The Mid-Pleistocene Transition (MPT) is commonly characterized as a change in both temperature and ice volume from smaller amplitude, 41-kyr variability to higher amplitude, ~100-kyr variability in the absence of any significant change in orbital forcing. Here we reassess these characteristics based on our new reconstructions of changes in global mean surface temperature (DGMST) and global mean sea level over the last 2.5 Myr. Our reconstruction of DGMST includes an initial phase of long-term cooling through the early Pleistocene followed by a second phase of accelerated cooling during the MPT (1.5-0.9 Ma) that was accompanied by a transition from dominant 41-kyr low-amplitude periodicity to dominant ~100-kyr high-amplitude periodicity. Changes in rates of long-term cooling and variability are consistent with changes in the carbon cycle driven initially by geologic processes followed by additional changes during the MPT in the Southern Ocean carbon cycle. The spectrum of our sea-level reconstruction is dominated by 41-kyr variance until ~1.2 Ma with subsequent emergence of a ~100-kyr signal that, unlike global temperature, has nearly the same concentration of variance as the 41-kyr signal during this time. Moreover, our sea-level reconstruction is significantly different than all other reconstructions in showing fluctuations of large ice sheets throughout the Pleistocene as compared to a change from fluctuations in smaller to larger ice sheets during the MPT. We attribute their longer period variations after the MPT to modulation of obliquity forcing by the newly established low-frequency CO2 variability. Specifically, prior to reaching their maximum size at the end of each ~100-kyr interval, ice-sheet response to periods of lower CO2 was modulated by higher obliquity, and vice versa, with the times of maximum ice-sheet growth only occurring when low CO2 combined with the next obliquity low. Ice sheets then began to melt in response to the next increase in obliquity, with the subsequent sequence of events and feedbacks leading to a termination. High-resolution ice-core CO2 records that extend beyond 0.8 Ma are needed to test this hypothesis. Otherwise, large ice sheets shared a common size threshold throughout the Pleistocene equivalent to sea level below -80 m that, when exceeded, resulted in a termination that was paced by the next increase in obliquity.

Changes in global mean sea level (GMSL) during the late Cenozoic remain uncertain. We use a reconstruction of changes in δ18O of seawater to reconstruct GMSL since 4.5 million years ago (Ma) that accounts for temperature-driven changes in the δ18O of global ice sheets. Between 4.5 and 3 Ma, sea level highstands remained up to 20 m above present whereas the first lowstands below present suggest onset of Northern Hemisphere glaciation at 4 Ma. Intensification of global glaciation occurred from 3 Ma to 2.5 Ma, culminating in lowstands similar to the Last Glacial Maximum lowstand at 21,000 years ago and that reoccurred throughout much of the Pleistocene. We attribute the middle Pleistocene transition in ice sheet variability (1.2 Ma to 0.62 Ma) to modulation of 41-thousand-year (kyr) obliquity forcing by an increase in ~100-kyr CO2 variability.

We use a recent reconstruction of global mean sea surface temperature change relative to preindustrial (ΔGMSST) over the last 4.5 Myr together with independent proxy-based reconstructions of bottom water (ΔBWT) or deep-ocean (ΔDOT) temperatures to infer changes in mean ocean temperature (ΔMOT). Three independent lines of evidence show that the ratio of ΔMOT / ΔGMSST​​​​​​​, which is a measure of ocean heat storage efficiency (HSE), increased from ∼ 0.5 to ∼ 1 during the Middle Pleistocene Transition (MPT, 1.5–0.9 Ma), indicating an increase in ocean heat uptake (OHU) at this time. The first line of evidence comes from global climate models; the second from proxy-based reconstructions of ΔBWT, ΔMOT, and ΔGMSST; and the third from decomposing a global mean benthic δ18O stack (δ18Ob) into its temperature (δ18OT) and seawater (δ18Osw) components. Regarding the latter, we also find that further corrections in benthic δ18O, probably due to some combination of a long-term diagenetic overprint and to the carbonate ion effect, are necessary to explain reconstructed Pliocene sea-level highstands inferred from δ18Osw. We develop a simple conceptual model that invokes an increase in OHU and HSE during the MPT in response to changes in deep-ocean circulation driven largely by surface forcing of the Southern Ocean. Our model accounts for heat uptake and temperature in the non-polar upper ocean (0–2000 m) that is mainly due to wind-driven ventilation, while changes in the deeper ocean (> 2000 m) in both polar and non-polar waters occur due to high-latitude deepwater formation. We propose that deepwater formation was substantially reduced prior to the MPT, effectively decreasing HSE. We attribute these changes in deepwater formation across the MPT to long-term cooling which caused a change starting ∼ 1.5 Ma from a highly stratified Southern Ocean due to warm SSTs and reduced sea-ice extent to a Southern Ocean which, due to colder SSTs and increased sea-ice extent, had a greater vertical exchange of water masses.

