I cannot think of a single positive impact over the long term. There may be some short term benefits to better crop yields in a few regions, but this is likely to be short lived as the rising heat content of our planet will increase the stress on these crops.
The positive effects are that carbon use creates wealth. As we become more advanced as a civilisation, we need more juice to run, and standards of living increase. China is undergoing an industrial revolution and this is lifting 40,000 households PER MONTH out of poverty and this is a major point of CONCERN for the IPCC because the Chinese are really ramping up how much carbon they use. Well, good, I want Chinese peasants to become Chinese middle class. I want Africans to become as wealthy as westerners are. The biggest threat to human society isn't global warming, its not storms its not sea levels. The biggest threat to us is POVERTY. Carbon use i.e. industrialisation is lifting hundreds of millions of people out of poverty this needs to continue.
Humans have worked for the benefit of humans and *usually* it comes as the expense of other organisms. However, the only one I can think of at the moment is that the use of fertilizers has greatly increases primary productivity in the ocean which causes greater fish populations which of course have been overfished....
They are huge and many, CO2 has already increased plant biomass (even taking deforestation into account) crop production is up, our planet is now greener and healthier.
Now if only the temperature would go up (as promised by AGW) we would have more temperate regions in the Northern Hemisphere, more places to live, more places to farm, more food for our 7 billion people.
Capturing and controlling water to improve the lives of humans.
Greenland becoming Green ONCE again.
P.S. Nothing Mankind can do will raise global temps.
