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Rising CO2 won't make trees grow more, study suggests (cbc.ca)
73 points by pseudolus 41 days ago | hide | past | web | favorite | 56 comments

A number of problems with this method of study, and the headline is misleading.

The real question is whether there will be more biomass and more photosynthesis in response to more CO2, not whether individual trees grow more. The climate skeptics don't say tree trunks will get wider, they say there will be more plant biomass.

And also this is adding in confounding variables and saying more CO2 could change precipitation in certain places, and thus cause lower growth. Other studies that show more growth with more CO2 are using greenhouses to control for confounding variables.

And even if you are trying to answer the question in terms of a holistic analysis, you can't just study one forest in Canada and then make any kind of conclusion.

You'd need to study trees from most of the major forests, and look at the density of the foliage in each.

> The climate skeptics don't say tree trunks will get wider, they say there will be more plant biomass.

Speculating that plant biomass might help mitigate global warming doesn't really deserve the label "climate skeptic." It's already confirmed that rising CO2 affects crop growth rates.

What do you mean by mitigate global warming? I think what they argue is that there will be winners and losers, and it isn't some kind of black and white issue.

For instance, the Saharan desert seems to be greening.

> For instance, the Saharan desert seems to be greening.

I hadn't heard about that, and my first few Googles came back without any good, definitive articles; do you have more information?

It's not. It's a false claim from climate skeptics based on this study:


The study is about sub saharan Africa, mostly the African savanna, and the identified drivers are human causes (decline in burned area, changes in herbivores density). They explicitly exclude CO2 levels increase as a driver.

>Factorial simulations with multiple global ecosystem models suggest that CO2 fertilization effects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%).


I acknowledge that CO2 contributes to fertilisation. I was refuting the claim about the saharan desert. Your new link doesn't seem to contradict me :

> CO2 fertilization effects explain most of the greening trends in the tropics

Yes, I accept your correction. I accept that the Saharan desert may be greening primarily for other reasons. It seems to have been on a long term trend towards greening irrespective of the Industrial Revolution, and there are other human activities that are greening it. Although, your link does leave open the possibility that rising CO2 explains up to 22% of the greening in the Sahara.

Incidentally the second (earlier) paper is cited by the first (newer) paper, so hopefully they are not in contradiction (since it was merely referenced and not responded to).

'mitigate' in this case means that some extra carbon gets captured, slowing whatever happens down.

It's unrelated to the specific positives and negatives.

Several of the reports I've seen have said we will get more biomass, but most of it will be plants we generally refer to as weeds.

"It is estimated that between 50% and 85% of the world's oxygen is produced via phytoplankton photosynthesis." Most 'plants' are in the water.

source: https://en.wikipedia.org/wiki/Phytoplankton

Makes sense, considering that most of the plants we have already match that description.

Most of the plants in residential areas, sure (but only if you think grass is a weed).

Not so much in reasonably well-run parks or woodlands.

This is what I dislike about the politically driven climate "science" or any science corrupted by politics. So much of it is intentionally misleading and many times outright false. They manipulate data, cherrypick time frames or simply outright lie.

The problem is exacerbated by the fact that special interest groups fund these misleading studies and are widely propagated by media with a particular interest.

Sounds like it's a bit more nuanced than the headline would suggest. More CO₂ only means more growth when CO₂ is the limiting factor on growth in the first place. Water, nutrients, and sunlight all play a role as well.

Bingo. While not usually the limiting factor in nature, it can still be a benefit when it is. Just ask all the 1000s of pot growers literally burning propane to produce it indoors and vent it to atmosphere. Grr.

I did not know they did that to produce CO2. Very interesting!

Is it ever the limiting factor though? I don't expect to see more animals if the O2 content of the air rises, except perhaps at high altitudes.

