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Thomas Homer-Dixon is executive director of the Cascade Institute at Royal Roads University and professor emeritus at the University of Waterloo.

Human beings have a natural optimism bias. For most of our species’ history, this bias has served us well, helping us persevere in the face of overwhelming odds. But when it comes to the climate crisis, our natural optimism could be our undoing. Our collective response to the crisis has been marked by denial, delay and delusion – denial of the problem’s seriousness, delay in doing anything significant about it and delusion about the efficacy of those things we’ve finally gotten around to doing.

One person who has railed against these tendencies is the renowned climate scientist James Hansen. Throughout his long career, Dr. Hansen has developed a reputation for being consistently ahead of the scientific curve in his assessment of climate change and its implications, most famously in the summer of 1988 when, as director of the NASA Goddard Institute, he brought public attention to global warming in testimony to the United States Senate. Now retired from NASA and based at Columbia University, he’s still vigorously engaged in climate science and policy advocacy.

In recent years, Dr. Hansen has argued that the scientific consensus, as reflected in the voluminous reports of the Intergovernmental Panel on Climate Change (IPCC), greatly underestimates the rate and magnitude of future warming. Earlier this month, he and 17 colleagues forcefully stated their case in a peer-reviewed paper, Global Warming in the Pipeline, published by a University of Oxford journal. I’d rank it as the most important scientific article I’ve read in the past decade.

If Dr. Hansen and his colleagues are right, the received wisdom of today’s supposedly informed climate cognoscenti – people such as David Wallace-Wells of The New York Timesis substantially wrong. Mr. Wallace-Wells and others tell us, with evident relief, that warming will likely peak somewhere around 2 to 3 C. The rapid decline in the cost of wind and solar power means we won’t burn all the world’s coal to get an an eventual rise in temperature of 4 C or even more. But Global Warming in the Pipeline shows that we don’t need to burn all our coal to get a 4 C rise in climate or hotter.

The paper makes two vital arguments undergirded by one striking empirical observation. The first argument is that Earth’s climate is much more sensitive to humanity’s carbon dioxide emissions than conventionally estimated. Taking into account feedbacks involving clouds, water vapour, snow cover and sea ice, “equilibrium climate sensitivity” – the eventual warming produced by a doubling of CO2 in the atmosphere – is likely around 4.8 C, rather than the IPCC’s best estimate of 3 C.

Greater climate sensitivity means that far more warming is “in the pipeline” than conventional models predict. Indeed, Dr. Hansen and his colleagues estimate that the atmosphere’s current concentrations of greenhouse gases are already producing a radiative effect (what scientists call “forcing”) equivalent to a doubling of CO2 and that this effect, if not reduced, could readily double or triple the 1.2°C the planet is already experiencing.

The article’s second key argument is that until recently a significant portion of human-caused greenhouse warming has been offset by our aerosol emissions – fine particles in the air that reflect sunlight and cool the planet. This effect is now declining, as key sources of pollution are cleaned up. The authors call aerosol cooling a “Faustian bargain,” because payment in greater global warming is coming due as we reduce pollution from shipping, vehicles, industry and power plants.

Finally, the striking empirical observation is that Earth’s energy imbalance (EEI) has recently soared. This imbalance arises as our planet receives more energy from the sun than it radiates as heat back to space, because our greenhouse gases are trapping heat in the atmosphere. The authors estimate that between 2005 and 2015, EEI averaged about 0.7 watts per square metre across Earth’s surface. From early 2020 to the middle of this year, they argue, it reached 1.36 watts per square metre, likely in part because lower aerosol emissions allowed more solar energy to reach Earth’s surface.

A 1.36-watt imbalance may seem trivial, but when added up across the planet’s entire surface, the total amounts to nearly a million Hiroshima bombs of extra energy injected into Earth’s atmospheric-ocean system – over and over, each and every day. Currently, most of this excess energy is melting the world’s glaciers and ice caps and heating the oceans, but it’s also supercharging the droughts, storms and heat waves now afflicting every corner of our world.

As Earth’s energy imbalance increases by about half a watt each decade, the authors argue, it’s accelerating Earth’s warming – from about 0.18 degrees C per decade between 1970 and 2010 to at least 0.27 degrees C per decade now. In a more recent commentary, Dr. Hansen and his colleagues go on to estimate that the world will at least temporarily cross the 1.5-degree C ceiling this coming year, in part because of the influence of El Nino, reaching about 1.7 degrees C of warming by 2030 and 2 degrees C “by the late 2030s.”

Now, to be clear, some prominent climate scientists vehemently disagree with Dr. Hansen and his team, especially with their claim that warming is accelerating – Michael Mann at the University of Pennsylvania being one. Ultimately, the dispute will be adjudicated by nature itself, as the warming rate is revealed in coming years.

But betting against Dr. Hansen would seem foolish, even if our optimism bias inclines us to do so, given his track record and the worldwide evidence of a spiralling climate crisis we’ve seen this past year.

So, it’s worth unpacking the broader implications of the paper. I believe there are four.

First, if Dr. Hansen and his colleagues are correct, warming will melt the world’s great ice sheets in Antarctica and Greenland much faster than the IPCC currently predicts, possibly entailing a rise of multiple metres in sea levels within the expected lifespan of coastal infrastructure being built now – that is, within the next century. Coastal communities should start planning for this change now.

Second, heating this century is likely to overwhelm many nature-based solutions to climate change. Fires and droughts will kill tree plantations intended to absorb carbon, while heating will weaken biological processes that practices such as regenerative agriculture must exploit to sequester carbon in soil.

Third, the most dangerous aspect of the climate problem is the long lag between emissions and full climate response. This lag facilitates denial, delay and delusion, and so increases the likelihood that some nations will ultimately attempt to “geoengineer” the atmosphere under emergency conditions – perhaps by using fleets of large aircraft to dump huge quantities of reflective sulfate particles into the stratosphere – with potentially catastrophic side-effects.

Lastly and most fundamentally, if James Hansen and his team are right, humanity’s responses to the climate crisis must be far more radical than currently planned. Incrementalism is now a waste of resources – and of time.

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