First of all, we know very well that CO2 increases planetary temperatures. For it not to do so, it would have to act differently in Earth's atmosphere than it does in a laboratory, or in Venus's atmosphere for that matter.
You are missing my point. The original poster was in effect arguing that if we don't know *everything*, we essentially know *nothing*. My point is that much of the limitations of our knowledge have to do with precision. What *precisely* will happen if global temperatures increase by, say, 0.8 degrees? It's unknowable *precisely* until it happens. By "precisely" I mean exactly what will happen in every region of the Earth. It's unreasonable to expect a scientific theory to predict *everything*. But one can predict some things, and one can certainly paint a pretty accurate "net" picture well before you can paint a finely detailed one.
As for some of the effects of warming being positive, I'll go further than you do. It will almost certainly include some positive effects, by which I mean effects that will benefit *some* people. But it will also include some effects that are *negative*. If you spent much time in nature studying it, you'd know that the bulk of effects will be unfortunate. It's not because of warmer temperatures per se; it would be true of rapidly cooling temperatures as well. It's rapid widespread change *in itself* that's a problem for the environment, not necessarily the direction of that change.
Life adapts to change; a very gradual warming would only move habitats around, on average to the north in latitude and to higher elevations. The problem with rapid change is that few species can move as rapidly has humans; in fact the differences in adaptability tip the balance of power toward weeds and pests.
For example there's a large grove of magnificent Canadian Hemlocks (Tsuga canadensis) a short walk from my house, many of them well over 30m in height. The milder winters we've been having over the last twenty years isn't a direct problem for these trees, many of which are centuries old and have seen many a mild winter. But a long string of mild winters favors a tiny insect called the wooly adelgid. The adelgid population has exploded after twenty years of unusually warm winters, and the number of them is astonishing. Quite literally every inch of the underside foliage on those giant trees is covered with adelgids.
Twenty years ago you could walk through this grove, look up and see hardly any sky. In mid-summer it was like walking into a refrigerator. Sometimes snow would persist on the ground there until early June. Today the sky is open leading to a weed choked understory where there used to be open old-growth forest floor. At this rate ten or twenty years this grove will be dead, as a secondary result of climate change. It's not the heat that will kill the grove, it's the change in range where it's safe from predation. The predator population can cross the continent in a few years, but it'll take thousands of years for a new grove to become established somewhere else.
Now hemlocks aren't going to go extinct. They'll just become very, very rare, like the American Chestnut. I've never seen an American Chestnut outside of an arboretum, but it was once the most common tree in North America. In its place we have millions of acres of crummy Norway Maples, which will likely replace the great hemlocks of this grove. If current warming trends continue we will see the emergence of larger, more uniform habitats, dominated by weed species.