Science Giants Square Off on Pace of Warming

James Hansen will probably go down in history with Isaac Newton as someone who fundamentally changed the way we think about physics and our planet.
Hansen has a new paper that makes some dramatic and sobering assertions relating to the high impact extremes we have seen in recent years, and expectations for near future.
Mike Mann has been, with some others, respectfully, pushing back.

James Hansen in Oxford Open Climate Change:

Equilibrium global warming for today’s GHG amount is 10°C, which is reduced to 8°C by today’s human-made aerosols. Equilibrium warming is not ‘committed’ warming; rapid phaseout of GHG emissions would prevent most equilibrium warming from occurring. However, decline of aerosol emissions since 2010 should increase the 1970–2010 global warming rate of 0.18°C per decade to a post-2010 rate of at least 0.27°C per decade. Thus, under the present geopolitical approach to GHG emissions, global warming will exceed 1.5°C in the 2020s and 2°C before 2050. 

Mike Mann at MichaelMann.net:

Let me preface my commentary by saying that I have nothing but the greatest respect for James Hansen. He has been a fundamental contributor to the advance of our science and a personal hero to me and many other climate scientists of my generation. I believe (and have stated) that he was wronged by the Nobel committee in not sharing in the 2021 Nobel Prize in Physics awarded for fundamental contributions to climate science. 

It has always been risky to ignore his warnings and admonitions. I say as much in my profile of Jim (a short excerpt of which is shown above) from my 2015 book Dire Predictions: Understanding Climate Change. So it with no pleasure whatsoever that I find myself in a position to have to criticize his latest work.

Jim and his co-authors are very much out of the mainstream with their newly published paper in the journal Oxford Open Climate Change. That’s fine, healthy skepticism is a valuable thing in science. But the standard is high when you’re challenging the prevailing scientific understanding, and I don’t think they’ve met that standard, by a longshot, for the following reasons:

1. Let’s take the title itself, “Global Warming in the Pipeline”. The latest science on this, including state-of-the-art models that deal with the complexities of the ocean carbon cycle, conclude that the “zero emissions commitment” or “ZEC” (how much warming is expected when emissions reach zero), is ZERO degrees warming, i.e. warming stops when carbon emissions reach zero. This understanding goes back more than a decade (see e.g. Matthews and Solomon, “Irreversible Does Not Mean Unavoidable”, Science, 2013 for a review of the science) (Watch this space for significant further developments regarding ZEC in the coming weeks!)

It is the basis of the concept of a “carbon budget” (i.e. the notion that there is a specified about of cumulative carbon emissions up to a given point in time that keeps warming below a specified level), including the widely-cited rule of thumb that we must reduce carbon emissions to zero by 2050 to avoid more than 1.5C warming. There is uncertainty about the effect of radiatively active shorter-term agents other than carbon dioxide (e.g. methane, black carbon, ozone, aerosols, etc), but the estimates are that in most scenarios these likely cancel out, and it pretty much comes down to just the carbon dioxide emissions (see e.g. this graphic from the IPCC (2018) special report on 1.5C and 2C warming:

So our best estimates today are that surface warming stops when carbon emissions stop, i.e.that there is no additional surface warming in the pipeline when emissions reach zero. The notion that there are decades of committed surface warming after emissions reach zero is based on outdated simulations that did not take into account the interactive role of the ocean carbon cycle. While the science on this is more than a decade old, this significant paradigm shift in our understanding of committed warming has still failed to be widely understood or recognized in much of the public discourse over climate science (see this op-ed I co-authored in the Washington Post about that last year). The point is that whether or not the 1.5C target is reachable is a matter of policy, not climate physics, at this point. It’s fine for Jim and his colleagues to explore scenarios where we do not act soon enough, andcarbon emissions are not lowered adequately to avert specific warming targets such as 1.5C or 2C, but it should be clear that the differences in their conclusions are a result of those policy and behavioral assumptions, not climate physics. 

2. The claim that the energy imbalance is increasing is not supported by ocean heat content data. As reported in the Guardian earlier this year, the latest estimates (by a group of scientists of which I am a member–see our publication here) demonstrate a very steady, rather than accelerating, increase in ocean heat content during the past few decades, as shown below. As I like to say, the truth is bad enough!

Worth looking at the full piece at the link.