&#62; The question is: by how much can we alter the planet's ecosystem before it is unsafe? The IPCC's models predict about a 2C temperature rise in the 2090s from the 1990s if we have 450 ppm in the atmosphere. That temperature rise is further predicted to be the point we had better not go past, to avoid more severe damage.<p>What definition of "unsafe" are we using?  What effects are you assuming?  (Ice has been growing in the artic since 2007.)  (I can't find my reference to the non-existent sea-level increases....)<p>As many have asked, what is the "correct" temperature?  Is it the one during the medieval warming period?  The one during the depths of the little ice age?  Now?  1934?<p>We know that Earth has had considerably higher CO2 concentrations in the past.  <a href="http://en.wikipedia.org/wiki/File:Phanerozoic_Carbon_Dioxide.png" rel="nofollow">http://en.wikipedia.org/wiki/File:Phanerozoic_Carbon_Dioxide...</a> (from <a href="http://en.wikipedia.org/wiki/Paleoclimatology" rel="nofollow">http://en.wikipedia.org/wiki/Paleoclimatology</a> )