Primary production refers to the basis of the entire food chain in plant growth through photosynthesis (it's a little broader than this, but that's the main idea). The total level of primary production can be measured by instruments on satellites and NASA have just published a new report summarising long term trends (more commentary including a short video can be found here. While primary production was rising fairly rapidly during the 1980s and 1990s, the last ten years have a slight decline in the total productivity of the globe's plant growth, largely due to more droughts. The full significance of this finding is not yet apparent, but it may signal a shift that scientists have predicted and feared: that the benefits of more carbon dioxide in the atmosphere to plant growth are being overwhelmed by disruptions to the hydrological cycle. Carbon dioxide is not simply "plant food", but also changes the climate, including precipitation patterns. And while (in small doses) it is necessary for life as we know it, it is possible to have too much of a good thing.
Indeed, a recent NAS study confirmed that for each degree Celsius the temperature rises, global crop yields from major grains like wheat, rice and corn will fall 5-15%. The recent Russian heat wave is estimated to have destroyed around 30% of Russia's entire wheat harvest. The heat wave cannot itself be directly blamed on global warming, but events like it become more common and more severe.
In years to come, it might not be climate change in the headlines, but food prices and political instability.
Give us this day our daily bread.