Carbon dioxide concentrations in the atmosphere are increasing every year. Each spring, CO2 concentrations drop as plants in the Northern Hemisphere turn green and begin photosynthesizing. Each fall, the amount of carbon dioxide in the atmosphere rises as plants shut down and soils keep respiring. On top of this annual cycle, each year CO2 concentrations drop a little less than they did the year before and rise a little bit more.
Over the past 50 years, CO2 concentrations have been increasing at an accelerating rate. In the 1960’s, the annual increase was less than 1 ppm per year. Over the past decade, CO2 concentrations have been increasing about 2 ppm per year.
The increasing CO2 concentrations in the atmosphere make me worry about grasslands. Grasses were forged under low CO2 concentrations. CO2 concentrations in late May of this year hit 400 ppm at the observatory in Hawaii where records have been kept since 1958. In early July, they were 397. They’ll probably dip down to 393 by September and then rise up to about 402 next May. You can see these patterns here.
Slowly and surely, year after year, the peaks and troughs creep higher and higher.
I worry about increasing CO2 concentrations for a number of reasons. One of those is that they might threaten grasslands throughout the world.
The threat isn’t direct. More CO2 helps plants grow better. But it helps some plants more than others.
The world’s 11,0000 grass species were forged under low CO2 concentrations. They show a number of adaptations that are especially good when CO2 concentrations are low. Many of them evolved C4 photosynthesis and they use silica to stiffen their cell walls. Both are good tricks for growing when CO2 concentrations are low.
Because grasslands expanded during these times of low CO2 concentrations, it stands to reason that there might be fewer grasslands when CO2 concentrations rise.
But who is favored more than grasses when there is more CO2 in the atmosphere?
Plants that don’t use C4 photosynthesis?
Plants that don’t use silica to build their walls?
Plants that use excess carbon to build carbon-intensive structures like wood?
Who is likely to be favored? Trees.
Trees need a lot of carbon in the air around them. There are no trees that use C4 photosynthesis. Few trees use silica to stiffen their walls. And wood is a luxury in a world where CO2 concentrations are low.
What does a grass look like when it is grown at low CO2 concentrations? Not that different than today. A tree? A shadow of itself.
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