In my two previous posts, I looked at two aspects of the Hansen et al pre-print, Global Warming in the Pipeline, forecasting the course of climate change this century from paleoclimate record and atmospheric aerosols. I have been trying to de-jargon the science and take it into the real world. In his own attempt to demystify and disarm his critics, Hansen writes to his newsletter subscribers:
Twitter-World “science.” We have been told that a bizarre criticism of the Pipeline paper appeared on Twitter, concluding that our paper was “wrong,” because we did not consider that GHGs in the atmosphere would soon be declining as human emissions decrease and the ocean and atmosphere absorb the human-made increase of GHGs.
We hope that even the non-scientist can see through such thinking. When Charney defined the gedanken problem: what is the equilibrium climate sensitivity (ECS) to 2×CO2 forcing, he knew that keeping CO2 fixed at that level and fixing certain boundary conditions such as ice sheet size was an idealized situation designed to help develop an understanding of climate change and climate feedbacks. Our conclusion that ECS is at least ~4°C, near the upper end of Charney’s estimated range, refers to that specific problem. Our study does more than that, however, we also address the delay in the climate response caused by the ocean’s thermal inertia. With the help of Earth’s paleoclimate history, we also address the question: what is Earth System Sensitivity (ESS) for the idealized case in which 2×CO2 forcing is kept constant until all fast and slow feedbacks are allowed to reach equilibrium. The answer we find is close to 10°C, for CO2 forcing alone. If we include the negative forcing by today’s aerosols, the response may be as low as 6–7°C, but ESS is ~10°C.
is still a bit obscure. Hansen’s key finding is in the last two
sentences — once carbon dioxide in the atmosphere crosses into the
territory of double pre-industrial levels (which the world did in 2022)
Earth’s average temperature will increase by 10 degrees Celsius (18
degrees Fahrenheit). Earth’s average temperature is 13.9 °C (57°F). It
has risen by 0.08°C (0.14°F) per decade since 1980. Achieving
equilibrium with CO2 today would rise the temperature of Earth to 24
degrees, 72% higher than today. In contrast, consider that we are only 5
degrees, or 36%, warmer today than in distant ice ages. So, take that
heating out twice as far as it went in the past 20,000 years and you can
imagine what some now living may experience in this lifetime.
Hansen points out that we are already past doubling for CO2 once you include all the other trace gases. Methane, for instance, has 84 times the potency of CO2 as an atmospheric insulator in the first 20 years but is transformed to less potent CO2 over a century, diminishing the average impact to about 28 times the potency of CO2. Atmospheric methane has increased 262% since pre-industrial times, to 1908 ppb, but since the start of the pandemic has been adding 18 ppb/y, double its historical rate.When the climate changes that quickly we’d like to know what the most likely outcomes are going to be for us. As I write this, there are thousands of incredibly knowledgeable and intrepid researchers around the world running complicated AI climate simulations through advanced supercomputers whose processing speeds are accelerating all the time. Their answers get better by the week and day, and the questions get more sophisticated. More and more data is being fed in from field instruments and newer generations of satellites and depth buoys — everything from seawater salinity to permafrost melt, and fugitive methane from pipelines and abandoned wells to the sunlight reflectivity of urban sprawl.
In December 2022 scientists at the Potsdam Institute for climate research in Germany teamed up with the Washington Post to produce a fascinating analysis of more than 1200 different scenarios in an attempt to establish whether any of them might actually keep global warming below 1.5 degrees by the end of the century.
The Potsdam team applied five key criteria to make their assessment.
- The rate of acceleration in technologies like direct air capture (DAC) and bioenergy with carbon capture (BECCS).
- The rate of acceleration in agriculture, forestry and other land use (AFOLU).
- Potential future carbon intensity of energy production.
- Potential changes in the global level of demand for energy itself.
- Trends in non-CO2 emissions, predominantly methane.
Most of the predictions push us up to somewhere between two and three degrees of warming where it would no longer be possible to maintain global civilization as we know it today, and the worst take us above 5 degrees and human extinction by the end of the century. Ninety-five percent of humanity is completely unaware of this and the 5% who are aware still party like it’s 1999 (in 1982).
The overwhelming scientific consensus is that we are suicidally stupid, but for sake of argument, let’s suppose we aren’t. Maybe we are just ignorant. Suppose we suddenly awaken to the threat and look up?
Bending the Curve
The Potsdam and Washington Post teams highlighted 230 of the original 1202 scenarios that — while vanishingly unlikely — would bend the curve enough to allow us an escape. Global emissions fell by about five percent during the pandemic and if we just held that course for the remainder of the century it could mark a pathway out of the dilemma.
I say “vanishingly unlikely,” based on numbers from globalcarbonbudgetdata.org showing emissions jumped back up in 2022. We are now back on the same pre-pandemic trajectory, so any scenarios predicting a dramatic fall in emissions by 2025 or 2030 look hopelessly unrealistic. Moreover, if you mention climate change to the average person on the street in China, India, Thailand, Latin America, or Africa, it is likely you will just get a blank stare. They don’t have any idea what you are talking about.
The Potsdam team then said there remain 112 Pathways that fall into two distinct categories: those that predict what’s known as a “high overshoot” before temperatures drop back down again and those that predict a “low overshoot” or even no overshoot at all. High overshoot involves traversing unpleasant tipping points like killing all the polar bears, tripling methane from permafrost, and burning away the Amazon rainforest. Hoping that we might get away with overshoot by employing some kind of as yet unproven silver bullet technologies — artificial trees or artificial plankton — a bit further down the line is arguably delusional and probably suicidal.
Can we build out renewable energy fast enough to avoid those nasty tipping points? What would that imply for the consumer lifestyles of a large and still growing human population? Would it necessarily imply that population will shrink, that consumerism will implode, or that our expectations for future cities built over rising oceans or colonies on Mars are either wildly unsustainable or delusional? We will look more closely at those questions next week.
Hansen, James E., Makiko Sato, Leon Simons, Larissa S. Nazarenko, Karina von Schuckmann, Norman G. Loeb, Matthew B. Osman et al. “Global warming in the pipeline.” arXiv preprint arXiv:2212.04474 (2022).
Steffen, Will, Johan Rockström, Katherine Richardson, Timothy M. Lenton, Carl Folke, Diana Liverman, Colin P. Summerhayes et al. “Trajectories of the Earth System in the Anthropocene.” Proceedings of the National Academy of Sciences 115, no. 33 (2018): 8252–8259.
Stoddard, Isak, Kevin Anderson, Stuart Capstick, Wim Carton, Joanna Depledge, Keri Facer, Clair Gough et al. “Three decades of climate mitigation: why haven’t we bent the global emissions curve?.” Annual Review of Environment and Resources 46, no. 1 (2021): 653–689.