This is the time of year when the preponderance of the plant life on the planet is dead or dormant -- most of it being in the northern hemispheric, because that's where most of the land is -- so the plant community isn't breathing much, so atmospheric CO2 levels are rising. And at Mauna Loa (MLO) they just hit a new high and crossed over 395 ppm for the first time:
The level will continue to rise until late May, though the exact amount varies, from this point in the year, from about 0.5 to 1.5 ppm.
So when will MLO CO2 first top 400 ppm? I suspect climate activist groups are already planning a huge campaign to mark the occasion -- it would be foolish for an activist not to.
The slope of the linear trend in the above graph is 1.88 ppm/yr, so assuming this year tops out at about 396 ppm, 400 ppm should be reached in either 2014 or 2015.
Topping 400 ppm will be possible in 2014, but not a sure thing, especially if this year tops out higher than 396 ppm. But to be safe that's what the 350.org's of the world need to plan for, though they might have to put their media package on hold in June 2014 and save it for the next year, when "400" will definitely happen.
(Yes, surely there will be some CO2 monitors somewhere in the world that will reach 400 ppm earlier, and probably some near strong CO2 sources already have. But Mauna Loa is the most famous monitoring site, so it will be the one that really matters.)