No abstract

O bservations of global average surface air temperature (SAT) show an unequivocal warming over the twentieth century 1 , however the overall trend has been interrupted by periods of weak warming or even cooling (Fig. 1). For example, warming largely stalled from the 1940s to the 1970s. Between 1975 and 2000 the overall upward SAT trend resumed, but it was not uniform, with a decade of accelerated warming from about 1975-1985 (ref. 2), as well as periods of little warming 3 . Since around 2001 a marked hiatus in global surface warming has occurred, raising questions about its cause, its likely duration and the implications for global climate change.Decadal periods of minimal warming, or even cooling, interspersing decades of rapid warming, are not inconsistent with a long-term warming trend; indeed this characterizes the interplay between steadily increasing greenhouse gas forcing and internally generated climate variability. Factors other than internal variability, such as volcanoes and changes in solar radiation, can also drive cooler decades against the backdrop of ongoing warming. Indeed, hiatus decades are expected to punctuate future warming trends, even under scenarios of rapid global warming 4,5 . Mechanisms proposed to explain the most recent observed hiatus include increased ocean heat uptake 2,3,6,7 , the prolonged solar minimum 4 and changes in atmospheric water vapour 8 and aerosols 9,10 . The cool surface waters of the eastern Pacific have also been linked to the global temperature hiatus 11 and consensus is building that the subsurface ocean, with its vast capacity for heat storage, is playing a significant role through enhanced heat uptake 2,3,7,12,13 . It remains unclear, however, where the bulk of anomalous ocean heat uptake has occurred, with the Pacific 2,3 , Atlantic 13,14 and Southern [14][15][16][17] Oceans all potential candidates. One notable aspect of the two most recent extended hiatus periods (1940-1975 and 2001-present), in contrast to periods of global SAT warming (1910SAT warming ( -1940SAT warming ( and 1976SAT warming ( -2000, is that they correspond closely to periods when the Interdecadal Pacific Oscillation 18-20 (IPO) has been in a negative phase (Fig. 1a). The IPO manifests as a low-frequency El Niño-like pattern of climate variability, with a warm tropical Pacific and weakened trade winds during its positive phase, and a cool tropical Pacific and strengthened winds during its negative phase. Recent analyses of climate model simulations suggest that hiatus decades are linked to negative phases of the IPO (refs 2,3,11). Here we examine the most recent hiatus in this context, particularly in relation to altered ocean dynamics and enhanced ocean heat uptake, and assess implications for the coming decades.To examine the ongoing hiatus compared with a period of warming, we start by considering climatic trends over the past two decades, spanning the transition from a period of global surface warming in the 1990s to the post-2000 hiatus. During this time the Pacific trade winds ...

The El Niño-Southern Oscillation (ENSO) cycle of alternating warm El Niño and cold La Niña events is the dominant year-to-year climate signal on Earth. ENSO originates in the tropical Pacific through interactions between the ocean and the atmosphere, but its environmental and socioeconomic impacts are felt worldwide. Spurred on by the powerful 1997-1998 El Niño, efforts to understand the causes and consequences of ENSO have greatly expanded in the past few years. These efforts reveal the breadth of ENSO's influence on the Earth system and the potential to exploit its predictability for societal benefit. However, many intertwined issues regarding ENSO dynamics, impacts, forecasting, and applications remain unresolved. Research to address these issues will not only lead to progress across a broad range of scientific disciplines but also provide an opportunity to educate the public and policy makers about the importance of climate variability and change in the modern world.

The 1997-98 El Nino was, by some measures, the strongest on record, with major climatic impacts felt around the world. A newly completed tropical Pacific atmosphere-ocean observing system documented this El Nino from its rapid onset to its sudden demise in greater detail than was ever before possible. The unprecedented measurements challenge existing theories about El Nino-related climate swings and suggest why climate forecast models underpredicted the strength of the El Nino before its onset.