To find out, we can look at how long it has taken “Nature” to lower them in the past.
For the past 420,000 years or more, CO₂ levels have (essentially) never gone above (about) 300 ppm
OK, we’re over 420 ppm right now. That could be a problem.
Back when they were at 300 ppm (and the world was significantly warmer than it is today) and people hadn’t been cutting down forests, and building cities, and interstate highways, and burning stuff. You know, back when “Nature” was in charge, and the planet was verdant, it took “Nature” about 1,000 years to lower CO₂ levels 1 ppm. We would like to lower them… oh, let’s say… 150 ppm?
Remember 350.org? That 350 figure was based on an initial “target" James Hansen et al, suggested, while fully understanding that levels would need to go lower than 350. The logic was, if we could figure out how to get them down to 350, we could use the same methods to lower them still further.
http://dx.doi.org/10.2174/1874282300802010217
Target Atmospheric CO₂: Where Should Humanity Aim?
Abstract: Paleoclimate data show that climate sensitivity is ~3°C for doubled CO₂, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO₂ for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO₂ was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO₂ fell to 450 ± 100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO₂ will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO₂ forcings. An initial 350 ppm CO₂ target may be achievable by phasing out coal use except where CO₂ is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO₂ is not brief, there is a possibility of seeding irreversible catastrophic effects.
…
❝… reduced from its current 385 ppm to at most 350 ppm, but likely less than that …❞
Unless Hansen et al were way off 16 years ago, and there’s little indication of that, “Net Zero” buys us very little.
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