Comments on New Article by James Hansen

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!)
(from Matthews & Solomon, 2013)
It is the basis of the concept of a “carbon budget” (i.e. the notion that there is a specified amount 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:
(from IPCC Special Report 15, 2018)
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, and carbon 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!
As nearly all (roughly 90%) of the global energy imbalance goes into ocean heating, a constant rate of increase in ocean heat content implies a constant, rather than increasing planetary energy imbalance, in contradiction of the claims by Jim and co-authors. Admittedly there is a range of estimates by different groups (both with respect to ocean heat content and other satellite-based measures of energy imbalance), but it is speculative at best to argue that there is a robust and statistically-significant increase in planetary energy imbalance. 
ADDENDUM (11/4):
Some individuals have questioned whether the observation above (i.e. that there is, by inspection, a steady trend in the Cheng et al ocean heat content data (OHC) over the past few decades) can be established statistically. Please see below. On the right is the OHC data with a best-fit linear trend from 1990-present (which includes the 2023 values today). A test for a quadratic (i.e. acceleration) term in the regression yields a null result to 4 significant digits, i.e. there is no statistically-significant "acceleration" term in the regression model, i.e. statistically speaking, the trend is indeed steady, not accelerating. Another approach is to look at the heating rate with time (essentially, the first derivative of the OHC with respect to time). If there is an increase in energy imbalance over time, we should see an increase in the heating rate. There is no evidence for this. The linear trend over the past few decades in heating rate is statistically zero. The distribution of 30 year linear trends (red lines) for ending dates of each of the past 10 years, starting with 2023 (thicker red line), is shown on the left. The trends are all statistically indistinguishable from zero, and indeed scatter about a zero trend mean. So, in short, the Cheng et al OHC data (I concede I have a preference for this version of OHC estimates as a co-author w/ Cheng et al) do not support a growing planetary energy imbalance. It remains an open problem as to how to reconcile the claims (e.g. in the new Hansen article) of an increasing planetary energy imbalance with the ocean observations. I look forward to further scientific work aimed at solving this puzzle. 
3. There is, furthermore, no statistical support for the claim that surface warming is currently accelerating. It is certainly true that the rate has increased since the 1970s, but that's related to changes in aerosol forcing at that time that are not relevant to the warming of the past few decades. Over the past few decades, there is no statistically-supportable evidence of an increase in the rate of surface warming. Surface warming has continued at a remarkably constant rate for the past few decades, as I recently showed in this thread on twitter (see plot below). The warming of the planet (and all of the worsening impacts associated with it including extreme weather events and intensified hurricanes) will continue until we bring carbon emissions to zero. The truth, once again, is bad enough.

4. The model simulations of the Intergovernmental Panel on Climate Change (IPCC) have accurately predicted the observed warming to date (see this NASA press release from 2020 for background). Even the anomalously-warm, El Nino-boosted provisional (year to date) 2023 global surface temperature is well within the predicted range of the models. Here is an updated version of the aforementioned NASA comparison including the provisional 2023 value (green vertical bar showing likely value and uncertainty range):

There is no evidence that the models are under-predicting human-caused warming. Indeed, the claim that warming is somehow accelerating now is no more accurate than the false claim, advanced by climate change deniers at the time, that global warming had “stopped” in the first decade of the 21st century.


5. There is no evidence that changes in ship-based aerosols have played any substantial role at all in recent warming trends. The existing literature on this (hat tip to Zeke Hausfather here) indicates a net effect on global surface temperatures of 0.05-0.06C: