What's realistic and what's not

Climate experts debate strategies for reducing atmospheric carbon and future warming

There's some discussion by some Cornell guys as to whether CO2 at 350, 450 or 500 ppm is "realistic."

Given that we're already well above 350ppm, that figure isn't coming back anytime soon:

Even if all CO2 emissions were to stop today, the gas already in the atmosphere would stay there for another century or two, maintaining warmth. But activists need to set firm goals.

"It's the best political strategy," Wolfe said of the 350 ppm goal. "If we allow slack, it will never happen."

Seems a silly suggestion, really. But is the planet already committed to a 2 degree rise?:

Part of the problem are delayed effects that have already committed the planet to warming on the order of 2.4 degrees Celsius (4.3 degrees Fahrenheit) by the end of the century, regardless of reductions in greenhouse gas emissions from today's levels. For example, as the ocean warms, it stores the heat and very slowly releases it to the atmosphere, creating a lag time in temperature equilibrium between the atmosphere and the ocean. Furthermore, due the ocean's great mass and heat capacity, it will take 1,000 years to reverse this century's warming and gradually reduce the heat already building up in the ocean, said Greene. Also, as pollution abatement strategies kick in this century, aerosols that now cool the atmosphere will decline, adding to warmth.

But, Greene added, the goal of 350 ppm can be reached and a calamitous warming halted if governments finance geo-engineering strategies that pull CO2 from the air and store it in the Earth.

For example, Greene and others advocate research to try to scale up simple machines already devised that draw CO2 from the atmosphere and then find ways to pump the gas into underground geological formations.

This (mechanically extracting enough CO2 from the atmosphere) sounds an extremely improbable solution to me.