Just for fun: Here's a tongue-in-check "example" of a negative feedback from ecoEnquirer.com, a site run by Roy Spencer. A good-natured article about the site is here.

[M]any people are now also having to deal with another sinister side effect of the extra CO2: plants are loving it. The extra plant "food" has caused a worldwide greening of the Earth, with vegetation sprouting to life in the spring weeks before it is supposed to. This unnatural disruption of Mother Nature's busy schedule has even caused some tropical species to expand their habitats poleward.

This worldwide greening has led to an unexpected problem for motorists in many countries. People are increasingly finding their cars, trucks, and SUVs covered in vegetation that has grown overnight into tangled masses that engulf their vehicles.