The Pacific Ocean heat engine: global climate's regulator

Abstract. Climate change is routinely represented as a smoothly changing signal surrounded by statistical noise. However, on decadal timescales, warming proceeds as a sequence of steady-state regimes punctuated by abrupt shifts. Here we present evidence that this process is regulated by a heat engine spanning the tropical Pacific Ocean. The eastern-central Pacific maintains steady-state conditions, collecting heat and delivering it to the Western Pacific warm pool. This acts as distributor, transporting heat upwards and to the poles. The heat engine is networked within the climate system, linking different oscillations and circulations as heat energy is dissipated. The process is self-regulating. Steady-state regimes will persist until they become unstable and need more or less power depending on the direction of forcing. Under greenhouse gas forcing, shifts initiated within the heat engine propagate broadly across the shallow ocean, followed by warming over land and at higher latitudes. The heat engine was in free mode during the early 20^th century, dominated by decadal variability. From the 1960s, it switched into forced mode, initiating a stepladder-like pattern of warming in regional and global climate. The most recent shift commenced in the warm pool in December 2012, ending the so-called hiatus (1997–2013). During 2014–15, surface temperatures warmed abruptly by ~ 0.25 °C globally and > 0.5 °C over northern hemisphere land and high latitudes. With increasing forcing, the heat engine will shift more frequently. Rapid decreases in greenhouse gas emissions will slow the process and potentially, could stabilise it. Managing unavoidable change requires developing the capacity to predict shifts in advance. Planning for rapid changes in extreme events is an urgent priority.