Tracking the movement of the tropics 800 years into the past

Posted: October 17, 2018 by oldbrew in climate, Natural Variation, research, volcanos

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.’

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

  1. cognog2 says:

    Interesting; but I expect the models will muck up the conclusions, if any.

    I am surprised that no one appears to have looked at the history of the Angar Wat site in Burma from a climate perspective. Lots of change – no CO2. Same with other derelict historic sites.
    Perhaps they have.

  2. oldbrew says:

    Cognog – see here:

    Climate as a contributing factor in the demise of Angkor,

  3. Graeme No.3 says:

    Wow! Fancy H. Lamb reporting this in (roughly) 1976. He must have been psychic or something.

  4. oldbrew says:

    Quote from the report:
    ‘Alfaro Sánchez and her colleagues found the tropical belt has expanded and contracted on its own long before industrial times.

    Internal variability in the Earth’s climate system affects the movement of the tropics, Trouet said.’

    So the said variability exists, and must have a cause or causes, which cannot include trace gases from humans 😎

  5. Jim says:

    It is human caused. Could a rabbit or a vulture make a tempreture scale, no, that takes a human, does a beaver count the tree rings it must cut thru to fell the tree? Or to notate the result? No, that’s human. Thank God for global warming, there would be no need for a beach, or a bikini.

  6. oldbrew says:

    Quote from the abstract:

    ‘We find that the tropical belt contracted (expanded) during positive (negative) phases of the El Niño Southern Oscillation and Pacific North American teleconnection patterns. The tropical belt also contracted significantly following major volcanic events that injected sulfur into the stratosphere. The longest period of persistent tropical belt expansion occurred in the late sixteenth century, during one of the coldest periods of the Little Ice Age. ‘

  7. oldbrew says:

    From another recent study: Solar forcing and climate variability during the past millennium as recorded in a high altitude lake: Lake Salda (SW Anatolia)…

    ‘Moreover, the Lake Salda records clearly show dry Dark Ages Cold Period (DACP), humid Medieval Climatic Anomaly (MCA), dry Little Ice Age (LIA), and humid Modern Warm Period (MoWP). These records suggest that the solar forcing, through its influence on the atmospheric circulation, is the main mechanism of climate change during the DACP, MCA, LIA and MoWP in this region.’

  8. The expansion and contraction of the various climate zones is visible manifestation of the negative system response to any forcing process that tries to move the energy content of the system determined by atmospheric mass and gravity.
    Such changes regulate energy loss to space so as to keep the atmosphere in hydrostatic equilibrium.

  9. cognog2 says:

    Thanks for the link on Angar Wat Oldbrew. Good reading and a good perspective on real climate change free of CO2 obsessions.

  10. Doonhamer says:

    Was the Earth’s tilt changing?
    Maybe “tropic” ,like so many words, does not mean what it used to.