This month’s open thread on climate topics. Please be constructive and keep nonsense to a minimum.

The post Unforced Variations: Sep 2026 first appeared on RealClimate.

A new paper undermines totally inaccurate claims in the DOE report.

Readers will recall the litany of cherry picks, misrepresentations and evasions in the DOE “Climate Working Group” (CWG) report that we discussed multiple times last year. In particular, we highlighted an egregiously bad presentation by Steve Koonin at UT Austin. In that presentation, there was a figure presented (around 29:00) providing (it seemed) evidence that CMIP6 climate models were missing out on a massive warming of the tropical lower stratosphere and that the models had (quote) “a fundamental misunderstanding … of how the atmosphere warms”.

This was not just an overstatement based on a biased comparison, but rather, it was totally wrong. Notably, while the DOE report made similar claims, it did not include this figure – which has not been published anywhere else either.

The new paper (Santer et al., 2026) demonstrates the errors clearly. First, they use the CMIP6 data for the 2020-2025 tropical MSU/SSU diagnostics (similar to the comparisons that we update every year) and compare that data to the satellite temperature data at multiple reported levels (LT, TT, LS, and SSU1-3) (note that TT is TMT corrected for the lower-stratospheric influence).

The recent cooling of the stratosphere is clearly visible in both the models and the observations (very consistently), as is the warming in the troposphere, and in both cases there is a significant offset from the control run trends (indicating that there is a clear forced trend). The only level where there isn’t a clear departure from the control run trends, is the lower stratosphere, but there the observations and models (historical and control) are still indistinguishable.

Very clearly, the observed trends in the tropical lower stratosphere are within ±0.1ºC/decade and the signals are expected to be small compared to the noise. On the other hand, the observed trends shown in Koonin’s (unpublished) figure are on average ~0.27ºC/dec (range of 0.1 to 0.33ºC/dec). These two calculations cannot both be correct. And indeed, John Christy confirmed (in the paper’s acknowledgements) that the data in Koonin’s graph was just wrong (though it’s not exactly obvious how).

Thus Koonin’s claims are not only based on cherry-picks but also on fundamentally incorrect comparisons. LOL.

The point here (and in the new paper) is not merely to correct the record, but rather to illustrate why assessments are done the way they are. Relying on half-baked bespoke calculations that have not been peer-reviewed (or even sanity-checked) is just a recipe for embarrassment. Assessments (like the NCA or the IPCC reports) have got better over the years at trying to minimise this kind of error and are now quite good at it (by relying only on published references and extensive peer review at all stages). The cos-playing being done by the CWG simply recapitulates all the pitfalls that led to the modern assessment process in the first place, making the case (again) that these efforts have to be serious to avoid wasting everyone’s time.

References

1. B.D. Santer, S. Solomon, D.W.J. Thompson, Q. Fu, and A.E. Dessler, "Misrepresentation of Human Climate Fingerprint in DOE Report", Earth's Future, vol. 14, 2026. http://dx.doi.org/10.1029/2026EF008831

The post Koonin’s Temperature of the LOL Stratosphere first appeared on RealClimate.

This month’s open thread on climate topics. The annual ranking horse race is in full swing, discussions of extreme weather abound, Arctic sea ice is reaching it’s annual minimum, and the future of evidence-based policy making seems dim. Oh well.

The post Unforced Variations: Aug 2026 first appeared on RealClimate.

There is a new El Niño out there and it was officially declared already on June 11 by NOAA. This is both unusually early in the year and very soon since the last El Niño in 2023-24. Another remarkable thing is the seasonal forecasts, which for a couple of months have indicated that it may well be at strengths by the end of the year that we have not seen before. All these three aspects combined seem to make this El Niño different to the previous ones.

One question is whether such El Niño events, which is a phase of the El Niño Southern Oscillation (ENSO) phenomenon, is changing because of global warming.

ENSO is a natural phenomenon, but so is also the greenhouse effect. We know that exploitation of fossil resources releases greenhouse gases, such as CO2 and methane, which strengthen the greenhouse effect. This is well-established knowledge.

The first time I was confronted with the question whether global warming also may affect ENSO was back in the 1990s, when I did my D.Phil. on mechanisms responsible for ENSO dynamics.

