Mapping climate change

The impact of climatic change on global agriculture is an area of concern for food security, policy, investment etc. While agricultural productivity continues to increase, climatic change introduces an unwelcome element of uncertainty and risk.

David Lobell (Stanford University) and co-workers have related changes in crop yields to observed climatic shifts over the past thirty years. They found that long-term temperature changes are having a greater impact on yields in major agricultural areas than shifts in rainfall patterns. A grain trader might find this fact surprising. There is a well-known rule of thumb that crops benefit from plenty of rain during the vegetative (growth) stages and drier conditions during the fruiting/harvest stages. Low yields or poor quality occur if these conditions are not met. Therefore yield variability is often associated with rainfall variability. The finding by Lobell and co-workers is an example of how short-term variability can mask a longer-term underlying trend.

Nowadays climatic forecasts are made using state-of-the-art physics based models such as CFSv2. Where there is sufficient historical data, these models can also be used to recreate the state of the atmosphere in the past. The maps below show the trends in 2m temperature over land during 1982-2010. Trends were extracted from CFSv2 Reanalysis data using R‘s raster package and plotted using ggplot2 (“trends” means linear regression coefficients although there is markedly non-linearity in many places.)

The upper map is for December-January-February (DJF) and the lower for June-July-August (JJA). Globally, DJF warmed by ~0.36oC/decade over land while JJA warmed somewhat less ~0.28oC/decade. Winter in North America and Scandinavia warmed by several degrees during the period 1982-2010, whereas summer warming was much weaker. There are even some places where winter and summer trends are reversed. The US breadbasket states show little warming (even some cooling) during the summer  growing season, but this is far from the case in Central Europe or Russia.

Corresponding maps for rainfall look quite different:


While regional trends can be large ~ ±50mm/decade, there was no significant net wetting or drying trend over land.

The spatial and temporal structure of climatic shifts during 1982-2010 is much more complex than headline global warming numbers suggest. For agriculture, the details matter a great deal.