I'll point out that the sunniest parts of western Quebec get fewer than 1900 hours of sunshine annually. [1] That's fewer than any point in the contiguous US. Cedars metabolize O2 -- not CO2 -- in the absence of sunlight. That makes this possibly one of the most biased and possibly misleading selections possible for both location and organism under study.

[1] https://www.currentresults.com/Weather/Canada/Quebec/sunshin...

Anecdote: When I artificially inflated the CO2 in my aquarium, my plants grew like BANANAS. Something like 4x the speed, and about 2x as big leaves, etc. It was quite something amazing.

In terms of geologic history the CO2 levels now are lower then they have been. It's not surprising that plants respond very positively to higher rates of CO2.

And it's been proven to be true many times.

I guess what the study is trying to address is the positive aspect of CO2 rising enough to counter the negative aspect of CO2 rising to the plant biomass.

I’d be curious how they explain the fact that greenhouses typically add co2 for faster growth?

( disclaimer I believe in global warming with all my heart)

Greenhouses are controlled environments where everything else is abundant too. In more natural environments, plants are limited by availability of water and nutrients in the soil.

On top of that, plant growth can be reduced by heat stress. They also have to contend with insects and disease, and may not have good defenses when climate change has brought invasive organisms to the area.

Sure, but what that suggests is that the existing established understanding is correct (or at least still plausible): even if the particular trees studied here aren't CO2-limited and don't grow more with more CO2, total plant biomass will or at least might (outside of climate change denial fantasies this doesn't mean increases in CO2 will automatically be neutralized by plant growth, because it doesn't imply that the size of the effect is necessarily sufficient to clear the additional CO2 once long-term equilibrium is reached, doesn't tell you how long it takes plants to reach the new equilibrium when CO2 levels have reached a new plateau, and even if it did all that wouldn't help when CO2 levels are constantly increasing and plant growth can't catch up.)

Sure, if everything else is ideal then many plants will grow more with more CO2. Even this study's authors don't deny that; they say that other factors will often not be ideal:

"climate change is generating warmer, drier conditions that could make them grow less in many places"

Overall, conditions are definitely getting warmer on average, and many places are also getting drier, sometimes with occasional very heavy rainfalls that wash away topsoil.

Another factor I didn't mention is forest fires, brought about by drier conditions, as we've been seeing in California.

I'm not taking issue with the study or it's authors, just the presentation of the article, which very heavily spins it, especially up front, as contradicting the existing understanding broadly, when it really doesn't.

It's important, because the fertilizer for anti-science attitudes exploited by groups like climate denialists is a false impression of radical flip-flops in science conveyed by popular media which uses exaggerated contradiction to overplay the impact of stories and draw eyeballs.

Based no my limited knowledge in this field, I believe that is because it is an enclosed and controlled system. You can increase all variables (CO2, water, nutrients) together; whereas, a plant in "the wild" cannot. Most things act more conservatively in Nature versus human controlled environments.

Some people should stop pretending that biology is easy and straightforward. Growing faster means taller, not necessarily wider.

Maybe it doesn't speed up cedar tree growth but it certainly affects algae and fern growth.


Plants may grow more easily, also wildfires may burn more easily, areas may get more water, also areas may get less water. Areas may be hotter, but also areas may be colder. The specific results that humanity gets depends on the particular weather patterns that we end up with, which are very difficult to predict, and thus the future holds a great many studies and predictions, most of which will be wrong in their specific results.

This is a subject of interest to scientists, but not to the general public, because nothing being studied here changes the central message in the slightest, which is that we are undoubtedly headed for climate apocalypse and we are accelerating into it rather than decelerating.

Well as far as Dan Britt's talk goes, trees haven't noticeably been adjusting for thousands of years.


Study means little as it takes a specific forest in Canada. We should probably look at Amazonian forests, California's redwoods, India's Vindhyas and Namibia's rainforests too.

The basics of photosynthesis are more or less identical across green plants. All the cool evolutionary shit has been going on in preserving water and capturing light, pointing at CO2 probably not being a limiting factor.

