A new look at climate ‘tipping points,’ where familiar patterns vanish forever

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CC/Camilo Mora

This detail from the researchers’ interactive map suggests that within three to four decades, the Twin Cities region will be permanently outside its historical temperature range — that is, the region’s coldest years will be  warmer than the hottest years in a record stretching back to 1860. The red line is for temperatures modeled on a business-as-usual scenario, in which greenhouse gas emissions continue to rise without major new efforts to curb them; the yellow line is for temperatures modeled on a scenario of aggressive global reductions.

Global warming has been likened to a slow-moving train wreck, a creeping forest fire, a saucepan of water warming imperceptibly until a bullfrog — that would be human settlement as we know it — sits calm and unawares until the moment of demise.

Partly this is because the timelines for arrival of new climate regimes can be long, indistinct and in some aspects debatable, climate mechanisms being so complex. Partly it’s because scientists are circumspect in presenting research findings, especially on a topic where a claque of industry-financed contrarians waits in perpetual ambush. And partly it’s because climate shifts are global and we can’t help but think local.

Every so often, though, a piece of research comes along to show with special clarity and power the fix we’re in, how rapidly we are hurtling toward a world entirely outside our experience of this one. Such a study appeared earlier this month in Nature, a journal whose prestige may assure that the authors’ new term “climate departure” becomes as commonplace as climate change, climate disruption and climate chaos.

“Climate departure” refers to a kind of tipping point at which the up-and-down fluctuations of annual temperature averages climb into a zone completely and permanently outside a “normal” range established in a record going back to 1860.

A new Minnesota in 2042

It’s the point at which the coldest years to come are warmer than the hottest years already experienced by anyone now alive, plus a few generations of ancestors. In much of Minnesota that point is projected to arrive about 30 years from now — in 2042, give or take five years.

In places like Indonesia and Papua New Guinea, data suggest the tipping point will come around 2020 — again, give or take five years. In Jamaica, 2023. In Nigeria, 2023. In Iceland, 2066.

Around the United States, Honolulu and Phoenix will reach their tipping points in 2043. Orlando and San Diego, 2046. New York, Philadelphia and Washington, 2047. Denver, 2048. Chicago, 2052. Seattle, 2055. Anchorage, 2071.

All of these predictions are based on a business-as-usual scenario for reining in greenhouse gas emissions — that is, no concerted new international efforts to reverse the atmospheric loading of the industrial age, which increasingly appears to be the safest bet.

But even given bold new action on climate, the dates of climate departure recede by only 20 to 25 years, because it takes so long for the impacts to work their way through climate systems already so distorted by globe-warming gases.

What happens after the tipping point is reached?

[C]limates without modern precedents could cause large and potentially serious impacts on ecological and social systems. For instance, species whose persistence is shaped by the climate can respond by shifting their geographical ranges, remaining in place and adapting, or becoming extinct. Shifts in species distributions and abundances can increase the risk of extinction, alter community structure and disrupt ecological interactions and the functioning of ecosystems.

Changing climates could also affect the following: human welfare, through changes in the supply of food and water; human health, through wider spread of infectious vector-borne diseases, through heat stress and through mental illness; the economy, through changes in goods and services; and national security as a result of population shifts, heightened competition for natural resources, violent conflict and geopolitical instability.

Although most ecological and social systems have the ability to adapt to a changing climate, the magnitude of disruption in both ecosystems and societies will be strongly determined by the time frames in which the climate will reach unprecedented states (my emphasis).

Tropics hit hardest

Geography-minded readers will have noticed a latitudinal progression in the departure dates cited above, from the tropics toward the poles, and might also assume that’s good news to some degree, because it puts off till last the full impact on polar ice sheets.

But it’s really bad news. This timing reflects the fact, first, that the climate record in the polar regions shows a wider band of variability in average annual temperatures, so it takes longer for the modeled trends to climb outside it.

Moreover, the impact of temperature changes in the tropics is magnified by two factors: They have more of the world’s zones of critical biodiversity, and are home not only to a disproportionate share of the world’s population, but also of its poorest peoples, least capable of coping with a hotter new world.

The Nature  paper’s chief author, Camilo Mora, is not himself a climatologist. His scholarship and labs at the University of Hawaii’s Manoa campus are devoted to studying the interactions of geography and biodiversity, and to bringing enormous masses of data to bear on certain questions about ecological health and conservation.

In this case, they aimed the data at a simple question said to have been raised by Abby Frazier, a doctoral candidate in Mora’s department: Rather than look at overall warming trends across the entire globe, or over smaller but still vast areas like the polar regions, couldn’t it be possible to predict the pace of change on a much finer scale — like a selection of the world’s largest cities?

The answer turned out to be yes, given enough graduate students and computing power, and the method was elegantly simple.

Plenty of data, and praise

Plenty of data was already publicly available in the form of forecasts generated from widely accepted climate models, some 39 in all.

Choosing mean annual near-surface air temperature as the variable to be mapped, the team overlaid the earth with a grid whose cells measured 100 kilometers on a side. It then computed a baseline band of temperature variability for each by plotting recorded temperatures from 1860 to 2005 (the hottest year on record, globally, so far).

Finally, it plotted for each cell the average of future temperatures predicted by the 39 climate models. A lot of math, but a clear — and conservative — comparison was now possible, pinpointing the year in which future temperatures at each location climb completely outside the pattern of past variability, never to return.

Although this paper has received little attention in Minnesota media, it attracted considerable coverage elsewhere and drew high praise from climate scientists and others who had no involvement with the work. A sampling:

“This paper is unusually important,” Jane Lubchenco, former head of the National Oceanic and Atmospheric Administration and now professor at Oregon State University, said in a statement quoted in USA Today, because it “connects the dots between climate models and impacts to biodiversity in a stunningly fresh way.”

“One can think of this year as a kind of threshold into a hot new world from which one never goes back,” Christopher Field, director of the Department of Global Ecology at the Carnegie Institution for Science, told the Associated Press. “This is really dramatic.”

The pace of change predicted for the tropics “immediately raises all sorts of alarms bells,” said Stuart Pimm, a conservation ecologist at Duke University, told the Los Angeles Times. “The greatest variety of life and biodiversity and the poorest people in world live in tropics, and the new climate shifts will be outside their parents’ and grandparents’ experience.”

Judith Curry, an earth scientist at Georgia Tech who has been publicly skeptical of climate models’ ability to make accurate predictions, told the Washington Post that she found the paper “compelling.”

And Michael Mann of Penn State observed, in comments to the AP, that the research “may be actually presenting an overly rosy scenario when it comes to how close we are to passing the threshold for dangerous climate impacts. By some measures, we are already there.”

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The Nature paper is available here, but you’ll have to buy or rent access beyond the abstract and charts (which are rather stunning). The team’s interactive map of tipping points across the globe, and other information about the research, is available from Camilo Mora’s lab.