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The IPCC Sixth Assessment Report

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Climate scientists are inordinately excited by the release of a new IPCC report (truth be told, that’s a bit odd – It’s a bit like bringing your end-of-(seven)-year project home and waiting anxiously to see how well it will be received). So, in an uncharacteristically enthusiastic burst of effort, we have a whole suite of posts on the report for you to read.

If/when we add some more commentary as we digest the details and we see how the report is being discussed, we’ll link it from here. Feel free to discuss general issues with the report in the comments here, and feel free to suggest further deep dives we might pursue.

Another dot on the graph

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So last week was the annual release of the temperature records from NASA, NOAA and Berkeley Earth. The Copernicus ERA5 data was released a few days ago, and the HadCRUT data will follow soon. Unlike in years past, there is no longer any serious discrepancy between the records – which use multiple approaches for the ocean temperatures, the homogenization of the weather stations records, and interpolation.

Depending on the product, 2021 was either the 5th, 6th or 7th warmest year, but in all cases, it is part of the string of warm years (since 2015) that have all been more than 1ºC warmer than the late 19th C.

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“Don’t Look Up”

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The highlight of the movie season for climate science has clearly been the release on Dec 24th 2021 of “Don’t Look Up”. While nominally about a different kind of disaster – the discovery of a comet heading to Earth on a collision course – the skewering of our current science-policy dysfunction transcends the specifics and makes a powerful metaphor for climate change, and even the ongoing COVID-19 pandemic.

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Forced responses: Jan 2022

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A bi-monthly open thread related to climate solutions.

PS. New year, new moderation policy. Please be substantive – sniping, insults, and tedious repetition will just be culled. We want to maintain a civil and productive discourse here, but the comment threads may need to be re-evaluated if that doesn’t happen.

Unforced Variations: Jan 2022

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This month’s open thread on climate science topics. Note that summaries of annual climate data from 2021 will start to appear in a couple of weeks, and updates to the model/observations comparisons will appear a week or so after that.

PS. New year, new moderation policy. Please be substantive – sniping, insults, and tedious repetition will just be culled. We want to maintain a civil and productive discourse here, but the comment threads may need to be re-evaluated if that doesn’t happen.

Making predictions with the CMIP6 ensemble

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The CMIP6 multi-model ensemble is a unique resource with input from scientists and modeling groups from around the world. [CMIP stands for the Coupled Model Intercomparison Project, and it is now in its 6th Phase]. But as we’ve discussed before (#NotAllModels) there are some specific issues that require users to be cautious in making predictions. Fortunately, there are useful ‘best practices’ that can help avoid the worst pitfalls.

A new paper by McCrystall et al that has just appeared in Nature Communications illustrates these issues clearly by having some excellent analyses of the changes in Arctic precipitation regimes at different global warming levels, and examining the sensitivity of their metrics to both local and Arctic warming, but unfortunately relying on the CMIP6 multi-model mean for their headline statements and press release.

The problem with the CMIP6 ensemble

Any use of CMIP future projections (either in CMIP6 or previously) requires some idea of what the scenarios represent (plausible storylines) of course, but in CMIP6 there is the (somewhat awkward) spread in climate sensitivities that is both greater than in previous CMIP iterations and also extends beyond the values constrained by multiple observations.

Histogram of CMIP6 climate sensitivities

There are notably eight models that have climate sensitivities greater than 5ºC (and more than a dozen greater than 4.5ºC) (and for fairness, two models with ECS under 2ºC). This is sufficient to skew the multi-model mean of the global mean surface temperature higher than the assessed projections seen in the latest IPCC report (Chapter 4).

Fig 4.11 from AR6 WG1
Projected temperatures (2081-2100 w.r.t. 1850-1900) from different scenarios in a) the ‘raw’ multi-model mean, and b) constrained mean using a weighted set of models. (Figure 4.11a,b AR6 WG1).

This was not the case for CMIP5. There, the spread of the models was similar to the constrained limits, and the multi-model mean was very close to the assessed projections, and so it was used widely (even by us) as a best guess. The bottom line is that for CMIP6 the multi-model mean is not the same as the expected prediction.

What difference does CMIP6 make for Arctic precipitation?

McCrystall et al do a lot of work in their paper that is great and appropriate. They look at the impacts at specific Global Warming Levels (GWLs):

Fig 7 McCrystall et al (2021)
Increase in rainfall in each season at specific GWLs in CMIP6 and CMIP5.

and in more detail at the sensitivity of the precipitation change per degree over the year:

Fig 4 from McCrystall et al (2021)
Sensitivity of model percentage change in Arctic precipitation per degree of Arctic (a) and Global (b) temperature change. (McCrystall et al, fig 4).

Both of these analyses incorporate all the models but don’t explicitly depend on the overall climate sensitivity. While there are some differences between the CMIP5 and CMIP6 models (particularly at the tails), the means are pretty close. That is, once you abstract out the global or Arctic level of warming, there isn’t that much difference between the projections from the CMIP5 or CMIP6 models (one could test that more formally of course).

