Forest emissions, wildfires explain why ancient Earth was so hot

This photo shows Nadine Unger with Yale University's omega supercomputer. -  Photo by Matthew Garrett/Yale School of Forestry & Environmental Studies
This photo shows Nadine Unger with Yale University’s omega supercomputer. – Photo by Matthew Garrett/Yale School of Forestry & Environmental Studies

The release of volatile organic compounds from Earth’s forests and smoke from wildfires 3 million years ago had a far greater impact on global warming than ancient atmospheric levels of carbon dioxide, a new Yale study finds.

The research provides evidence that dynamic atmospheric chemistry played an important role in past warm climates, underscoring the complexity of climate change and the relevance of natural components, according to the authors. They do not address or dispute the significant role in climate change of human-generated CO2 emissions.

Using sophisticated Earth system modeling, a team led by Nadine Unger of the Yale School of Forestry & Environmental Studies (F&ES) calculated that concentrations of tropospheric ozone, aerosol particles, and methane during the mid-Pliocene epoch were twice the levels observed in the pre-industrial era – largely because so much more of the planet was covered in forest.

Those reactive compounds altered Earth’s radiation balance, contributing a net global warming as much as two to three times greater than the effect of carbon dioxide, according to the study, published in the journal Geophysical Research Letters.

These findings help explain why the Pliocene was two to three degrees C warmer than the pre-industrial era despite atmospheric levels of carbon dioxide that were approximately the same as today, Unger said.

“The discovery is important for better understanding climate change throughout Earth’s history, and has enormous implications for the impacts of deforestation and the role of forests in climate protection strategies,” said Unger, an assistant professor of atmospheric chemistry at F&ES.

“The traditional view,” she said, “is that forests affect climate through carbon storage and by altering the color of the planet’s surface, thus influencing the albedo effect. But as we are learning, there are other ways that forest ecosystems can impact the climate.”

The albedo effect refers to the amount of radiation reflected by the surface of the planet. Light-colored snowy surfaces, for instance, reflect more light and heat back into space than darker forests.

Climate scientists have suggested that the Pliocene epoch might provide a glimpse of the planet’s future if humankind is unable to curb carbon dioxide emissions. During the Pliocene, the two main factors believed to influence the climate – atmospheric CO2 concentrations and the geographic position of the continents – were nearly identical to modern times. But scientists have long wondered why the Pliocene’s global surface air temperatures were so much warmer than Earth’s pre-industrial climate.

The answer might be found in highly reactive compounds that existed long before humans lived on the planet, Unger says. Terrestrial vegetation naturally emits vast quantities of volatile organic compounds, for instance. These are critical precursors for organic aerosols and ozone, a potent greenhouse gas. Wildfires, meanwhile, are a major source of black carbon and primary organic carbon.

Forest cover was vastly greater during the Pliocene, a period marked not just by warmer temperatures but also by greater precipitation. At the time, most of the arid and semi-arid regions of Africa, Australia, and the Arabian peninsula were covered with savanna and grassland. Even the Arctic had extensive forests. Notably, Unger says, there were no humans to cut the forests down.

Using the NASA Goddard Institute for Space Studies Model-E2 global Earth system model, the researchers were able to simulate the terrestrial ecosystem emissions and atmospheric chemical composition of the Pliocene and the pre-industrial era.

According to their findings, the increase in global vegetation was the dominant driver of emissions during the Pliocene – and the subsequent effects on climate.

Previous studies have dismissed such feedbacks, suggesting that these compounds would have had limited impact since they would have been washed from the atmosphere by frequent rainfall in the warmer climate. The new study argues otherwise, saying that the particles lingered about the same length of time – one to two weeks – in the Pliocene atmosphere compared to the pre-industrial.

Unger says her findings imply a higher climate sensitivity than if the system was simply affected by CO2 levels and the albedo effect.

“We might do a lot of work to reduce air pollution from road vehicle and industrial emissions, but in a warmer future world the natural ecosystems are just going to bring the ozone and aerosol particles right back,” she said. “Reducing and preventing the accumulation of fossil-fuel CO2 is the only way to ensure a safe climate future now.”

The modeling calculations were performed on Yale University’s omega supercomputer, a 704-node cluster capable of processing more than 52 trillion calculations per second.

Comprehensive analysis of impact spherules supports theory of cosmic impact 12,800 years ago

This is UCSB Earth Sciences professor emeritus James Kennett. -  Courtesy photo
This is UCSB Earth Sciences professor emeritus James Kennett. – Courtesy photo

About 12,800 years ago when the Earth was warming and emerging from the last ice age, a dramatic and anomalous event occurred that abruptly reversed climatic conditions back to near-glacial state. According to James Kennett, UC Santa Barbara emeritus professor in earth sciences, this climate switch fundamentally — and remarkably — occurred in only one year, heralding the onset of the Younger Dryas cool episode.

The cause of this cooling has been much debated, especially because it closely coincided with the abrupt extinction of the majority of the large animals then inhabiting the Americas, as well as the disappearance of the prehistoric Clovis culture, known for its big game hunting.

“What then did cause the extinction of most of these big animals, including mammoths, mastodons, giant ground sloths, American camel and horse, and saber- toothed cats?” asked Kennett, pointing to Charles Darwin’s 1845 assessment of the significance of climate change. “Did these extinctions result from human overkill, climatic change or some catastrophic event?” The long debate that has followed, Kennett noted, has recently been stimulated by a growing body of evidence in support of a theory that a major cosmic impact event was involved, a theory proposed by the scientific team that includes Kennett himself.

