Not trying to be a skeptic for its own sake, but its worth noting that, buried deep in the article, you can find reference to an "unorthodox publishing practice" -- it hasn't yet been peer reviewed.
As a layperson I don't know what I'm supposed to make of these results. It looks like a bunch of prestigious authors held a press conference to convince non-specialist reporters that the end of civilization is a distinct possibility. But I know from my own research that you can convince the general public of anything as long as you can give a good presentation. Famous names and bold statements don't necessarily equal good science.
Maybe the sixteen co-authors could have...oh, wait what?
The thing you have to remember is that this is all speculation. If they're right, 16 authors can say they were the founders of a new model. If they're wrong... Well there's always another model to publish. The fact that a bunch of friends and colleagues can agree on something doesn't mean there is a wider consensus.
The arXiv is not mainstream in may fields, so they may not have thought of putting it there. I am sure they would have figured something out, but glacncing at the subject areas on the main page, it isn't immediately obvious where a study such as their should go. Physics/Nonlinear science, Statistics? Occasionally a meta-climate study appears in the astrophysics listings, but those are generally papers studying any interplay between astrophysical events and (often historical) climate change.
It would be great if more fields used arXiv. Some do have their own similar resources, separate from arXiv (the cryptography community comes to mind). But I am not aware if there are any associated with climate research.
If this were some no name scientist then you're point would be taken. But James Hansen - that's pretty authoritative. Regardless of where he publishes, people should pay attention.
>"The process of peer review and publication in the interactive scientific journal Atmospheric Chemistry and Physics (ACP) differs from traditional scientific journals. It is a two-stage process involving the scientific discussion forum Atmospheric Chemistry and Physics Discussions (ACPD), and it has been designed to utilize the full potential of the Internet to foster scientific discussion and enable rapid publication of scientific papers."
I'm not sure how well respected or wide-spread this "interactive" peer review process is, or if the study can also be submitted for more "traditional" peer review in the future.