How much on a global scale and how do you prove it? Feedback mecahnisms seem very poorly understood. Positive feedback is ASSUMED yet unproven. Could just as easily be negative feedbacks.
The temperature has not been correlated very well at all to CO2 levels in the last 100 years. CO2 levels have been rising in a clean straight line while the temperature has times of cooling and stagnation, punctuated with surges. Comparing a plot of temperature and CO2 levels illustrates that there are plenty of other mechanisms in place.
CO2 isn't the most effective and efficient greenhouse gas, and typically in the past, as the temperature went up, CO2 went up.
The difference is that we are burning fuels that release lots of CO2, and CO2 is a greenhouse gas, regardless of it not being as powerful as methane or even water vapor. And whatever bit of warming the CO2 we add to the atmosphere performs, it has all sorts of effects, such as warming permafrost, which then releases lots of methane currently trapped in the soil. And the methane causes warming, which increases the water vapor in the air, which increases warming, increasing CO2, and or methane, which increases CO2 or water vapor...or other greenhouse gasses, I just use those two as examples.
So, CO2 doesn't have to be the strongest greenhouse gas, and doesn't have to directly drive all warming, as much as the CO2 we add to the atmosphere causes a greenhouse effect, which then causes further warming, in a nice, positive feedback cycle, with other gases causing as much or more warming, which can then increase other gases, and CO2 itself...and round and round we go.
You can keep spitting into the wind if you'd like, but humans are affecting the climate. At best, we're preventing some future ice age that would destroy civilization. If you want to go with that argument, fair enough. Perhaps "plants grow better" with a slightly higher CO2 concentration. But as of right now, increasing the CO2 in the atmosphere without the oceans being able to absorb it all, will lead to more warming. And if the oceans warm considerably enough, you could reasonably expect the CO2 previously absorbed by the ocean to be released too.
And I'm not worried about plants being able to breathe in the new environment, or being uncomfortable about the extra heat. I do worry, though, about the problems it creates around the planet as currently habitats are changed, not necessarily for the better. Adding in extra heat and water isn't going to affect every habitat for the better, and can disrupt the natural flora and fauna that live there. Which affects the humans who live there.
If there is some sort of negative feedback mechanism like the ocean, increasing the amount of CO2 absorption, good, but the positive feedback in re: warming is pretty solid. Increased temperatures mean increased water vapor and methane, which are both substantially more effective greenhouse gasses than CO2, meaning that CO2 doesn't have to directly drive every temperature increase, as it's still effective nudging it along.
And we can't rely on the oceans being able to continue absorbing all of the CO2 we're dumping into the atmosphere. It can't be negotiated with, and it just doesn't give a shit. That is why people are concerned.