I am a global change scientist. I’ll admit we are a bit of a mythical beast. Most people have never met one. We don’t show up on TV or radio much. People assume we are all like Al Gore. Some people like to tell us we are wrong.
What’s important (and why I’m writing here) is that global change scientists have learned some interesting things over the years.
I, for one, have learned a lot about grass.
Everyone who reads this depends on grass for their income in one way or another.
Yet, grass still holds mysteries.There are over ten thousand species of grass in the world. Why so many?
Most grasses do not taste bad. Almost all plant leaves have chemicals that deter animals, but most grasses do not. Why not?
Grasses were around during the age of dinosaurs, but grasslands are a relatively recent phenomenon. What caused grasslands to suddenly expand?
It turns out that the history of grasses** cannot be separated from CO2 concentrations in the air. Neither can its future.
**Global change scientists spend about as much time thinking about the past as the future. Understanding how things used to work helps us understand how things are going to work.
When ice sheets were at their greatest extent 20,000 years ago, the CO2 concentration of the atmosphere was about 180 parts per million (ppm). When we first brought cattle to the Great Plains, it was about 280 ppm. Today it’s 400 ppm. By 2100 it likely could be 700 ppm.
What does all this mean for grasslands and the world’s 1 billion cattle? How different were the grasslands of the settlers compared to today? What are they going to be like at the turn of the next century?
One way we start to answer this question is to run experiments. One clever experiment was in Texas, funded by the USDA and the Department of Energy. Global change scientists constructed long linear chambers that looked like hoop houses used for growing plants in winter. Air was blown through the chambers and plants scrubbed the CO2 out of the air. At one end, CO2 concentrations were elevated to 550 ppm. At the other end, they were 200 ppm. So the experiment simulated grasslands growing in atmospheres from 60 years in the future to 20,000 years ago.
If you had to choose, which grassland would you want your cattle to graze? The grassland of the mammoths? Of the settlers? Or your grandkids?
In this experiment, your grandkids’ grasslands produced about 50 percent more plant biomass aboveground than the mammoth’s.
But, the grass had about 25% lower protein concentrations.**
**Why this happens is a bit complicated. Plants can photosynthesize more, which dilutes out the protein in their leaves and roots. This then leads soils to change and reduce the amount of N being supplied to plants. Whether it is better to have a greater quantity of lower quality grass is another question for another day.
So, as CO2 concentrations go up, the quantity of grass rises, but the nutritional quality declines.
All the extra CO2 accumulating in the atmosphere reduces protein concentrations in grass.
As scientists, we’re built to be skeptical. That was just one experiment. Experiments can always give unintended results. Is there any real evidence that this has happened?**
**How many people reading this routinely monitor the diet of pastured cattle and would know whether protein concentrations have changed over the past 25 years?
As for real evidence, researchers ** went into the Kansas State Herbarium and sampled grassland species collected from the region from as far back as 1876. They then measured the nitrogen concentrations of the grasses going back over 125 years.
**I got to help on this project a little bit. It was mostly others.
In their data, they found that plants collected in 1926 had nitrogen concentrations that were 40% higher than today. In terms of protein, those are declines from 15% to 11%.
Same pattern as the experiment.
So, when the next news story comes around showing the latest increase in atmospheric CO2 concentrations, think protein. And as that line traces upward, realize that all that CO2 is pushing your grass protein down.
Joseph Craine studies the functioning of nutrient-limited grasslands. At the heart of it all is understanding grass: how it's built and how it works. Craine earned his B.S. from Ohio State University and his Ph.D. from the University of California, Berkeley.