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Dr. Zeke Hausfather Profile picture
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19 Aug, 5 tweets, 2 min read
Theres been a lot of debate about which simple metrics are best when comparing climate impacts of CH4 and CO2. The new IPCC AR6 report has a great figure (7.22) that compares different approaches. Ultimately, you want to come as close as possible to the actual temperature (GSAT):
Its pretty clear the optimal approach is GWP*. However, GWP* is a bit more complex and lacks constant equivalence (e.g. CH4 is always x times worse than CO2). Unfortunately given differing atmospheric lifetimes constant equivalence is not very accurate. carbonbrief.org/guest-post-a-n…
If you are going to use GWP-20 or GWP-100, the IPCC AR6 figure suggests that GWP-20 will significantly overestimate near-term warming of continued emissions of CH4, while GWP-100 will modestly underestimate warming.
I think this makes a reasonable case for using GWP-100 over GWP-20 when evaluating emissions of CH4 over time. GWP is only really ideal for evaluating a single one-time flux, so if at all possible its far better to use GWP* or cumulative radiative forcing for these purposes.
For more details on why CH4 and CO2 differ in their climate effects (and why its important to mitigate both!), see my recent thread:

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More from @hausfath

Dr. Zeke Hausfather Profile picture
13 Sep
As the world adopts climate policies and the price of clean energy falls, we have and will continue to move away from some of the worst climate outcomes of 4C+ warming. But this should not distract us from our ultimate goal of getting emissions to net-zero thewellnews.com/in-the-news/we…
A decade ago the world seemed on track for a very dark climate future. Global emissions were increasing at 3% per year, China was building a new coal plant every three days, and the idea that emissions could double or triple by 2100 did not seem that far-fetched.
Today things have changed markedly. Global coal use peaked back in 2013, and the International Energy Agency’s (IEA) most recent World Energy Outlook suggests that coal is now in “structural decline.” Global emissions are still increasing, but at a rate of only 1% per year.
Read 15 tweets
Dr. Zeke Hausfather Profile picture
2 Sep
For every 1C of warming the world experiences, saturated air contains 7 percent more water vapor on average.
Per the IPCC AR6: "The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis (high confidence), and human-induced climate change is likely the main driver."
"Event attribution studies and physical understanding indicate that human-induced climate change increases heavy precipitation associated with tropical cyclones (high confidence)."
Read 6 tweets
Dr. Zeke Hausfather Profile picture
27 Aug
One under-appreciated finding in the IPCC AR6 is a lot more certainty around future warming.

Previously IPCC only gave "likely" warming ranges (e.g. a 2 in 3 chance of falling in the range). New report gives "very likely" 9 in 10 ranges. Here is a rough like-to-like comparison: Image
The IPCC AR5 future warming projections were nominally based on the 90th percentile of CMIP5 models, but the assessed range of climate sensitivity was much wider than the range in CMIP5 models, so these were treated "likely" (66th percentile) ranges.
The AR6, on the other hand, bases its warming projections on a combination of observationally-constrained CMIP6 models and a simple energy balance model using the new transient climate response (TCR) and equilibrium climate sensitivity (ECS) values in the report. Image
Read 9 tweets
Dr. Zeke Hausfather Profile picture
11 Aug
The recent IPCC report had a big focus on methane (CH4) – and rightly so. We should work to cut methane emissions quickly, but not at the expense of cutting CO2.

Methane is temporary, while CO2 is forever.

A quick thread: 1/13
Methane is a strong greenhouse gas – over 100x more effective at trapping heat than CO2 while its in the atmosphere. Its responsible for around 28% of positive radiative forcing (and historical warming). 2/
However, methane has a short atmospheric lifetime. Most of the methane we emit this year will be gone from the atmosphere in around a decade. Methane interacts with hydroxyl radicals (OH) in the atmosphere, and ultimately breaks down into (mostly) CO2 and H2O. 3/
Read 14 tweets
Dr. Zeke Hausfather Profile picture
11 Aug
There's grim news in the IPCC report, but also reasons for hope. We're flattening the curve of future emissions, and the darkest climate futures a decade ago are much less likely now. We can both celebrate progress and acknowledge how far we have to go: thebreakthrough.org/issues/energy/… 1/
A decade ago the world seemed on track for a particularly grim climate future. China was building a new coal plant every three days; global emissions were increasing at a rate of 3% per year and increased by 31% between 2001 and 2010. 2/
Scenarios where global carbon emissions tripled by the end of the 21st century with coal use increasing sixfold seemed plausible to many. Researchers argued that “business as usual” would likely lead to a world 4ºC or 5ºC above pre-industrial levels by 2100. 3/
Read 18 tweets
Dr. Zeke Hausfather Profile picture
10 Aug
In my latest piece at @CarbonBrief, I take a deep dive into what the IPCC AR6 says about when the world will likely pass 1.5C and 2C, and how the new estimates of the remaining carbon budgets compare to those in the 2018 IPCC SR15 special report: carbonbrief.org/analysis-what-… 1/14
When we talk about passing a particular warming level like 1.5C or 2C we are not referring to an individual year (or month). Any given year may be ~0.2C warmer or cooler than average due to natural variability from El Nino and La Nina events. 2/
The AR6 reports on the 20-year period in which temps exceed 1.5C (e.g. 2021-2040), and suggests using the midpoint of that range (e.g. 2030) as the year when long-term average temps passes that level. We used a similar approach in an earlier analysis: carbonbrief.org/analysis-when-… 3/
Read 14 tweets

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