In June 2009, Anthony Watts reposted an article by Lawrence Solomon that pointed out that the Earth is greener than it has been in decades if not centuries.
See also NASA's animations of this Earth (the map of its bio-product), for example the low-resolution one.
In less than 20 years, the "gross primary production" (GPP) quantifying the daily output of the biosphere jumped more than 6%. About 25% of the landmass saw significant increases while only 7% showed significant declines.
Note that the CO2 concentration grows by 1.8 ppm a year, which is about 0.5% a year. It adds up to approximately 10% per 20 years. In other words, the relative increase of the GPP is more than one half of the relative increase of the CO2 concentration. The plants also need solar radiation and other things that haven't increased (or at least not that much) which is why the previous sentence says "one half" and not "the same as".
Because the CO2 concentration in 2100 (around 560 ppm) may be expected to be 50% higher than today (around 385 ppm), it is therefore reasonable to expect that the GPP will be more than 25% higher than it is today. Even by a simple proportionality law, assuming no improvements in the quality, transportation, and efficiency for a whole century, the GPP in 2100 should be able to feed 1.25 * 6.8 = 8.5 billion people, besides other animals.
Of course, in reality, there will be lots of other improvements, so I find it obvious that the Earth will be able to support at least 20 billion people in 2100 if needed. On the other hand, I think that the population will be much smaller than 20 billion, and perhaps closer to those 8.5 billion mentioned previously.
Back to the present: oxygen
Now, in September 2009, Anthony Watts mentions a related piece of work that some Danish researchers just published in Nature:
In two different contexts, they are forced to conclude that an increased concentration of oxygen in the oceans and the atmosphere led to cooling.
The authors say a couple of things about the ice ages that are manifestly incorrect. They say that the oxygen concentration could have been the key driver behind the temperature swings during the glaciation cycles: a higher amount of oxygen allowed the organisms to consume more CO2 and other greenhouse gases that reduced the temperature by a weaker greenhouse effect.
That's clearly incompatible with the fact that the temperature was changing roughly 800 years before the concentration of the greenhouse gases. The temperature variations couldn't have been an effect caused by the greenhouse gases, not even if you try to add oxygen in the sequence of all the correlated phenomena.
However, it's plausible that the oxygen levels influenced the temperature more directly (which consequently influenced the concentrations of trace gases, via outgassing).
A simple additional comment I can make is that the higher concentrations of oxygen may be increasing the albedo (reflectivity) of the oceans and the landmass by adding life forms which may be optically brighter than the dead soil and oceans and/or the life forms that don't need oxygen (or because of another inequality in the energy balance of photosynthesis and/or breathing).
Even if that is the case, it remains largely unknown whether the oxygen variations in the glaciation periods were sufficient to drive the temperatures (I guess that they're not) and even if they were sufficient, it would remain to be seen what was their cause.