The Game of Climate Whac-a-Mole Will Hit the Tropics First

A new paper details when just about anywhere on the world can expect to have inarguable proof that global warming isn't a debate topic but a reality.
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A new paper details when just about anywhere on the world can expect to have inarguable proof that global warming isn't a debate topic but a reality.


At the airport departure gate, the two most pressing questions are when and where (unless you have baggage issues). In the climate change debate, those are also two key questions (and by using the word "debate" when there really isn’t among scientists we know there’s a baggage issue there, too).

A major new paper (available here) answers those questions: When can we expect warming to exceed the records highs we’re comfortable with, i.e. the natural variability that gets talked about so much by skeptics, and where will we see these temperatures go off the rails first.

The answers: In the lifetime of most the people already living on Earth, and the tropics.

“Most of the climate change discussion up to now has been about avoiding this two degrees of warming and focusing on the magnitude of the change,” said Abby G. Frazier, one of 14 co-authors of the meta-analysis appearing today in the journal Nature. “Our paper is really helping us to understand change through this lens of timing.”

The team considered using a more aggressive mitigation scenario, but since not all models used it, they opted not to chance putting in more error by leaving it out. Plus, there was realistically no political chance of seeing the scenario in action.

To do so, the team, led by Camilo Mora of the University of Hawai’i geography department, created an index that draws from almost 40 existing climate models drawn from institutions in a dozen different countries. They then compared that with historical records of high and low average temperatures from between 1860 and 2005 to set the boundaries of what would be considered normal variability, and determined when temperatures would exceed those limits and never look back. Mora is a specialist in data-driven examinations of the nexus between people and biodiversity.

When looking at climate change, all eyes are usually on the poles, the polar bears and penguins becoming the fuzzy or the feathered face of global warming. But this analysis finds that the first place to experience out-of-the box temperatures, up to a decade before the rest of the world, will be the tropics.

“Most of the conversation has been focused on these absolute changes,” Mora said, referring to the wider ranges seen closer to the poles, “but here we come along and say, ‘You know what? The tropics are going to be the most impacted by these changes, even if they’re not as big as at the high latitudes.’”

While the tropics are already, well, tropical, their temperature range actually is pretty stable. So even small absolute changes are going to exceed existing records, both at land and sea. This is a particular problem for tropical plants and animals—and remember that the tropics are the most biologically diverse places on Earth—since “small but prompt” change gives species used to a stable climate no chance to adapt or migrate in time. It’s very easy to exceed what a species can tolerate at these latitudes; despite those sad pictures of solitary polar bears, species at higher latitudes generally can tolerate more variability than can tropical species.

People, of course, also live in the tropics, between one and five billion of them depending on what scenario used for estimates. In general, climate change’s extreme effects tend to get amplified in cities and so new extremes will be much more detrimental here. Plus, as Frazier pointed out, a lot of countries in the tropics tends to have a lower GDP as well, and so a lower capacity to respond. (That these countries on the whole are also less responsible for pumping out greenhouse gases is an irony not lost on the researchers.)

“When we were focusing on the Arctic,” Frazier reflected, “we tended to think the tropics were fairly safe, but that’s not what we’re finding.”

When an area departs from normal variability depends on the scenario used. This research drew from two of the four scenarios commonly used in climate science, a we-do-nothing plan and one where a moderate effort is made to reduce greenhouse gases in the atmosphere so that warming is stabilized by the year 2100, with carbon concentrations of 538 parts per million (it’s at 395 ppm right now). Under the do-nothing scenario, the world departs from the historical record in 2047, while in the do-something scenario that occurs two decades later, in 2069.

As we’ve seen with the tropics, not every place on the globe will "depart" simultaneously. The paper’s underlying dataset looks at a lot of desolate places on both land and see, but let’s look at cities. Mexico City will depart in 2031 (or 2050 under the stabilizing scenario); Mumbai in 2034 (2051), Washington in 2047 (2071), London in 2056 (2088), and Moscow not until 2063 (2092).

Based on the do-nothing scenario, a sampling of where select places on Earth will permanently exceed historical temperatures.


