Roy M. Huffington Department of Earth Sciences

Research: SMU-led fossil study finds carbon dioxide link to global warming 22 million years ago

Fossil leaves from Ethiopia

Variations in the concentration of atmospheric carbon dioxide affect carbon fixation during photosynthesis and can be measured on a preserved fossil leaf like this one from Ethiopia.

Fossil leaves from Africa have resolved a prehistoric climate puzzle — and also confirm the link between carbon dioxide in the atmosphere and global warming.

Research until now has produced a variety of results and conflicting data that have cast doubt on the link between high carbon dioxide levels and climate change for a time interval about 22 million years ago.

But a new study has found the link does indeed exist for that prehistoric time period, say SMU researchers. The finding will help scientists understand how recent and future increases in the concentration of atmospheric carbon dioxide may impact the future of our planet, they add.

The discovery comes from new biochemical analyses of fossil leaves from plants that grew on Earth 27 million years ago and 22 million years ago, said geologist Tekie Tesfamichael, an SMU postdoctoral fellow in Earth Sciences and a lead scientist on the research.

The new analyses confirm research about modern climate — that global temperatures rise and fall with increases and decreases in carbon dioxide in our atmosphere — but in this case even in prehistoric times, according to the SMU-led international research team.

Carbon dioxide is a gas that is normally present in the Earth’s atmosphere, even millions of years ago. It’s dubbed a greenhouse gas because greater concentrations cause the overall temperature of Earth’s atmosphere to rise, as happens in a greenhouse with lots of sunlight.

Recently greenhouse gas increases have caused global warming, which is melting glaciers, sparking extreme weather variability and causing sea levels to rise.

The new SMU discovery that carbon dioxide behaved in the same manner millions of years ago that it does today has significant implications for the future. The finding suggests the pairing of carbon dioxide and global warming that is seen today also holds true for the future if carbon dioxide levels continue to rise as they have been, said Tesfamichael.

“The more we understand about the relationship between atmospheric carbon dioxide concentrations and global temperature in the past, the more we can plan for changes ahead,” said Tesfamichael.

“Previous work reported a variety of results and conflicting data about carbon dioxide concentrations at the two intervals of time that we studied,” he said. “But tighter control on the age of our fossils helped us to address whether or not atmospheric carbon dioxide concentration corresponded to warming — which itself is independently well-documented in geochemical studies of marine fossils in ocean sediments.”

The researchers reported their findings in Geology, the scientific journal of the Geological Society of America. The article is “Settling the issue of ‘decoupling’ between atmospheric carbon dioxide and global temperature: [CO2]atm reconstructions across the warming Paleogene-Neogene divide.”

Co-authors from the Roy M. Huffington Department of Earth Sciences in Dedman College are professors Bonnie Jacobs, an expert in paleobotany and paleoclimate, and Neil J. Tabor, an expert in sedimentology and sedimentary geochemistry. Other co-authors are Lauren Michel, Tennessee Technological University; Ellen Currano, University of Wyoming; Mulugeta Feseha, Addis Ababa University; Richard Barclay, Smithsonian Institution; John Kappelman, University of Texas; and Mark Schmitz, Boise State University.

— Written by Margaret Allen

> Read the full story at the SMU Research blog: smuresearch.com

Research: SMU scientists help solve the mystery of climate and leaf size

Conifer needlesWhy is a banana leaf a million times bigger than a common heather leaf? Why are leaves generally much larger in tropical jungles than in temperate forests and deserts? The textbooks say it’s a balance between water availability and overheating – but researchers have found that it’s not that simple.

SMU paleobotanist Bonnie F. Jacobs has contributed work to a major new study that provides scientists with a new tool for understanding both ancient and future climate by looking at the size of plant leaves.

The study, published in the Sept. 1, 2017 issue of Science, was led by Associate Professor Ian Wright from Macquarie University, Australia. The study’s findings reveal that in much of the world the key factor limiting the size of a plant’s leaves is the temperature at night and the risk of frost damage to leaves.

Jacobs said the implications of the study are significant for enabling scientists to either predict modern leaf size in the distant future, or to understand the climate for a locality as it may have been in the past.

“This research provides scientists with another tool for predicting future changes in vegetation, given climate change, and for describing ancient climate given fossil leaves,” said Jacobs, a professor in SMU’s Roy M. Huffington Department of Earth Sciences in the Dedman College of Humanities and Sciences.

“Now we can reliably use this as another way to look at future climate models for a specific location and predict the size of plant leaves,” she said. “Or, if we’re trying to understand what the climate was for a prehistoric site tens of millions of years ago, we can look at the plant fossils discovered in that location and describe what the climate most likely was at that time.”

Wright, Jacobs and 15 colleagues from Australia, the U.K., Canada, Argentina, the United States, Estonia, Spain and China analyzed leaves from more than 7,600 species, then pooled and analyzed the data with new theory to create a series of equations that can predict the maximum viable leaf size anywhere in the world based on the risk of daytime overheating and night-time freezing.

The researchers will use these findings to create more accurate vegetation models. This will be used by governments to predict how vegetation will change locally and globally under climate change, and to plan for adaptation.

“The conventional explanation was that water availability and overheating were the two major limits to leaf size. But the data didn’t fit,” says Wright. “For example the tropics are both wet and hot, and leaves in cooler parts of the world are unlikely to overheat.”

“Our team worked both ends of the problem – observation and theory,” he says. “We used big data – measurements made on tens of thousands of leaves. By sampling across all continents, climate zones and plant types we were able to show that simple ‘rules’ seemingly operate across the world’s plant species, rules that were not apparent from previous, more limited analyses.”

Jacobs contributed an extensive leaf database she compiled about 20 years ago, funded by a National Science Foundation grant. She analyzed the leaf characteristics of 880 species of modern tropical African plants, which occurred in various combinations among 30 plant communities. Jacobs measured leaves of the plant specimens at the Missouri Botanical Garden Herbarium, one of the largest archives of pressed dried plant specimens from around the world.

She looked at all aspects of leaf shape and climate, ranging from seasonal and annual rainfall and temperature for each locale, as well as leaf shape, size, tip, base, among others. Using statistical analyses to plot the variables, she found the most prominent relationship between leaf shape and climate was that size increases with rainfall amount. Wet sites had species with larger leaves than dry sites.

Her Africa database was added to those of many other scientists who have compiled similar data for other localities around the world.

— Written by Margaret Allen

> Read the full story from the SMU Research blog