Even the one neat CO2 trick I remember was really a hack to preserve water: some desert plants have a mechanism to store a day’s worth of CO2, allowing them to close pores and prevent evaporation.

Anyway, if you’re looking for a counterexample, I’d switch environments altogether and try again in bacteria, algae, and maybe grasses. While I doubt there would be much of a difference to trees, the first two are at least a few degrees further away from them in the deity’s org chart. All three categories also strike me as possibly being more relevant than trees in terms of total biomass, although that’s just a hunch that may be completely wrong.

Different groups of green plants have significantly different photosynthetic cycles (notably C3, C4, and CAM) which respond very differently to changes in CO2 concentration. For C4 plants (about 3% of plants) the current CO2 concentration is at the point of saturation. C3 plants (85% of plant species, including most trees and grasses, and staple crops such as wheat) have rates of photosynthesis proportional to CO2 concentration to about 1000ppm or so. (The desert plants you mentioned use CAM cycle, but overall their contribution to CO2 consumption is small.)

What is the right amount of CO2 to have in the atmosphere scientifically speaking?

Depends on what your utility function is.

If it's "as close to the nature right before apes became humans", it's easy to answer (though hard to justify on any grounds other than quasi-religous belief in Gaia or something). If it's "planet with more plants and animals" it could be quite different, and any honest scientist would answer it "we don't really know, but here are some considerations..."

An amount similar to levels before human impact.

There have been times "before human impact" when atmospheric CO2 was _way_ higher than it is now, though. You'll have to be more specific.

Non sequitur.

Do plants in cities grow faster than plants in rural areas?

Cities tend to have more air pollution, but the difference in CO2 and O2 are neglible.

I once ran the numbers for O2, which is proportional, and it came out to roughly 10m of high in the atthmosphere. I. e. climb a tree in the countryside and you’re at O2 pressure similar to a city.

I’ve heard this argument before from climate change deniers. From a laypersons point of view I wonder why would you assume more CO2 makes trees grow more? More oxygen doesn’t make humans grow more, in fact it becomes toxic.

We "assume" that because we can demonstrate it to happen. Place a tree in an enclosure, increase the CO2 level but otherwise keep things the same as outside the enclosure. Trees within the enclosure grows faster than the ones outside the enclosure.

This is not surprising, as we already know trees and plants consume CO2 to produce cellulose and other plant materials.

This is heavily exploited in greenhouses which are CO2 enrichened to produce better yields - albeit used mostly for growing vegetables, not trees.

Apparently more oxygen does make bugs grow larger.


A better analogy is CO2 in plants vs. food in humans. And more food definitely makes humans grow more.

Trees are made of carbon, humans are not made of oxygen (humans are also made of carbon).

By mass, trees contain more oxygen than carbon. So do humans. Water is 89% oxygen by mass. Cellulose is 49% oxygen by mass. Dry wood contains a bit more carbon than oxygen, but living trees do not.

Putting a lot of shit around the foot of a plant can make it grow faster and better. Not so much humans. I understand your point but your argument is terrible.

It's also so trivially easy to refute. If CO2 was a significant stimulant for plant growth, they'd be sucking it out of the air as fast as we were putting it out, and the whole issue would be moot.

I'm not sure that's a good argument. It's entirely plausible that plants need CO2 to exist at a particular partial pressure to absorb it. In the same way people wouldn't drag the O2 level in a sealed room down to 0%.

Humans can't bring the level near 0% because they would die long before that. Humans need between 19% and 24% O2. Below 19%, humans can't exert much energy without passing out. Above 24% increases damage from free radicals.


then again the original comment is also in either bad faith or ignorance since he's comparing humans to plants when they have completely different metabolic systems.

The headline is clearly wrong. Maybe they should have said "current trees". The tree tech has already been invented to circumnavigate present restrictions in tree biological restrictions. Very clickbait-y.

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