Green hydrogen has emerged as a potentially important pathway in decarbonizing the hard-to-abate sectors, including freight, dispatchable power, and industry. Many organizations predict that green hydrogen will become cost competitive with fossil fuels as production costs fall. However, most published green hydrogen cost estimates do not consider storage and distribution costs and how they vary across sectors. We estimate the carbon abatement cost of green hydrogen across major sectors in the United States, considering each sector's storage and distribution requirements. At current delivered prices, green hydrogen is a prohibitively expensive abatement strategy, with carbon abatement costs of $500-1,250/tCO2 across sectors. If production costs reduce to $2/kgH2, low-cost carbon abatement opportunities will remain limited to sectors already using hydrogen (e.g., ammonia) unless storage and distribution costs decrease. Our findings suggest that green hydrogen's potential is narrower than suggested, emphasizing the need for diverse technological options to decarbonize hard-to-abate sectors.

The decreasing cost of electricity worldwide from wind and solar energy, as well as that of end-use technologies such as electric vehicles, reflect substantial progress made toward replacing fossil fuels with alternative energy sources. But a full transition to clean energy can only be realized if numerous challenges are overcome. Many problems can be addressed through the discovery of new materials that improve the efficiency of energy production and consumption; reduce the need for scarce mineral resources; and support the production of green hydrogen, clean ammonia, and carbon-neutral hydrocarbon fuels. However, research and development of new energy materials are not as aggressive as they should be to meet the demands of climate change.

Carbon isotopes in ancient marine carbonates have long been thought to record organic carbon burial rates (Knoll et al., 1986). However, the carbon isotopic composition (δ 13 C) of carbonates has been relatively stable for the last three billion years, suggesting limited variability in the oxygen production associated with organic carbon burial. This suggests that oxygen production far outpaced oxygen consumption during oxygen-poor portions of Earth history, given that the rate of oxidative reactions are themselves sensitive to atmospheric oxygen levels. However, an unbalanced oxygen budget is at odds with overwhelming evidence for billions of years of low-oxygen conditions during the Precambrian and early Paleozoic. We propose that the δ 13 C of carbonates is in fact not a reliable record of oxygen production. Rather, it is controlled by oxygen-sensitive precipitation of isotopically depleted authigenic carbonates, a negative feedback that regulates the δ 13 C of platform carbonates and partially decouples that record from the rate of organic carbon burial on geologic timescales.

The Chicxulub impact (in the northern Yucatan Penninsula, Mexico) marks the Cretaceous-Paleogene (K-Pg) boundary and is implicated in one of the five major extinctions. Researchers have examined ejecta from the Chicxulub impact, and most recently a drill core from the crater itself, yet the processes and chemical reactions occurring in the impact vapor plume are poorly constrained. Rounded carbonate particles, identified as accretionary lapilli, have been found thousands of kilometers from the impact crater and may be a unique record of plume conditions. We present carbon (delta C-13), oxygen (delta O-18), and clumped (Delta(47)) isotope ratios of lapilli from the Brazos River, Texas (USA), as well as from foraminifera and a mudstone. Unaltered lapilli delta C-13 and delta O-18 values covary, ranging from -9.38 parts per thousand to -2.10 parts per thousand and from -7.72 parts per thousand to -5.36 parts per thousand, respectively, and they are distinct from mudstones, foraminifera, and secondarily altered lapilli in the same section. Clumped isotope temperatures [T(Delta(47))] from the lapilli range from 66 degrees C to 539 degrees C and average 155 +/- 46 degrees C (1 standard deviation), with sedimentary and fossil carbonates recording clement, shallow ocean-like T(Delta(47)). These data are consistent with petrography and hypothesized vapor plume formation, and we argue that the delta C-13 and delta O-18 values result from target rock decarbonation. Atmospheric temperatures >100 degrees C extending >1800 km from the Chicxulub crater imply an uninhabitable zone within seconds to minutes of the impact that was 10x larger in diameter than the crater itself.

Climate change is a key problem of the 21st century. China, as the largest emitter of greenhouse gases, has committed to stabilize its current emissions and dramatically increase the share of electricity production from non-fossil fuels by 2030. However, this is only a first step: in the longer term, China needs to aggressively strive to reach a goal of zero-emissions. Through detailed discussions of electricity pricing, electric vehicle policies, nuclear energy policies, and renewable energy policies, this book reviews how near-term climate and energy policies can affect long-term decarbonization pathways beyond 2030, building the foundations for decarbonization in advance of its realization. Focusing primarily on the electricity sector in China - the main battleground for decarbonization over the next century – it provides a valuable resource for researchers and policymakers, as well as energy and climate experts.

