I think that we strongly underestimate not only probability of runaway global warming but also fragility of our environment because of anthropic bias.

May be runaway global warming is long overdue in our planet and small human actions could trigger it.

I wrote an article about it several years ago, but I want to rewrite it completely. The current version is here: "Why anthropic principle stops to defend us: observation selection, future rate of natural disasters and fragility of our environment" http://www.slideshare.net/avturchin/why-anthropic-principle-stopped-to-defend-us-observation-selection-and-fragility-of-our-environment