Without action, how much will the Earth warm?

The ‘business as usual’ story is misleading

By Michael Mann

A new commentary in the journal Nature by Zeke Hausfather and Glen Peters is making the rounds today.

The commentary is similar in content and outlook to a previous piece written by Hausfather on the website of the "Breakthrough Institute" a month ago, arguing that "business as usual" burning of fossil fuels will likely only lead to 3C warming, rather than the considerably higher range of 3-5C warming typically cited based on past IPCC projections. The latter piece was relied upon heavily in a recent Wall Street Journal op-ed by Breakthrough Institute founder Ted Nordhaus that is highly dismissive of the need for rapid reduction in global carbon emissions. The new piece has predictably led to some distorted headlines, for example this one by the BBC: "Climate change: Worst emissions scenario 'misleading' " which, itself, is ironically rather misleading.

Let me provide some context and caveats about this new commentary. First of all, it is just that-a commentary, not a peer-reviewed scientific article. That must be kept in mind by anyone somehow thinking this overthrows conventional scientific thinking. It doesn't. It's basically an opinion piece.

The most recent peer-reviewed article I’m familiar with that covers this ground is a 2016 article by Rogelj et al in Nature. However, it comes up with higher numbers than Hausfater and Peters for “current policies”, with a central estimate of 3.2C warming, and as much as a 17% chance of warming in excess of 4.1C taking into account both the physical uncertainties and the scenario uncertainties. But that study used a simple climate model (MAGICC) that doesn’t account for non-linearities and, most importantly of all, doesn't include so-called “carbon cycle feedbacks”, that is to say, the feedback mechanism by which global warming can actually release more CO2 (or e.g. methane), adding further to the warming. Indeed, this deficiency applies to all studies that are based on specifying CO2 concentrations rather than emissions, and it applies to the current commentary by Hausfather & Peters.

We have seen a sobering example of the importance of these feedback mechanisms here in Australia where I am currently on sabbatical. In the catastrophic fires that have engulfed the continent (which were exacerbated and amplified by unprecedented heat and drought made possibly by climate change), roughly twice as much carbon escaped into the atmosphere as was produced by all of fossil fuel burning in Australia over the last year.

These sorts of amplifying “carbon cycle” feedback mechanisms (and this is just one example--there are many others including, for example, the potential release of frozen methane in the Arctic with warming) are not accounted for in the simple sorts of projections that Hausfather and others are using here. It is very likely that these feedback mechanisms will add substantially to the warming over the next century.

Combine that with the fact that the most recent (“CMIP6”) IPCC climate models seem to be showing the potential for considerably greater warming than the previous generation ("CMIP5") of IPCC models, and in my view, it is very difficult to rule out warming in excess of 4C under "business-as-usual" climate policies. Only with very strong mitigation efforts and rapid reduction of carbon emissions can we avoid such a scenario with a high degree of confidence.

Finally, let's not forget that even a 3C warmer world would be catastrophic. Here in Australia, we're already seeing the catastrophic impacts of less than half that much planetary warming.

Most of this critical context has been lost in the recent discussion.

There is some good news here. The numbers show that escalating efforts around the world to decarbonize our economy are starting to pay dividends. We're starting to bend that emissions curve downward. But we need to reduce emissions by a factor of two over the next decade and bring them down to zero in a matter of a few decades if we are to avert catastrophic climate change impacts. We have to get off fossil fuels far more quickly than we're on track to do under current policies.

This latest commentary doesn't change that at all.

Dr. Michael E. Mann is Distinguished Professor of Atmospheric Science at Penn State, with joint appointments in the Department of Geosciences and the Earth and Environmental Systems Institute (EESI). He is also director of the Penn State Earth System Science Center (ESSC).

This article was first published on michaelmann.net on 29 January, 2020.