The ocean stores the bulk of excess anthropogenic heat in the Earth system. The ocean heat uptake efficiency (OHUE) – the flux of heat into the ocean per degree of global warming – is therefore a key factor in how much warming will occur in the coming decades. In climate models, OHUE is well-characterised, tending to decrease on centennial timescales; in contrast, OHUE is not well-constrained from Earth observations. Here OHUE and its rate of change are diagnosed from global temperature and ocean heat content records. OHUE increased from $0.57\pm0.06$W/m$^2$K to $0.7\pm0.02$W/m$^2$K over the past five decades. This increase is attributed to steepening heat content gradients in the ocean, and corresponds to $\sim$4 years’ delay until temperature targets such as 1.5$^\circ$C or 2$^\circ$C are exceeded.