Increases in greenhouse gas, particulate pollution emissions drive regional drying around the globe

LLNL scientists and collaborators used fingerprinting — the process of separating the relative roles of natural and human influences on global climate — to identify how greenhouse gas and particulate pollution emissions drive regional drying around the globe.
Credit: Lawrence Livermore National Laboratory

Despite devastating impacts of drought on human and natural systems, the reasons why long-term regional drying occurs remain poorly understood.

Research led by Lawrence Livermore National Laboratory (LLNL) scientists has identified two signatures or “fingerprints” that explain why arid conditions are spreading worldwide, and why the Western United States has tended toward drought conditions since the 1980s while the African Sahel has recovered from its prolonged drought. The research appears in the July 6 edition of Nature Climate Change.

The team of U.S. and Canadian scientists found that since 1950, human-generated greenhouse gasses and particulate  have influenced global changes in temperature, precipitation and aridity in two distinct ways, each yielding a different fingerprint pattern.

The fundamental goal of fingerprinting is to separate the relative roles of natural and human influences on . While it has helped to identify a discernible human influence in observational records of temperature, , rainfall and snowpack, this approach based on fingerprinting has not been used very often in aridity research.

Using a novel fingerprinting analysis focusing on the joint fluctuations in temperature, rainfall and aridity in  model simulations and observations, the team identified two large-scale mechanisms that have been driving global aridity change.

“The climate we are experiencing is influenced by many factors,” said LLNL climate scientist Celine Bonfils, lead author of the paper. “In nature, the atmosphere is very noisy but it also responds to external factors that act at different paces and places, a bit like how different musical instruments contribute to a song, each with their own tonal signatures, rhythms and placements within the song.”

These external factors include the well-known 11-year fluctuation in the sun’s energy; the episodic drumming of volcanic eruptions that shade the Earth from the sun for one to three years; the accompanying choir of particulate aerosols emitted from Europe, North America, China and India that shade the Earth and alter its cloud properties; and the resounding crescendo of the gradual accumulation of greenhouse gasses. Because each of these external factors has a characteristic timbre, like that of each of the musical instruments, this team of scientists has been able detect their distinct individual resonances in climate observations.

In this study, the team found two major forced changes in the climate system that emerge from the “background noise” produced by random internal climate processes. “It is a bit like capturing two different songs playing simultaneously out of a noisy background,” Bonfils said.

One song is louder and clearer: The dominant fingerprint is characterized by global warming, intensified wet-dry patterns of precipitation, and progressive large-scale continental aridity—all largely driven by a slowly evolving increase in .

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