The biggest contribution scientists can make to #scicomm related to the newly released IPCC Sixth Assessment report, is to stop talking about the multi-model mean.
[Read more…] about #NotAllModelsClimate Science
A Tale of Two Hockey Sticks
Two decades ago, the so-called “Hockey Stick” curve, published in 1999 by me and my co-authors (Mann, Bradley and Hughes, 1999), was featured in the all-important “Summary for Policy Makers” (SPM) of the 2001 IPCC Third Assessment report. The curve, which depicted temperature variations over the past 1000 years estimated from “proxy data such as tree rings, corals, ice cores, and lake sediments”, showed the upward spiking of modern temperatures (the “blade”) as it dramatically ascends, during the industrial era, upward from the “handle” that describes the modest, slightly downward steady trend that preceded it.
The Hockey Stick became an icon in the case for human-caused climate change, and I found myself at the center of the contentious climate debate (I’ve described my experiences in “The Hockey Stick and the Climate Wars”).
Featured two decades later now in the AR6 SPM is a longer Hockey Stick with an even sharper blade. And no longer just for the Northern Hemisphere, it now covers the whole globe. The recent warming is seen not only to be unprecedented over the past millennium, but tentatively, the past hundred millennia.
The relevant statements in the SPM and Technical Summary are:
A.2.2 Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years (high confidence). Temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6500 years ago13 [0.2°C to 1°C relative to 1850– 1900] (medium confidence). Prior to that, the next most recent warm period was about 125,000 years ago when the multi-century temperature [0.5°C to 1.5°C relative to 1850–1900] overlaps the observations of the most recent decade (medium confidence). {Cross-Chapter Box 2.1, 2.3, Cross-Section Box TS.1}
SPM AR6
Global surface temperature has increased by 1.09 [0.95 to 1.20] °C from 1850–1900 to 2011–2020, and the last decade was more likely than not warmer than any multi-centennial period after the Last Interglacial, roughly 125,000 years ago.
Cross Section Box TS.1
As the new IPCC report lays bare (you can find my full commentary about the new report at Time Magazine), we are engaged in a truly unprecedented and fundamentally dangerous experiment with our planet.
References
- M.E. Mann, R.S. Bradley, and M.K. Hughes, "Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations", Geophysical Research Letters, vol. 26, pp. 759-762, 1999. http://dx.doi.org/10.1029/1999GL900070
AR6 of the best
Half a dozen takeaways from a first read of the new IPCC AR6 report.
As climate scientists we tend to look at the IPCC reports a little differently than the general public might. Here are a few things that mark this report out from previous versions that relate to issues we’ve discussed here before:
- Extreme events are increasingly connected to climate (duh!)
- Sea level rise is a big deal
- Use, abuse and misuse of the CMIP6 ensemble
- The radiative forcing bar chart has gone full circle
- Droughts and floods are complicated
- Don’t mention the hiatus
There are other things that will get the headlines (the expected time before we get to 1.5ºC or 2ºC, the headline SLR numbers, the ‘unprecedented rate’ statement, constraints on climate sensitivity, carbon cycle feedbacks, the implications for the carbon budgets etc.), and other things worth noting – for instance, the much better and more direct graphics that they have clearly worked on a lot. As usual, most of the headlines will also focus on the Summary For Policy Makers (SPM) which was approved word by word by the governments over the last two weeks (full disclosure, I was advising the US delegation), but the full report will be worth dipping into over the next few months (there is a lot there to digest!).
1. Extremes: Back in 2012, the literature assessed by AR5 connecting changes in extremes to climate change was scant. As we wrote at the time, attribution of single events was difficult and experimental. But as was exemplified by the recent reaction to the PNW heatwave, things have moved on considerably. This has allowed the IPCC authors to produce regional assessments of past changes in heat extremes, intense precipitation and agricultural/ecological drought in drying regions (see below for a discussion on what that means), and produce assessed projections of a whole suite of what they call Climate-impact Drivers (CIDs) – which includes floods, marine heatwaves etc. People who (even a couple of weeks ago) were quoting the AR5 statements on extremes as if that was current are going to have to update their talking points (that is, of course, if they care about correctly reflecting the most up-to-date science…).