It was also the time seasonal forecasting was established at the European Centre for Medium Range Forecasting (ECMWF), and it was very much focused on ENSO. Since then, the seasonal forecasts for ENSO have made impressive progress and are now quite accurate (see the graphic above).

One indicator central for ENSO is NINO3.4 which provides a measure of how much the average sea surface temperature (SST) over the region 5°S-5°N/120°W-170°W deviates from normal conditions. The latest seasonal forecasts for NINO3.4 are shown in the graphics above.

Jacob Bjerknes was a pioneer on ENSO and the first to realise that El Niño events were coupled with changes in the atmosphere, the Southern Oscillation. Since ENSO indeed involves a coupling between the ocean and the atmosphere, there are several ways that global warming plausibly may affect how ENSO behaves.

The mechanisms that drive the ENSO dynamics include cloud formation, how winds respond to temperature changes, and how Rossby and Kelvin waves propagate in the ocean. In my D.Phil thesis from 1997, I found that the propagation Kelvin waves is affected by changes in the temperature structure. There have been many studies since then, and different global climate model studies (GCMs) have pointed in different directions.

One problem is that the GCMs may not have had detailed descriptions of small-scale parts of the system with sufficiency accuracy to give a robust result, and a quote from the Intergovernmental Panel on Climate Change, assessment report 6 by working group 1 (IPCC AR6 WG1) sums it up:

CMIP6 models are able to reproduce most aspects of the spatial structure and variance of the El Niño–Southern Oscillation (ENSO) and Indian Ocean Basin and Dipole modes of variability (medium confidence). However, despite a slight improvement in CMIP6, some underlying processes are still poorly represented.

In this case, CMIP6 refers to the World Climate Research Programme’s (WCRP) Coupled Model Intercomparison Project phase 6. While the IPCC AR6 WG1 concludes that

There is no consensus from models for a systematic change in amplitude of ENSO sea surface temperature (SST) variability over the 21st century in any of the SSP scenarios assessed,

I still think that this question is not resolved, since there are also studies suggesting that the models indicate that a global warming may result in stronger El Nino events, e.g. Cai et al., (2014) and Fredriksen et al., (2020).

References

1. W. Cai, S. Borlace, M. Lengaigne, P. van Rensch, M. Collins, G. Vecchi, A. Timmermann, A. Santoso, M.J. McPhaden, L. Wu, M.H. England, G. Wang, E. Guilyardi, and F. Jin, "Increasing frequency of extreme El Niño events due to greenhouse warming", Nature Climate Change, vol. 4, pp. 111-116, 2014. http://dx.doi.org/10.1038/nclimate2100
2. H. Fredriksen, J. Berner, A.C. Subramanian, and A. Capotondi, "How Does El Niño–Southern Oscillation Change Under Global Warming—A First Look at CMIP6", Geophysical Research Letters, vol. 47, 2020. http://dx.doi.org/10.1029/2020GL090640

The post This new El Niño is different first appeared on RealClimate.

This month’s open thread on climate topics. Impending El Niño, fossil fuel funded wedges, retirements, heat waves, and the same old, same old, from the usual suspects. Try to discuss these things calmly!

The post Unforced Variations: July 2026 first appeared on RealClimate.

This month’s open thread on climate topics. Please be respectful and substantive.

The post Unforced Variations: June 2026 first appeared on RealClimate.

The 4th UArctic congress on the Faroe islands finished with the message that the Arctic Council is still alive. It has overcome recent setbacks with difficulties concerning two of its member states, Russia and the US. The Arctic Council represents 8 nations together with indigenous peoples and has observers from around the world, and this wide-ranging diversity of course coloured the congress. It was allegedly the largest scientific conference on the Faroe islands ever, as it was combined with the Oceans Connectivity Conference.

I participated at the congress as a member of the Arctic Monitoring and Assessment Programme’s (AMAP) climate expert group to present the Arctic Climate Update reports for 2024 and 2026 (in progress).

Some of the topics that interested me included climate change, so-called “climate interventions”, and the Atlantic meridional overturning circulation (AMOC) which also has been discussed previously by Stefan. I noted that the question concerning changes to the AMOC is still being discussed, and scholars have different opinions on whether it will collapse or if it merely will weaken to a more modest degree.

