International audienceEl Niño–Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Niño events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO’s impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions

Sea surface temperature (SST) across much of the tropics has increased by 0.4 degrees to 1 degrees C since the mid-1970s. A parallel increase in the frequency and extent of coral bleaching and mortality has fueled concern that climate change poses a major threat to the survival of coral reef ecosystems worldwide. Here we show that steadily rising SSTs, not ocean acidification, are already driving dramatic changes in the growth of an important reef-building coral in the central Red Sea. Three-dimensional computed tomography analyses of the massive coral Diploastrea heliopora reveal that skeletal growth of apparently healthy colonies has declined by 30% since 1998. The same corals responded to a short-lived warm event in 1941/1942, but recovered within 3 years as the ocean cooled. Combining our data with climate model simulations by the Intergovernmental Panel on Climate Change, we predict that should the current warming trend continue, this coral could cease growing altogether by 2070.

Ice shelves modulate Antarctic contributions to sea-level rise 1 and thereby represent a critical, climate-sensitive interface between the Antarctic ice sheet and the global ocean. Following rapid atmospheric warming over the past decades 2,3 , Antarctic Peninsula ice shelves have progressively retreated 4 , at times catastrophically 5 . This decay supports hypotheses of thermal limits of viability for ice shelves via surface melt forcing 3,5,6 . Here we use a polar-adapted regional climate model 7 and satellite observations 8 to quantify the nonlinear relationship between surface melting and summer air temperature. Combining observations and multimodel simulations, we examine melt evolution and intensification before observed ice shelf collapse on the Antarctic Peninsula. We then assess the twenty-first-century evolution of surface melt across Antarctica under intermediate and high emissions climate scenarios. Our projections reveal a scenario-independent doubling of Antarctic-wide melt by 2050. Between 2050 and 2100, however, significant divergence in melt occurs between the two climate scenarios. Under the high emissions pathway by 2100, melt on several ice shelves approaches or surpasses intensities that have historically been associated with ice shelf collapse, at least on the northeast Antarctic Peninsula.Antarctic ice shelves have undergone widespread and accelerated thinning and retreat in recent decades in response to coupled atmospheric and oceanic forcing [3][4][5]9,10 . On the Antarctic Peninsula (AP), this recession has been particularly pronounced and punctuated with near-uniform, abrupt collapses of Larsen A, Prince Gustav, and Larsen B ice shelves occurring since 1995 ( Fig. 1). Across this region, recent atmospheric warming has exceeded global average rates 2 and current surface melting levels are unprecedented over the past millennium on the northeast AP (ref. 11). This warming and melt intensification has directly led to an expansion of meltwater ponding, and the resultant hydrofracturing is considered a leading mechanism of AP ice shelf collapse 3,5,12 .All Antarctic ice shelves experience surface melting today 7,8 , yet ocean-induced basal melting at present dominates ice shelf mass losses, particularly outside of the AP (refs 9,10). Nevertheless, surface melt intensities approach those of the AP elsewhere in Antarctica (Fig. 1c), meltwater ponding exists beyond the AP (refs 13,14), and strong basal melting can hasten ice shelf destabilization 4,10 . The question therefore arises, are recent ice shelf dynamics on the AP indicative of forthcoming changes elsewhere in Antarctica? Understanding the present-day and future viability of all Antarctic ice shelves requires an improved characterization of the sensitivity of ice shelves to temperature change, a better historical context for AP melt acceleration and ice shelf collapse, and robust projections of future pan-Antarctic change.Air temperature is often used to parameterize surface melt owing to several important physical linkages with the sur...

In part III of a three-part study on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, the authors examine projections of twenty-first-century climate in the representative concentration pathway 8.5 (RCP8.5) emission experiments. This paper summarizes and synthesizes results from several coordinated studies by the authors. Aspects of North American climate change that are examined include changes in continental-scale temperature and the hydrologic cycle, extremes events, and storm tracks, as well as regional manifestations of these climate variables. The authors also examine changes in the eastern North Pacific and North Atlantic tropical cyclone activity and North American intraseasonal to decadal variability, including changes in teleconnections to other regions of the globe. Projected changes are generally consistent with those previously published for CMIP3, although CMIP5 model projections differ importantly from those of CMIP3 in some aspects, including CMIP5 model agreement on increased central California precipitation. The paper also highlights uncertainties and limitations based on current results as priorities for further research. Although many projected changes in North American climate are consistent across CMIP5 models, substantial intermodel disagreement exists in other aspects. Areas of disagreement include projections of changes in snow water equivalent on a regional basis, summer Arctic sea ice extent, the magnitude and sign of regional precipitation changes, extreme heat events across the northern United States, and Atlantic and east Pacific tropical cyclone activity.

Decadal variations of very small amplitude [;0.38C in sea surface temperature (SST)] in the tropical Pacific Ocean, the genesis region of the interannual El Niñ o-Southern Oscillation (ENSO) phenomenon, have been shown to have powerful impacts on global climate. Future projections from different climate models do not agree on how this critical feature will change under the influence of anthropogenic forcing. A number of attempts have been made to resolve this issue by examining observed trends from the 1880s to the present, a period of rising atmospheric concentrations of greenhouse gases. A recent attempt concluded that the three major datasets disagreed on the trend in the equatorial gradient of SST. Using a corrected version of one of these datasets, and extending the analysis to the seasonal cycle, it is shown here that all agree that the equatorial Pacific zonal SST gradient has strengthened from 1880 to 2005 during the boreal fall when this gradient is normally strongest. This result appears to favor a theory for future changes based on ocean dynamics over one based on atmospheric energy considerations. Both theories incorporate the expectation, based on ENSO theory, that the zonal sea level pressure (SLP) gradient in the tropical Pacific is coupled to SST and should therefore strengthen along with the SST gradient. While the SLP gradient has not strengthened, it is found that it appears to have weakened only during boreal spring, consistent with the SST seasonal trends. Most of the coupled models included in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report underestimate the strengthening SST gradient in boreal fall, and show almost no change in the SLP gradient in any season. The observational analyses herein suggest that both theories are at work but with relative strengths that vary seasonally, and that the two theories need not be inconsistent with each other. * Lamont-Doherty Earth Observatory Contribution Number 7260.