For example, IPCC finds that Northern Europe, Western Central Europe and Eastern Europe all show an observed increase in heavy precipitation events, such as the one causing the recent massive flooding in Germany and Belgium. Western Central Europe is one of the few regions where both an increase in extreme precipitation and in drought have already been documented. The IPCC notes that it’s the most rare and severe extremes which are expected to show the biggest percentage increase in frequency (see this past RealClimate post).
2. Sea Level Rise: The previous IPCC reports, notably AR4 and AR5 (to a lesser extent), have had a hard time dealing with SLR. This has been due to multiple issues, including a historical lack of comprehensive literature to assess, very uncertain observations of ice sheets, and difficultlies in blending different lines of evidence. In this report they’ve tried much harder to put the data together more coherently, there is more evidence, and they haven’t shied away from being explicit about the low-likelihood/high-impact possibilities (mostly associated with a collapse of WAIS). Literally, the sea level projection runs off the page… (Stefan will have a more detailed assessment later).
3. Use, Abuse and Misuse of the CMIP6 ensemble: I’ll discuss this in more detail in another post, but I want to commend the IPCC authors for dealing with the increased spread in the CMIP6 ensemble climate sensitivity in two very sensible ways. Firstly, the use constrained projections for all the temperature (and sea level) time series out to 2100 allows them to downweight (effectively to zero) the high (and low) ECS models that are outside of the assessed range (note this would not have made much (if any) difference in CMIP5). Secondly, they choose to focus on the patterns of change, not for certain time-periods, but for specific “Global Warming Levels” (GWLs). That is to say, what the expected pattern of rainfall (for instance) might be when the global mean temperature reaches 1.5ºC, or 2ºC or 4ºC etc. This allows them to include all the models (including good models with improved climatologies that happen to have high ECS like the NCAR CESM2 or the HadGEM3 models). Additionally, the GWL impacts plots neatly divorce the limited scenarios that were used in CMIP6 from the ability to assess impacts. Thus if policy-makers or others want to explore the impacts of other scenarios that might reach specific warming levels earlier or later than any one of the SSPs, they can do so easily, without having to rerun the models.
4. The radiative forcing bar chart has gone full circle: Almost every IPCC report has a version of the radiative bar chart showing the contributions over the historical period of all the different forcings (greenhouse gases, aerosols, solar, etc.). Every iteration has changed in trivial and sometimes substantive ways (I wrote a history of this a few years back), for instance, it’s oscillated from a vertical or horizontal presentation for no apparent reason, and the individual components have followed the scientific views of what was important. In this SPM it appears in Fig. SPM 2c and has gone back to being vertical. They have stuck with the contributions by emission (as opposed to concentration – something we pushed for last time), but the novelty here is that they are plotting the estimated temperature impact from each of the contributions (using the radiative forcing, the assessed climate sensitivity and an simple impulse-response model). Oddly enough this is most reminiscent of the very first bar chart that appeared in Hansen et al. (1981) which can be seen here.
5. Droughts and floods are complicated: The picture on droughts and floods is more complicated than most people think. First, there is a great deal of regional variation, second the historical metrics we use to assess drought (such as the Palmer Drought Severity Index) don’t perform very well in a changing climate, and third the attribution of meteorological or agricultural or ecological drought varies as well. Analyses that average over too wide an area, or that look at the wrong metrics, will come to erroneous conclusions. The IPCC authors went to a lot of trouble to disentangle this and the assessment in Fig. SPM3 of evidence for observed drought changes focuses specifically on agricultural and ecological drought (based on soil moisture), as opposed to hydrological drought (runoff) or meteorological drought (based on rainfall). This is because we don’t see strong attribution in total rainfall amounts, but we do in evaporative demand (which depends on temperature). Thus when we have a longterm precipitation anomaly (such as in the American South West (WNA in the hexagon plot above)), we can’t (yet) attribute the rainfall change, but we can attribute the soil moisture change. Floods are also complicated because they too don’t solely depend on a single factor (such as intense precipitation) – but instead are a function of prior state of soil moisture, water management practices and other hyper-local effects. Work here will continue to advance, but the picture is clear only in a few regions so far.
