Researchers at the University of Rochester have identified a key mechanism for methane production in the open ocean, driven by phosphate scarcity. The discovery, published in Proceedings of the National Academy of Sciences, suggests warming oceans could boost these emissions, creating a potential climate feedback loop. This resolves a long-standing puzzle about methane in oxygen-rich surface waters.
A team led by Thomas Weber, an associate professor in the Department of Earth and Environmental Sciences at the University of Rochester, along with graduate student Shengyu Wang and postdoctoral research associate Hairong Xu, analyzed global datasets and computer models to pinpoint the process. Certain bacteria generate methane while breaking down organic material, but only when phosphate—a vital nutrient—is scarce. “This means that phosphate scarcity is the primary control knob for methane production and emissions in the open ocean,” Weber said. The findings explain why surface ocean waters, which are rich in oxygen, still release methane into the atmosphere—a phenomenon that has puzzled scientists for years. Methane, a potent greenhouse gas, could see increased emissions as climate change alters ocean dynamics. Warming from the surface downward heightens the density gap between surface and deep waters, slowing the upwelling of nutrients like phosphate. “Climate change is warming the ocean from the top down, increasing the density difference between surface and deep waters,” Weber explained. “This is expected to slow the vertical mixing that carries nutrients like phosphate up from depth.” With less phosphate at the surface, methane-producing microbes could proliferate. This mechanism sets up a feedback loop: warmer oceans lead to more methane, which drives further warming. The process is not yet factored into most climate models. “Our work will help fill a key gap in climate predictions, which often overlook interactions between the changing environment and natural greenhouse gas sources to the atmosphere,” Weber noted.