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Nate Cohn in New Republic: Last decade warmest on record and hiatus due to “laundry-list of mitigating factors like a prolonged La Nina, […] modest volcanic activity, and ebb in solar activity.” [For general commentary]

March 20, 2014 7:17 am0 comments


The following excerpts are from a fairly recent article in the New Republic, titled “Explaining the Global Warming Consensus — Grapping with climate change nuance in a toxic political environment,” by Nate Cohn. (Click on the previous link to read the full article on the New Republic’s website.) This article was critiqued by an article in The Economist, titled “A Cooling Consensus,” which I have also posted for commentary by the CCNF Scientist Community.

Explaining the Global Warming Hiatus — Grappling with climate-change nuance in a toxic political environment

By Nate Cohn

Even as scientists asserted an incontrovertible consensus on climate change, a funny thing has happened over the last 15 years: Global warming has slowed down. Since 1998, the warmest year of the twentieth century, temperatures have not kept up with computer models that seemed to project steady warming; they’re perilously close to falling beneath even the lowest projections.

Some people are playing the hiatus as good news: “Apocalypse perhaps a little later,” the Economist put it. But in a political environment where vast swathes of the American right reject even the premise of global warming—and where prominent right-wing pols suggest it’s an enormous fraud—this inconvenient news could easily lead to still more acrimony over the subject. Especially since scientists themselves aren’t entirely sure what the evidence means. If scientific models can’t project the last 15 years, what does that mean for their projections of the next 100?

It might seem like a decade-long warming plateau would cause a crisis for climate science. It hasn’t. Gerald Meehl, a Senior Scientist at the National Center for Atmospheric Research, has seen hiatus periods before. They “occur pretty commonly in the observed records,” and there are climate models showing “a hiatus as long as 15 years.” As a result, Isaac Held, a Senior Research Scientist at NOAA’s Geophysical Fluid Dynamics Laboratory, says “no one has ever expected warming to be continuous, increasing like a straight line.” Those much-cited computer models are composed of numerous simulations that individually account for naturally occurring variability. But, Meehl says, “the averages cancel it out.”

Global Annual Mean Surface Air Temperature Change. Credit: NASA Goddard Insitute for Space Studies

Graph showing global annual mean surface air temperature change since 1880. Source: NASA Goddard ISS (republished here by Michael Quirke).

The phenomena that most clearly causes the Earth’s temperature to rise and fall are El Nino and La Nina. During El Nino, heat is brought to the surface of the eastern Pacific, raising global air temperatures. The reverse happens during La Nina. Conveniently, the “hiatus” is said to begin in 1998, when a historic El Nino produced the warmest year of the twentieth century. That starting point amounts to cherry-picking: The 2000s were warmer than the 1990s, and the nine warmest years on record have occurred since 1998.

But all this leaves a big question, one that scientists have been trying to answer: If the atmosphere is warming more slowly than projected, where did the heat go?

There are two ways to create a global-warming hiatus: The heat can go somewhere other than the atmosphere, or there might be less heat in the climate system than scientists predicted.


The most obvious culprit is the ocean, which absorbs 90 percent of the heat added to the climate system. With the oceans holding so much heat, the focus on mean surface temperature as the measure of global warming misses much of the point. Minor shifts between the oceans and the air could keep the planet heating up, even while slowing the pace of atmospheric warming. And that wouldn’t necessarily be good news, since warmer oceans would raise sea levels, change the climate, and hurt the ocean’s ecosystem.

But sea surface temperatures and the upper ocean heat content didn’t increase over the last decade, not by enough to account for what Trenberth called “missing heat”—the heat that greenhouse gas emissions should have trapped in the Earth’s climate system, but couldn’t be found.

Scientists speculated that the heat might be hidden in the deep ocean, beneath 700 meters, where there are not reliable temperature measurements. Without good data, they couldn’t prove heat was going into the deep oceans. So Meehl and his colleagues turned to computer models. Their approach was straightforward: Look at naturally occurring hiatus decades in climate models, and see where the computers put the heat. In the simulations, the deep ocean warmed by 19 percent more during hiatus periods, even as sea surface temperatures cooled in the tropical Pacific, creating a pattern similar to the prolonged La Nina-like pattern of the last decade. They then plugged in the conditions of the last decade and found that  “the models produced roughly 20 percent less warming than the free running models.”

El Nino/La Nina Global Surface Temperature Influence, by

El Nino / La Nina Global Surface Temperature Influence graph by (originally republished in the New Republic article by Mr. Cohn).

