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Dr. Craig Idso claims

increased levels of CO2 reduce the negative effects of a number of plant stresses including: high salinity, low light, high and low temperatures, insufficient water, air pollution, and protects against herbivores i.e. being eaten by animals and insects.

Notably, Dr. Idso emphasized that the increased CO2 levels allow plants to produce the same amount of crop yield with less water. Moreover, plants are able to grow in dry areas where it had been previously too dry to exist. A collateral benefit to this is that the increased vegetation reduces the effects of soil erosion.

Is that true?

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    A rebuttal to this can be found [here](http://www.skepticalscience.com/co2-plant-food.htm), along with links to several studies. Basically, CO2 *does* increase size in most plants, but there are other side effects that decrease overall well-being. – Is Begot Jul 10 '14 at 15:07
  • @Geobits Thank you Geobits, but that seems to confirm some of the claims, aside from some digressions about temperature and ozone. I don't think I saw any discussion about stress or reduced water requirements, only size. Still, thank you! –  Jul 10 '14 at 15:16
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    @Gracchus, the SkS article does give an explicit counter example regarding insect pest stress that is apparently exacerbated by additional CO2. The article clearly implies that increased CO2 is a net benefit to global crop production, which certainly isn't likely to be the case, given the evidence in the SkS article. –  Jul 10 '14 at 15:28

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"CO2 is plant food" is a fairly common skeptic argument (this answer is adapted from the SkepticalScience article on that topic). The basic point is that additional CO2 is good for plants, provided they are in an environment where CO2 is a limiting factor in growth, i.e. they already have adequate water, sunlight, nutrients etc. This is why additional CO2 is sometimes introduced into greenhouses, where watering, sunlight and feeding are already well optimized. However outside greenhouses, CO2 is not generally a limiting factor. Quite often the low availability of water (e.g. due to drought) or poor soils (due to over-farming) is what prevents productivity, and adding extra CO2 is of little or no use to plants. CO2 however, is a greenhouse gas, and can be expected to produce warming, which may have a negative effect on water availability, and for some plants temperature is a limit on where they can grow. Simplistic arguments are generally unlikely to be true on this one as there are many factors that are involved.

As an example where Dr Idso's argument clearly is not the case, consider this study:

Insects Take A Bigger Bite Out Of Plants In A Higher Carbon Dioxide World

Date: March 25, 2008

Source: University of Illinois at Urbana-Champaign

Summary: Atmospheric carbon dioxide levels are rising at an alarming rate, and new research indicates that soybean plant defenses go down as carbon dioxide goes up. Elevated carbon dioxide impairs a key component of the plant's defenses against leaf-eating insects, according to a new article.

There are references to other studies at the SkS article mentioned above (especially the advanced version).

The problem is that you can always cherry pick studies to support a point of view, but the best course of action depends on taking a broader view of the scientific literature, and consider what is likely to happen in the real-world biosphere (i.e. you need to consider the local limiting factors around the world on a family-by-family basis). The IPCC WG2 report is probably a good place to start. I'm much more familiar with WG1 issues, rather than WG2, but this article suggests that the IPCC do not agree with Dr Idso.

I should however point out that Dr Idso is presenting this work at a conference for climate skeptics only, funded by the Heartland Institute (a political lobbying organisation). It isn't the sort of place he is likely to get a serious scientific peer review of his work that will point out the errors.

  • Thank you Dikran Marsupial! It's clear high CO2 plants of the type in the study suffer more attacks by pests, but did it come to any conclusions as to overall health? It may have produced less cumulative or per volume defensive hormones, but it didn't say if high CO2 plants were on net larger & stronger or not than the low CO2 plants. Thank you so much in advance! –  Jul 10 '14 at 16:09
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    Individual papers are unlikely to come to a conclusion on overall health as science generally investigates depth first, rather than breadth. For the big picture you need to look at surveys and reports (such as the IPCC WG2). The point is that while the primary effects of CO2 may be a benefit to some plants, the indirect effects (temperature, humidity etc.) are likely to mean that crop production is harmed overall. As usual, the IPCC is a good place to check out skeptic claims. –  Jul 10 '14 at 16:38
  • If that paper is inconclusive to overall health, why offer it as evidence of decreased well-being? The only valid conclusion it can make is that higher CO2 levels produce healthier beetles dependent upon those soybeans, and those soybeans do not produce hormones associated with defense. It could still be argued from those isolated facts that higher CO2 concentrations are beneficial to plant well-being because it supports dependent life, and the plants are so healthy that they find no need to produce defensive hormones, possibly out of necessity to cull dying leaves. –  Jul 10 '14 at 21:08
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    If cherry-picking is an issue it applies to both sides of the argument. The Science Daily story, for example, is both cherry-picking and (if the description is accurate) not a particularly well-done experiment. Yes, things are complicated and the simple argument given by some skeptics is bollocks, but CO2 *is* good for plants and to address the question of whether other effects cancel it out we need rather better analysis than this experiment. – matt_black Jul 10 '14 at 21:28
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    @Gracchus and matt_black. My answer clearly states that the ScienceDaily article is a counter-example against Idso's claims, that show that he is at best cherry picking. I didn't say that this one paper was a definitive answer, and instead directed you to the IPCC WG2 report. I specifically said not to look at isolated facts, but to look at a survey of the literature. It is dissapointing that both of you have ignored the key point and instead fixated on the Science Daily story. –  Jul 11 '14 at 06:33
  • Note that I again referred you to the IPCC report in my initial comment above. You can find it here: http://ipcc-wg2.gov/AR5/report/final-drafts/ –  Jul 11 '14 at 06:43