Looking at the CO2 levels in that chart, I got to wondering how uncomfortable humans would be in that 7000ppm period. It turns out, most of us wouldn't even notice (some of the more sensitive people might get a little sleepy, though).
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On a side note, I recently realized that ignoring industrialization, every single living thing on this planet is a net carbon sink (though some less so than others). Plants (and any other photosynthetic organism) absorb CO2 from the atmosphere and produce sugars, cellulose and other hydrocarbons (along with much needed (by non-photosynthetic organisms) O2). Although non-photosynthetic organisms produce CO2 as a metabolic byproduct, they are still net carbon sinks, otherwise they would not grow and/or multiply: the carbon in all those protein chains has to come from somewhere, and that somewhere is indirectly (though food) from the atmosphere.
My conclusion: without human intervention, CO2 will continue being sucked out of the the air to the point where photosynthesis collapses. This will cause massive species die-offs as food supplies dwindle. All that will be left living is fungoids and maybe deep sea dwellers that don't rely on food from the surface. CO2 levels might rise again after that, and photosynthetic organisms might make a comeback.
Save the planet! Produce more CO2! (I'm about 3/5 serious. I'm sure there are flaws in my thinking).