Now, in one of the most comprehensive related investigations ever, the group has documented a wide distribution of microspherules widely distributed in a layer over 50 million square kilometers on four continents, including North America, including Arlington Canyon on Santa Rosa Island in the Channel Islands. This layer — the Younger Dryas Boundary (YDB) layer — also contains peak abundances of other exotic materials, including nanodiamonds and other unusual forms of carbon such as fullerenes, as well as melt-glass and iridium. This new evidence in support of the cosmic impact theory appeared recently in a paper in the Proceedings of the National Academy of the Sciences.

This cosmic impact, said Kennett, caused major environmental degradation over wide areas through numerous processes that include continent-wide wildfires and a major increase in atmospheric dust load that blocked the sun long enough to cause starvation of larger animals.

Investigating 18 sites across North America, Europe and the Middle East, Kennett and 28 colleagues from 24 institutions analyzed the spherules, tiny spheres formed by the high temperature melting of rocks and soils that then cooled or quenched rapidly in the atmosphere. The process results from enormous heat and pressures in blasts generated by the cosmic impact, somewhat similar to those produced during atomic explosions, Kennett explained.

But spherules do not form from cosmic collisions alone. Volcanic activity, lightning strikes, and coal seam fires all can create the tiny spheres. So to differentiate between impact spherules and those formed by other processes, the research team utilized scanning electron microscopy and energy dispersive spectrometry on nearly 700 spherule samples collected from the YDB layer. The YDB layer also corresponds with the end of the Clovis age, and is commonly associated with other features such as an overlying “black mat” — a thin, dark carbon-rich sedimentary layer — as well as the youngest known Clovis archeological material and megafaunal remains, and abundant charcoal that indicates massive biomass burning resulting from impact.

The results, according to Kennett, are compelling. Examinations of the YDB spherules revealed that while they are consistent with the type of sediment found on the surface of the earth in their areas at the time of impact, they are geochemically dissimilar from volcanic materials. Tests on their remanent magnetism — the remaining magnetism after the removal of an electric or magnetic influence — also demonstrated that the spherules could not have formed naturally during lightning strikes.

“Because requisite formation temperatures for the impact spherules are greater than 2,200 degrees Celsius, this finding precludes all but a high temperature cosmic impact event as a natural formation mechanism for melted silica and other minerals,” Kennett explained. Experiments by the group have for the first time demonstrated that silica-rich spherules can also form through high temperature incineration of plants, such as oaks, pines, and reeds, because these are known to contain biologically formed silica.

Additionally, according to the study, the surface textures of these spherules are consistent with high temperatures and high-velocity impacts, and they are often fused to other spherules. An estimated 10 million metric tons of impact spherules were deposited across nine countries in the four continents studied. However, the true breadth of the YDB strewnfield is unknown, indicating an impact of major proportions.

“Based on geochemical measurements and morphological observations, this paper offers compelling evidence to reject alternate hypotheses that YDB spherules formed by volcanic or human activity; from the ongoing natural accumulation of space dust; lightning strikes; or by slow geochemical accumulation in sediments,” said Kennett.

“This evidence continues to point to a major cosmic impact as the primary cause for the tragic loss of nearly all of the remarkable American large animals that had survived the stresses of many ice age periods only to be knocked out quite recently by this catastrophic event.”

Mystery of Infamous ‘New England Dark Day’ Solved by Tree Rings


Black day of 1780 caused by distant wildfires, MU experts say



At noon, it was black as night. It was May 19, 1780 and some people in New England thought judgment day was at hand. Accounts of that day, which became known as ‘New England’s Dark Day,’ include mentions of midday meals by candlelight, night birds coming out to sing, flowers folding their petals, and strange behavior from animals. The mystery of this day has been solved by researchers at the University of Missouri who say evidence from tree rings reveals massive wildfires as the likely cause.



“The patterns in tree rings tell a story,” said Erin McMurry, research assistant in the MU College of Agriculture, Food and Natural Resources Tree Ring Laboratory. “We think of tree rings as ecological artifacts. We know how to date the rings and create a chronology, so we can tell when there has been a fire or a drought occurred and unlock the history the tree has been holding for years.”



Limited ability for long-distance communication prevented colonists from knowing the cause of the darkness. It was dark in Maine and along the southern coast of New England with the greatest intensity occurring in northeast Massachusetts, southern New Hampshire and southwest Maine. In the midst of the Revolutionary War, Gen. George Washington noted the dark day in his diary while he was in New Jersey.


Nearly 230 years later, MU researchers combined written accounts and fire scar evidence to determine that the dark day was caused by massive wildfires burning in Canada.



“A fire comes along and heat goes through the bark, killing the living tissue. A couple of years later, the bark falls off revealing the wood and an injury to the tree. When looking at the rings, you see charcoal formation on the outside and a resin formation on the top that creates a dark spot,” said Richard Guyette, director of the Tree Ring Lab and research associate professor of forestry in the MU School of Natural Resources.



The researchers studied tree rings from the Algonquin Highlands of southern Ontario and many other locations. They found that a major fire had burned in 1780 affecting atmospheric conditions hundred of miles away. Large smoke columns were created and carried into the upper atmosphere.



“This study was a unique opportunity to take historical accounts and combine them with modern technology and the physical historical evidence from the tree rings and solve a mystery with science,” McMurry said.



The study – “Fire Scars Reveal Source of New England’s 1780 Dark Day” – was published in the International Journal of Wildland Fire.