As University geosciences professor Daniel Sigman was growing up in southern California, he considered structure to be an entirely man-made phenomenon. It was only later, as an undergraduate at Stanford University, that he became interested in the earth as a source of this kind of order. It was then that he realized that some of the most elegant structures and systems known to man are those created by and observed in nature.
"I've always been very interested in working systems, mechanistic systems," Sigman said. The natural environment is on the frontier of these systems, he said, and is among the most complex.
As a scholar, he has been developing techniques which can be used to create and refine theories about the systems that have regulated the planet's climate over the last several million years.
Just lucky?
Scientists believe these elaborate systems have kept the earth stable enough to support life continuously for roughly three and a half billion years.
Indeed, as far back as the fossil records reliably go — about 600 million years — there has never been an extinction on this planet down to the unicellular level.
While some higher species have come and gone, the earth has never been void of life.
So the big question is, How did the earth's inhabitants get so lucky? Were we just fortunate to wind up on a planet whose conditions simply didn't fluctuate too far from those necessary for life?
Many scientists think it's more than just luck. They believe there are mechanisms working behind-the-scenes — like those keeping the levels of nutrients even and preventing irreversible climate change — that keep the earth inhabitable.
These climate control mechanisms fall into two main categories. First, there are negative feedback systems, which act a bit like cruise control on a car. Once they are set to a certain level, they fight to maintain that level, adding gas as the car slows and easing up as the car goes down a hill.
The other type of feedback, so-called positive feedback, has none of this stability. In cases of positive feedback, as conditions move away from equilibrium, the system pushes things further from that equilibrium and the process spirals out of control unless some other process intervenes.
Scientists in the field are actively trying to figure out how various negative feedbacks have regulated positive feedbacks, keeping the earth stable enough to allow for the continuity of life while explaining the types of variation observed over smaller time periods.
And while they're not there yet, Sigman has added some important pieces to the puzzle — helping gather data which can be used to suggest or evaluate different feedback models.
Nitrogen and the ecocycle
A main facet of Sigman's research involves measuring the amount of nitrogen that was present in the oceans during different time periods.
Nitrogen is an essential element in the oceanic ecocycle, but its supply can vary greatly over short time periods.
While oxygen can remain dissolved in a useful form for more than a million years, the entire nitrogen supply in the ocean would disappear in about 5,000 years if not replenished. Thus scientists are searching for a negative feedback system that may keep nitrogen levels stable.
To measure nitrogen levels, Sigman has refined a technique to examine a phenomenon known as nitrogen fractionization.
Certain organisms, which live in the first few meters of open ocean, can subsist by consuming either oxygen or nitrogen which are dissolved in the water. When atmospheric oxygen levels are low, the organisms are forced to use nitrogen.
Nitrogen comes in two slightly different varieties, or isotopes, one much rarer and a bit heavier than the other. The surface organisms can use both types of nitrogen, though the reactions involved in consumption are slightly more effective on molecules containing the lighter variety because the bonds it makes are easier to break.
Thus, by studying the ratios of the two sorts of nitrogen in samples from the ocean floor, Sigman has gained insight on how rates of processing and prevalence of elements varied over millions of years.
By looking at core samples from the floor of the ocean, "we can get a sense of what was going on at the surface," he said.
Global warming
Sigman has also explored whether carbon dioxide may be a part of a positive feedback mechanism that explains some of the environmental temperature variations scientists have observed. This positive feedback may help explain recent cycles between ice ages and more temperate periods.
Scientists have long known about certain irregularities in the earth's motion around the sun. The axis on which the earth rotates wobbles in two different ways, and the path of the earth's orbit also slightly varies its shape. Geoscientists noticed that the rate of temperature fluctuations on the earth seemed to have a correlation with the rates of these three irregularities — particularly the two wobbling motions.
However, over the last three million years, the temperature fluctuations associated with wobbling have been larger than what one would expect from these motions alone.
Sigman developed a possible positive feedback mechanism which would amplify theses small changes to the scale of those observed. He found that when temperatures fell, carbon dioxide became trapped in the deep ocean. This decrease in carbon dioxide lessened the greenhouse effect, further cooling the earth.
Slight increases in temperature had the opposite effect, raising the amount of atmospheric carbon dioxide. Once the gas was in the air, it trapped more solar energy, further heating the earth and releasing even more greenhouse gas.
The refinement of this mechanism is just another small step as geologists try to piece together a theory which would fully describe the glacial and interglacial periods in the earth's recent past.
Real-world applications
When asked about where all of this data could be applied, Sigman joked that attempts to successfully model climate change had "been a 30 year project for about 100 years now."
However, he said, "in the next 10 years I'm hopeful we'll be able to develop and understand the fundamental processes that are driving glacial [and] interglacial cycles.
"We're not altogether prepared for the complexity we're likely to see in the years that are coming," he added. "But this is why we need to work so hard to ask simple questions to try to understand what is a relatively complex system in the simplest possible way."
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