Ancient climates show high stakes of human push on climate now

by Elizabeth McCarthy
Oct 24, 2013


Courtesy of Adam Hudson


Geosciences Ph.D. candidate Adam Hudson, of the University of Arizona, digs up shells for radiocarbon dating from a dried out lake in Oregon. The dating will help reconstruct periods of glaciation when water filled the lake. 

PALEO - Doug and Aaron in field

Courtesy of Doug Boyle

Scientists study glacial moraines of the now dry area of Convict Lake in the Great Basin to find clues to climate change.

PALEO - Boyle slide

Courtesy of Doug Boyle

Ancient shifts in the Earth's thermal equator caused massive changes in temperature and precipitation. Scientists are trying to find out if this could happen again today. Click on the image to enlarge it.

PALEO - Aaron Putnam

Elizabeth McCarthy/MEDILL

Aaron Putnam, climate scientist at Columbia University, presented research on glacial melt in the Great Basin of the western U.S. during a period of climate change 16,000 years ago. This research can help us understand climate change today.

With the Earth in a warming climate at stake, preparing for the future means understanding the past. That’s where paleoclimate scientists come in. 

Paleoclimatology is the study of ancient climates. Climatologists such as Aaron Putnam explain that 800,000 years of climate records locked in the Earth’s ice show that carbon dioxide levels are far higher now than during the natural climate shifts over all that time. 

Carbon dioxide, a greenhouse gas rising with fossil fuel emissions, drives global warming. Studying natural climate variations and climate events in the past can give us clues as to how today’s climate may react to unprecedented levels of the greenhouse gas linked to rising temperatures and calving glaciers.

Putnam, with the Lamont-Doherty Earth Observatory of Columbia University, researches glacial history all over the world. At the recent Comer Conference on Abrupt Climate Change in Wisconsin he presented his research reconstructing the glacial and climate history of the Great Basin region of the Western U.S., a desert area sandwiched between the Rocky Mountains and the Sierra Nevadas. Putnam sampled boulders deposited by glaciers to determine their age, revealing when and how fast glacial retreat happened in basin once dotted with lakes.

Geochemist Wally Broecker, a veteran paleoclimate scientist at Lamont-Doherty, coined the term “global warming,” and said natural climate shifts cause massive climate changes, but this time human beings are the drivers of global warming.

“Here we are giving climate an equally large push by adding carbon dioxide. So the question is what are the possible consequences,” Broecker said. 

Paleoclimate scientists are looking for the answers to how all the factors involved in climate change work together.

“By comparing glaciers in the lakes we can begin to tease out fundamental aspects of ice age climate and abrupt climate change in the western U.S.,” Putnam said. By understanding how the climate works and how glaciers retreat on a natural level, he said, we can place modern global warming in that context.

“The mountain glaciers are the best temperature records on the planet. And the closed basin lakes are the best recorders of precipitation,” Broecker said. These records tell us how climate shifts affected temperature and rainfall in these regions, and they can be compared to similar regions in other parts of the world, building a picture global climate change. 

Experts think an imbalance in the temperature contrast between the hemispheres caused a southward shift in the thermal equator, the warm belt that encircles the Earth, ultimately driving the pulse of carbon dioxide into the atmosphere.

Records like those developed by Putnam in the Great Basin allow other researchers, such as climate-modeler Doug Boyle, a hydrologist at the University of Nevada who also presented at the Comer Conference, to create computer-based models of the impacts of historic and current climate shifts on hydrologic systems.

For Boyle, computer-based modeling is a crucial component to paleoclimate research. “It’s not enough to go out and look at something,” he said. Modeling helps create a picture of causality, using records to figure out the “why” behind a particular event or system. 

Putnam’s research, combined with computer modeling from Boyle and other colleagues, can produce a temperature curve for the last ice age in the two polar hemispheres, to compare glacier records globally.

Putnam’s research suggests that by about 16,000 years ago, glaciers in both the Southern and Northern Hemispheres had retreated to essentially their post-ice age positions. But the retreat happened earlier in the Southern Hemisphere, and when glacier melt there had already stabilized, Northern Hemisphere mountain glaciers underwent recession. 

This warming was followed by another shift in the thermal equator during the period called the Younger Dryas, about 13,000 years ago, according to Putnam. But this shift seems to have had little effect on the mountain glacier record, a mystery scientists are still trying to solve, he said. 

So what caused the thermal equator to shift and drive massive climate change? According to Putnam, one leading theory is that the ocean had a lot to do with it. 

The ocean exhibits a bi-polar, seesaw pattern of oceanic and atmospheric circulation between the Northern and Southern Hemispheres. Something jarred the ocean system into a different mode of operation, the thermal equator shifted and the southern ocean warmed, releasing the carbon dioxide that warmed the Earth. The systems are so complex scientists are not yet sure exactly how all these mechanisms work together.

But scientists do know carbon dioxide levels are much higher now than during the last warming cycle. 

All of this could have serious implications for the warming we’re seeing today. Today, the Northern Hemisphere is warming faster than the Southern, glaciers are melting, and it could mean the thermal equator is shifting again. We could see less rainfall in certain parts of the world, bringing drought, and more in others, resulting in floods. 

While today’s climate change is largely driven by human activity, Putnam points out that there is great value in understanding natural variations in climate. 

“These natural climate changes aren’t anything to scoff at,” Putnam said. “I know that global warming’s the big story, but civilizations have risen and fallen in response to natural climate oscillations over the past 10,000 years, so that in itself is a reason to understand it.”