et al. were able to more specifically state “from 601 to 900 AD the regional mean crop yield had a significant (P < 0.01) negative trend with the rate of ‑0.24% per decade,” as the overall climate cooled. But superimposed upon this long-term linear decreasing crop yield trend, they found there were several periods of warming, which ultimately enabled them to confirm “high crop yield occurred generally under the warming climate background.” In fact, they were able to quantitatively conclude that “crop yield increased by 6.9% per 1°C warming,” which they say “was slightly less than the estimation of 7.5% grades per 1°C warming from Su et al. (2014).”
It would appear, therefore, that global warming—if it is ever to begin again following its current 18-year hiatus—may very well prove providential in terms of boosting crop yields and enhancing food security, which is a much more optimistic assessment than that presented by the IPCC.
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