Rising carbon dioxide levels are making the world greener. But that’s nothing to celebrate.
“Global greening” sounds lovely, doesn’t it?
Plants need carbon dioxide to grow, and we are now emitting 40 billion tons of it into the atmosphere each year. A number of small studies have suggested that humans actually are contributing to an increase in photosynthesis across the globe.
Elliott Campbell, an environmental scientist at the University of California, Santa Cruz, and his colleagues last year published a study that put a number to it. Their conclusion: plants are now converting 31 percent more carbon dioxide into organic matter than they were before the Industrial Revolution.
Climate change denialists were quick to jump on Dr. Campbell’s research as proof that increased carbon dioxide is making the world a better place.
“So-called carbon pollution has done much more to expand and invigorate the planet’s greenery than all the climate policies of all the world’s governments combined,” the Competitive Enterprise Institute declared shortly after the study came out.
“The best messages are positive: CO2 increases crop yields, the earth is greening,” wrote Joseph Bast, the chief executive officer of the Heartland Institute, in an October 2017 email obtained by EE News.
In June, Mr. Bast co-authored an opinion piece in The Wall Street Journal in which he cited Dr. Campbell’s work as evidence of the benefits of fossil fuels. Our unleashing of carbon dioxide contributes “to the greening of the Earth,” he said.
Recently I talked Dr. Campbell, and as it turns out, he feels people like Mr. Bast are drawing the wrong lessons from his research. Here are four reasons he believes nobody should be celebrating “global greening.”
More Photosynthesis Doesn’t Mean More Food
Yes, we now get far more food from each acre of farmland than we did a century ago. But extra carbon dioxide only accounts for a small fraction of the increase.
“A 30 percent increase in photosynthesis does not translate into a 30 percent increase in strawberries off the land,” said Dr. Campbell.
While photosynthesis does pull carbon dioxide out of the atmosphere, much of that gas goes right back into the air. The reason: At night, the chemical reactions in plants essentially run backward. In a process known as respiration, plants pump out carbon dioxide instead of pulling it in.
“Part of the story is that photosynthesis is going up, and part of the story is that so is respiration,” said Dr. Campbell.
While the increase in photosynthesis is greater than that of respiration, the ultimate benefit to crops has been small — and it doesn’t explain our modern agricultural revolution.
“The driving factor has to be the fertilizers, the seed varieties, the irrigation,” Dr. Campbell said.
Extra Carbon Dioxide Can Make Plants Less Nutritious
A number of studies indicate that plants that grow in extra carbon dioxide often end up containing lower concentrations of nutrients such as nitrogen, copper and potassium.
As more carbon dioxide gets into the atmosphere, the problem will grow. “There’s definitely strong evidence that quality will be affected,” said Dr. Campbell.
It’s not clear why this happens. In a paper published in the journal Current Opinion in Plant Biology in June, Johan Uddling of the University of Gothenburg in Sweden and his colleagues speculated that microbes are to blame.
Just as carbon dioxide speeds up photosynthesis, it may also increase the rate at which soil microbes take up nutrients, leaving less for plants to suck in through their roots.
If we eat food that lacks nutrients, we become more vulnerable to a host of diseases. Recently, a team of researchers at Stanford University studied how future changes to crops could affect the world’s health. The findings were grim.
In Southeast Asia, for example, the researchers estimated that the rate of iron deficiency may rise from 21.8 percent to 27.9 percent by 2050.
Deficiencies in iron and other nutrients could make millions of people more vulnerable to diseases including malaria and pneumonia, leading to many premature deaths.
More Plants Won’t Prevent Climate Change
It’s not just strawberries and other crops that are taking in extra carbon dioxide. So are the forests, grasslands and other wild ecosystems of the world.
When scientists take into account both extra photosynthesis and respiration, they estimate that plants remove a quarter of the carbon dioxide we put in the atmosphere.
“That’s on par with what China emits,” said Dr. Campbell. “And China is the biggest global polluter.”
Even more remarkably, the plants have been scrubbing the same fraction of carbon dioxide out of the air even as our emissions explode.
“Every year we build more power plants, and every year the plants take out more CO2,” Dr. Campbell said.
But that isn’t cause to celebrate. It’s a bit like hearing that your chemotherapy is slowing the growth of your tumor by 25 percent.
Despite global greening, carbon dioxide levels have climbed over the past two centuries to levels not seen on Earth for millions of years. And the carbon dioxide we’ve injected into the atmosphere is already having major impacts across the planet.
The six warmest years on record all occurred after 2010. The weather has already become more extreme. Sea levels have risen. The oceans are acidifying.
If plants keep on absorbing only a quarter of our carbon dioxide in the future, then we can expect all these trends to get stronger.
In other words, if global greening isn’t saving us now, we can’t rely on it to save us in the future.
Global Greening Won’t Last Forever
There’s still a lot that Dr. Campbell and his colleagues don’t understand about global greening. Most importantly, they don’t know how long it will last.
As temperatures rise and rainfall patterns change, plants may stop soaking up extra carbon dioxide.
“Plants are quietly scrubbing the air of one China’s worth of carbon. What frightens me is knowing this can’t go on forever,” said Dr. Campbell. “If respiration catches up with photosynthesis, this huge carbon reservoir could spill back into our air.”
“There’s a wild card out there.”