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Dr. Zeke Hausfather Profile picture
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3 Nov, 5 tweets, 2 min read
The folks at @ClimateRsrc have a new projection that updated NDCs and net-zero commitments will put the world on track for a best-estimate of below 2C warming by 2100.
This seems pretty reasonable to me; we were on track for a best-estimate of 2C to 2.1C based on net zero commitments prior to COP26: carbonbrief.org/unep-current-c…
Of course, the challenge is that long-term net zero commitments are doing all the heavy lifting here. 2030 NDCs alone still likely put us on track for around 2.4C warming.
It's easy to pledge to do something in 30 to 50 years; the real test is the extent to which they are reflected in enhanced near-term ambition, and in that department results are mixed given the relatively modest updates to many countries 2030 NDCs.
We should also emphasize the uncertainties here; as the @ClimateRsrc analysis shows the combination of emissions pathway and climate system uncertainties mean that this could end up anywhere between ~1.4C and 2.6C. Central estimates are convenient, but rarely tell the full story

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More from @hausfath

Dr. Zeke Hausfather Profile picture
4 Nov
CO2 emissions are flatting. This is good news, but it does not mean global warming will stop. Rather, it means that warming continues at the same rate rather than accelerating. To stop the world from warming we need zero emissions.

This is the brutal math of climate change.
We have become more confident in recent years that when emissions do reach zero (or net-zero), global warming will likely stop. There does not appear to be substantial additional warming "locked in" or "in the pipeline". carbonbrief.org/explainer-will…
At the same time, once we reach zero emissions global temperatures will not fall for many centuries to come. Reducing global temps would require humans to actively remove CO2 from the atmosphere. E.g. getting back to 1970 temps requires sucking out all our emissions since 1970.
Read 4 tweets
Dr. Zeke Hausfather Profile picture
4 Nov
Big news: recent CO2 emissions have been revised notably downward in the just-released @gcarbonproject dataset. The revisions – due to a major reassessment of land-use – suggest emissions have likely been flat rather than increasing over past decade: carbonbrief.org/global-co2-emi… 1/19
Global CO2 emissions come from a combination of fossil fuel emissions (~90% in recent years) and land use change (LUC) emissions (~10%) from deforestation and soil carbon loss. While fossil emissions have an uncertainty of +/- 5%, LUC emissions are much more uncertain. 2/
The Global Carbon Project (GCP) substantially revised their best estimate of LUC emissions in their newly released dataset. Rather than a 35% increase in LUC emissions since 2000 – as the data previously showed – the new version has a roughly 35% decrease instead. 3/
Read 20 tweets
Dr. Zeke Hausfather Profile picture
3 Nov
How to show everyone that you do not understand physics with one crappy analogy.
For what its worth, we have been projecting future warming since the first climate models in the late 1960s/early 1970s. We can look back to see how well they have performed. It turns out our models generally did a good job:
In case folks are interested, climate models are not trying to predict year-to-year variability which is mostly dominated by semi-stochastic El Nino and La Nina cycles. They are trying to project long-term responses to changes in radiative forcing from CO2 and other GHGs.
Read 4 tweets
Dr. Zeke Hausfather Profile picture
27 Oct
Planting trees is great. It helps restore ecosystems, sequesters some carbon, and has many other co-benefits.

But a ton of carbon absorbed by trees its not equivalent to avoiding a ton of CO2 emissions, and we need to stop pretending that it is. grist.org/wildfires/cali…
The warming effects of CO2 emissions last thousands of years, and a ton of CO2 that we avoid emitting is a ton of CO2 that is never emitted.

A ton of CO2 absorbed by planting trees, on the other hand, is temporarily rather than permanently removed from the atmosphere.
If we reforest an area that would have otherwise never been reforested, keep trees intact in that location for thousands of years to come, and don't have any secondary land use impacts, then its equivalent to mitigating CO2 emissions. But thats rarely – if ever – the case today.
Read 6 tweets
Dr. Zeke Hausfather Profile picture
26 Oct
If emissions had peaked back in 2000 we would be skiing down a bunny slope toward 1.5C. Today we face a double black diamond, and in a few years it will be a cliff.

We are almost certainly going to overshoot 1.5C and need large-scale permanent carbon removal to get back down.
Scenarios commonly used to limit warming to 1.5C include a lot of late century negative emissions – sucking a quarter to a half of current emissions out of the atmosphere each year by 2100 – in order to make near-term reductions more plausible:
There is, of course, no guarantee that negative emissions of that scale will pan out, but any chance of ultimately limiting warming to 1.5C likely requires both speeding up emissions reductions significantly and building a robust carbon removal industry later in the century.
Read 8 tweets
Dr. Zeke Hausfather Profile picture
26 Oct
The new UNEP Emissions Gap report provides up-to-date estimates of likely warming outcomes associated with current policies, NDCs, and net-zero pledges. Between UNEP, @climateactiontr, and @IEA we are getting a clearer picture of how much the world is on track to warm: 1/6
Note that UNEP does not provide a lower bound to its warming ranges, just 50th, 66th, and 90th percentiles. For this reason the uncertainties are expressed as "up to" the 90th percentile rather than a 10th-90th percentile range. 2/
A technical detail: I've also estimated the 90th percentile of combined emissions and climate system (e.g. sensitivity, carbon cycle feedback) uncertainties by adding the two in quadrature, assuming independence. 3/
Read 8 tweets

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