A couple of scientists at the University of Montana say they have detected a small but non-negligible decline in global terretrial “net primary production.” NPP is basically a way of measuring plant growth — how much carbon they’re removing from their surroundings and turning into biomass. To my mind, there are two noteworthy aspects to their research, which just appeared in Science. Both led to me to the phrase that is the title for this post, although each use carries distinct meanings.
First, “Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009” begins by pointing out it used be thought that as carbon dioxide levels in the atmosphere rise, plants were expected to grow more. That idea has long since been undermined by evidence that there are other growth-limiting factors, although it still sees the light of day in pseudoskeptical arguments against doing something about anthropogenic climate change in the form of “CO2 is plant food.”
Indeed, write the authors, Maosheng Zhao and Steven W. Running, NPP “increased from 1982 through 1999, in part due to eased climatic constraints on plant growth.” But not anymore.
The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon.
That works out to a decline about about 1%, which doesn’t sound like much. But the idea that NPP isn’t rising anymore, despite an annual growth rate in CO2 emissions of 2 or 3%, is troubling. If these findings, which are based on data from satellite, are verified by other researchers, we can add falling NPP — with all the disastrous consequences for agricultural output and wildlife habitat — to the list of things to worry about if warming trends continue. This could prove to be an important piece of research. News of a worst kind.
The second thing that caught my attention is the first graph in the paper. Here it is. Please look closely at it before drawing any conclusions:
On first glance, it looks like NPP and CO2 levels are closely correlated. Which they are, which is why Zhao and Running plotted them in this fashion. It’s the sort of thing that good scientists do: choose a visualization technique that best expresses what one is trying to communicate. The troublesome part is the decision to invert the CO2 scale. This makes sense if the primary message to be conveyed is absolute correlation between plant growth rates and CO2 levels. Of course, the correlation is negative — the more CO2 rises, the more NPP falls. The graph’s legends states this explicitly and the right-hard y-axis is inverted. But given how easy it is to misinterpret even data that’s presented straight-ahead, you can pretty much guarantee that this graph, either in whole or cleverly edited or cropped, will find its way into presentations and blogs as evidence that NPP and CO2 are positively correlated and so is plant food after all.
So 9 out of 10 for good intentions, but minus several million for poor style.
Perhaps it is unfair to ask every scientist to take into account how their work will be distorted by those with political agendas when crafting their papers. Surely they should be encouraged to always use the most precise and scientifically appropriate methods, those that maximize the signal-to-noise ratio, to describe their work? There’s merit to that argument. But I can’t help thinking about Randy Olson’s book, Don’t Be SUCH a Scientist. On a matter as important as climate change, it wouldn’t hurt to think a bit beyond the lab when writing up one’s research.
Zhao, M., & Running, S. (2010). Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009 Science, 329 (5994), 940-943 DOI: 10.1126/science.1192666