Credit: USGS
It turns out the influence of natural drivers of tropical belt expansion and contraction is ‘poorly understood’. The authors say: ‘Our results warn of potential socio-economic consequences of future variations in tropical belt width driven by natural climate variability or stratospheric aerosol injections, whether volcanic or artificial.’
H/T Phys.org
For the first time, scientists have traced the north-south shifts of the northern-most edge of the tropics back 800 years, reports a University of Arizona-led international team.
The movement of the tropical boundary affects the locations of Northern Hemisphere deserts including the Sonoran, Mohave and Saharan. Those deserts sit just north of the tropical belt, which includes the subtropics.
Before now, scientists had information about the location of the tropical belt going back to around 1930, when reliable instrumental record-keeping began.
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 while a postdoctoral researcher at the UA Laboratory of Tree-Ring Research.
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, said Alfaro Sánchez, currently a postdoctoral researcher at the Centre de Recerca Ecològica i Aplicacions Forestals in Barcelona, Spain
Co-author Valerie Trouet said, “Our results suggest that climate change was one of the contributing factors to those societal disruptions.”
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, said Trouet, a UA associate professor of dendrochronology.
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 said.
Trouet said 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.
The team’s research paper, “Climatic and volcanic forcing of tropical belt northern boundary over the past 800 years,” is scheduled for online publication in Nature Geoscience on October 15. Names of the additional co-authors are at the bottom of this release.
Other researchers have documented that the tropics have been expanding northward since the 1970s, Alfaro Sánchez said.
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 said.
Continued here.
Nature article: Climatic and volcanic forcing of tropical belt northern boundary over the past 800 years
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October 17, 2018 at 03:57AM
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