Man-Made Climate Change

New study identifies human contribution to atmosphere circulation changes

A new study published in this week’s issue of Nature is the first to show that human activity is altering the circulation of the tropical atmosphere and ocean through global warming.

Scientists widely agree that the climate has warmed over the past century and that human activities, such as burning fossil fuels, have significantly contributed to this global warming. This study tapped historical records that date back to the mid-19th century as well as simple theory and state-of-the-art computer model simulations to detect and attribute these climate changes. The conclusion was that the principal loop of winds that drives climate and ocean behavior across the tropical Pacific is slowing down and causing the climate to drift towards a more El Niño-like state. This could have important implications for the frequency and intensity of future El Niño events and biological productivity in tropical oceans.

In their paper, titled “Weakening of Tropical Pacific Atmospheric Circulation Due to Anthropogenic Forcing,” the researchers identify a 3.5 percent weakening that has occurred since the mid-1800s in this air system known as the Walker circulation. They also cite evidence that it may weaken another 10 percent by 2100.

“There is an indication that the slowdown may be intensifying,” said Dr. Gabriel A. Vecchi, lead author from NOAA’s Geophysical Fluid Dynamics Laboratory. “The trend since World War II is larger than that over the entire record, and the long-term trend is larger than what is expected from natural climate variability. This is why we employed a very long observational record — to be able to accurately detect and attribute these changes.”

The study does send mixed signals on the future of El Niño/La Niña. “While we can’t predict with certainty how the frequency or intensity of El Niño-related weather events will respond to global warming, our study does suggest that the climate as a whole is slowly moving towards a more El Niño-like state,” said Dr. Brian Soden, a co-author from the University of Miami Rosenstiel School of Marine and Atmospheric Science. “Additionally, this slowdown has modified the structure and circulation of the tropical Pacific Ocean, which is a source of nutrients to one of the most biologically productive regions of the world’s oceans. This has implications to the well-being and proliferation of marine life in tropical oceans.”

“The Walker circulation is fundamental to climate throughout the globe: its variations are closely linked to those of the El Niño/Southern Oscillation and monsoonal circulations over adjacent continents, and variations in its intensity and structure affect climate all over the globe,” wrote Vecchi, Soden, and their co-authors Andrew T. Wittenberg, Isaac M. Held, Ants Leetmaa, and Matthew J. Harrison, also from the NOAA Geophysical Fluid Dynamics Laboratory in Princeton, N.J. The Walker circulation spans almost half the circumference of the Earth.

This study found a weakening of the Walker circulation in historical observations that corresponds closely to what theoretical and modeling studies expect from an increase in greenhouse gases. This agreement provides increased confidence in model projections of future climate change in the tropics.

Rosenstiel School is part of the University of Miami and, since its founding in the 1940s, has grown into one of the world’s premier marine and atmospheric research institutions.