[erikpukinskis]

I would rather we strengthen our forests than add more pollution. I guess if forestry is considered geo-engineering then I support it. But I don't think "we're already adding pollution, so we might as well add more pollution to try to net out carbon" is a sound argument.

[philipkglass]

Reducing forest clearance and engaging in afforestation could be enough had anthropogenic CO2 emissions started dropping significantly in the 1990s. But they have continued to rise. Forestry changes aren't nearly enough to offset anthropogenic emissions from other sectors. They may be part of the solution but additional measures are needed.

Geoengineering is on the table now not because it is an easy shortcut, but because the world has failed to do enough in other ways. It's better for patients with prediabetes to change diet than develop full blown type 2 diabetes, but if diet doesn't change fast enough it's better to prescribe insulin than just let them die. Industrial civilization has discounted decades of warnings about changing its energy "diet" and will soon need more drastic measures.

I'm a little optimistic because renewable energy has become cheaper faster than I expected. I'm pessimistic because the world still isn't reducing fossil use fast enough (or at all, really -- so far the best news is "the percentage growth rate is slowing.") Even when the economics start to favor non-combustion energy sources, legacy fossil industries have often delayed the transition by obtaining government support to resist the economic pressures. So I believe that the world can transition to low-emissions energy but I also believe that it's not happening fast enough.

Even worse, the climate perturbation from anthropogenic emissions can trigger a dangerous positive feedback loop that will release even larger quantities of GHGs from natural stores as forests burn more frequently and permafrost thaws. I think that if people get the problem under control (as opposed to just suffering the effects, with no softening of the blow), it's going to involve 3 major prongs:

- Transition to non-fossil energy sources

- Geoengineering via solar radiation management, as a temporary bandaid to prevent runaway warming feedback

- Geoengineering via enhanced silicate weathering, as a thermodynamically stable fix for the excess CO2 added to the environment

Solar radiation management can be phased out as atmospheric CO2 levels drop. But with silicate weathering alone, I fear that thawing permafrost will outpace even the most ambitious CO2 drawdown efforts.

The second two prongs are still highly controversial and advocating for them tends to get one lumped in with climate denialists. I think that most people concerned about climate are going to come around eventually, though. The IPCC already has. We clearly aren't going to avert feedback loops by 2030 via emissions-reductions alone.

[erikpukinskis]

A lot of assertions there. You lost me at “Forestry changes aren't nearly enough to offset anthropogenic emissions from other sectors”. Can you explain your math there? Is there a limit to how much carbon we can warehouse in trees (living and milled)?

[philipkglass]

If you keep harvesting trees and store them in a way that they won't rot or burn, there is no practical limit on the total amount of carbon that can be sequestered that way. There is still a limit on the rate at which carbon can be sequestered that way. To stabilize concentrations of atmospheric CO2, the sequestration rate must match current rates of anthropogenic CO2 emissions. If significant feedbacks kick in, the sequestration rate needs to be higher than current anthropogenic emission rates.

Here's one of the more optimistic studies I have seen about forestry-based approaches to curbing CO2:

https://arstechnica.com/science/2017/10/co%e2%82%82-benefits...

The authors estimate that afforestation and other positive land use changes could provide up to 37% of the CO2 reductions needed through the year 2030 in order to stay under 2 degrees of warming. The other 63% has to come from elsewhere.

[graeme]

Forests are helpful but not sufficient. The world was once covered in forests and all the oil/coal etc was in the ground.

Now we have reduced forest cover and burned underground carbon. Reforestation only solves the former.

[erikpukinskis]

We can harvest forests without reducing canopy coverage and store the wood. You can even grow your forests on top of wood landfills.

[supergauntlet]

I'd agree with you if we could do what we need with just forests. But unfortunately we can't. Some other form of reducing carbon is a core part of any long term plan.

It's just bad enough now that forestry is no longer a valid option on its own.

[erikpukinskis]

Do you have data on the global capacity for intensive carbon storage forestry techniques? What are you basing that on?