September 1, 2007
How cold was it then? How hot is it now? Those questions, on which so much science (and politics) depends, are less straightforward than they might sound. Take the fact that, according to the U.S. National Climate Data Center, the earth’s surface temperature averaged about 58°F in 2006, which was about one degree above the average for the 20th century. That number is high compared to the recent past, but what about more distant times? And what does it tell us, if anything, about what causes global warming or what the future might bring?
This is where paleoclimatologists such as David Lea come in. The MIT-trained Lea is a professor of Earth Science at UC Santa Barbara and leader in research on prehistoric global warming and cooling. His work has filled gaps in the long-term temperature record and has played a key role in setting the context for today’s observations and debates. It also sheds light on how climate change occurs and what we might expect in our own century.
Lea’s focus is on the record of climate change encoded in the chemical archive of tropical oceans. By measuring trace levels of magnesium in the shells of tiny marine organisms deposited on the seabed, he has been able to construct a record of water temperatures going back 1.4 million years.
Tracing Ancient Temperatures
Climate change leaves records on both land and sea. Tree rings show variations in the amount of rain and snow. The temperature of long-ago rainwater is reflected in the proportion of oxygen and hydrogen isotopes in rock formations precipitated from water – such as stalactites and stalagmites in caves. In ocean environments, impurities in the fossilized calcite shells of organisms such as foraminifera (see photo on page 10) are clues to the temperature of the water at the time these tiny creatures lived. Lea measures one such trace element, magnesium, which precipitates in calcite at different levels depending on water temperature.
The amount of magnesium in the shells of foraminifera is very low. Each shell weighs about 10 micrograms, and the magnesium is only 0.25% of that. But Lea works with an apparatus, the inductively coupled plasma mass spectrometer, that is precise enough to detect differences in the level of magnesium corresponding to differences of 2° F (or roughly 1° C) in the temperature of the water when the shells were formed. His key findings on ancient tropical sea temperatures were based on samples taken from the sea floor at two widely separated sites in the Pacific Ocean, near the Galapagos Islands and northeast of New Guinea.
His most important discovery, announced in 2000, was that the water near the Equator in the eastern and western Pacific cooled significantly – by about 5° F – during ice ages. This was not what most scientists expected, says Lea: “Ten or 15 years ago, people would have been inclined to say the tropics then were about the same as they are now.” What made this data especially striking was the timing of the temperature change. Lea found that the cooling of tropical seas preceded the buildup of ice on land by about 3,000 years. This ran counter to the most popular idea of how ice ages occur – in a sequence that starts with slight variations in the earth’s angle toward the sun, followed by the advance of glaciers and further cooling from feedback effects, such as the reflection of solar energy from the spreading layer of snow and ice. Instead, the root cause of the cooling had to be something that preceded the glaciation, several thousand years before.
David Lea says global temperatures are warmer today than at any time since the last ice age ended some 10,000 years ago. And they seem to be not far from their highs of the past one million years.
In other words, the “How cold was it?” question turned out to be complicated in its own way. Just when and where the cooling occurred made a crucial difference. Lea says his data from the equatorial Pacific got him thinking. “One of the ideas that really captured my imagination was that the tropics are very responsive to greenhouse gas abundance in the atmosphere.” (Evidence from ice core samples shows that the level of carbon dioxide, a greenhouse gas, fell before the last ice age). That insight led Lea to study the tropical oceans for clues into the impact that greenhouse gases are having on global climate today.
It is in the waters near the Equator that climate trends are most directly linked to global climate change, he says. The public may hear mainly about shrinking glaciers and sea ice in polar regions or heat waves and other extreme weather in the temperate zones, but Lea says the much less volatile tropical seas are the best places to look for evidence that the earth’s atmosphere, as a whole, is heating up. One reason is that large expanses of the equatorial Pacific and Indian oceans are essentially static. They have little of the weather patterns that would cause sharp changes in temperature by drawing in or pushing out the colder air and water from higher latitudes. They are far from human influences such as the local warming of urban “heat islands.” And as large, deep pools, they have great thermal inertia; it takes a lot of heating or cooling, over a long time, to change their temperature much. Lea and fellow researchers, including James Hansen at NASA’s Goddard Institute for Space Studies, say they have found a slow but significant rise in water temperature – just over one degree Fahrenheit over the past century – in these regions.
So what about that big question, “How hot is it?” On the basis of recent data and the chemical record from tropical oceans, Lea says global temperatures are warmer today than at any time since the last ice age ended some 10,000 years ago. And they seem to be not far from their highs of the past 1 million years. Lea cautions that temperature records get increasingly uncertain with age; he notes that scientists’ recent estimates of ocean temperatures at the time of the dinosaurs range from 77° F to 104° F. And he says the ability of climatologists to predict what happens next is far from reliable – though that fact is less than reassuring. On the subject of sea levels he says, “My sense is that we’re probably not going to see a rise of more than a meter in this century,” but he says scientists have already been surprised by the speed with which the Arctic sea ice is shrinking. Melting sea ice does not raise sea levels, but if ice on land melts sooner than expected, the rise in sea levels could also be faster and more destructive – and not without precedent: “We know, for example, that 125,000 years ago, sea level was four to six meters higher than it is today.”
Lea faces this uncertain future with a mix of optimism and concern. He believes that human activity now fuels global warming and that, by the same token, human ingenuity – including the energy efficiency and solar-power research being conducted at UCSB – is capable of solving the problem in the long run. “But to some extent we have to divide [our efforts] between the short term and the long term.” It will be some time until the needed technology comes of age, and in the meantime Lea says there’s no getting around the need to conserve. “We’re emitting, as a global community, 7 billion tons of carbon a year,” he says. “You don’t want to be too negative because it doesn’t inspire people, but you need to tell people that this is a serious problem.”