Is the Greening of the Planet a Good Thing?

The Cato Institute has a not peer-reviewed article about the greening of the planet, which deniers are hawking. It's as biased as you'd expect, and takes it as unquestionably obvious good that this is good thing:

First of all, it's simply wrong say this result is "the opposite of what the IPCC expected" -- here is an article in Science from 2002 that predicted the high latitude northern greening seen over the prior 20 years. And here is an excerpt from the IPCC 1st Assessment Report (1990), Scientific Asessment, Chapter 10 section 10.2.1.1.1 page 289:

That's hardly all the IPCC has to say about plants under higher CO2 and climate change; it's worth reading the FAR's entire section 10.2 to see all the many issues and questions that were raised even then.

Nor is this CATO study the first; there have been studies before showing this; CO2's fertilization effect is hardly a surprise or unexpected.

But is this greening an obviously good thing? No.

For several reasons:

• greening lowers the albedo of the planet, a positive feedback on global warming. Such changes can have signficant impacts on climate, as Roger Pielke Sr has often pointed out.
• although a given plant may use less water as CO2 increases -- which also has effects on ecosystems, such as changing soil moisture and higher runoff -- more plants require more water, at a time when humans already use more than 50% of the world's complement of freshwater.
• CO2-fertilized plants extract more minerals from soil.
• weeds are plants too, and they compete with crops.
• more CO2 means, of course, higher temperatures, with accompanying changes in precipitaiton patterns, evapotranspiration, soil moisture, etc. that also affect plants. In fact, a 2007 study found evidence that warming is counteracting the CO2-fertilization effect for some major crops: “Global scale climate–crop yield relationships and the impacts of recent warming,” David B Lobell and Christopher B Field, Environ. Res. Lett. 2 (2007) 014002  http://iopscience.iop.org/1748-9326/2/1/014002.
• elevated CO2 reduces the quality of plant tissues: "Elevated CO2 reduces the nitrogen concentration of plant tissues," M. Francesca Cotrufo et al, Global Change Biology, Volume 4, Issue 1, pages 43–54, January 1998  http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2486.1998.00101.x/abstract. This changes the nutritional value of plants, for insect herbivores and for humans.
• The Duke Free-air CO2 enrichment (FACE) experiment found evidence that growth due to enhanced CO2 was limited by nitrogen availability. 

Plants on planet Earth were doing just fine before the Industrial Revolution (except for maybe trees in North America and Europe, which were decimated by man for fuel and building materials.) How they respond to enhanced CO2 is complicated, and hardly the unalloyed good deniers are trying to make it out to be.

(Obligatory snark: You'll notice there are no plants on Venus, where CO2 is 965,000 ppm.)

This 2010 article in Nature is a good summary of all the many factors. It concludes:

Current evidence suggests that that the concentrations of atmospheric CO2 predicted for the year 2100 will have major implications for plant physiology and growth. Under elevated CO2 most plant species show higher rates of photosynthesis, increased growth, decreased water use and lowered tissue concentrations of nitrogen and protein. Rising CO2 over the next century is likely to affect both agricultural production and food quality. The effects of elevated CO2 are not uniform; some species, particularly those that utilize the C4 variant of photosynthesis, show less of a response to elevated CO2 than do other types of plants. Rising CO2 is therefore likely to have complex effects on the growth and composition of natural plant communities.