Whither elevated CO2 research?

Biocon FACE site in Minnesota.

The list of major global changes is short. The world is becoming warmer, CO₂ concentrations are rising, N deposition is increasing, agriculture continues to increase, and the world’s flora is becoming homogenized.

The list of major global change research efforts is even shorter. For all intents and purposes its now all about climate. Our research into understanding the extent and implications of N deposition is greatly diminished and comparably small. Land use we largely quantify with remote sensing but do little else. Invasive species are approached piecemeal, but not with any major initiatives.

The surprising fall from grace has been research into elevated CO₂. The politics behind this are one thing, but there is no indication that this remains a major research question. Do we understand the effects of elevated CO₂ on ecosystems? Not even close. Does it seem like we do? For some, yes.

The reason the energy into elevated CO₂ research has waned derives from the early focus of the research. Early research centered less on testing mechanistic hypotheses than on quantifying response ratios. The modeling community wanted to know how much will photosynthesis and NPP increase. For all intents and purposes, the Ainsworth and Long (2005) meta-analysis of responses of photosynthesis to elevated CO₂ was, for some, the last important paper on the topic. I’m oversimplifying, but--with increases in elevated CO₂, photosynthesis goes up 25%, stomatal conductance goes down about the same amount.

With that synthesis, with those response ratios, so withered the major impetus into elevated CO₂. And with that, whither the research?

Without a major, overarching question, it is next to impossible to generate the funding necessary to sustain the research. The previous search for response ratios has fractured research into components that seek out other response ratios, but none of the remaining response ratios generate that much excitement.

In short, to reinvigorate CO₂ research, look at the fundamental unanswered questions and question whether the current response ratios can be trusted. (This is largely what fueled a large round of research shifting from chambers to FACE technology.)

Here’s what we know:

Elevated CO₂ increases water use efficiency.

Elevated CO₂ increases nitrogen use efficiency.

Given these two points, here’s what we don’t know:

Elevated CO₂ increases/decreases the relative limitation of water vs. nitrogen.

This is the major unanswered question for elevated CO₂ research, because it’s one of the major question we have left for modern CO₂ concentrations. If we do not understand the relative limitation of water and N to productivity and C storage now, and we know less about how CO₂ will alter the balance, then we really can’t trust the average response from modern experiments if the relative availability of water and N will change in the future. And right now, from our experiments, we just do not know whether CO2 increases photosynthesis because more water is left in the soil or less N is needed to grow. Considering that if more water is left in the soil more N should be made available to plants, we might know what the average response is today, but not what it will be tomorrow. Without knowing why, we can't know what.

Surely, there are other major questions to go forward with, but without something as large as this, elevated CO₂ research will continue to wither.