There’s been quite a lot of recent discussion about warming commitments. It started with an article by Bob Berwyn called Net Zero Emissions Would Stabilize Climate Quickly Says UK Scientist, followed soon after by one saying [w]arming already baked in will blow past climate goals, study finds. The first article is (I think) based on a recent multi-model analysis which suggests that the most likely value of Zero Emission Commitment (ZEC) on multi-decadal timescales is close to zero. The second article is reporting on results from another recent paper suggesting that [g]reater committed warming after accounting for the pattern effect.
So, why are we being presented with what appear to be inconsistent results? The simple answers is that we’re not really being careful enough to define what we mean by a warming commitment. The first article, and paper, are considering what would happen when we get emissions to zero. The second article, and paper, are essentially considering what would happen if atmospheric greenhouse gas concentrations remained at today’s levels. These are clearly two different scenarios.
When we get emissions to zero, the first paper indicates that – on multi-decade timescales – the zero emission warming commitment (ZEC) would be close to zero. On the other hand, if atmospheric CO2 concentrations were to remain constant, then we would continue warming to equilibrium. At today’s atmospheric CO2 concentrations, this would lead to additional warming of around 0.5oC or even more, according to the paper being highlighted in the second article above. However, it is important to realise that constant concentrations require continued emission, as illustrated by the second figure in this Steve Easterbrook post.
I should also stress that our understanding that there is little warming commitment associated with zero emissions has been understood for quite some time. The first paper to point this out was probably Matthews and Caldeira (2008), followed by Solomon et al. (2009), and Cao and Caldeira (2010). There’s also a Realclimate post pointing this out in 2010, the Steve Easterbrook post I mentioned above from 2013, and a post I wrote in 2016.
There are, however, a number of important caveats. That the zero emission warming commitment is probably small probably only applies on multi-decade timescales. The models that demonstrate this typically don’t include slower processes (such as ice sheet retreat, sea level rise, permafrost release) that may lead to additional warming on longer timescales.
Also, even though there is probably little commited warming on multi-decade timescale once we get emissions to zero, without negative emissions global surface temperatures will remain at an elevated level (relative to pre-industrial times) for a very long time. It does, however, indicate that our future warming depends mostly on future emssions. We can still influence how much future warming we are likely to experience, even if we can’t turn everything off right now.
So, I think it’s good that there is more recognition that the ZEC is probably small. It does address claims that there’s nothing we can do to avoid a lot of future warming and does illustrates that, in the context of future warming, most of the inertia is societal, rather than inertia in the climate system.
Net Zero Emissions Would Stabilize Climate Quickly Says UK Scientist, article by Bob Berwyn.
Warming already baked in will blow past climate goals, study finds, Associated Press article.
Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO2, MacDougal et al. (2020).
Greater committed warming after accounting for the pattern effect, Zhou et al. (2021).
Stabilizing climate requires near‐zero emissions, Matthews and Caldeira (2008).
Irreversible climate change due to carbon dioxide emissions, Solomon et al. (2009).
Atmospheric carbon dioxide removal: long-term consequences and commitment, Cao and Caldeira (2010).
Climate Change Commitments, Realclimate (2010).
How Big is the Climate Change Deficit?, Steve Easterbrook (2013).
Committed Warming, my post from 2016.