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Climate change is a highway, not a cliff, and we can still take the exit ramp.
By Michael E. Mann | September 14, 2023
Credit: Antonio Rodriguez / Adobe
Climate models are fuzzy, rather than clear crystal balls. They provide important guidance, in many respects our best guidance, drawing upon the laws of physics, chemistry, and biology to make quantitative, rigorous projections of our potential futures. That’s a whole lot better than relying upon hunches, opinions, and wild speculation. The overall warming of the planet, for example, is very much in line with early climate model predictions. But when it comes to some key climate change impacts, such as ice sheet collapse, sea level rise, the retreat of arctic sea ice, ocean conveyor slowdown or collapse, western North American drought, and the increase in extreme weather events, the absence or poor representation of importance processes in the models leads to a systematic underestimate of the rate and magnitude of the changes.
Such nuanced views struggle to gain currency in a political economy where hot takes, hyperbole, and polarizing commentary best generate clicks, shares, and retweets. I often encounter, especially on social media, individuals who are convinced that the latest extreme weather event is confirmation that the climate crisis is far worse than we thought, and scientists and climate communicators are intentionally “hiding” the scary truth from the public. It is the sort of conspiratorial thinking that we used to find among climate change deniers, but increasingly today we see it with climate doomists. Such sentiment emerged, for example, during the mid-June 2022 heat wave, where one individual tweeted at me and my climate scientist colleague Katharine Hayhoe: “Again we see that climate science as often presented to the public is too conservative, avoids what at the time are deemed worse [sic] case scenarios. BUT these are becoming our reality TODAY.”
This is not true, or at best partly true. I responded, “Actually, the warming of the planet is very much in line with early climate model predictions. Some impacts, such as ice sheet melt and sea level rise, and the slowdown of the ocean ‘conveyor belt’ are exceeding those predictions.” Current policies alone likely keep warming below 3°C (5.4°F), nowhere near the “worst-case” scenarios. That doesn’t mean that some impacts aren’t unfolding earlier and more dramatically. They are. As the great Stephen Schneider counseled decades ago, it’s neither “end of the world” or “good for you.” The collective evidence supports neither fatalism nor complacency.
It is also important to recognize that climate change isn’t a cliff that we go off at certain thresholds of planetary warming such as the oft-discussed 1.5°C (2.7°F) warming level, though it is often framed that way. Climate action isn’t a binary case of “success” or “failure.”
A better analogy is that it’s a dangerous highway we’re going down. We need to take the earliest exit ramp possible. Dangerous climate change impacts, as we have seen, are already being felt — in the form of devastating droughts, heatwaves, wildfires, floods, and superstorms. Supply chains have been disrupted through a combination of a pandemic — which is likely at least in part a result of ecological destruction — and more extreme weather, sometimes with disastrous consequences, such as shortages of baby formula. Extreme heat is leading to substantial decreases in worker productivity, costing the US economy alone nearly 100 billion dollars a year. Dangerous climate change cannot be avoided. It’s already here.
So, it’s a matter of how bad we’re willing to let it get. Worse impacts can be avoided if we limit the warming below 1.5°C (2.7°F). But if we miss that exit off the carbon emissions highway, 2°C (3.6°F) is certainly preferable to 2.5°C (4.5°F). And if we miss that exit, 2.5°C (4.5°F) is certainly preferable to 3°C (5.4°F). Consider, for example, the matter of species extinction. The IPCC estimates as much as fourteen percent of species could be lost at 1.5°C (2.7°F) warming and eighteen percent at 2°C (3.6°F). Tragic for sure, but greater rates of extinction are expected from other unchecked human activities, including habitat destruction and human exploitation of animals.
However, the number climbs to twenty-nine percent at 3°C (5.4°F), thirty-nine percent at 4°C (7.2°F), and forty-eight percent at 5°C (9°F). Half of all species would, by any reasonable standard, constitute a sixth extinction event rivaling the great extinctions of Earth’s geological past. But that is avoidable in a scenario of meaningful climate action.
Despite the breathless claims of climate-driven mass extinction that one sees all too often in today’s headlines, we are not yet remotely committed to such a future. We can avoid catastrophic climate impacts if we take meaningful actions to address the climate crisis. Yes, that’s an important “if.” But the science actually tells us it’s doable. One of the important developments in climate science over the past decade is the recognition that greenhouse warming depends on cumulative carbon emissions up to a given point in time. This has led to the concept of the carbon budget, which determines how much additional carbon we can afford to burn and still limit warming to below a particular level.
