Unsurprising bad news: Plants are getting worse at absorbing carbon dioxide


Of the many arguments against treating climate change like the serious problem it is, one of the most foolish has been the attempt to claim that it’s actually a good thing that CO2 levels are rising. On some occasions, this argument is made by people who say it’s a good thing to have the temperature rise. Back in the late 1890s, Svante Arrhenius apparently felt this way, but that’s perhaps understandable given that he expected the warming to take a couple thousand years, and Sweden can be fairly cold. More often, what we get is the grade-school-science argument that “CO2 is plant food”:

In case it needs to be said, this has never been a valid argument. It’s on par with saying that because plants like water, it would be better for all of them if the entire planet was flooded, or because they like sunlight, it would be better for them if they sky was never, ever cloudy. Still, photosynthesis remains our best method for pulling CO2 out of the atmosphere, and wild plants and algae are responsible for a large majority of the photosynthesis going on around the world.

Unfortunately, it has always been more likely than not that a warmer planet would actually reduce the degree to which plants pull carbon out of the air. The problem is that the pores through which plants take in and emit gases are the same ones through which they lose water. Lower rainfall and higher temperatures mean that plants must reduce their rate of photosynthesis, or they will dry out and die. Different plants have adaptations to deal with this, but with global weather patterns changing, and average temperatures rising, many of them are increasingly out of their comfort zones.

While some plants have been taking up more carbon as it has become more abundant, other factors have been pushing the other direction, and uptake as a proportion of atmospheric concentration is declining:

“In this study, by analyzing the best available long-term data from remote sensing and state-of-the-art land-surface models, we have found that since 1982, the global average CFE [CO2 fertilization effect] has decreased steadily from 21 percent to 12 percent per 100 ppm of CO2 in the atmosphere,” said Ben Poulter, study co-author and scientist at NASA’s Goddard Space Flight Center. “In other words, terrestrial ecosystems are becoming less reliable as a temporary climate change mitigator.”

What’s Causing It?

Without this feedback between photosynthesis and elevated atmospheric CO2, Poulter said we would have seen climate change occurring at a much more rapid rate. But scientists have been concerned about how long the CO2 Fertilization Effect could be sustained before other limitations on plant growth kick in.

For instance, while an abundance of CO2 won’t limit growth, a lack of water, nutrients, or sunlight – the other necessary components of photosynthesis — will. To determine why the CFE has been decreasing, the study team took the availability of these other elements into account.

“According to our data, what appears to be happening is that there’s both a moisture limitation as well as a nutrient limitation coming into play,” Poulter said. “In the tropics, there’s often just not enough nitrogen or phosphorus, to sustain photosynthesis, and in the high-latitude temperate and boreal regions, soil moisture is now more limiting than air temperature because of recent warming.”

In effect, climate change is weakening plants’ ability to mitigate further climate change over large areas of the planet.

In a lot of ways, this changes nothing. For those who’ve been paying attention, it has long been clear that saving ourselves and the species on which we rely will require immediate, drastic action from humanity. What this study does provide is confirmation of what we strongly suspected, and underscores the urgency of the situation. Earth’s natural rapid CO2 sinks – terrestrial plants, diffusion into the ocean, and algal photosynthesis – all have limits, both on how fast they can absorb carbon, and on how much they can absorb. They have been providing us with a “cushioning” effect for generations. They’ve been absorbing a lot of the carbon we’ve been digging up and burning. We know this because as the quoted article mentions, the CFE is a known phenomenon, and because the rising acidity of the world’s oceans has been caused by the absorption of a vast amount of CO2 as the relative atmospheric concentration has risen.

