Looking briefly at the recent IPCC report it presents a number of pathways for keeping global warming below 1.5 degrees celsius by 2100. These follow a spectrum from enormous reductions in consumption through to a business-as-usual situation, albeit with enormous carbon capture and storage (CCS) and higher risk of temporary overshoot of the 1.5 degree target. The latter pathway is described as pathway 4 in the policymaker summary and pathway S5 in chapter 2.

What isn't described in close proximity to this pathway is (1) the level of risk associated with the temporary overshoot, and (2) the likely economic requirements of implementing all the CCS the pathway needs (and hence the likelihood of actually being able to implement it).

Does anyone know further details on this pathway (likely encompassing both (1) and (2) above)?

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    You've got two different questions here: please edit this to pick out one of them, or to consolidate it into a single question: for example, you might ask whether there exists an economic impact assessment of the pathways that included not just the costs of implementation but also the damage costs of overshoot. – EnergyNumbers Nov 15 '18 at 14:21

I don't know about CCS, sequestration never seems to get close to scaling up for a reasonable cost. (A flagship “clean coal” plant is a flailing mess.) Carbon-capture, however, might have a new price ceiling set: CBC.ca: carbon from air to fuel, but also Vox.com: Sucking carbon out of the air won’t solve climate change, but it might fill in a few key pieces of the clean energy puzzle..

On the other hand, forests are a great way of CC: Nasa (2014) - Tropical forests absorb 1.4 billion metric tons of carbon dioxide.

"Global Forest Watch estimates that they could potentially provide 23 percent of the climate change mitigation needed to keep warming under two degrees by 2030" (link)

This is an interesting read on policies for GHG reduction: Designing Climate Solutions: A policy guide for low-carbon energy, by Hal Harvey.

As for overshooting, there's a general consensus (but by no means proven) that even the predicted rise is going to be bad as well as that there may be natural systems that are currently absorbing GHGs and heat (e.g. deep ocean water) more than might be known, and once at capacity there may be a runaway result. When I read interviews with some of the IPCC contributors that seems to be the general but cautiously stated opinion.

"[W]e can’t say the 2C target is an absolute guarantee of human safety, while missing it will necessary cause catastrophe. But if global average temperatures increase by over 2C, some hard-to-control changes may be triggered. For example, changes in the global water cycle may affect the world’s resources and ecosystems; parts of ice sheets may melt and thus contribute to sea level rise, and risks of extreme events will increase. And those changes will get worse and be subject to a sustained influence the greater the warming will be." (Thomas Stocker, co-chair of IPCC Working Group 1)

Coral reefs and climate change: "Limiting global average temperature to well below 2°C above pre-industrial levels...provides the only chance for the survival of coral reefs globally."

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