because increased CO2 will lead to increased plant growth, the build-up of CO2 must therefore be a net benefit rather than a net harm to our societyDoes this type of argument make sense? To understand why it doesn't, realize that in any environmental disaster, some groups always benefit. Consider the destruction of New Orleans by Katrina. Few people consider this event to be a net benefit to our society. However, it is easy to pick out small groups that greatly benefitted from it. The owners of demolition companies, construction companies, construction material companies, etc., are all making money hand over fist.
Thus, one could make the argument:
Katrina's destruction of New Orleans was a highly beneficial event because it greatly stimulated the construction industry on the Gulf Coast.This is, of course, extremely misleading and most people would agree is wrong. While some benefitted from the destruction of New Orleans, the net result, considering all harms and benefits, was clearly negative.
So when people argue that climate change must be good because plants grow better at higher CO2, think about the Katrina example. Plants might indeed do better(*), but that tells us nothing about all of the other harms and benefits. As I discussed in a previous post:
Clearly, some people will benefit from warming, while others will suffer. There are a lot of dimensions to this (economic, moral, etc.), but I'll just give the broadest answer. For small warming (e.g., 1 deg C over the next 100 years), current thinking is that harms and benefits are largely comparable, although it is estimated that harms still outweigh the benefits. As the warming increases, the harms get much bigger, and begin dominating over benefits somewhere around 2-3 deg C of warming. That's why 2-3 deg C is often referred to as a tipping point or threshold for dangerous anthropogenic interference. Warmings much greater, say 5 deg C, would be a calamity of Biblical proportions ... real Wrath of God stuff.Make no mistake. Estimates of harms and benefits are highly uncertain. However, there is clearly a significant risk of serious net harms over the next century as the climate warms. People arguing that there is no risk of serious harms are either dishonest or disconnected from reality.
* For the record: While most plants do grow better as atmospheric CO2 increases, there's dispute about whether this means that plants will grow better under global warming scenarios. While CO2 is definitely going up, which will help plant productivity, there are other changes that might not benefit plants, like changes in precipitation patterns, soil moisture, migration of invasive species, etc. Overall, it's unclear how much plant productivity will actually increase.