There is a slightly clearer difference in their fig. 5 in the SON period where the sensitivity of precipitation to the open water amount seems stronger in CMIP6 than in CMIP5, but mostly everywhere else the sensitivities seem comparable (even though the magnitude of change in some of the CMIP6 models is much larger). So far so good.

However, the main conclusion that the authors highlight in the press release and which was picked up in the newspaper coverage was the claim that rain would dominate Arctic precipitation ‘decades earlier’ than previously thought. This is based on the last row of figure 8:

The decade of transition from a snow-dominated precipitation regime to a rain-dominated regime for CMIP6 [first column] and CMIP5 [second column] multi-model ensemble means for Sep/Oct/Nov. (McCrystall et al. (2021), Fig 8)

This shows the results from the raw ensemble means, and uses the absolute times in the output. Given the excessive warming in some models, which will have an earlier transition than the other CMIP6 or CMIP5 models, this result is mostly due to the inclusion of the high ECS models, not anything fundamental about the processes involved. There is some evidence of differences in this metric in the GWL diagnostics, but the signal is much smaller than in fig 8. Ideally, one might like to compare this result to the constrained subset of the CMIP6 models that were used in the IPCC AR6 Fig. 4.11 or those that match the constrained ECS values, to disentangle the impact of (presumably) better process modeling in CMIP6 from the impact of a wider spread in ECS. However, that is not (yet) possible (since what was done is not yet clear). So, unfortunately, we have a widely telegraphed conclusion that purports to be because of some advances in the science, but mostly relies on the excessive and unrealistic ECS in some of the models used.

Lessons to be learned

As I stated when the IPCC report came out, the CMIP6 ensemble has killed the notion of model democracy (‘one model, one vote’). It wasn’t really strictly correct in CMIP5 or previously, but it was close enough to serve as a useful heuristic. This is no longer the case in CMIP6. The default behavior in the community has to move away from considering the raw model ensemble mean as meaningful. The analyses that make sense are the impacts related to GWL, the sensitivity of any impact to the global mean, and various flavours of weighted or subsetted means that would allow the impacts to be coherent with the assessed projections of global mean temperature from AR6.

Providing these recipes to the broader community should be a high priority for the CMIP community or for any projects that are using the CMIP6 data as input.

References

  1. M.R. McCrystall, J. Stroeve, M. Serreze, B.C. Forbes, and J.A. Screen, "New climate models reveal faster and larger increases in Arctic precipitation than previously projected", Nature Communications, vol. 12, 2021. http://dx.doi.org/10.1038/s41467-021-27031-y

Net Zero/Not Zero

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At the COP26 gathering last week much of the discussion related to “Net-Zero” goals. This concept derives from important physical science results highlighted in the Special Report on 1.5ºC and more thoroughly in the last IPCC report that future warming is tied to future emissions, and that warming will effectively cease only once anthropogenic CO2 emissions are balanced by anthropogenic CO2 removals. But some activists have (rightly) pointed out that large-scale CO2 removals are as yet untested, and so reliance on them to any significant extent to balance out emissions is akin not really committing to net zero at all. Their point is that “net-zero” is not zero and hence will serve as a smokescreen for insufficient climate action. To help sort this out some background might be helpful.

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Forced responses: Nov 2021

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A bi-monthly open thread related to climate solutions. This month will start off with COP-26 and many targets and plans and mechanisms will be proposed and discussed. Look out for the updated impacts of the evolving NDCs such as this one from Climate Resource, suggesting that the world could be on track for just a little less than 2ºC warming (relative to the pre-industrial) (if everyone does what they pledge and we are lucky with respect to climate sensitivity). Please be respectful and constructive.

Unforced variations: Nov 2021

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This month’s open thread. The first two weeks will be dominated by COP-26, and various science updates that will be announced there, including this year’s Global Carbon Project report. Curiously, there is some archival interest in the climategate affair possibly in connection to COP-26 (a BBC dramatization “The Trick“, a BBC radio series on the security aspects “The Hack that Changed the World”, and a couple of months ago, a podcast episode of “Cheat!”). Please stick to science-related topics on this thread.

A science-based move to climate change adaptation

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All countries in the world urgently need to adapt to climate change but are not yet in a good position to do so. It’s urgent because we are not even adapted to the present climate. This fact is underscored by recent weather-related calamities, such as flooding in Central Europe and heatwaves over North America. It’s also urgent because the oceans act like a flywheel, making sure that cuts in emission of greenhouse gases will have a lagged effect on global warming.

Climate change adaptation was addressed in the Paris Agreement from 2015, the Climate Adaptation Summit in January 2021, and will be one of four key priorities during the upcoming COP26. Proper climate adaptation of course needs meteorological and climatological data for mapping weather-related risks to prepare us for future extreme weather. However, I would argue that the climate research community has not had a visible presence during any of these meetings. Instead the summits have been dominated by politicians and NGOs.

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