2°C is Dead; Long Live 2°C!

A review of James Hansen's recent article on the acceleration of global warming

James Hansen's research group's new paper is out, and it’s been gaining attention thanks to a follow up article in The Guardian in which Hansen says that the 2°C global warming limit set by the Paris Agreement is fundamentally dead, because of an acceleration in the rate of global warming. In this post I will recap the main findings of the paper and then provide a bit of a contextual review of the paper…

The article, titled “Global Warming Has Accelerated: Are the United Nations and the Public Well-Informed?” reiterates previous arguments the team has made about the “Faustian Bargain” associated with humanity's reductions of cooling aerosols (which have hitherto masked the warming effect of GHGs).

The authors assert that the rate of global warming is accelerating, and that it will continue to increase at a faster rate than the IPCC expects. Why do they believe this? First, they not how all the various natural climate forcings - from volcanoes to changes in the sun's energy - etc., are all well known. So are the anthropogenic forcings of GHGs…. but, they note the anthropogenic forcing with the greatest uncertainty is Aerosols. Aerosol forcing is complicated: First by reflecting and absorbing sunlight, but then also by the indirect affect they have on clouds. Also, the cooling effect of aerosols is stronger in cleaner air. The paper then takes all this information to argue that the IPCC's modelled forcing for aerosols is off.

This in turn presents a serious problem: If the IPCC's estimates for the cooling power of aerosols is too low, that implies that the IPCC's estimates for the WARMING power of GHGs is.... also too low. In other words, the expected warming from GHGs is UNDERESTIMATED. The IPCC’s existing estimate of climate sensitivity is 3°C. But if that is the case, they note, then we would not have seen the amount of warming we did in 2023. In reality, as aerosols are reduced we are seeing more warming than the climate models expected.

One telltale sign, they note, is the unplanned experiment occurring over the oceans as a result of new Shipping Fuel regulations, which reduced the amount of sulfate aerosols. They argue this unmasked GHG warming AND reduced Earth's reflectivity (via cloud feedbacks).

So, the first main conclusion of the paper is that because climate sensitivity and aerosol forcing is stronger than IPCC estimates, we're already passed +1.5 °C of warming, and MUST reduce GHGs ASAP! Specifically, we need to stop burning fossil fuels, as fossil fuel emissions will determine the future of global warming.

Failing to reduce GHGs could cause us to reach a Point of No Return; namely seen in the collapse of the West Antarctic ice sheet, which could be kickstarted by a shutdown of the AMOC, and cause several meters of sea level rise. We're 20-30 years away from this, they say [more on this exceptional claim below].

A final conclusion is that humanity can avoid “Point of No Return" by phasing out fossil fuels, scaling up nuclear power, modernizing technologies, carbon pricing, and possibly even using Solar Radiation Modification (aka geoengineering). The article concludes by placing hope in young people.

Contextual Review

I now feel it's important to provide a bit of *academic* context on Hansen's article, as it might shape how people interpret some of the claims made within it, such as the one above about AMOC collapse being "likely in the next 20-30 years” (and on The Guardian’s headline about 2°C being ‘dead’).

First, this is not your typical research article, nor your typical journal. Environment: Science and Policy for Sustainable Development brands itself as "peer-reviewed magazine" specifically giving voice to "highly respected researchers and practitioners who provide a broad range of international perspectives and offer appropriate actions at local and global levels." It's a venue for folks like James Hansen - a highly respected expert - to discuss his ideas for a broad audience in "accessible" terms.

This is important contextual information because Hansen and colleagues are not here reporting the specific results of a novel study. Rather, they are reporting a broad 'update' of work that they've been carrying out and thinking about (and writing about) for the last couple years. The work offers a whole bunch of policy recommendations (from carbon pricing to nuclear energy scale-ups to Solar Radiation Modification), which are entirely normative - that is, they would never be acceptable claims for a peer-reviewed study because they don't necessarily logically flow from the main findings of the paper (regarding how aerosols forcing changes our understanding of climate sensitivity).

The article itself is highly unusual for a typical peer-reviewed article - it is even highly unusual for this specific publication (which proposes a word limit of 5000 words for articles; Hansen et al's text is closer to 50,000 words, 10X the limit, which is one of many signs that they've afforded Hansen unusual 'privileges' that the vast majority of authors would not get, based on who he is).

That is, most articles of this nature would have been flatly rejected by editors and peer reviewers, merely because of it's length - you get the sense of this after the abstract itself, which is abnormally long for an article abstract. The abstract, in turn, hints at some of the major 'liberties' that Hansen took in writing this piece - a massive tome covering everything from assessments of climate forcings to criticisms of the IPCC, to policy proposals and even normative statements about hope in younger generations. Another highly usual feature: the article contains an Epilogue, which itself is multiple pages and contains a host of new ideas not thoroughly covered in the article itself.

All this to say, there is NO WAY this would be published as a peer-reviewed article in a typical peer-reviewed journal: It is too large, has too many moving parts, doesn't present novel findings (rather reiterates findings the team has already reported), and contains too many unsubstantiated claims.

This is not to take away from the legitimacy or the veracity of the article's main points - it is indeed co-authored by some of our world's best climate scientists - and these people know what they're talking about. Rather, my point is you just wouldn't see this published as a typical peer-reviewed article (which may be a problem with academic publishing model, but that's another matter).

And this relates to the claim made about the AMOC. Taken out of context, one might assume that this was a study that evaluated the weakening of the AMOC current and came up with an estimate of when the current might collapse. But this is not quite right. In fact, you may be surprised to learn that the article PDF itself (what the printed magazine article itself looks like) DOES NOT INCLUDE ANY MENTION OF THE AMOC! This is because all mention of the AMOC in the article is either found in the ABSTRACT (which is not included in the Magazine PDF), or the SUPPLEMENTARY MATERIALS document (also not part of the actual Magazine article PDF)...

The Supplementary Materials itself is an additional 23 page document, and there you WILL find a discussion of AMOC and where the "20-30 years" assessment comes from. It is indeed an extensive assessment. Nine pages carefully making the team's estimate based on existing observations around warming and projections of ice melt, drawing upon research by AMOC experts. This estimate is then used to back up the claim in the abstract, but typically that's not how it works (usually claims made in the paper need to be substantiated in the paper itself, which is the primary focus of peer reviewers).

So, is the "20-30 years" estimate accurate? It could be ... but it could also be way off! I think it's best to think of it as an educated guess by highly-respected experts in the field, based upon their own projections drawing from the available evidence.

What I would NOT consider it, however, is the result of a Peer-Reviewed Study specifically on the timing of an AMOC collapse. Those studies *do exists*, and their findings range significantly (from a likely mid-Century current shut down to a mid-Century current *tipping point* to a late Century *tipping point* to even some research that doesn't expect a full shutdown at all). One of the things that a Peer-Reviewed study of that nature requires authors to do is situate their own estimates within the literature, to offer an uncertainty assessment, and justify claims and methods. This article does *some of that* in the Supplementary Resources document, but the tone is mostly "existing climate models are doing this wrong; our approach is better and this is what we find".

And this all comes back to The Guardian’s headline about the paper, and the claim that 2°C is now dead. Well, it may be, but that’s not really stated anywhere in the actual paper! That is a quote from Hansen when being interviewed by the reporter. He told the journalist that “the 2C target is dead, because the global energy use is rising, and it will continue to rise,” but this claim is not directly from the actual paper. It may be implied, because the 2°C would be reached by the year 2045 in this new analysis, unless geoengineering tools are used to artificially cool the climate. Anyway, just felt a bit of context was in order!