Now a team of scientists led by UC Santa Barbara's Francis Macdonald has published a study suggesting that tectonic activity may be the culprit. They found that long-term trends in Earth's climate are set by the presence or absence of collisions between volcanic arcs and continents in the tropics. The results appear in the journal Science.
"There've been a few hypotheses but no agreements as to why we have warmer or colder climates on these very long timescales," said Macdonald, a professor in the Department of Earth Science.
Macdonald - Timescales - Periods - Minimum - Trends
And when Macdonald says "long timescales," he's talking about 10 million-year periods, at a minimum. These are broad climatic trends, the backdrop against which natural and human-made fluctuations play out. Scientists have a relatively good understanding of what factors influence the climate on a thousand-year timescale, according to Macdonald.
On any scale, though, the primary agent of climate change is carbon dioxide (CO2). The question is what factors influence the amount of CO2 in atmosphere. Some processes produce CO2, while others absorb it. Scientists call these sources and sinks.
Debate - Geologists - Sources - Sinks - Climate
The debate among geologists is whether sources or sinks affect the climate more. "Some have argued that CO2 sources, like volcanism, have driven climate change on long timescales, while others have argued that, no, it's the sinks that have caused climate change on these timescales," said Macdonald.
He believes it's mostly the sinks, specifically vast deposits of rock that absorb CO2 through chemical reactions. But these carbon sinks are not distributed evenly across the surface. For instance, greater Indonesia is only 1-2 percent of the Earth's exposed land area, but accounts for roughly 10 percent of the current geologic carbon sink.
Activity - Sinks - Number - Factors - Water
The activity of these sinks depends on a number of factors. Water is important for the chemical reactions and also washes the end results away into the oceans,...
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