How hot will it get? 18,000-year-old mystery could answer today’s biggest climate question

By Kevin Stark

A mystery from some 18,000 years ago directly impacts how scientists understand the threat of climate change today.

The Earth, shivering though the end of an ice age, rapidly warmed in the Southern Hemisphere, just like it is warming globally today.

At the Comer Conference on abrupt climate change, climatologist Jeff Severinghaus, presented a possible answer to the mystery.  “Just what in the heck caused the rapid period of warming?” said the professor of geosciences of the Scripps Institution of Oceanography in San Diego.

If scientists can answer this question, they can resolve a degree and a half of uncertainty in current climate modeling, the difference between projections that show a fairly habitable planet or an Earth with  massive drought that could kill “millions,” Severinghuas said.

“This means a really radically different planet,” he said.

Severinghaus’ hypothesis is that a “stripping of clouds” that normally cover the ocean over the equator caused a dramatic warming in ancient global temperatures in the Southern Hemisphere, a period of time that offers a mirror for the current abrupt period of climate change.

Clouds above the equator that typically reflect heat from the sun dissipated and Earth was rapidly heated by the ocean absorbing the sun’s rays. “It is the color of the planet,” Severinghaus said. “Just as simple as that. White clouds reflect sunlight. Dark ocean absorbs sunlight.”

Jeff Severinghaus presenting his theory of what caused a mysterious 18,000-year rapid warming of the Earth's atmosphere (Kevin Stark/Medill)
Jeff Severinghaus presenting his theory of what caused a mysterious 18,000-year rapid warming of the Earth’s atmosphere (Kevin Stark/Medill)

At the time, glaciers were melting in the Northern Hemisphere. The theory is that cold fresh water gushing into the ocean in the North pushed a large rain belt into the Southern Hemisphere, disrupting tropical atmospheric circulation and displacing low-level clouds with thunderstorms.

The low-level clouds typically reflect sunlight back into space. Without this band of clouds, the dark-colored ocean would absorb heat from the sun—just like a “parking lot on a hot summer day,” Severinghaus said.

Currently, the modelers and the climatologists are debating whether this rain belt that typically appears as thunderstorms over the Eastern Pacific Ocean in the Northern Hemisphere could form further south.

Scientists call the cloud band the Intertropical Convergence Zone, but sailors call it the “doldrums.” Severinghaus said that moisture records from ice cores and a now-dry lake bed in Bolivia are evidence that massive storms dumped rain and snow 18,000 years ago.

Today’s scientific models need to be pinned on data that “you can hang your hat on,” Severinghaus said. He and his colleagues are paleo-climatologists, which means they reconstruct ancient earth history to better predict the scientific understanding of where  Earth’s climate is heading now.

Some modelers don’t think that the cloud shift is plausible but the “paleo-evidence is screaming at us that it did,” Severinghaus said. The more we understand the past, the more we can understand about the future, he said. A full understanding of this dramatic period of global warming—informed by accurate data—can provide more clarity for policy-makers globally.

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Going deeper: Jeff Severinghaus explains his cloud theory. (Kevin Stark/Medill)

Today’s World

The low-level clouds currently cover a huge area of the Eastern Pacific Ocean, reflecting heat into to space.

If these clouds were to dissipate in the future because of ice melting in Antarctica and Greenland—in the same way that Severinghaus proposes happened centuries ago—the Earth could get hotter faster.

This is a threatening global future.

For decades, climate scientists like Severinghaus have been urging the public to burn less fossil fuels, which emits the greenhouse gas carbon dioxide and holds heat in the atmosphere. Scientific models show that by doubling the rate of carbon dioxide in the atmosphere compared to the pre-industrial age,  global temperatures will rise to the point that causes widespread drought, extreme weather, and poses an threat to human life as we know it.

As Earth weathered ice ages and hot spells across the past 1 million years, ice cores show that carbon dioxide levels ranged from just under 200 parts per million during the cold snaps to nearly 300 parts per million during warm cycles. Since the industrial age, CO2 levels have risen to the current levels of approximately 400 parts per million. A doubling from pre-industrial levels would bring us to about 600 parts per million and levels are rising at just over 2 ppm per year.

With that amount of carbon spooled in the atmosphere, Severinghaus said the atmosphere will rise by 2 degrees Celsius. In addition, many climate experts agree that 2 degrees of temperature rise is the tipping point where changes in crops, water supply and disease can already cause widespread suffering.

But for nearly 4 decades, global temperature predictions have always been packaged with 1.5 degrees of uncertainty, effectively spinning the heads of policymakers who say that they need precise figures to craft meaningful legislation. So some models show the atmosphere warming by as much as 4 degrees Celsius (about 7 degrees Fahrenheit) above pre-industrial levels.

But climate skeptics use the uncertainty—inherent and common in modeling—to discredit the science and as a reason for inaction. Researchers are working feverishly to reduce the uncertainty.

Climate Models

Severinghaus researches ancient climate, but the same grouping of equatorial clouds that interests him has puzzled present-day scientists who use models to make predictions about the Earth’s climate future.

Whether or not this cloud bank is able to “see-saw” between hemispheres is “the single biggest conflict in all the climate models,” Severinghaus said.

Some models show this cloud bank disappearing in the future (a result of that 4 degrees of warming) while other models do not (2 degrees). The difference of the two models is why predictions about future global climate always include uncertainty.

Climate skeptics and reluctant policymakers often point to uncertainties in warming projections as evidence that immediate action on global warming can wait.

If proven correct, Severinhaus’ hypothesis could go a long way to reducing the uncertainty about how fast the Earth is currently warming, which is linked to greenhouse gases building in the atmosphere due to fossil fuels use.

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Scientists Sidney Hemming and Guleed Ali (Lamont-Doherty Earth Observatory Columbia University) respond to Jeff Severinghaus’ cloud theory. (Kevin Stark/Medill)

Comer Conference

At the fall climate conference hosted by the Comer Family Foundation, Severinghaus’ put the theory before his colleagues. The foundation supports widespread climate research and the conference brought together top climate scientists who pursue their research across the globe.

There was widespread “enthusiasm for the idea,” said Sidney Hemming, a professor of earth and environmental studies at Lamont-Doherty Earth Observatory Columbia University. “I liked it,” she said.

Severinghaus stressed that his idea is only a theory and the requires more research, but the methodology for that research is not yet clear.

Guleed Ali is a doctoral student working with Hemming, said he wonders how the cloud system functions, and he said he also does not know how science will test the hypothesis.

“How would you measure this, a ‘sunburn index?’” he said.

Photo at top: Ice cores from the West Antarctic Ice Sheet. Scientists use air pockets in these ice cores to measure the presence of carbon dioxide and other atmospheric gases over hundreds of thousands of years.  (Jeffrey Severinghaus)