The new IPCC 6th Assessment Report (AR6) provides an unprecedented degree of clarity about the future of our planet, and the need to reduce – and ultimately eliminate – our emissions of greenhouse gases.

In this thread I take a look at some key findings from the report: 1/27

Perhaps most importantly, this report gives us a much clearer view of our climate future.

It does this by narrowing the range of climate sensitivity – which had remained largely unchanged since 1979 at "likely" between 1.5C and 4.5C warming if atmospheric CO2 is doubled. 2/

The new AR6 report gives a "likely" (e.g. 67% chance) climate sensitivity range of 2.5C to 4C, a full 50% reduction in uncertainty relative to the likely range given in the AR5. The AR6 "very likely" (~90% chance) range is 2C to 5C, compared to 1C to 6C in the AR5. 3/

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This reflects the recent findings of Sherwood et al 2020 (that I coauthored) which found that combining three different independent lines of evidence – from paleoclimate, observations, and physical process models – narrowed the range of sensitivity: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019RG000678 4/

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This narrower sensitivity range is both good news and bad news. The bad news is we are much less likely to get lucky and have climate change on the milder side of what we expected. This sensitivity revision cuts the legs off the lukewarmer argument: https://www.carbonbrief.org/guest-post-why-low-end-climate-sensitivity-can-now-be-ruled-out 5/

The good news is that it suggests that very high sensitivity outcomes of 5C+ as is found in some of the new CMIP6 models is very unlikely (albeit not possible to fully rule out).

And speaking of the new climate models, the new AR6 report takes a novel approach here. 6/

Instead of simply using the average of all the models and their spread – as the AR5 does – the AR6 combines two separate approaches to project future warming. In the first, they use CMIP6 models weighted by how well they agree with observations. 7/

High sensitivity models tend to do poorly in reproducing historical temperatures, so get less weight in the resulting analysis. The net effect is to make CMIP6 future projections in AR6 (and weighted sensitivity) quite similar to the CMIP5 models in AR5 https://www.sciencemag.org/news/2021/07/un-climate-panel-confronts-implausibly-hot-forecasts-future-warming 8/

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At the same time, the AR6 uses a simple energy balance model (e.g. emulator) tuned to the new climate sensitivity values featured in the report to create a set of future projections somewhat independent from the CMIP6 models. This figure compares the approaches: 9/

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The two approaches are than averaged together to create assessed warming projections that are used widely throughout the report. This is a *big* departure from the approach taken in the AR5. 10/

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Speaking of warming projections, the AR6 features two new scenarios that were not in the AR5: the SSP1-1.9 scenario designed to limit 2100 temperatures below 1.5C, and the SSP3-7.0 scenario in which emissions continue to increase and roughly double by the end of the century. 11/

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SSP3-7.0 provides a somewhat more plausible high-end emissions pathway than SSP5-8.5 (RCP8.5's successor), which the AR6 suggests "has been argued to be implausible to unfold". Unfortunately AR6 does not use SSP4-6.0 much, which is most comparable to current policy outcomes. 12/

So, what does the AR6 project future warming will be, and how does this compare to the warming ranges in the AR5? The mean projected warming is 0.1C-0.3C higher in the AR6. The ranges on first glance seem similar, but hide a key difference! 13/

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The AR5 gave "likely" future warming ranges. This meant there was a roughly 1 in 3 chance that future warming would be outside this range – either above or below. The AR6, on the other hand, give very likely ranges, with only a 1 in 10 chance of outcomes outside that range! 14/

This is a critically important change, and reflects substantially more confidence in future warming outcomes than we had only a decade ago. This increase in confidence is driven primarily by advances in our understanding of climate sensitivity. 15/

The AR6 includes a major focus on when the world will pass 1.5C and 2C warming levels. They argue we will most likely pass 1.5C sometime in the early 2030s. These ranges are similar to those in our @CarbonBrief analysis: https://www.carbonbrief.org/analysis-when-might-the-world-exceed-1-5c-and-2c-of-global-warming 16/

These exceedance dates are nearly a decade earlier than the best-estimate found in Chapter 1 of the 2018 IPCC Special Report on 1.5C (SR1.5) – which were based on assumed continuation of historical trends – but are quite similar to those found in Chapter 2 of the SR1.5. 17/

The world is most likely to pass 2C somewhere between early 2040s and early 2050s in higher emissions scenarios (SSP2-4.5, SSP3-7.0, and SSP5-8.5). There are also wide uncertainties for both 1.5C and 2C reflecting diffs across models. 18/

Note that I will have a more more detailed analysis of both exceedance years and remaining carbon budgets in the AR6 over at @CarbonBrief. 19/

The AR6 reflects major advances in our ability to attribute extreme events to climate change since the AR5. No longer do we have to talk in generalities about climate change influencing all events; we can estimate how much worse a given heatwave was made by human activity. 20/

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The AR6 makes it crystal clear that our best estimate of the human contribution to warming is all of it. Natural forcings along would have led to flat or slightly cooling temperatures, and multidecadal variability is unlikely to play a major role in modern warming. 21/

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A few other more minor points:
Our historical temperature records have been revised to cover more of the Arctic and correct for biases in the ship-to-buoy measurement transition over the ocean. This has increased our estimate of historical warming by around 0.1C. 22/

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The IPCC features a lot of analysis on temperature outcomes – 1.5C, 2C, 3C, 4C – rather than solely focusing on emissions scenarios. This is helpful, as high sensitivity and carbon cycle feedbacks can potentially result in 3C or 4C warming under a moderate emissions scenario. 23/

The AR6 discusses the results of the new Zero Emissions Model Intercomparison Project (ZECMIP), and its finding that our best estimate is that the world will stop warming once we reach net-zero emissions: https://www.carbonbrief.org/explainer-will-global-warming-stop-as-soon-as-net-zero-emissions-are-reached 24/

Finally, my own modest contribution to Chapter 1 of the report: the AR6 finds that historical climate models and past IPCC projections have done a good job projecting real-world temperatures in the years after they were published: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL085378 25/

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All of this is just the tip of the proverbial iceburg, and there are many other key findings not included in this thread. We will publish a comprehensive Q&A later at @CarbonBrief, as well as analysis of remaining carbon budgets and 1.5C/2C exceedance years, so stay tuned! 26/

And of course, behind all the detailed scientific analysis a simple message rings loud and clear: it's real, it's us, experts agree, it's bad, but there is still time to do something about it. Our climate future is, ultimately, up to us to decide. https://www.ipcc.ch/assessment-report/ar6/ 27/