Source: Mars Society
"Mars now - and what the earth will look like eventually."
[Lovelock] argues that because the earth is already struggling to keep itself cool, our extra increment of heat is particularly dangerous, and he predicts that we will soon see the confluence of several phenomena. ... Some or all of these processes will be enough, Lovelock estimates, to tip the earth into a catastrophically hotter state ... (aus B. McKibben, "How close to catastrophy?", New York Review of Books, Volume 53, Number 18, November 16, 2006)

Flannery estimates that when we take account of other stresses on species imposed by human beings, the alternative scenario will lead to the eventual extinction of 20 percent of today's species, while continuing with business-as-usual will cause 60 percent to become extinct.
['business-as-usual': emissions of CO₂ continue to increase at the current rate for at least fifty years,
'alternative': CO₂ emissions level off this decade, slowly decline for a few decades, and by mid-century decrease rapidly]
(aus J. Hansen, "The Threat to the Planet", New York Review of Books, Volume 53, Number 12, July 13, 2006)