Unfortunately for humans and today’s ecosystems, that concentration needs to be under 350ppm. As we charge full-steam ahead toward and past 400ppm, we become the dominant forcing mechanism to the Earth’s climate system. We have already seen effects from forcing that took place decades ago and they are becoming more frequent, more extreme and more expensive.
December 2010’s CO2 concentration was measured at 389.69ppm.
2010’s average CO2 concentration was 389.78ppm. 2010 will be the last year in which the annual concentration was below 390ppm for quite some time. Even if greenhouse pollution were to magically stop tomorrow, the concentration would continue to rise for years. Getting the concentration to decline will require WWII-era efforts around the globe. The longer we wait, the longer it will be before the climate returns to the “normal” conditions found throughout our species’ existence.