Hollywood Hysteria or Scientific Reality?
Much hype is made about the impact of climate change from both sides of the ideological spectrum. But what does the actual science say? These NASA researchers break it down.
Published March 1, 2005
By Sheri Fink
Academy Contributor
From the cover stories of popular science magazines to the content of popular Hollywood movies, the possibility of abrupt, catastrophic climate change has stirred the public imagination. But how real is the threat? At NASA’s Goddard Institute for Space Studies, Gavin A. Schmidt and Ronald L. Miller are attempting to answer that question by creating climate models, testing them against evidence from historical climate records, and then using the models in an effort to predict the climate of the future.
The Greenland ice core offers clues to the history of climate change. Calcium content and methane levels correlate with the sharp temperature changes during abrupt climate changes. “You can count the layers in these ice cores. It’s like tree rings; you can see one year after another,” says Schmidt.
The idea that abrupt climate change is even a possibility in our relatively climatically stable Holocene era comes by way of a single example. Recorded in the Greenland ice core, it dates to the very end of the last ice age, roughly 12,000 years ago.
“This is the poster child for abrupt climate change,” says Schmidt, “extremely cold going to extremely warm very, very quickly. When this was first discussed, people had no idea that the climate could change so rapidly.”
The period was named the Younger Dryas because of its reflection in European Dryas flower pollen records. Various other climate records also show the event – from caves in East China to sediments in Santa Barbara and Cariacao Basins to ice cores in the Andes to cave deposits around the Mediterranean.
Flow and Flux in the North Atlantic
“You can see a clear signature of this event almost everywhere in the globe,” says Schmidt. However, the effect is largest near the North Atlantic. “That kind of points you to something that’s happening in the North Atlantic as a possible cause or trigger for what’s going on,” he says.
The circulation of the ocean is driven not only by wind, but also by the water’s salt content and density. The two factors interact in a complex way.
Schmidt sums up the ocean’s overturning circulation – also known as thermohaline circulation – as “warm water that rises along the surface and cold salty water that remains underneath. That transport makes it much warmer in the North Atlantic than it is for instance in the North Pacific.”
The process is self-sustaining. “It’s warm in the North Atlantic because those currents also bring up salt. That salt is heavy, which causes water to sink, and this motion causes the water to release heat.”
He points out that the system also has the potential for different states, however. If for some reason the currents ceased, then the water would not be as salty. It would not sink, and the surroundings would stay cold.
Researchers recently developed a paleoclimate measure that correlates with the residence time of water in the North Atlantic. “In the Younger Dryas,” says Schmidt, “there was a big dip in how much water was being exported – or the residence time of water in the North Atlantic.” This indicates that the North Atlantic overturning circulation was much reduced at the time of that rapid climate change.
An Explanation for Abrupt Climate Change?
The last ice age was characterized by many examples of rapid climate change. Changes in ocean circulation provide a possible explanation.
“We have reasons to believe that ice sheets aren’t particularly stale,” says Schmidt. “Every so often, if they get too big, they start melting at the base.”
An iceberg calving that landed in the ocean and melted would produce a large freshwater pulse. “As you make the ocean fresher and fresher and fresher, then you get less and less formation of that deep water. As that reduces, then there’s less salt being brought up from the lower latitudes,” he explained.
“At one point it’s just too fresh, and then nothing’s being brought up anymore.” In that case, the only stable solution, Schmidt says, is the slowing of the thermohaline circulation.
Could a reduced overturning actually cause abrupt climate change? The answer isn’t clear yet, but there is a correlation. “When we have a weak circulation, it seems like the climate in a lot of cases is very cold,” says Ron Miller.
Why Worry Now?
On top of this instability, humans have dramatically changed the atmosphere’s composition over the past 150 years. And that’s cause for some concern. The energy absorbed by greenhouse gases is balanced by evaporation, which should lead to an increase in rainfall.
“It’s predicted by every climate model,” says Miller. That rainfall could be a source of just enough fresh water to tip the scales, stilling the ocean and, perhaps, making the atmosphere colder. Indeed, a study of ocean salinity shows that in the past decades, the ocean has gained extra fresh water. “The question is, how much cooling do we get?” asks Miller. “Where is this cooling happening? Is it global, and how important is it compared with the warming caused by greenhouse gasses?”
Miller and Schmidt are using a general circulation model to predict the answers to these questions. First, the model was tested to see how well it could predict climatic occurrences of the past century. A rough grid was superimposed on the planet, and within each grid cell, the changes in water vapor, liquid water, momentum, energy, and other factors were observed.
Next, positive and negative forcings – the atmospheric conditions expected to warm or cool the planet, such as solar irradiance and tropospheric and stratospheric aerosols – were calculated or estimated and added to the model. “It’s tracking the observed global average temperature surprisingly well, and we’re really quite proud of this,” says Miller.
Miller admits to a few kinks in regional predictions. Still, he says, “we have a lot of confidence that the model is good at reproducing 20th century climate trends. That gives us some confidence that we can actually make predictions in the future.”
No Cause for Alarm, Yet
After estimating the atmospheric conditions of the next century – no easy task in and of itself – the researchers took the model out for a spin to see what could be expected over the next 100 years. The results indeed predict a slowing of the thermohaline circulation corresponding to a cooling in some areas. “But it’s swamped globally by the warming expected from the greenhouse gases,” says Miller. “So clearly there’s no evidence for any sort of ice age.”
Other researchers have created their own models. All of these point to various drops in the freshwater ocean circulation, but all agree with Miller and Schmidt’s conclusions. “The models give no indication we’re going to see any climate surprises or ice ages in the next 100 years or so,” says Miller.
The terrible tsunami of Dec. 26, 2004 has left no one to doubt the power of oceanic change, in this case due to an undersea earthquake. Still, those kept awake at night by the imagined catastrophic aftermath of thermohaline circulation slowing, as depicted in the film The Day After Tomorrow, may rest easier. Meanwhile, the scientists are continuing to refine their models and studying other factors that may have led to rapid climactic change in the past.
Also read: Climate Change: A Slow-Motion Tsunami
About the Author
Sheri Fink, M.D., Ph.D, is a freelance journalist. Her award-winning book War Hospital: A True Story of Surgery and Survival (PublicAffairs, 2003) was published in paperback in December 2004.