The conventional wisdom was once that warming would continue on for decades even if we stopped emitting carbon into the atmosphere due to the sluggishness of the oceans, which continue to warm up even after CO2 stops increasing. This is known as committed warming. But committed warming is only half of the story, an artifact of simplistic early climate modeling experiments in which CO2 levels are kept fixed after the hypothetical cessation of emissions.
Later, more comprehensive simulations with interactive ocean carbon cycle dynamics revealed that CO2 levels actually drop after emissions cease as the oceans continue to draw carbon down from the atmosphere. That decrease in the greenhouse effect cancels out the committed warming, and the result is an essentially flat line. In other words, global temperatures stabilize quickly once net carbon emissions drop to zero.
As a consequence, we can calculate the carbon budget for a particular global temperature stabilization target. To keep surface temperatures below 1.5°C (2.7°F), for example, carbon emissions have to be brought to zero within three decades, and we have to get halfway to zero within a decade. There are some confounding factors. For example, when coal burning ends, there is a drop in cooling sulfate aerosol pollution, which leads to warming. But that warming is largely offset by a decrease in other warming factors, including greenhouse gases like methane and black carbon from fossil fuel burning. These additional factors all nearly cancel as well.
There are scenarios where global temperatures exceed a given target such as 1.5°C (2.7°F), rise as high as 2°C (3.6°F) or so by mid-century, and then come back down and stabilizes below 1.5°C (2.7°F). This is called overshoot, and a shorter-duration, small overshoot is favorable, from a climate-impact standpoint, to a longer-duration, large overshoot. Once again, there are no absolutes. The less, and shorter duration, the warming, the better. But the most comprehensive and authoritative assessment of risk across all sectors — health, food, water, conflict, poverty, and the natural ecosystem — by the IPCC in 2018 basically concluded that we don’t want to warm the planet beyond 1.5°C (2.7°F), and we really don’t want to warm it beyond 2°C (3.6°F). And if we do happen to overshoot those targets, we want to keep the duration of overshoot to a minimum.
Where do we stand in this effort? Scientists have evaluated the upwardly revised commitments made at the United Nations Climate Change Conference (COP26) in Glasgow in late 2021 and have determined that they would likely keep warming below 2°C (3.6°F). That’s substantial progress compared with the roughly 4°C (7.2°F) warming that we were headed toward prior to the 2015 Paris summit. But it’s still a lot riskier than limiting warming to 1.5°C (2.7°F). Moreover, it’s one thing to make commitments, and something else entirely to keep them. As my colleague Susan Joy Hassol and I explained in a Los Angeles Times op-ed published at the completion of COP26, the goal of limiting warming to 1.5°C (2.7°F) is still alive but “only if the hard work begins now” (emphasis added).
Among other things, a pathway to 1.5°C (2.7°F) requires there be no new fossil fuel infrastructure at a time when pipelines continue to be built. A handful of fossil fuel companies — including ExxonMobil and Gazprom (Russian state fossil fuel company) — are planning for new projects that will produce about 200 billion barrels of oil and gas. That’s the equivalent of a decade of emissions from China, the world’s largest producer of carbon pollution (the United States, meanwhile, is the world’s greatest cumulative producer of carbon pollution).
Holding policymakers, opinion leaders, and corporations accountable is essential. For while citizens themselves now overwhelmingly support concerted climate action, they can’t effect the needed changes themselves. We, as individuals, can of course make climate-friendly choices as consumers. But we cannot impose subsidies for the renewable energy industry or remove them for the fossil fuel industry, price carbon, or block major fossil fuel infrastructure projects. It is only our elected policymakers who are in a position to do that.
Michael E. Mann is a geophysicist and climatologist at the University of Pennsylvania, the recipient of the 2022 APS Leo Szilard Lectureship Award for his contributions to public understanding of climate change, and the author of six books.
This article is an excerpt from Our Fragile Moment: How Lessons from Earth's Past Can Help Us Survive the Climate Crisis, by Michael E. Mann. Copyright © 2023. Available from PublicAffairs, an imprint of Hachette Book Group, Inc.
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