Summit County Citizens Voice

Could forest-clearing in beetle-killed stands tilt regional climate toward dry conditions?

New research tracks rainfall patterns in Central America 

By Summit Voice

SUMMIT COUNTY — As if the pine beetle outbreak weren’t bad enough, there’s new evidence to suggest that widespread forest clearing can change precipitation patterns on regional scale, tilting climate toward drought conditions.

The findings by NASA climatologist Ben Cook suggest ancient Meso-American civilizations of the Mayans and Aztecs likely amplified droughts in the Yucatán Peninsula and southern and central Mexico by clearing rainforests to make room for pastures and farmland.

Converting forest to farmland can increase the reflectivity, or albedo, of the land surface in ways that affect precipitation patterns.

“Farmland and pastures absorb slightly less energy from the sun than the rainforest because their surfaces tend to be lighter and more reflective,” Cook said. “This means that there’s less energy available for convection and precipitation.”

Cook and colleagues reached their conclusions by comparing Central American rainfall patterns from pre-Columbian (before 1492 C.E.) and post-Columbian periods. The pre-Columbian era saw widespread deforestation on the Yucatán Peninsula and throughout southern and central Mexico. During the post-Columbian period, forests regenerated as native populations declined and farmlands and pastures were abandoned.

The results are unmistakable: Precipitation levels declined by a considerable amount — generally 10 to 20 percent — when deforestation was widespread. Precipitation records from stalagmites, a type of cave formation affected by moisture levels that paleoclimatologists use to deduce past climate trends, in the Yucatán agree well with Cook’s model results.

The effect is most noticeable over the Yucatán Peninsula and southern Mexico, areas that overlapped with the centers of the Mayan and Aztec civilizations and had high levels of deforestation and the most densely concentrated populations. Rainfall levels declined, for example, by as much as 20 percent over parts of the Yucatán Peninsula between 800 C.E. and 950 C.E.

Cook’s study supports previous research that suggests drought, amplified by deforestation, was a key factor in the rapid collapse of the Mayan empire around 950 C.E.

During the peak of Mayan civilization between the years 800 and 950, the land cover reconstruction Cook based his modeling on indicates that the Maya had left only a tiny percentage of the forests on the Yucatán Peninsula intact.

But after Europeans decimated native populations, natural vegetation covered nearly all of the Yucatán. In modern times, deforestation has altered some areas near the coast, but a large majority of the peninsula’s forests remain intact.

“I wouldn’t argue that deforestation causes drought or that it’s entirely responsible for the decline of the Maya, but our results do show that deforestation can bias the climate toward drought and that about half of the dryness in the pre-Colonial period was the result of deforestation,” Cook said.

Cook presented his research in early December at the annual meeting of the American Geophysican Union in San Francisco, where other researchers discussed significant droughts that affected the Northeast, including a three-year dry spell in the 1960s that had major impacts on the region.

New studies by NASA paleoclimatologist Dorothy Peteet shows that far more severe droughts have occurred in the Northeast — at least three major dry spells have hit region in the last 6,000 years.

The longest, which corresponds with a span of time known as the Medieval Warm Period, lasted some 500 years and began around 850. The other two took place more than 5,000 years ago. They were shorter, only about 20 to 40 years, but likely more severe.

Peteet analyzed sediment cores collected from several tidal marshes in the Hudson River Valley to gain an understanding of historic precipitation patterns. Using X-ray fluorescence spectroscopy they search for characteristic elements — such as bromine and calcium — that are more likely to occur at the marsh during droughts.

“People don’t generally think about the Northeast as an area that can experience drought, but there’s geologic evidence that shows major droughts can and do occur,” Peteet said. “It’s something scientists can’t ignore. What we’re finding in these sediment cores has big implications for the region.”

Fresh water from the Hudson River and salty water from the Atlantic Ocean were both predominant in Piermont Marsh at different time periods, but saltwater moves upriver during dry periods as the amount of fresh water entering the marsh declines. Peteet’s team detected extremely high levels of both bromine and calcium, both of them indicators of the presence of saltwater and the existence of drought, in sections of the sediment cores corresponding to 5,745 and 5,480 years ago.

During the Medieval Warm Period, the researchers also found striking increases in the abundance of certain types of pollen species, especially pine and hickory, that indicate a dry climate. Before the Medieval Warm Period, in contrast, there were more oaks, which thrive wetter conditions. They also found a thick layer of charcoal demonstrating that wildfires, which are more frequent during droughts, were common during the Medieval Warm Period.

“We still need to do more research before we can say with confidence how widespread or frequent droughts in the Northeast have been,” Peteet said.

There are some gaps in the cores Peteet’s team studied that she plans to investigate in greater detail. She also expects to expand the scope of the project to other marshes and estuaries in the Northeast and to collaborate with climate modelers to begin teasing out the factors that cause droughts to occur in the region.

Filed under: climate and weather, Environment, Snow and weather, Summit County news Tagged: | deforestation, drought, Forest health, forests, Mayan civilization, Medieval Warm Period, Yucatán Peninsula