Person:

Lin, Marena

Wheat yields have increased approximately linearly since the mid-twentieth century across the globe, but stagnation of these trends has now been suggested for several nations. We present a new statistical test for whether a yield time series has leveled off and apply it to wheat yield data from 47 different regions to show that nearly half of the production within our sample has transitioned to level trajectories. With the major exception of India, the majority of leveling in wheat yields occurs within developed nations—including the United Kingdom, France and Germany—whose policies appear to have disincentivized yield increases relative to other objectives. The effects of climate change and of yields nearing their maximum potential may also be important.

In an earlier study, a weaker trend in global mean temperature over the past 15 years relative to the preceding decades was characterized as significantly lower than those contained within the phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensemble. In this study, divergence between model simulations and observations is estimated using a fixed-intercept linear trend with a slope estimator that has one-third the noise variance compared to simple linear regression. Following the approach of the earlier study, where intermodel spread is used to assess the distribution of trends, but using the fixed-intercept trend metric demonstrates that recently observed trends in global mean temperature are consistent () with the CMIP5 ensemble for all 15-yr intervals of observation–model divergence since 1970. Significant clustering of global trends according to modeling center indicates that the spread in CMIP5 trends is better characterized using ensemble members drawn across models as opposed to using ensemble members from a single model. Despite model–observation consistency at the global level, substantial regional discrepancies in surface temperature trends remain.