While everybody at the congress agreed on climate change, the question concerning climate interventions, formally known as “geoengineering”, was highly controversial. I was invited to a side meeting organised by Ocean Visions on rescuing the sea ice. 

I expressed my doubts about the idea of geoengineering in the Arctic, as the region seems to have a semi-permanent cloud-cover that moderates the sea ice albedo during summer. The strongest Arctic warming by far, after all, takes place during the winter and the Polar Nights, in darkness when albedo cannot play a role. 

The sea ice does nevertheless play an important role for the warming, as retreating sea ice exposes a warmer ocean surface underneath. But sea ice is not necessarily the top amplifying feedback mechanism in the Arctic, according to AMAP’s Adaptive Actions in a Changing Arctic report for the Barents Sea region from 2017 (p.65). There are also some preliminary analyses suggesting that low-level cloudiness increases where the sea ice retreats. However, the cloud cover in different reanalyses and satellite observations are not entirely consistent, and more work is needed to address this question. 

Other reasons why spraying particles into the air is a bad idea is that it doesn’t deal with the oceanic acidification, which is particularly a problem for the cold oceans in the Arctic. It’s a big concern for marine ecosystems. Furthermore, it completely ignores the fact that the global hydrological cycle is entangled with the greenhouse effect and is also affected by our exploitation of fossile resources.

Finally, geoengineering opens a can of worms when it comes to conspiracy theories that may pave way for new unnecessary conflicts. We have plenty of such examples from the past: chemtrails, Haarp, Climate denial, 5G-COVID, and anti-vaxxers.

The post “The Arctic Council is not dead” first appeared on RealClimate.

The fantasy version of the normal updating of scenarios for a new round of CMIP simulations doing the rounds is bad faith BS.

As climate folk will know, the community is currently embarking on a new round of climate model simulations to support analyses and projections for the next IPCC report (due in 2028/9). This new effort has been dubbed CMIP7, because it is the sixth iteration of the CMIP effort (IYKYK), that started in the late 1990s. For each of these iterations, a new set of projections has been formulated for the modeling groups to use and the ones for this round were just published (van Vuuren et al., 2026). So far, so totally normal.

Why do scenarios need updating? Why can’t we use the three scenarios that Hansen et al. (1988) first came up with in the early 1980s? Three reasons. First the scenarios need to be continuous with the trajectories of the observed changes. The ‘join’ point was 1984 for Hansen’s original scenarios, then 2000 for the CMIP3, 2005 for CMIP5, 2014 for CMIP6, and it will be 2023 for CMIP7. As you can imagine, things have changed over the last 40 years (the Montreal Protocol, the Clean Air Acts, renewable energy price falls, fracking, the Paris Agreement, actual climate policies, reversal of climate policies, etc.). All of these things are the result of humans behaving in ways that humans behave and which are not easily predictable ahead of time. This is why future simulations have long been described as ‘projections’ and not predictions.

Second, the rationale for future scenarios has shifted in light of what we (as a society) are doing. At the beginning it made sense to think about a spread of baseline scenarios where no climate policy was enacted: “Business as usual” so to speak. But now? we have already done things and so ‘business’ is no longer ‘usual’. Now, ranges based on ‘current policies’, ‘current aspirations’ and ‘possible backsliding’ are perhaps more useful. Additionally, we are now much closer to 2100 than we used to be (also obvious, but often forgotten) and so scenarios need to be extended out further.

Third, what we are making scenarios for has expanded enormously. In 1984 there were only concentrations of well-mixed greenhouse gases, the solar cycle and the occasional volcano to project, but now, we have emissions of GHGs including CO₂, plus all of the halogenated gases, the short lived climate forcers (CH4, aerosols, NOx, SO₂/SO₄), land use change (deforestation, irrigation, agricultural shifts), possible anthropogenic impacts on dust and fire, and freshwater inputs from melting glaciers and ice sheets that are not otherwise represented in models. Did I mention nitrogen inputs, solar particle fluxes, and volcanic emissions of water vapor as well as sulfates? Keeping this all coherent and up-to-date is an enormous undertaking.

This all means that, duh, of course the scenarios would be updated for CMIP7.

