Guest Post: Carbon Dioxide and Plants

A colleague of mine, Gunnar Schade, who unlike me actually does research on the interaction of carbon dioxide and plants, thought my brief comments didn’t do justice to the subject.  So I invited him to write a guest blog post.  He did.  Thanks, Gunnar!  Now I’m inviting you to read it.

Carbon Dioxide, Climate, and the Biosphere

Gunnar Schade, Texas A&M University

John’s post on Three Simple Facts About Carbon Dioxide from a few weeks ago highlighted some important and quite easy to understand facts about CO2. The third fact, however, is not as “simple” as the headline suggests, and promptly drew a few more comments than the first two. As usual, the devil is in the details, and while John’s third fact and the associated passages from the last IPCC report that he cited are not controversial per se, it is often the things that are not highlighted or mentioned that can cause one to misunderstand, misinterpret, or even misrepresent that third fact:


To most of us, this is trivial, because we know that virtually all plant mass we see was once atmospheric CO2. But leaving it at that is oversimplifying the complexity of possible vegetation-climate feedbacks. For starters, if we focus for a moment just on C3-plants and CO2, I think it is useful to state that

  • the increase of net assimilation (the process of acquiring CO2 out of the air and incorporating it into plant mass) itself it not linear in CO2; it saturates between 500 and 1500 ppm depending on plant species, and
  • the slope of that response curve under current ambient CO2 is not proportional, i.e. there is no doubling of assimilation for a doubling of atmospheric CO2.

I illustrate these points in the graph below, which shows data from our Oaktree Project measurements for several post oak leaves from June last year:

Graph of CO2 uptake vs. CO2 concentrationNotice how the net assimilation vs. ambient CO2 concentration curve trails off towards a flat response above about 500 ppm. The shape of the curve is very similar between the leaves, while one assimilates more efficiently than the other. I included two lines to represent the approximate slope of the curve in the region 300-600 ppm ambient CO2. Both lines show a much lower than double increase of assimilation for the prescribed doubling of CO2 (note that current CO2 levels are nearing 400 ppm, while preindustrial levels were just below 300 ppm), and the higher CO2 becomes, the lower that slope.

This means that even if all plants on earth showed this particular response, it is not efficient in removing the extra atmospheric CO2 provided, and becoming less and less efficient the more CO2 is offered, facts conveniently omitted by people who want to mislead.

[Editor’s note: But wait, you might say, CO2’s effect on climate is logarithmic, so the plant response doesn’t have to keep up.  Nice try, but the plants’ uptake decays faster than logarithmic, so that it plateaus at 500-1500 ppm while CO2’s radiative effect is still continuing to increase. – John N-G]

But that is not all. Maybe even more important is the fact that

  • growth in most ecosystems on Earth is not limited by ambient CO2 concentrations, but rather by soil water or soil nitrogen (more generally: nutrient) availability.

Plants need both water and nitrogen aside from CO2 for growth. The equivalent is nutrition: Your child will not grow on carbohydrates alone, it needs proteins, fat, iron, etc.

So this further reduces the biosphere’s ability to uptake CO2-carbon in response to increasing ambient CO2 concentrations. Only in situations, where there are little to now other limitations on growth, such as in modern industrial agriculture providing irrigation and fertilization, does increased ambient CO2 unfold its full potential to spurt plant growth.

Roughly as a result of the above, the IPCC concluded, cited by John, that plant yields will increase somewhere between 0-25% (10-25% for C3 plants) when doubling CO2. There will be large geographical and species-related differences, with well-nurtured woody plants representing the high end number. [Updated 2-24 to correct editing errors. – John N-G]

But then there also are another two important caveats in that IPCC statement: The first is the word unstressed. We have seen first-hand a very important stress factor on terrestrial plants, drought. Increased plant, particularly tree mortality during drought can nullify years to decades of increased growth in a single year. The “death toll” of the current drought is not yet firmly established, but the numbers coming out are staggering. All the carbon that is now in standing dead and decaying biomass as a result of the drought will eventually return to the atmosphere.

The second caveat is the IPCC statement over the short to medium term. To understand what is meant, one has to know that the last IPCC report did not yet include several newer findings from (mostly) modeling studies that considered climate change in global dynamic vegetation models. As the current changes in atmospheric CO2 pushed the biosphere-atmosphere carbon balance way out of equilibrium, the expected, and observed, uptake into the terrestrial biosphere is very complex, and considered to be transient.

Why is that? Once carbon enters into living plants, say trees, it is distributed into several compartments, such leaves and wood. As time goes by, leaves drop, branches are shed, and eventually the tree dies and its carbon enters different compartments of the soil. The so-called residence times in these terrestrial biosphere compartments (aside from changing now that it gets warmer) vary strongly geographically and as a function of compartment. At some point in the future, depending on factors including the absolute amount and rate of warming, as well as the plant growth response itself (aka how much extra carbon has come in), the terrestrial biosphere as a whole will become a net source to the atmosphere again.

The scientific debate is about when that will be (mid century? end century? next century?), not if. So while the IPCC (and John) is right, in the long term the message is more of a mixed bag, and this is likely going to be reflected better in the next IPCC report.

John Nielsen-Gammon