The tropics are widening rapidly, but humans may not be entirely to blame — yet

  • An uptick in tropical expansion the past few decades would seem to suggest that some unknown factor, perhaps as a result of human activities, is driving the widening of the tropics. But a study led by Paul Staten, an atmospheric sciences professor and researcher at Indiana University Bloomington in the United States, finds that that is not necessarily the case.
  • Staten and his colleagues determined that the tropics have been widening at an average rate of about 17 miles, or 0.2 degrees latitude, per decade in both the Northern and Southern hemispheres, which is not outside of what climate models predict.
  • Staten and team state in the study that no hidden forcing is required in order to explain the tropical expansion we’ve already observed — our current models, which take into account natural variation and manmade global warming, can account for the 0.2 degree-per-decade expansion rate they established.

Scientists have observed the tropics expanding toward Earth’s poles in recent decades, which was projected to happen as increasing concentrations of greenhouse gases in the atmosphere continue to heat up the planet. Some observational studies have found that the tropics are widening much faster than climate models predicted, however.

Any widening of the tropics can have significant impacts for the roughly half of the global population who live there — from shifting rain belts and growing desertification to more severe and frequent droughts and wildfires. So it’s an important question: Is anthropogenic climate change causing even more rapid poleward expansion of the tropics than expected? And if so, why?

An uptick in tropical expansion the past few decades would seem to suggest that some unknown factor, perhaps as a result of human activities, is driving the widening of the tropics. But a study led by Paul Staten, an atmospheric sciences professor and researcher at Indiana University Bloomington in the United States, finds that that is not necessarily the case.

“[S]ome studies claim that the observed tropical widening outpaces that expected from modern climate change, suggesting that some ‘hidden forcing’ may be unaccounted for. Here we strive to resolve this apparent mystery by synthesizing results from the growing body of literature on the quantification, attribution and underlying processes of tropical widening,” Staten and team write in a paper detailing their findings, published in the journal Nature Climate Change late last month. “We review metrics, causes, observations and simulations of tropical widening, and find that the widening of the global mean tropical belt may not be predominantly human-induced.”

In contrast to previous estimates of tropical widening made since the beginning of the satellite era in the late 1970s, which ranged from 0.25 to 3 degrees of latitude per decade, Staten and his colleagues determined that the tropics have been widening at an average rate of about 17 miles, or 0.2 degrees latitude, per decade in both the Northern and Southern hemispheres — though they add that the rate can vary significantly from year to year and location to location.

“If we compare the observed trends of how the tropics have widened to modeling trends, it’s actually not outside of what the models predict,” Staten said in a statement.

Staten and team state in the study that no hidden forcing is required in order to explain the tropical expansion we’ve already observed — our current models, which take into account natural variation and manmade global warming, can account for the 0.2 degree-per-decade expansion rate they established. “Including recent evidence, it is fair to assert that the natural swings in decadal atmospheric and oceanic variability may have driven at least as much of the observed expansion as human activity,” the researchers write.

Staten said that this should give us more confidence in predictions based on current climate models. “Climate change should continue to expand the tropics over the next several decades,” he said, adding: “But the expansion may not continue at the rapid rate we’ve seen; at times it may even temporarily contract.”

The researchers focused on five factors that influence the widening of the tropics, including increasing greenhouse gas emissions; ozone depletion in the stratosphere over the South Pole; aerosols from volcanic eruptions; pollution like soot and ozone in the troposphere; and natural variation, such as changes in sea surface temperatures due to El Niño and La Niña events.

Due to how complex these factors are, the researchers note, it’s actually quite difficult to discern between natural and manmade causes of tropical widening. But if we don’t do something soon to rein in greenhouse gas (GHG) emissions and pollution, the human factor will become much more readily apparent.

“Although discerning a forced signal in the observational record is challenging, the detection of such a signal in the future is not so much a question of if, but when, if humanity continues on the business-as-usual path of GHG emissions,” the researchers write in the study. “In the near term, natural variability muddles the widening signal of increasing GHG concentrations. But models project that the forced tropical widening will break out of the envelope of natural year-to-year variability some time in the middle of this century.”

Australia’s Lake Hume is on the fringes of the tropics and could be affected by the expansion of desert areas associated with widening of the tropics. Photo Credit: Tim J. Keegan.

CITATION

• Staten, P. W., Lu, J., Grise, K. M., Davis, S. M., & Birner, T. (2018). Re-examining tropical expansion. Nature Clim Change, 8(9), 768-775. doi:10.1038/s41558-018-0246-2

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