People (hi Roger!) acting as if the publication of new CMIP7 scenarios is some huge policy shift or an admission that previous scenarios are no longer ‘official’ are just bull-shitting. This is something that was planned for and expected for literally years. Previous scenario sets were used in previous rounds, a new set will be used for the new round – that is all there is to it.

Oh noes!

The supposed focus of the ire are the high end scenarios of RCP85 (CMIP5) and SSP5-85 (in CMIP6). The reasons why these were set up in the first place (back in 2007!) was that IPCC wanted to span what had appeared in the literature before then – going beyond (in sophistication) what the (CMIP3) SRES projections had done. But the IAM folks involved decided (correctly) that they didn’t have the time to start from scratch, and so they decided to split up the task – come up with a spread that covered 99% of published scenarios with ‘representative’ concentration pathways (RCPs – gettit?) for the climate models to use quickly (in CMIP5), and back-fill plausible socio-economic pathways later (to be used in CMIP6). Thus the CMIP5 models (which were run in 2007-9 or so) used the RCPs (including RCP85 – which was so-called because it reached 8.5 W/m² of direct radiative forcing from GHGs in 2100).

For climate modellers, the reasons why the pathways are the way they are is a secondary concern – if they were only to be given CO₂ levels (and other GHGs etc.), the basic need is just for a low, middle and high scenario that encompass our most ambitious climate policy pathways, a worst case scenario (‘Burn it all!’), and something in the middle. When it came time for CMIP6 (2016-2019 or so), the SSPs (that had been promised a decade earlier) were ready, and so they were used. But for the purposes of the climate modellers, the drivers underlying the SSPs were not really that relevant. A climate model really doesn’t care how cooperative or antagonistic regional economic blocks are – it just responds to the resulting emissions. That there is a need for high end scenarios should be obvious – where are the tipping points in the system? what are the impacts of a 2ºC warmer world? what about 3ºC or 4ºC? Are these worth avoiding perhaps? Having seen these results, the answer (IMO) is definitely yes!

In the last few years, a number of people have pointed out that assumptions underlying the highest SSPs don’t look as plausible as they used to seem (this is also true of the lowest projections, but people seem less bothered by that). Note this is many years after all the models that were ever going to use them were run. But this is less of an issue that some people portray. Climate impacts rely on a chain of calculations – a specific set of emissions, a resulting concentration pathway, and a modelled sensitivity. Similar impacts can arise with lower emissions, but greater carbon cycle feedbacks and higher sensitivities, and given that each of these steps are quite uncertain, it is not really worthwhile for climate models (or modelers) to get too attached the specific storylines the IAM folks put together. Hence the collective shrug from climate modelers around the RCP85 ‘dialogue’ in the last couple of years.

Let me give two examples why high end scenarios are important: Impacts on ice sheets are a very important part of the climate change and have yet to be fully integrated into the standard climate models. So independent efforts with ice sheet models were set up using the output from the CMIP5 and CMIP6 models – they used two scenarios, RCP85 and RCP26 to bracket possibilities in ISMIP6 (Seroussi et al, 2020). Interestingly, they found that, particularly for Greenland, that none of the models had melt rates as high as observed even with RCP85 forcing. Thus for a situation where the (ice sheet) models are insufficiently sensitive, a higher than expected forcing might give you a more likely outcome.

A second example is the use of ‘global warming levels’ in the last IPCC assessment. These were averages of the models when they reached particular temperature levels (2ºC, 3ºC, 4ºC etc.), but for that to work, enough models had to reach those temperatures in order to make an average – and in practice for 3ºC and 4ºC, this was only possible with SSP5-85 scenarios. Other assessments used the higher signal-to-noise ratios in the high end scenarios to estimate sensitivities across many systems that would have been noisier and more uncertain if that was not available. How the models got there is basically irrelevant. The new high end runs will also be used for this (note that H gets to 8.5 W/m² only about 20 years after RCP85).

Note that even the harshest critics of RCP85 will admit (in academic circles at least) that these are legitimate uses. However, some of the more stupid commentaries equate the mere mention of RCP85/SSP585 with scientific misconduct, claiming that counting the number of times the ‘naughty’ words of RCP85 appear in publications or assessments is a damning indictment of the entire field’s integrity. This is so dumb and lazy that I find it hard to credit.

