The site has been a little quiet, but since this blog has been going long enough to have commented on the release of the IPCC AR5 WG1 report, it only seems right to comment on the release of the IPCC’s AR6 WG1 report. In case anyone doesn’t know, the IPCC is the Intergovernmental Panel on Climate Change, AR6 is the sixth Assessment Report, and WG1 is Working Group 1, the Physical Science Basis.
I haven’t read much of the main report, but have read through the Summary for Policy Makers (SPM). The bottom line is that the big picture hasn’t really changed; it’s real, it’s us, there’s strong agreement amongst relevant experts, the impacts could be really severe, we can still do things to limit the impact.
However, I thought I would highlight some of the things that caught my eye, and maybe others can provide other explains in the comments. One of the key highlights in AR5 was that it was extremely likely that most of the observed warming was anthropogenic, and that hasn’t changed: The likely range of total human-caused global surface temperature increase from 1850–1900 to 2010–2019 is 0.8°C to 1.3°C, with a best estimate of 1.07°C (i.e., it’s probably all human-caused).
What does seem to have changed is there seem to be stronger statements about extreme events, in particular heatwaves and extreme precipitation, but also droughts, flooding and tropical cyclones. For example, there is a pretty clear statement that climate change [is] already affecting every inhabited region across the globe with human influence contributing to many observed changes in weather and climate extremes.
The AR6 report also presented a narrower likely range for equilibrium climate sensitivity (2.5oC to 4.5oC) and – unlike AR5 – presents a best estimate of 3oC. It also presents a slightly narrower range for the Transient Climate Response to Cumulative Emissions (TCRE) of 0.27oC to 0.63oC per 1000 GtCO2, with a best estimate of 0.45oC (or 1.65oC per 1000 GtC).
There also seem to be stronger statements about many of the changes, and the current state, being unprecedented over many centuries to, potentially, thousands of years. The report now specifically states that it is more likely than not that no multi-centennial period after the Last Interglacial (roughly 125,000 years ago) was warmer globally than the most recent decade.
There’s plenty more that I could highlight, but overall it seems to be mostly strengthening the basic understanding that we’ve had for quite some time now. Maybe we’ll see less of the “climate change isn’t influence extreme events” narrative that some like to promote, but I wouldn’t bank on it.
If you want to read some other takeaways from AR6 WG1, Realclimate has a new post. As I said above, maybe others could highlight their takeaways in the comments and, if interested, make any others points they might like to make.
IPCC AR6 WG1 report – Link to the AR6 WG1 report.
IPCC AR5 WG1 post – my post about the release of the AR5 WG1 report in 2013.
AR6 of the best – Realclimate post about the release of the AR6 WG1 report.