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News Article

Tree-rings help track the movement of the tropics

New research finds that periods of tropical expansion were associated with severe droughts

The position of the tropical boundary affects the locations of deserts in the Northern Hemisphere that are situated just north of the tropical belt.  This includes the Saharan, Mohave and Sonoran deserts. Until now, information about the location of the tropical belt only dated back to 1930, when reliable and instrumental record-keeping began.  However, new research by an international team of scientists, led by the University of Arizona has identified the north-south shift of the northern most boundary of the tropics.  

On standard maps, the tropical belt extends roughly 30 degrees north (latitude) to 30 degrees south (latitude), although the study shows that from the year 1203 until 2003, the northern boundary of the tropics shifted by up to 4 degrees north and south of the 30th parallel north.   

"Movement of the limit of the tropics is associated with changes in precipitation regimes," said study co-author postdoctoral researcher Raquel Alfaro Sánchez. 

The team found that between 1568 to 1634, the tropics expanded to the north, which coincided with severe droughts and human societal disruptions, including the end of the Ming Dynasty in China and the collapse of the Ottoman empire in Turkey.  According to the researchers, the findings suggest that climate change was one of the contributing factors to these historical events. 

In order to track the northern boundary of the Earth’s tropical belt over the past 800 years (AD 1203-2003), the researchers used the growth rings of trees from five different mid-latitude regions across the Northern Hemisphere, including Arkansas, the American West, the Tibetan Plateau, Turkey and northern Pakistan.  Using this data, the researchers were also able to connect rare natural events, such as volcanic eruptions with subsequent changes in climate.   

Huge volcanic eruptions emit vast quantities of fine particles and aerosols into the atmosphere, which cool down the Earth.  The Tambora eruption in Indonesia in 1815, caused such worldwide cooling, by several degrees, that 1916 became known as “the year without summer.” 

According to study co-author and Professor of Dendrochronology Valerie Trouet, the contraction of the tropics was evident after volcanic eruptions such as Tambora. Understanding how aerosols affect climate is important because some researchers have suggested sending particles such as these into the atmosphere, as a geoengineering solution to global warming. 

Other researchers have reported the expansion of the tropics northward since the 1970’s, but Trouet and the team wanted to devise a longer history of the movement of the tropical zone. 

Tree rings are frequently used in the reconstruction of past climate and the changes in climate over time.  To deduce how the tree-ring records reflected changes in the tropical belt, the team analysed the growth rings from 1930 to 2003 and compared this natural climate record to instrumental records of changes in the tropical belt.  The researchers focused on Hadley cells, large atmospheric convective cells that circumnavigate the globe that are important drivers of atmospheric circulation. 

By understanding how changes in these Hadley cells correlated with changes in tree rings, the team used multiple growth-ring chronologies to see how the tropics expanded and contracted up to 800 years ago. 

"This is the first reconstruction that went back to pre-industrial times," Trouet said. "To know what the natural climate variability is, we need to go farther back in time than the last 150 years." 

The team found that the tropical belt had expanded and contracted on its own accord many years before industrial times.  They explain that internal variability in the Earth’s climate system plays an important role in the movement of the tropics.  The longest period of persistent tropical belt expansion took place during the late sixteenth century, during one of the coldest periods of the Little Ice Age.  The current recorded expansion since the 1970’s is thought to be partly linked with the rise in atmospheric greenhouses gases, according to a separate report. 

The researchers found that past severe droughts were linked with long periods of tropical expansion and their results warn of potential socio-economic consequences of future variations in tropical belt width driven by natural climate variability or aerosol injections into the stratosphere. 

Journal Reference 

R. Alfaro-Sánchez, H. Nguyen, S. Klesse, A. Hudson, S. Belmecheri, N. Köse, H. F. Diaz, R. K. Monson, R. Villalba, V. TrouetClimatic and volcanic forcing of tropical belt northern boundary over the past 800 yearsNature Geoscience, 2018; DOI: 10.1038/s41561-018-0242-1

Article details

  • Author(s)
  • Stephanie Cole
  • Date
  • 31 October 2018
  • Source
  • University of Arizona
  • Subject(s)
  • Dendrochronology