Observed multi-decadal increase in the surface ocean's thermal inertia

The ocean surface layer plays a pivotal role in Earth's climate by absorbing excess heat from the atmosphere, thereby regulating global temperatures. Here we document an observed and very notable increase in the persistence of sea surface temperature around the global ocean over the past four decades. This prolonged memory is attributed to two key factors: a deepening of the surface mixed layer and a decrease in the damping rates due to processes such as vertical entrainment, mixing and damping to the atmosphere. Our findings have great relevance to the observed increase in the duration of marine heatwaves and associated heightened thermal threats to marine organisms. Our study also suggests that the ocean's ability to sequester heat is weakening, a trend that may accelerate as the 21st century proceeds.