In a paper published in this month’s issue of the journal Nature Geoscience, UCSB geologist Lorraine Lisiecki linked slow but regular warping of Earth’s orbit to climate changes over the past million years.

Earth’s climate history can be determined by scientists such as Lisiecki by examining the shells of a microscopic organism known as foraminifera. Over the last three decades, scientists have used the organisms to determine the climate conditions at the time which the foraminifera were alive.

“The ratio of different elements and isotopes in these shells tell us about the chemical composition and temperature of the ocean at the time that the organism was alive,” Lisiecki said.

According to Lisiecki, the remains of these ancient organisms are layered on the ocean floor. Since each layer corresponds to a certain period of time in Earth’s history, the fossils within them provide an excellent record once they are retrieved.

“By drilling into the sea floor we can get fossil shells from organisms that lived millions of years ago,” Lisiecki said.

As Lisiecki compared this climate record to the history of changes in Earth’s orbit — as mathematically derived by astronomers — she found that the climate changes indicated by her analysis correlated to changes in the eccentricity of Earth’s orbit. The eccentricity is a measure of how much the orbit deviates from being circular.

Specifically, it had been known that periods of substantial glacier growth occur every 100,000 years. Lisiecki found that these instances of glacier growth, or glaciations, and changes in the planet’s orbital eccentricity happened to coincide.

“The clear correlation between the timing of the change in orbit and the change in the Earth’s climate is strong evidence of a link between the two,” Lisiecki said. “It is unlikely that these events would not be related to one another.”

In addition to these conclusions, further analysis resulted in Lisiecki making the unexpected discovery that the most dramatic periods of glacier formation correlated to the weakest changes in the eccentricity of Earth’s orbit, and vice versa.

According to Lisiecki, the results weaken the theory that orbital changes were directly responsible for climate change. Lisiecki described how her results offered some insights into a long-standing debate between geologists concerning the causes behind long term climate change.

“Scientists don’t fully understand why the Earth’s climate has been so unstable over the past million years,” Lisiecki said. “Some scientists think it is because of the orbital changes, which does not agree with the results of my study. Some scientists think that it might be because large ice sheets, like we had 20,000 years ago at the peak of the last ice age, are physically unstable. Finally, others think that the instability might be caused by changes in carbon dioxide which result from unstable interactions between ocean circulation and climate.”

In addition, Lisiecki said her conclusions were on the scale of hundreds of thousands of years, and that climate change in the past century could not be explained by her findings. Therefore, Lisiecki said, recent climate change is probably due to human pollution.

“The current changes in climate and CO­2 are not consistent with what scientists would expect due to orbital forcing or natural climate changes,” Lisiecki said. “The expected natural climate response is a very gradual cooling trend over the next 90,000 years or so. In contrast, the recent rapid increases in global temperature are definitely caused by greenhouse gas emissions from human activities.”