What microscopic fossilized shells tell us about ancient climate change

 At the end of the Paleocene and beginning of the Eocene epochs, between 59 to 51 million years ago, Earth experienced dramatic warming periods, both gradual periods stretching millions of years and sudden warming events known as hyperthermals.

Driving this planetary heat up were massive emissions of carbon dioxide (CO2) and other greenhouse gases, but other factors like tectonic activity may have also been at play.

New research led by University of Utah geoscientists pairs sea surface temperatures with levels of atmospheric CO2 during this period, showing the two were closely linked. The findings also provide case studies to test carbon cycle feedback mechanisms and sensitivities critical for predicting anthropogenic climate change as we continue pouring greenhouse gases into the atmosphere on an unprecedented scale in the planet's history.

"The main reason we are interested in these global carbon release events is because they can provide analogs for future change," said lead author Dustin Harper, a postdoctoral researcher in the Department of Geology & Geophysics. "We really don't have a perfect analog event with the exact same background conditions and rate of carbon release."

But the study published Monday in the Proceedings of the National Academy of Sciences, or PNAS, suggests emissions during two ancient "thermal maxima" are similar enough with today's anthropogenic climate change to help scientists forecast its consequences.

The research team analyzed microscopic fossils -- recovered in drilling cores taken from an undersea plateau in the Pacific -- to characterize surface ocean chemistry at the time the shelled creatures were alive. Using a sophisticated statistical model, they reconstructed sea surface temperatures and atmospheric CO2 levels over a 6-million-year period that covered two hyperthermals, the Paleocene-Eocene Thermal Maximum, or PETM, 56 million years ago and Eocene Thermal Maximum 2, ETM-2, 54 million years ago.

The findings indicate that as atmospheric levels of CO2 rose, so too did global temperatures.

"We have multiple ways that our planet, that our atmosphere is being influenced by CO2 additions, but in each case, regardless of the source of CO2, we're seeing similar impacts on the climate system," said co-author Gabriel Bowen, a U professor of geology & geophysics.

"We're interested in how sensitive the climate system was to these changes in CO2. And what we see in this study is that there's some variation, maybe a little lower sensitivity, a lower warming associated with a given amount of CO2 change when we look at these very long-term shifts. But that overall, we see a common range of climate sensitivities."

sources-science daily