Tree rings show how the tropics shifted 800 years into the past

Studying shifts of the northern-most edge of the tropics revealed that periods of tropical expansion coincided with severe droughts, an article led by Raquel Alfaro Sánchez (CREAF) found.

Where do the tropics begin and end? Have they always occupied the same extension? The climate in the tropics is dominated by the tropical rain belt. A belt of clouds and rains delimited approximately by the Tropic of Capricorn in the south, and the Tropic of Cancer in the north, but that is not static: it oscillates during the year creating a rainy season and another dry season.

A team of researchers led by Raquel Alfaro Sánchez, postdoctoral researcher at CREAF, published in Nature Geoscience a study that has measured for the first time the movement of the tropical limit in the Northern Hemisphere during the last eight hundred years. On a standard map, the tropical belt spans roughly 30 degrees north latitude to 30 degrees south latitude.

However, the new research reveals that from the year 1203 to the year 2003, the northern edge of the tropics fluctuated up to 4 degrees north and south of the northern 30th parallel.

"Movement of the limit of the tropics is associated with changes in precipitation regimes," said Raquel Alfaro Sánchez, who led the research team.

From 1568 to 1634, the tropics expanded to the north, the team found. That time period coincides with severe droughts and other disruptions of human societies, including the collapse of the Ottoman empire in Turkey, the end of the Ming Dynasty in China, and near abandonment of the Jamestown Colony in Virginia, says Alfaro Sánchez.

“Our results suggest that climate change was one of the contributing factors to those societal disruptions,” says coauthor Valerie Trouet, an associate professor of dendrochronology at the University of Arizona.

"From 1568 to 1634, the tropics expanded to the north, the team found. That time period coincides with severe droughts and other disruptions of human societies, including the collapse of the Ottoman empire in Turkey, the end of the Ming Dynasty in China, and near abandonment of the Jamestown Colony in Virginia, says Alfaro Sánchez..

It is well known that the tropics are not a fixed line. However, until now there were no data on the evolution of these invisible geographical boundaries before 1930, when scientifically accurate recording systems began to be used.

Tree rings as tracking devices

To track the northern boundary of the Earth’s tropical belt from 1203 to 2003, the team used the annual rings of trees from five different locations throughout the Northern Hemisphere. Researchers can figure out annual precipitation years into the past because each annual growth ring of a tree reflects the climate that year.

Having an 800-year history also allowed the researchers to connect rare events such as huge volcanic eruptions with subsequent changes in climate, says Trouet.

Massive volcanic eruptions cool the Earth because of all the fine particles and aerosols thrown into the atmosphere. The 1815 Tambora eruption in present-day Indonesia caused such cooling worldwide that 1816 was known in Europe as “the year without summer,” the team writes.

“We can see the contraction of the tropics after volcanic eruptions such as Tambora,” Trouet says.

Learning how aerosols affect climate is important because some researchers have proposed sending such particles into the atmosphere as a geoengineering solution to global warming.

Other researchers have documented that the tropics have been expanding northward since the 1970s, Alfaro Sánchez says. Because computer models of current and future climate models also show expansion of the tropical belt, but not as much as is actually occurring, the researchers wanted to develop a longer history of the movement of the tropical zone, Trouet says.

Researchers use tree rings to reconstruct past climate and climate changes for many locations around the globe. Those climate reconstructions extend hundreds of years into the past. To track past tropical belt movements, the researchers used existing tree-ring chronologies from five locations:

• Arkansas
• The American West
• The Tibetan Plateau
• Turkey
• Northern Pakistan

To discern how the tree-ring records reflect changes in the tropical belt, the team looked at tree rings from 1930 to 2003 and compared the trees’ natural archive of climate to instrumental records of changes in the tropical belt.

Article:

Alfaro-Sánchez, R., Nguyen, H., Klesse, S., Hudson, A., Belmecheri, S., Köse, N., ... & Trouet, V. (2018). Climatic and volcanic forcing of tropical belt northern boundary over the past 800 years. Nature Geoscience, 1. doi:10.1038/s41561-018-0242-1