Follow the movement of the tropics 800 years ago

The movement of the tropical border affects the locations of the deserts of the northern hemisphere, including Sonoran, Mohave and Sahara. These deserts are just north of the tropical belt, which includes the subtropics.

Until now, scientists had information about the whereabouts of the tropical belt dating back to around 1930, when reliable record keeping began.

On a standard map, the tropical zone extends about 30 degrees north latitude at 30 degrees south latitude.

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

"The movement of the tropical boundary is associated with changes in rainfall patterns," said Raquel Alfaro Sánchez, who led the research team while he was a postdoctoral researcher at the UA Research Laboratory on tree rings.

The team found that from 1568 to 1634, the tropics extended to the north. This period coincides with severe droughts and other upheavals in human societies, including the collapse of the Ottoman Empire in Turkey, the end of the Ming Dynasty in China and the near abandonment of the colony from Jamestown, Virginia, said Alfaro Sánchez, currently a postdoctoral researcher. at the Ecological Research Center for Forests in Barcelona, ​​Spain

Co-author Valerie Trouet said, "Our results suggest that climate change was one of the factors contributing to these societal upheavals."

To track the northern boundary of the Earth's tropical belt from 1203 to 2003, the team used annual tree rings from five different locations in the Northern Hemisphere. Researchers can understand the years of annual rainfall in the past because each annual tree growth ring reflects this year's climate.

Its 800-year history has also allowed researchers to link rare events such as huge volcanic eruptions to subsequent climate changes, said Trouet, an associate professor of dendrochronology.

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

"We can see the contraction of the tropics after volcanic eruptions such as Tambora," said Trouet.

According to Trouet, it is important to learn how aerosols affect the climate, as some researchers have proposed sending such particles into the atmosphere as a geoengineering solution to global warming.

The team's research paper, "Climate and Volcanic Forcing of the Northern Limit of the Tropical Belt in the Last 800 Years", is to be published online in Nature Geoscience October 15th. The names of the other co-authors are at the bottom of this release.

Other researchers have documented that the tropics have spread northward since the 1970s, said Alfaro Sánchez.

Because computer models of current and future climate models also show an expansion of the tropical belt, but not as much as what is actually happening, the researchers wanted to develop a longer history of movement of the tropical zone, said Trouet.

Researchers use tree rings to reconstruct past climate and climate change for many parts of the world. These climatic reconstructions extend over hundreds of years. To follow the movements of the tropical belt, Alfaro Sánchez and his colleagues used tree ring chronologies from five locations: Arkansas, the American West, the Tibetan Plateau, Turkey and northern Pakistan.

To determine how tree ring records reflect changes in the tropical belt, the team examined tree rings from 1930 to 2003 and compared the natural records of tree climate to instrumental recordings of changes in the tropical belt. .

The researchers focused on the changes in Hadley cells, the huge atmospheric convection cells that travel around the world in the tropics. Trouet said that Hadley's cells are an important engine of atmospheric circulation.

Knowing how changes in Hadley's cells correlated with changes in dark circles, the team then used several chronologies to identify trees to see how the tropics developed and contracted 800 years ago.

"It's the first reconstruction that goes back to the pre-industrial era," said Trouet. "To know the natural variability of the climate, we need to go back in time over the past 150 years."

Alfaro Sánchez and his colleagues found that the tropical belt had spread and shrunk by itself long before industrial times.

The internal variability of the Earth's climate system affects the movement of the tropics, said Trouet.

The current expansion of the tropical belt recorded since the 1970s is partly due to the increase in greenhouse gases in the atmosphere, reported other researchers.

The current expansion of the tropics can have significant impacts on society, as the team found that severe droughts were associated with persistent periods of tropical expansion, Alfaro Sánchez said.

Explore further:
According to a study, the tropics expand as expected by climate models

More information:
R. Alfaro-Sánchez et al, Climate and volcanic forcing of the northern limit of the tropical belt during the last 800 years, Nature Geoscience (2018). DOI: 10.1038 / s41561-018-0242-1