New research suggests that greenhouse gas emissions generated by humans may be influencing long-term climate patterns, which could lead to persistent drought conditions across the western United States.
The study highlights a connection between human activities and a climate pattern known as the Pacific Decadal Oscillation (PDO), which is linked to ongoing aridity affecting California and the Colorado River Basin.
“The drought conditions and ocean patterns we observe aren’t merely natural fluctuations. They’re largely driven by human actions,” said Jeremy Klavans, a postdoc at the University of Colorado Boulder.
The PDO involves natural cycles in the North Pacific that typically change every few decades, with phases characterized by warm or cold ocean waters along the US West Coast and Japan. However, since the 1990s, the PDO has remained in an unusually prolonged negative phase, which has led to cooler, less humid air and reduced precipitation in the western US.
Currently, about 93% of this region is experiencing drought, while 70% is facing severe dry conditions, according to the researchers. Previous studies indicate that this trend has been evident for around 200 years.
Traditionally, scientists have viewed PDOs as natural phenomena, but the research indicates that, if accurate, the pattern should have switched to a positive phase after the strong El Niño event in 2015. Instead, the PDO briefly shifted before returning to its negative state.
“For over three decades, the PDO has followed a consistent downward trend, leading to stable yet damaging climate impacts,” the authors noted.
In their investigation, Klavans and his team analyzed over 570 climate simulations to compare past conditions with potential future scenarios.
The findings showed that from 1870 to 1950, PDO changes were largely driven by natural internal factors. However, since the mid-20th century, more than half of those changes have been attributed to human-induced greenhouse gases and aerosols.
By refining previous models to eliminate errors, the researchers concluded that human emissions are now responsible for nearly all of the forced changes in the PDO.
While the study focused on the western United States, Klavans suggested that the methodology could also apply to other regions, mentioning, for example, the North Atlantic Oscillation and its impact on drought conditions in Spain.
Klavans cautioned that if greenhouse gas emissions continue to rise, the PDO may remain in a negative phase, potentially prolonging drought for at least another 30 years.
With this understanding, the authors hope that their findings will assist water managers in developing improved strategies for water management. “Water planners can establish new expectations and make critical investments in infrastructure. This drought is not going away,” Klavans added.