6. Don’t mention the hiatus: Readers will probably remember the prominence of the ‘hiatus’ in the discussions around the AR5 report (written in 2013) (see here, here, or here). Due in part to (IMO) an over-reliance on a single temperature record (HadCRUT4), and (it turns out) non-climatic biases in the ocean temperature records, the trends from 1998-2012 got a specific call out in the AR5 SPM:
In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and interannual variability (see Figure SPM.1). Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade)5. {2.4}
Section B.1, SPM AR5
The observed reduction in surface warming trend over the period 1998 to 2012 as compared to the period 1951 to 2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from natural internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. There is medium confidence that natural internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of natural internal variability. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (dominated by the effects of aerosols). {9.4, Box 9.2, 10.3, Box 10.2, 11.3}
Section D.1, SPM, AR5
Now however, the updates to the historical warming, the use of four datasets instead of one, and of course, the series of record breaking years subsequently (2014, 2015, 2016/2020), the issue of variability in decadal trends is no longer so salient. The shifts in the quoted trends (1998-2012 is now 0.12ºC/decade, 1951-2012 is 0.13ºC, HadCRUT5) underlines the trivialness of the issue. To be fair, there is one mention of the hiatus in the AR6 Technical Summary:
The observed slower global surface temperature increase (relative to preceding and following periods) in the 1998–2012 period, sometimes referred to as ‘the hiatus’, was temporary (very high confidence). The increase in global surface temperature during the 1998–2012 period is also greater in the data sets used in the AR6 assessment than in those available at the time of AR5. Using these updated observational data sets and a like- for-like consistent comparison of simulated and observed global surface temperature, all observed estimates of the 1998–2012 trend lie within the very likely range of CMIP6 trends. Since 2012, global surface temperature has warmed strongly, with the past five years (2016–2020) being the hottest five-year period between 1850 and 2020 (high confidence). {2.3.1, 3.3.1, 3.5.1, Cross-Chapter Box 3.1}
AR6 Cross-Section Box TS.1
Let this episode stand as a clear reminder for assessment reports not to get ahead of the science…
And so to bed…
Finally, let me finish up with a couple of personal observations. This was the first IPCC report where I was involved in the SPM approval process, and while that was frustrating at times, the vast majority of delegates were obviously focussed on getting the best summary consistent with the science that they could. Obviously, some countries had specific sensitivities, but seeing the negotiations on how those issues could be finessed while sticking to language that the authors approved of was impressive. In particular, the chairing of the meeting by Valerie Masson-Delmotte was a masterclass in effective meeting strategies. The virtual nature of the proceedings means that this was undoubtedly the SPM approval session with the lowest carbon footprint which might serve as a model for future efforts. However, the ‘all time zone’ nature of the proceedings and the very interrupted nature of my resulting sleep patterns has left a mark on pretty much everyone involved. Forgive me if I sleep in for the rest of this week…
References
- J. Hansen, D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and G. Russell, "Climate Impact of Increasing Atmospheric Carbon Dioxide", Science, vol. 213, pp. 957-966, 1981. http://dx.doi.org/10.1126/science.213.4511.957
We are not reaching 1.5ºC earlier than previously thought
Guest commentary by Malte Meinshausen, Zebedee Nicholls, and Piers Forster
Of all the troubling headlines emerging from the release of the Intergovernmental Panel on Climate Change (IPCC) WG1 report, one warning will surely dominate headlines in the next days and weeks: Earth is likely to reach the crucial 1.5℃ warming limit in the early 2030s.
In 2018, the IPCC Special Report on 1.5C warming stated in its summary for policy makers that the world was likely to cross the 1.5℃ threshold between 2030 and 2052, if current warming trends continue.
