A 16-year study found that we’re at a point where more CO2 won’t keep increasing plant production, but higher temperatures will decrease it

A new study by scientists at Stanford University, published in the Proceedings of the National Academy of Sciences, tested whether hotter temperatures and higher carbon dioxide levels that we’ll see post-2050 will benefit the kinds of plants that live in California grasslands. They found that carbon dioxide at higher levels than today (400 ppm) did not significantly change plant growth, while higher temperatures had a negative effect.

The oversimplified myth of ‘CO2 is plant food’

Those who benefit from the status quo of burning copious amounts of fossil fuels love to argue that more carbon dioxide in the atmosphere will benefit plant life. It’s a favorite claim of climate contrarians like Matt Ridley and Rupert Murdoch.

Rupert Murdoch (@rupertmurdoch) World growing greener with increased carbon. Thirty years of satellite evidence. Forests growing faster and thicker.

It seems like a great counter-argument to the fact that carbon dioxide is a dangerous pollutant – a fact that contrarians often dispute. However, reality is far more complicated than the oversimplification of ‘CO2 is plant food.’ Unlike in the controlled environment of a greenhouse, the increasing greenhouse effect on Earth causes temperatures to rise and the climate to change in various ways that can be bad for plant life. We can’t control all the other variables the way we can in a greenhouse.



So far, as contrarians like Rupert Murdoch love to point out, the plant food effect has won out. Earth has become greener in recent decades (although that trend may now be reversing). The situation is not unlike a human diet – at relatively low calorie levels, more food is beneficial. But as calorie intake continues to rise, at a certain point it’s no longer benefiting the human body. More food is good, but only up to a certain point, as the global obesity epidemic makes clear.

The experiment

The Stanford scientists set up 132 plots of flowers and grass in California and introduced varying levels of carbon dioxide, temperature, water, and nitrogen. The scientists conducted the experiments over 16 growing seasons between 1998 and 2014. They found that only higher nitrogen levels resulted in higher plant productivity, while higher temperatures caused it to decline.



While this experiment was specific to California grasslands, other studies have similarly undermined the ‘more CO2 is great’ myth. For example, a 2012 paper found that higher temperatures are detrimental to French corn yields. While French corn production has increased steadily in recent decades due to a combination of technological improvements and CO2 fertilization (the former far more than the latter), yields have leveled off in recent years, and were particularly low when struck by heat waves.

A significant reduction in maize yield is found for each day with a maximum temperature above 32°C, in broad agreement with previous estimates. The recent increase in such hot days has likely contributed to the observed yield stagnation.

Another study published in Nature Climate Change last week concluded that higher temperatures will cause wheat production to decline. Just a 1°C rise in global temperature will decrease wheat yields by about 5% (approximately 35 million tons). Climate change is bad news for several of our staple crops.



Carbon pollution and rapid climate change are dangerous

The evidence thus suggests we’re at or near the point where rising atmospheric CO2 levels will no longer benefit overall plant growth, while the rising heat that comes along with that carbon are generally detrimental to plant productivity.



There are also many ways in which dumping more carbon pollution into the atmosphere has negative effects, on plants and also animal species. Climate change is causing increased heat waves, flooding, and other extreme weather events; national security threats; ocean acidification; sea level rise; wide-scale species extinctions; and so on.

There will certainly be some positive climate change outcomes as well, but all evidence suggests the negatives will far outweigh the positives. Cherry picking one possible positive outcome and ignoring all the negatives as an excuse to maintain the status quo is simply a failure of basic risk management. And with a threat as dangerous as global climate change, engaging in proper risk management is incredibly important. Failure is simply not an option.

