"the only way to achieve that is gradual moving back to agricultural society, which allows overall rate of photosynthesis to be higher than natural emmision of CO2."

There is another way.

How can the greenhouse effect be reversed back to pre-industrial levels?

Right now conservation reserve land in the US absorbs 1/3 of our CO2 emmisions. Expand that effect with new conservation land, like a Prairie National Park encompassing 1000s of square miles of northern prairie that doubles as a site for 1000s of very large scale 1000 foot wind generators.

That could absorb 2/3rds of present cO2 production. Then cut CO2 with plugin hybrids that use 1/10nth of the fuel we now use. Use distributewd renewable energy generation and storage to replace fossil and nuclear power plants.

Global climate disaster is reversible in a 20 year time span if thease steps are taken to actually reduce the amont of greenhouse gases.

Turning fertilizer and manure and other waste runnoff into biogas that is used in fuel cell/turbine power plants would eliminate a huge amount of methane, 20 times worse as a GHG, from the ecosystem. This extra nitrogen runnoff acting on lake and river sediment is releasing this methane now, with a huge greenhouse gas effect.

Then the cO2 from the fuel cell power plant is used to grow algae in solar collectors to produce biodiesel that runs in fuel cell backup generators in electric cars and powdered cellulose that runs in the fuel cell power plant.

And here's another huge effect that has so far gone unrecognized. Each vehicle containing a fuel cell/microturbine backup generator of around 20 kw can supply power for 30 homes when it is plugged into the grid and into a natural gas or biogas source.

100s of millions of these cars would solve the problem of storage AND backup generation for variable renewable energy sources like wind, wave, and solar power. A gas line could plug into your car at home or work, then another line could take the cO2 emmited back to a solar collector algae system for sequestration.

100 million cars x 20 kw generating capacity per car= 2 million mw of generating capacity. Present generating capacity in the uS is around 600,000 mw. So even 100 million of these cars could generate over three times the present electric power we now use. At 3 to 5 time present efficiency levels.

At the same time this plan would eliminate over 80% of present cO2 emmisions plus a yet to be calculated huge amount of methane emmissuons due to nitrogen runnoff from waste.

This along with the increase in conservation land could reverse global climate disaster within 20 years. The other alternative, the status quo, will stop the Gulf Stream conveyor and plunge the uS and europe into an ice age. If WW3 over oil with terrorists using nuclear weapons doesn't get us first.

A distinct possibility given nuclear proliferation combined with petrodollars that support terrorism.

http://thefraserdomain.typepad.com/energy/2006/12/teacher_saves_t.html#comment-26079953

No new coal plants are needed and this proves it. Conservation can more than free up enough capacity to power plugin cars.

Adsorption cooling works well in really hot climates with a lot of solar energy, but in most places only geothermal cooling using circulating pumps is needed. In cloudy hot humid climates where ground cooling or adsorption cooling won't do the job, heat pumps that dump heat to the ground are the best way to reduce cooling load.

That solar cogeneration of electricity and heat is the key. Add in solar collector algae biofuel and fuel cell/turbine replacement for coal and natural gas power plants and burning coal and gas as backup for wind and solar becomes unecessary.

Biogas from manure, farm, human, and landfill waste used to power fuel cell/turbines at 75%, that's the first step. That prevents the release of methane from manure and chemical fertilizer runnoff (organic fertilizer that builds the soil is a byproduct of biogas digestors), and methane is 20 times worse than CO2 as a greenhouse gas. That huge reduction of methane release from lake and river sediment and landfills makes up for 20 times greater cO2 releases.

That cO2 is then sent into algae solar collectors which produce biodiesl for vehicles and powdered dry algae to send through the fuel cell. The biodiesl CO2 release has already been offset many times over by cutting methane release.

And the biogas adds enough CO2 into the algae system to make up for cO2 lost from the cycle from the biodiesl used in cars.

Lastly the fuel cell/turbine runs on natural gas, coal, or algae filtered from fertilizer polluted lakes and rivers. Helping to clean the present fertilier runnoff out of the ecosystem.

That provides for a transition to all renewable energy as solar collector area and biogas digestor capacity is expanded. Until then cO2 from present coal and natural gas plants can be captured by solar collector algae biofuel cogeneration, that also produces electricity and heat.

The problem of fertlizer and manure runnoff into lakes and rivers is huge, largely unrecognized, but easily mitigated. I wonder if any estimates of the percentage of GHG production caused by this nitrogen pollution is available?

New parameters may be needed to estimate the effect of biogas digestion on climate change.

75% renewable energy use in 10 years, 100% in 20 years. That's a better goal.

The devestation from GHG caused climate disaster without that better performance may inundate every major coastal city on earth. And the melt loss of the arctic ice impelled  water conveyor that powers the gulf stream will plunge the US and Europe into an ice age.

That will cost orders of magnitude more than this more ambitious renewable energy goal. In fact the energy revolution will pay its own way, and even more, in economic success. Jobs, tax base, and lowered costs from climate related damage.