In this latest AR6, a more comprehensive assessment was undertaken to estimate when a warming level of 1.5℃ might be reached. As a result, some early media reports suggest 1.5ºC warming is now anticipated 10-years earlier than previously assumed (AFR, THE TIMES).
We want to explain here why that is not backed up by a rigorous comparison of the SR1.5 and AR6 reports. In fact, the science in the previous SR1.5 report and the new AR6 report are remarkably consistent.
[Read more…] about We are not reaching 1.5ºC earlier than previously thoughtThe IPCC Sixth Assessment Report
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.
- AR6 of the Best. Half a dozen takeaways from the report from Gavin
- New (8/13): Sea Level Rise in AR6 from Stefan
- A Tale of Two Hockey Sticks by Mike
- #NotAllModels discusses the use (and mis-use) of the CMIP6 ensemble by Gavin
- We are not reaching 1.5ºC earlier than previously thought from guest authors Malte Meinshausen, Zebedee Nicholls and Piers Forster
- New (8/12): Deciphering the SPM AR6 WG1 Code by Rasmus
- New (8/12): A deep dive into the IPCC’s updated carbon budget numbers from guest author Joeri Rogelj
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.
Realclimate redesign
After more than 15 years with basically the same layout, the Realclimate website – while still functional – has become increasing anachronistic both in appearance and ‘under the hood’. In order to take advantage of more up to date web-site designs and new features that have been developed since the early 2000s (!), we need to upgrade the site, and while we are at it, update the theme and design, while maintaining an aesthetic link to the original.
With this post, we can reveal the new layout.
The differences are not related to content but to appearance – all of the material is still here – but the new layout is much fresher and includes much better support for people reading the site on their phones or iPads. We are using larger typefaces, cleaner menus and a more content-focused presentation for posts.
All of this is intended to improve the reader/commenter experience, but it’s inevitable that we have missed some aspects and/or some of the design features may be suboptimal for some. We will endeavor to fix any issues or problems that you find. So please comment below if you like it or hate it, if you find anything that’s broken or if you have any specific requests for tweaks. We’ll try and iterate over the next few days to get it right.
Thanks!
Unforced Variations: Aug 2021
This month is IPCC month – the Sixth Assessment Report from Working Group 1 is out on Monday August 9. We’ll have some detailed comments once it’s out, but in the meantime, feel free to speculate widely (always considering that IPCC is restricted to assessing existing literature…).
Open thread – please stick to climate science topics.
Climate adaptation should be based on robust regional climate information
Climate adaptation steams forward with an accelerated speed that can be seen through the Climate Adaptation Summit in January (see previous post), the ECCA 2021 in May/June, and the upcoming COP26. Recent extreme events may spur this development even further (see previous post about attribution of recent heatwaves).
To aid climate adaptation, Europe’s Climate-Adapt programme provides a wealth of resources, such as guidance, case studies and videos. This is a good start, but a clear and transparent account on how to use the actual climate information for adaptation seems to be missing. How can projections of future heatwaves or extreme rainfall help practitioners, and how to interpret this kind of information?
[Read more…] about Climate adaptation should be based on robust regional climate informationRapid attribution of PNW heatwave
Summary: It was almost impossible for the temperatures seen recently in the Pacific North West heatwave to have occurred without global warming. And only improbable with it.
It’s been clear for at least a decade that global warming has been in general increasing the intensity of heat waves, with clear trends in observed maximum temperatures that match what climate models have been predicting. For the specific situation in the Pacific NorthWest at the end of June, we now have the first attribution analysis from the World Weather Attribution group – a consortium of climate experts from around the world working on extreme event attribution. Their preprint (Philip et al.) is available here.
[Read more…] about Rapid attribution of PNW heatwaveUnforced Variations: July 2021
This month’s open thread for climate science. Probably a good time to discuss attribution for extreme heat, wildfires, hurricane intensity and intense precipitation.