I was unaware of this petition. It may take me awhile to look into it. I'll reply when I am satisfied that I understand it.
Carefully read the supporting article in the Economist. You'll see there are a lot of assumptions made. Will there be enough land to support 11 billion people? How firm is that estimate? What will be the effect on biodivirsity of such a large population? Forest cover is said to have decreased radically in one part of the article but not decreased in another part. Long-term data are used in one part and short-term in another part, depending on what sort supports the argument the authors are making. Greenhouse gases are a problem. The extent of it and its best solution are disputed. And so. And that's just my impression, as a non-specialist, on first reading.
In this article Dr. Seitz is cited as saying the IPCC altered it's report to, falsely, indicate a consensus of opinion. I remember reading somewhere that the IPCC denied this charge and cited evidence to the contrary..
Take a look at this month's Scientific American. They review the work of another distinguished Global Warming opponent - rather unfavorably.
I don't know who's correct. I just cite these as examples of why it's so difficult for non-specialists to make sense of it all.