"Somewhere, something incredible is waiting to be known" Carl Sagan
sábado, 5 de noviembre de 2011
Ancient Environment - Could fish fossils help us better predict climate change?
By measuring an element present in fish fossils, researchers at the University of Missouri may have found a key to better understanding how climate change works.
Can the study of fossil fish predict climate change? [Credit: Flickr/moonlightunicorn]
The researchers measured neodymium, an element that indicates where sea water originally came from.
The ratio of two isotopes of neodymium varies in different areas, leaving a sort of signature on the water. Fish teeth and bones pick up the same signature from the water where the fish died and fell to the sea floor. Because of that, the ratio of the neodymium isotopes can act as a natural tracking system for water masses, according to Ken MacLeod, a professor of geological science at the university.
This tracking system shows that, in a prehistoric time called the Late Cretaceous Epoch, the deep oceans circulated differently than many scientists had previously thought. That’s important because the Late Cretaceous Epoch was a greenhouse climate – one with high levels of atmospheric carbon dioxide – and researchers also found that the timing of a change in the ocean-circulation patterns closely matched the timing of climate change. Understanding how the oceans circulated during prehistoric climate change could help us better predict how climate change might affect us in the future.
“As we look at ancient climates, modern climates and future climates, the same processes are active at all times,” MacLeod says. “It’s largely the same water, the same physics, the same chemistry. … [Understanding the circulation patterns of the past] is vitally important to understanding the climate dynamics of the future.”
Approaching Prehistoric CO2 Levels
And not too far into the future, either, at least in geological terms. “We’re surprising close” to the Cretaceous levels, MacLeod says. How close? Within 90 years, by most predictions. As MacLeod puts it, “We’re on the bottom edge of the levels that were thought to have existed in the Cretaceous and rapidly heading toward the estimated values during the greenhouse times, including the Late Cretaceous.”
In just a few decades, humans have managed to cause the kind of atmospheric changes that it took geological climate cycles millions of years to accomplish. Carbon-dioxide levels in the atmosphere have grown 38 percent to approximately 389 parts per million today from 280 parts per million in 1850, the pre-industrial benchmark for climate change. In 2009, a study at the University of California at Los Angeles concluded that these carbon-dioxide levels haven’t been seen on Earth for 15 million to 20 million years.
Predictions for 2100 range from 600 parts per million to 1,000 parts per million, levels similar to those 65 million to 71 million years ago. The estimate for atmospheric carbon dioxide for that period is two to four times preindustrial levels, or roughly between 560 parts per million and 1120 parts per million.
Most models set up to try to figure out what happened during the Late Cretaceous climate changes conclude that water sank from the surface to the bottom of the ocean around Antarctica or the North Pacific. But the fish fossils show that this sinking actually happened in the North Atlantic. Warm water moved into the North Atlantic while the cool waters from the North Atlantic flowed out.
At the same time, temperatures on the land around the North Atlantic also warmed while the rest of the globe cooled. While it’s not yet clear whether the ocean circulation pattern caused the climate changes or vice versa, it’s clear the two are correlated and that circulation in the deep ocean is “a major controlling variable” in climate dynamics, MacLeod says.
What We Don’t Know
He cautions against assuming that we’d see the same circulation patterns as those in prehistoric times if the atmospheric carbon-dioxide levels reach those same highs again. After all, many things have changed since the Late Cretaceous Epoch. But the study’s conclusions certainly raise such questions.
“We do wonder if [the sinking pattern we found] is a general characteristic of greenhouse times,” MacLeod says. “Is there a characteristic greenhouse circulation pattern? Finding a generality like that – if this is how greenhouse circulation works as opposed to the circulation in icehouse times — would change our fundamental understanding of greenhouse dynamics.”
The new evidence about how the oceans circulated indicates that models showing a different circulation pattern – which is most of them – probably are missing a crucial factor in their calculations, MacLeod says. “We would argue that the models that don’t reproduce this mode of circulation are missing something about the way that circulation in the Cretaceous period works,” he says. “And the way that it’s correlated with temperature changes lead us to believe that it’s pretty important.”
Armed with more certainty about the circulation patterns of the past, scientists will be able to better test how well the climate-change models are working, MacLeod adds. In other words, if a model doesn’t come up with the Late Cretaceous patterns of circulation that the fish fossils show actually occurred, scientists will have less confidence in that model. Meanwhile, MacLeod says,“if the model is accurately giving us the way things work in the Cretacous period, it should work 100 years from now.”
The University of Missouri researchers now aim to use their findings to test various climate-change models. The team has submitted a grant proposal and expects to find out in the next three to five months if it will get funding for the project, MacLeod says. “We hope that we’re going to get the models and the data to concur,” he says. “We hope that we have a model that reproduces the processes that we think occurred, that we can find completely separate kinds of evidence – modeling and measuring chemical components on fish fossils – that both get the same result.”