We need to plant more trees, why 100% renewables isn't enough !

Google used to have a team called RE< C focused on investing tens of millions of dollars in to breakthrough clean power technologies (on top of the over $1 billion they'd spent directly on wind and solar). They gave that up over 5 years ago, not because they didn't think it would work, but because they figured out that even if they hit their ambitious goals it wouldn't be enough.

The thing with greenhouse gasses is that they don't make the planet warmer when they get in the atmosphere, they change the atmosphere so that the planet will get warmer and warmer until it hits a new equilibrium. We're over 400ppm CO2 in the atmosphere and we're nowhere near equilibrium. We could switch 100% to renewables today and the planet would still see significant warming for the rest of our lives. Even if we halted every single source of manmade emissions not only would the planet still warm significantly, but more carbon would end up in the atmosphere as things like permafrost melting and releasing methane or warming soils release carbon.

The simple fact is that nothing we do to reduce emissions now, no matter how fast or drastic is a solution. It'll make the eventually problem less bad, but if all we do is transition to renewables we're going to face catastrophic climate change. There's only two things that count as an actual solution, as something we need to do to avoid significant warming:

• Geoengineering - spraying stuff in the air or making the ocean more reflective or something. This stuff is very much a last ditch emergency option. Since we have zero experience doing it, and it would need to be done on a huge scale with unknown side effects.
• Carbon sequestration. We need to take the carbon we've put in the atmosphere and get it back in the ground. People have been talking about this somewhat seriously for 30-40 years, and we're really not any closer to a breakthrough now than we were then. We could hope that in the next 30-40 years we might make a breakthrough, but if we don't it could be too late.

Right now there's really only one solution that's a real option, fortunately it's a "technology" we have a lot of experience with, and if we get started soon it's well within a realistic possibility. We should be reforesting huge amounts of the land that we've cleared over the last 200 years.

Unfortunately there's a few downsides to this:

• It would require people to actually change their life style a little bit. Not a lot, but a little each year to make room for the new forests. Right now it seems like the most people are willing to buy some LED lights and maybe a Prius.
• We're actively doing the opposite. We're cutting down significant tracts of forest every years, and then we're doing things like raising cattle on that land, who will belch out methane for their entire lives.
• From 2000 to 2013 we cut down 7% of the world's forests Mostly to make room for more livestock.
• Carbon in all the world's forests is about 638 GtC
• All manmade carbon emissions add up to about 10 GtC per year
• If instead of spending time and energy to cut down forests, we'd been re-planting forests and had added 7% more over the last 13 years we would've eventually sequestered the equivalent of 50% of human GHG emissions from that period.

If everyone on earth suddenly decided that climate change was important and we started large scale reforestation tomorrow, it's possible that by 2100 we could be back down below current carbon levels in the atmosphere. Essentially, at the extreme end we'd be able to sequester 100% of man made emissions. Here's the possible range of climate change scenarios we're looking at. 

None of those include any significant carbon sequestration projects. Even if we didn't hit that goal of sequestering 100% of emissions, any significant amount would put us towards the bottom, or even below those predictions. Sequestering 50% of emissions is a totally reasonable and possible goal and it would put us in a much better place than even the most optimistic model.