An interesting article in Grist.  How do you get the CO2 coming out of our tailpipes and smokestacks back out of the air once it's been mixed together?  Scientists and entrepeneurs are looking for a majic techno fix that does this.  The main problem is that it takes huge amounts of energy, and with our present energy mix, where would that energy come from?  Burning more fuel, producing more CO2.

The key to unscrambling the CO2 from the air is photosynthesis, plants supplying the titanic amount of solar derived photochemical energy.  But methane is 21 times worse as a GHG than CO2.

So if an amount of methane  eqivalent to 5% of our CO2 emissions, could be eliminated, the human carbon footprint would go to zero.  Offset by the methane emission prevented.

 But as you say, just getting to zero won't stop GHG climate change.  By using the methane (from biogas digestors running on manure, garbage, sewage, and waste biomass) to backup a renewable power grid powering geo heat exchange heating/cooling (36% of GHG comes from heating/cooling buildings) and plugin hybrid vehicles; most CO2 emissions can be eliminated.  That sends GHG on a downward trend.

Then by using organic fertilizer from the biodigestion process the carbon sink activity of farm land can be restored, photosynthesis will then "unscrmble" the atmosphere.  Prairie stores 1.8 tons of CO2 per year per acre.

It all starts with recycling waste with  biodigestion.  A well understood, profitable process.  Subsidizing distributed generation using farm biogas and using it as a tractor, train, and truck fuel could get us past the 5% figure in a few years.

It's the quickest techno fix available.  Branson ought to award his prize today, to a foundation that applies the 25 million prize for GHG elimination to actual projects around the world.   Barack should make frequent campaign stops at farm biogas projects to highlight this subsidy deserving technology.

10 cents for every kwh of energy provided to the grid should come out of exxonmob's pocket (the oil industry gets 13 billion in subsidies per year) and go straight to farmers using these biogas energy systems.

How long would it take to get to 5% of CO2 emissions given this sort of biogas gold rush?  A few years.  Especially with diesel prices bankrupting farmers, biogas is a great substitute.  farm economy saved!  Without increasing food prices toconsumers.  In fact revenue from biogas would save failing family organic (and other) farms.