Surprises from Past Could Spell Longer Droughts, Rising Sea Levels

Abrupt climate changes could lead to decade-long droughts and massive sea-level rise, according to a University of Arizona geoscientist who studies the climate of the distant past.

Factors that influence the climate system, such as natural changes in the Earth’s orbit or rising carbon dioxide emissions from cars and power plants, can result in dramatic climate shifts, says Jonathan Overpeck, a professor of geosciences and director of the UA Institute for the Study of Planet Earth. Scientists studying natural climate records, such as the variations in tree rings and gas bubbles trapped in the polar ice caps, find ample evidence of these types of rapid changes in the past — sometimes occurring in a decade or less.

“The climate system rather than responding gradually to a change in climate forcing, has the ability to cross thresholds and change without warning,” Overpeck told attendees this morning (Dec. 6) at the annual meeting of the American Geophysical Union.

(A climate “forcing” is something that triggers climate change, like increased greenhouse gases or changes in the sun’s irradiance.)

Overpeck was one of 11 scientists serving on the National Research Council’s committee on abrupt climate change, which published the first major study of the issue last year. Since that time, Overpeck notes, there has been a lot of attention on shifts in ocean circulation and rapid cooling events seen during the last ice age, when glaciers covered large parts of Earth’s land mass 80,000 to 10,000 years ago.

Perhaps more relevant to the future, Overpeck says, are abrupt changes that occur during warm-climate intervals, like today. Paleorecords for the past 10,000 years show rapid shifts in the frequency and strength of droughts, floods, and tropical storms.

Decades of Drought
In earlier research, Overpeck found that droughts as severe as the 1930s dustbowl?the worst dry spell in U.S. history?occur once or twice per century in North America. He and other researches have also found evidence of “megadroughts,” that last a decade or longer, striking North America, as well as other parts of the world, at least once or twice a millennium.

For example, Overpeck and colleagues used lake sediments and tree-rings to reconstruct rainfall in part of West Africa for the past eight centuries. They found long periods of drought prior to 1800, including a 120-year spell of exceptionally dry conditions from 1640 to 1760.

Drought also gripped large parts of North America for nearly 25 years in the 1600s, Overpeck notes. A few centuries earlier, he says, there is evidence of even more frequent and longer droughts, some lasting more than 100 years.

Extended droughts may have caused the collapse of several ancient civilizations, Overpeck says. He cites correlations between intense drought and the demise of the classic Mayan civilization 1100 years ago on Mexico’s Yucatan peninsula, as one example.

Since abrupt climate changes are difficult to predict, even modern-day societies may be ill-equipped to deal with megadroughts in the future. In the United States, reservoir levels and coping strategies are only good enough to handle 5 to 6 years of drought, Overpeck says.

“It would be wrong to assume that any part of the world is free from risk,” he says.

Changes in El Ni?o
Scientists are not sure what caused megadroughts in the past, but abrupt changes in El Ni?o patterns could play a role, Overpeck says. This climate phenomenon, known as the “El Ni?o-Southern Oscillation” or “ENSO” to meteorologists, is marked by a change in sea-surface temperatures in the Pacific Ocean.

At one extreme of ENSO, El Ni?o events brings rain and often flooding to parts of the United States, while leaving places like Australia and Indonesia bone dry. Its sister event, La Ni?a, often has opposite effects and is associated with drought in the American Southwest.

Variations between extreme states of the ENSO system orchestrate year-to-year climate variability in many parts of the world, Overpeck explains. In fact, climate forecasters often look to ENSO to help make predictions. But the records from corals and lake sediments show that the frequency and duration of El Ni?o has made abrupt shifts in the past.

“You go across some climate threshold and all the sudden atmospheric circulation shifts into another mode where La Ni?as are much more common and they start getting longer,” Overpeck says. “That’s happened in the past and that had the net effect of making droughts in the Southwest longer.”

Rapid shifts in El Ni?o patterns might also impact the frequency and intensity of tropical storms and floods. The paleorecord shows periods in the past when big hurricanes were more common, Overpeck says. Large floods occurred more frequently in the past, as well.

Sea-level rise
While Overpeck believes megadroughts might be the biggest “climate surprise” in our future, he also notes that abrupt changes in sea-level could have devastating effects.

“The last time the Earth was warmer than today was the last interglacial period, 130,000 years ago,” Overpeck says. “We know at that time the Earth was only a fraction of a degree warmer than it is today, yet sea level was between 3 and 6 meters higher (9.9 to 19.8 feet).

Scientists estimated this figure by analyzing coral reefs that jut above our current sea level around the globe. Overpeck also points to ice core evidence that shows the Greenland ice sheet was substantially melted back at that time.

The Intergovernmental Panel on Climate Change predicts global temperatures will rise 3 to 10 degrees Fahrenheit by the end of the century. The group also estimates that the Greenland ice sheet could melt between .1 and .35 centimeters per year (.04 to .14 inches).

But Overpeck has compiled data that suggests this 1.7 million square kilometer expanse of ice could melt much faster, between 2 to 5 centimeters each year (.8 to 2 inches).

“This is a perfect example of an abrupt change,” Ovepeck says. “No one thinks that the Greenland ice sheet can melt that fast, but the paleorecord shows pretty good evidence that it did in the past, and thus might in the future.”

Overpeck and researchers Jeffrey T. Kiehl and Bette Otto-Bliesner from the National Center for Atmospheric Research in Boulder, Colo. are using a new climate model to look at how climate change might affect the ice sheet. The model simulates conditions from 130,000 years ago, as well as future climate if carbon dioxide levels double or triple. Their preliminary data shows that the Earth may be warm enough in 100-150 years to melt the Greenland ice sheet.

The resulting sea-level rise could have dire consequences, Overpeck says.

“Many islands will be submerged around the world, even with 1 meter of sea-level rise,” he says. “This could include whole island nations such as the Maldives, the Marshall Islands, and the Tuvalu.”

Rising carbon-dioxide levels and greenhouse warming may not only speed up the arctic melting, but it may also affect other abrupt climate shifts, Overpeck says.

“The most alarming thing is that we think global warming will act to increase the probability of abrupt climate shifts happening in the future,” he says. “It really makes sense for us to slow the rate of human-induced climate change in order to reduce the threat of one or more devastating climate surprises.”