To be clear, this departure is when the average temperature for the full year exceeds the extreme highs and lows, and not every month, and certainly not every day. However, the paper notes that a time when even every month breaks records nonetheless will occur later in the century.

The team considered using a more aggressive mitigation scenario, but since not all models used it, they opted not to chance putting in more error by leaving it out. Plus, there was realistically no political chance of seeing the scenario in action; “It would take massive global cooperation that started yesterday to reach those levels of CO2 in the atmosphere,” commented Ryan Longman, another co-author and like Frazier a student of Mora's.

Meanwhile, the data collection involved was truly massive—21 different modeling centers in 12 countries that produced 39 different models using seven variables like precipitation, evaporation, the temperature of the ocean’s surface temperature and its pH. What was surprising, said Frazier, was how small the standard error between models was—“all of these models from countries that don’t even speak the same language are giving the same results.”

She noted that the scientists ran the data with two different models, one that used the existing directly collected natural historical record, and another that attempted to factor out the effects—the widening of the boundaries—that existing human-caused climate change has already caused since 1860. Ultimately, they stuck with the historical data alone, with its anthropogenic effects, to keep the results robust.  “It makes it that much more conservative because we’re already including the effects of climate change in our past bounds,” she explained.

“We tried to be conservative across the board,” Mora said. Besides choosing the record in which human activity has already pushed out the natural boundaries, conservative actions included using a large time frame “which makes it very hard to get out of those bounds” and looking for the year where temperatures permanently exceeded the historical bounds.

The latter was important because under natural variability, individual years, even a couple in a row, may bust out of the historical record but then come back. But for this paper, predictions were “looking into future, at a year where climate never goes back to those historical bounds,” he said.

Being conservative is a cardinal scientific trait, but given the debate over climate change it’s now a cardinal political virtue, too. “We were more focused on doing it and doing it right, letting the data speak for itself,” Longman explained. “Better to take a conservative path to cover all your bases.”

When you let the numbers do the talking, Mora interjected, “you realize right away that there is no controversy about climate change.”

And yet there is. Take the recent kerfuffle over a leveling off of global temperature rise in the past decade—cited by climate change skeptics that the scientific models are wrong.

Mora counters that climate models are derived from known laws of physics that we understand well that provide generalities about things we know will happen as a result of putting additional greenhouse gases in the atmosphere and oceans. So modeling is solid, even if individual models have issues—hence the advantages of combing 39 different models.

He offers an analogy of a piece of paper release a few feet above the ground. "You know that a constant force acting on this piece of paper is gravity, that ultimately will lead this paper to land on the floor. However, there are going to be instances where other forces are going to be stronger than gravity and as a result you’re going to see this piece of paper phasing up and down in its trajectory to the floor."

"Climate is the same. We know that a consistent factor on the climate is CO2 and the effect of this CO2 is to cause warming. However, there are going to be instances in which some forces are going to be stronger than CO2 and you’re going to see the climate going in opposite directions. However, given the consistency of the CO2’s effect on climate, you can expect that the climate will eventually go back to warming, just like gravity will lead to the piece of paper landing on the floor."

One piece of paper has already hit the floor, so to speak. The pH of the ocean is below the boundaries of the past 150 years, and has been since 2008. This ocean acidification finding actually surprised the researchers, who doubled back to make sure their determination was correct.

“The discussion was whether this was right or not,” Mora explained, “but it actually made a lot of sense. Oceans have been absorbing lots of carbon put out by humans.”

As a result of these warming trends, even setting aside protected areas for plants and animals under siege may not help them face higher temperatures. “Unfortunately,” the Nature paper notes, key conservation strategies such as protected areas, which may ameliorate the extent of several anthropogenic stressors, are unlikely to provide refuge from the expected effects of climate change because protected areas with biodiversity hotspots will experience unprecedented climates at the same time as non-protected hotspot areas.”

Which raises the question: Given the inevitability of our departure, is there a point of struggling for a later flight?

Frazier will have none of this counsel of despair and inaction. “We buy ourselves time, and that’s the more optimistic way of looking at it rather than why should we bother. Pushing back by 20 years can buy more time for us to adapt, for technological advances, and buy time for species as well. What’s really hitting them is how fast this is happening.”