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Nice summary of an important thread here. I wanted to add a couple of thoughts. @Peters_Glen @KevinClimate https://twitter.com/RadReduction/status/1351880197167955968
As one of three authors of the first comprehensive analysis of a 2C warming limit back in 1989, I have a soft spot in my heart for carbon budgets, which were an important counterpoint for many years to the prevailing benefit-cost analysis approach.
Krause, Florentin, Wilfred Bach, and Jon Koomey. 1989. From Warming Fate to Warming Limit: Benchmarks to a Global Climate Convention. El Cerrito, CA: International Project for Sustainable Energy Paths. [ http://www.mediafire.com/file/pzwrsyo1j89axzd/Warmingfatetowarminglimitbook.pdf]
I document some of the intellectual history here: Koomey, Jonathan. 2013. "Moving Beyond Benefit-Cost Analysis of Climate Change." Environmental Research Letters. vol. 8, no. 4. December 2. [ http://iopscience.iop.org/1748-9326/8/4/041005/]
As years have passed and humanity has dithered on climate, I've concluded that we need to act as if there is no more carbon budget, and that we need to reduce emissions as much as possible and as fast as possible, starting now.
This matches Glen's admonition to move "as fast as possible", but I wanted to add an additional constraint to that formulation. The world needs to get to net zero emissions by mid century, and adequacy of our efforts needs to be measured against that benchmark.
The power of the warming limit approach is that it defines the appropriate sense of urgency, but if you just say "as fast as possible" you open yourself up to predatory delay by people who use politics to delay action (reducing what's possible in practice).
So when we use the word "possible" nowhere should the idea of "political feasibility" come into play. By possible I mean PHYSICALLY possible. That defines the appropriate level of effort, not any artificially constrained view of what's possible because of politics.
We can use the "net zero by 2050" benchmark as a metric of adequacy of effort. If your proposed policies wouldn't get us there by 2050, your proposal is inadequate. We need such a benchmark to hold people's feet the fire.
The carbon budget used to be the way we defined adequacy of action because the paths to hitting any particular budget were so tightly constrained. Now that we've just about used up any reasonable budget, "net zero by 2050" will need to cover it.
The politics is hard. The fossil fuel industry is the most powerful industry in human history, with about $5T/yr in revenues in 2010 (10X more than tobacco). The physical reality, though is that we need to reduce emissions to net zero by 2050.
If politics gets in the way of what is necessary, we need to change politics to make what is necessary possible. So in a sense I'm arguing for a merger of the approaches discussed in the previously cited thread.
Here's the mantra: Reduce emissions as fast as necessary to achieve net zero emissions by 2050 or before. If your actions in any year don't put us on a path to net zero by 2050, then do more next year. Lather. Rinse. Repeat.
We need the urgency created by the physical reality of the climate system to define adequate effort. That's what the carbon budget used to bring to these discussions, but now it all boils down to "net zero emissions by 2050". /fin