But wait!

The funny thing is that there are real issues with the way this whole endeavor has grown up. First, because CMIP is the only (serious) climate projection game in town, as climate change has become more salient, CMIP projections have been used to inform a far wider array of science than was imagined back in 2007. Not all of those uses are optimal. For instance, the ERA5 reanalysis still uses CMIP5 projections of solar forcing from 2008 which didn’t turn out to be so good at matching what actually happened. Consultants and banks have used CMIP6 projections as if they were real predictions, and the adaptation community have often assumed that specific CMIP6 pathways are the most likely outcomes.

All of these misuses are compounded by the fact that it appears to need a decade to update these pathways in the light of new science and societal decisions and changes. This is way too long – annual updates of the process should be achievable if funders prioritized it.

And finally, there are of course far more scenarios that would be interesting to explore in climate models (policy specific scenarios, delta scenarios (where only one thing changes at a time), in-between scenarios, annually updated scenarios etc.) than can possibly be performed given existing computational capacity. This is (right now) prohibitive, but at the rate that faster, more efficient machine-learning emulators are advancing, it might not be for much longer.

A serious critique of the climate modeling enterprise would be focused on these issues (for instance), rather than tilting at RCPs.

References

1. D. van Vuuren, B. O'Neill, C. Tebaldi, L. Chini, P. Friedlingstein, T. Hasegawa, K. Riahi, B. Sanderson, B. Govindasamy, N. Bauer, V. Eyring, C. Fall, K. Frieler, M. Gidden, L. Gohar, A. Jones, A. King, R. Knutti, E. Kriegler, P. Lawrence, C. Lennard, J. Lowe, C. Mathison, S. Mehmood, L. Prado, Q. Zhang, S. Rose, A. Ruane, C. Schleussner, R. Seferian, J. Sillmann, C. Smith, A. Sörensson, S. Panickal, K. Tachiiri, N. Vaughan, S. Vishwanathan, T. Yokohata, and T. Ziehn, "The Scenario Model Intercomparison Project for CMIP7 (ScenarioMIP-CMIP7)  ", 2025. http://dx.doi.org/10.5194/egusphere-2024-3765
2. J. Hansen, I. Fung, A. Lacis, D. Rind, S. Lebedeff, R. Ruedy, G. Russell, and P. Stone, "Global climate changes as forecast by Goddard Institute for Space Studies three‐dimensional model", Journal of Geophysical Research: Atmospheres, vol. 93, pp. 9341-9364, 1988. http://dx.doi.org/10.1029/JD093iD08p09341
3. H. Seroussi, S. Nowicki, A.J. Payne, H. Goelzer, W.H. Lipscomb, A. Abe-Ouchi, C. Agosta, T. Albrecht, X. Asay-Davis, A. Barthel, R. Calov, R. Cullather, C. Dumas, B.K. Galton-Fenzi, R. Gladstone, N.R. Golledge, J.M. Gregory, R. Greve, T. Hattermann, M.J. Hoffman, A. Humbert, P. Huybrechts, N.C. Jourdain, T. Kleiner, E. Larour, G.R. Leguy, D.P. Lowry, C.M. Little, M. Morlighem, F. Pattyn, T. Pelle, S.F. Price, A. Quiquet, R. Reese, N. Schlegel, A. Shepherd, E. Simon, R.S. Smith, F. Straneo, S. Sun, L.D. Trusel, J. Van Breedam, R.S.W. van de Wal, R. Winkelmann, C. Zhao, T. Zhang, and T. Zwinger, "ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century", The Cryosphere, vol. 14, pp. 3033-3070, 2020. http://dx.doi.org/10.5194/tc-14-3033-2020

The post Scenarios, schmenarios… first appeared on RealClimate.

This month’s open thread on climate topics. El Niño coming (shades of 2023), competitive predictions, ill-advised geo-engineering ideas, and massive shifts in renewable energy roll out. Surely something there to discuss…

The post Unforced Variations: May 2026 first appeared on RealClimate.

Somewhat belated open thread for this month! (Oops). Please stick to climate related topics, and remain respectful.

The post Unforced Variations: Apr 2026 first appeared on RealClimate.