Our global climate is a massive system, and it takes a huge amount of energy to “move” it. For over a century now, the organic and inorganic parts of that system have been absorbing staggering amounts of CO2 and heat as we have extracted and burned hundreds of billions of tons of carbon that was buried over hundreds of millions of years. It is no exaggeration to say that what we face right now is the biggest problem in human history. Without the benefit of hindsight, it’s impossible to say if we’ve ever faced a greater risk of extinction, but the literal scale of the problem, and the sheer mass of matter we need to move are unlike anything our species or its immediate ancestors have ever dealt with. The only consolation is that we are also at a point where our technical ability to tackle such problems is at its peak. As I have said, and as I will keep saying, we have everything we need to deal with this problem. What we lack is the organization and structure required to do so.


Despite everything happening in the world right now, life goes on, and I’m still required to spend money in order to live. My work is supported by a group of wonderful people over at patreon.com/oceanoxia, and I would be immeasurably grateful if you would consider joining their ranks. How much you give, and for how long are entirely under your control, and every little bit helps a great deal, as my household is very short on money right now. Thank you for reading, and take care of yourselves.

Comments

  1. says

    “CO2 fertilization” has long been known to be far more complicated than people usually think. Even before the saturation effects you describe, the impact of CO2 enrichment was shown to do things like making seed coats much thicker (thus reducing germination rates), increasing starch content in leaves (thus reducing nutrient value for humans and pests), contributing to structural tissues or defensive compounds. (a great review article on this was published by Fahkri Bazzazz of Harvard in 1992). Plant systems for allocating photosynthates are complex and diverse.

  2. StevoR says

    Yup. Then there’s the small issue of plants inlcuding a lot of species we call weeds and find problematic rather than beneficial to us – something that’s an issue in its own right due to human actions but is being exacerbated here.

  3. klatu says

    Fuck. Whenever I hear anything about ocean acidification I lose hope…

    I really really don’t see humanity stopping global warming before it’s “too late”. By too late I mean a majority of our land masses becoming unfit for human life for a very long time to come.
    Yes, we have the technology and the tools to reverse course. But we simply do not cooperate well enough nor do we think rationally enough. Not where it counts.

    A very pessimistic, slightly drunk, prediction:
    Once enough tipping points have been tipped and enough feedback loops have started, the warming process will accelerate to unmanageable levels. We will achieve an average global temperature in excess of 4°C by the end of the century. Which may well translate to an unstoppable 8-10°C in a few hundred years. At which point our species, along with a majority of life on Earth, is very likely permanently done for.
    But before we kick the bucket, there will be water and food shortages, which combined with an ever expanding population explosion and shrinking habitable zones, can only lead to wars and genocides on an unprecedented scale. Fascism seems to be our go-to response to discontentment, so… yeah: Climate refugees will be shot on sight (or worse) everywhere humans can still live. It will be “homo homini lupus” all over the planet.
    And maybe in a few thousand years, the few surviving humans will eventually expand again, re-invent capitalism and do the whole spiel again.

    I share your values. I just don’t share your optimism(?). Personally, I feel absolutely powerless when it comes to this crisis. I don’t think I’m alone in that. And to be perfectly honest, the constant messages of hope ring false. I wish scientists in particular were less concerned about sounding “alarmist”. People won’t even lift a finger when entire continents are on fire. Maybe because they keep being told that “we can still do something”? Except, as you have noted yourself, individual actions have an extremely low ceiling. But I also don’t see any kind of system change on the horizon. Economic growth is still the predominant world religion.
    In a fucked up way, this current pandemic has arrived at the exact right moment. This could have been a test run. How can we sustain a severely slowed-down economy? How to live without mass transit of people and goods? Are our lives really poorer without so much excess consumption and luxury? Nobody is asking these questions. Instead, most people are clamouring for exotic vacations and a “return to normal”. I’m so glad Christmas is finally over because the constant griping over social distancing was driving me nuts. People are extremly slow to make even the tiniest concessions for a common cause. Too slow to stop the apocalypse, basically.

    And now I’m just rambling.

    Sorry. Not here to rag on you, Abe. You do stellar work and I’m thankful for every one of your posts. I just needed to get my frustration off my chest.

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