The carbon fertilization effect (CFE) is in principle simple: the larger amount of carbon dioxide in the atmosphere that has resulted from rising anthropogenic emissions should help the growth of plants, which use carbon dioxide during photosynthesis. The effect ought to increase crop yields – and that is some good news for farmers, amid the overwhelmingly gloomy forecasts for other aspects of climate change.

In detail, however, carbon fertilization is not so simple. It tends to have a greater effect on plants whe