More proof of the deep ocean hypothesis: Over the last decade, more than 3,000 Argo floats—basically, seaborne thermometers—were deployed across the world’s oceans to measure oceanic heat content. The floats can measure temperatures down to 2000 meters. They found that more than 30 percent of ocean warming over the past decade occurred beneath 700 meters—potentially covering most of the missing heat.

But analysis was limited by the absence of historical data: Without knowledge of how much heat was trapped in the deep ocean in the past, scientists couldn’t prove much. Just last month, an intriguing paper in the journal Geophysical Research Letterssuggested a way around this problem. Using a model that projects ocean temperatures back to 1961, its trio of authors found that the surge in deep ocean heat intake over the last decade was indeed historic.

But other scientists think that the heat is missing because it never made into Earth’s climate system. The idea that heat might not have made it relates to the concept of “forcing.” The term refers to the forces that add or remove heat from the climate system. The best known example of forcing is the Greenhouse effect, where greenhouse gases in the atmosphere trap heat that might otherwise radiate into space. But there’s negative forcing, too—i.e., other pollutants that reflect energy back into space.

The sun itself is a major factor in forcing. Over an average of eleven years, the sun’s energy output ebbs and wanes, subtly influencing earth’s climate. The last solar maximum was in 2000, but a prolonged solar minimum has kept the sun even dimmer than usual. According to Kevin Trenberth of the National Center for Atmospheric Research, lower levels of solar radiation account for 10 to 15 percent of the hiatus.

Explaining the rest is more difficult. Susan Solomon, an MIT professor best known for research on the ozone hole, has focused on stratospheric water vapor and aerosols. Water vapor is a greenhouse gas, and satellite data shows stratospheric water vapor decreasing since 2000—meaning less heat is getting trapped.

Working in the opposite way is an increase in stratospheric aerosols, which counter greenhouse gases and serve as a giant parasol—reflecting energy away from the Earth. But even there, the theory means rethinking former givens. Until recently, scientists believed large volcanic eruptions were the culprit when stratospheric aerosol levels rose. But stratospheric aerosol levels have risen since 2002, even though there hadn’t been a large volcanic eruption since 1991. Some initially attributed the trend to increased coal burning from South and East Asia. The data hints at another explanation: a wave of smaller volcanic eruptions, previously assumed to be too small to contribute to stratospheric aerosol. Ground and satellite-based observations show a correlation between increases in stratospheric aerosol and this decade’s smaller volcanic eruptions—like Monserrat, Ruang in Indonesia, and Manam in Papua New Guinea.


[T]he combination of imperfect data, overlapping explanations, and continued uncertainty mean that scientists cannot discount the possibility that they have overestimated the climate’s “sensitivity” to additional greenhouse gas emissions. For Held, the last 10 to 15 years “make it more plausible that the size of climate response to greenhouse gas increase is on the lower side of what models have been projecting over the last 10 or 20 years rather than over the high side.” Held is not alone.

In the end, the so-called scientific consensus on global warming doesn’t look like much like consensus when scientists are struggling to explain the intricacies of the earth’s climate system, or uttering the word “uncertainty” with striking regularity.


But the “consensus” never extended to the intricacies of the climate system, just the core belief that additional greenhouse gas emissions would warm the planet. The greenhouse effect is truly undeniable—just consider Venus, where 96.5 percent of the atmosphere is composed of carbon dioxide, and the average surface temperature is more than 860 degrees Fahrenheit. Conversely, without greenhouse gases, Earth’s average surface temperature would be 0 degrees Fahrenheit.

And once you concede the existence of the greenhouse effect, it’s tough to dispute the role of greenhouse gas emissions in warming the planet: Over the last ten years, increased carbon emissions added trapped about as much heat in the climate system as small volcanic eruptions, the solar cycle, and stratospheric water vapor combined to deduct. But since the start of the industrial age, carbon dioxide has added nearly seven times as much positive forcing, and that number will keep growing with additional carbon emissions.3 So here’s what’s clear: Over the longer term, temperatures will increase. As Held puts it, “warming over 100 years isn’t that sensitive to fluctuations.”

“I don’t see how you can argue against it,” Solomon observed after declaring that “carbon dioxide will be king over the long run.”


The last decade is proof of climate change, not a cause for reflexive skepticism. It was the warmest on record, despite a laundry-list of mitigating factors like prolonged La Nina, a wave of modest volcanic eruptions, and an ebb in solar activity. As those attenuating factors subside, climate scientists anticipate another round of rapid warming.

Click here to read the full article on the New Republic’s website.

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