In a zonally averaged and simplified sense, there exists an upper oceanic Hadley Cell in the Pacific: during the winter season subduction occurs in the subtropics and extra-tropics and this water ventilates the tropical thermocline where it upwells and returns to the subducting regions through surface flow (e.g., Wyrtki & Kilonsky, 1984). Significant interior pycnocline exchange occurs between the subtropics and tropical thermocline (Johnson & McPhaden, 1999). Building on the observational evidence of Deser et al., (1996), it has been hypothesized that temperature anomalies originating at the sea-surface in the subtropics can be propagated via this subsurface pathway and interact with the equatorial thermocline, changing the character and sensitivity of the El NiñoSouthern Oscillation (ENSO) (Gu & Philander, 1997; Zhang et al., 1998). Tritium and He tracer data indicate that the ventilation time-scale of the tropical thermocline is on the order of decades (Fine et al., 2001; Jenkins, 1996). It is therefore a logical extension to hypothesize that the intergyre exchange between the extra-tropical subduction zones and the tropical thermocline could determine the decadal-scale climate character of the tropical Pacific (Gu & Philander, 1997), as well as other important processes.

The alkalinity concentration swing (ACS) is a new process for direct air capture of carbon dioxide driven by concentrating an alkaline solution that has been exposed to the atmosphere and loaded with dissolved inorganic carbon. Upon concentration, the partial pressure of CO2 increases, allowing for extraction and compression. Higher concentration factors result in proportionally higher outgassing pressure, and higher initial alkalinity concentrations at the same concentration factor outgas a higher concentration of CO2. Two desalination technologies, reverse osmosis and capacitive deionization, are examined as possible ACS implementations, and two corresponding energy models are evaluated. The ACS is compared to incumbent technologies and estimates for water, land, and energy requirements for capturing one million tonnes of CO2 per year are made. Estimates for the lower end of the energy range for both approaches compare favorably to other approaches, such as solid sorbent and calcining methods.

Otavi Group is a 1.5-3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran-Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S-N crustal stretching (770-655 Ma), followed by post-rift thermal subsidence (655-600 Ma). Rifting under southern Swakop Group continued until 650-635 Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S-N plate divergence. Rift related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770-717 Ma; (2) glacial early Cryogenian/Sturtian, 717-661 Ma; (3) non-glacial middle Cryogenian, 661-646 +/- 5 Ma; (4) glacial late Cryogenian/Marinoan, 646 +/- 5-635 Ma; and (5) non glacial early Ediacaran, 635-600 +/- 5 Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585 measured sections (totaling >190 km of strata) and > 8764 pairs of delta C-13/delta O-18(carb) analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick 'catch-up' depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5-9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3-8.8 m Myr(-1), despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters <= 1.6 km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9 m regional average thickness, n = 140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement <= 90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over >= 0.6 km of paleobathymetric relief. Late Tonian shallow-neritic delta C-13(carb) records were obtained from the 0.4-km-thick Devede Fm (similar to 770-760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (similar to 737-717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4-8 parts per thousand VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746 Ma volcanics and shows two negative excursions bridged by heavy delta C-13 values. The negative excursions could be correlative with Russoya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic delta C-13 records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the subMarinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0-3.5 parts per thousand. Early Ediacaran neritic delta C-13 records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (<=+ 11 parts per thousand) with extreme point-topoint variability (<= 10 parts per thousand) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform delta C-13 records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9 +/- 1.5 parts per thousand. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60-0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding >= 1.85 km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.

Combining data from Argo and the TAO buoy array we present new observations of variability in the Pacific Ocean. Argo profiles reveal the development of a thickness anomaly in the lower levels of the ventilated thermocline of the South Pacific in 2010. Data through 2017 show this anomaly propagating as a baroclinic wave westward and towards the equator. Theory suggests that this wave will reduce the velocity of the equatorial undercurrent (EUC) when it reaches the equator, transitioning the equatorial Pacific to a warm state. This is supported by TAO array observations that show a past decadal shift in EUC strength around 2000, as well as radiocarbon coral measurements which suggest a similar change in the 1970s, both of which align with phase changes in Pacific decadal variability. Using model simulations with enhanced vertical resolution in the thermocline, we affirm this link between the subtropical south Pacific thermocline and the EUC, which also manifests in eastern Pacific sea surface temperatures. We combine these results to hypothesize a mechanism that may explain some of the decadal variability observed in the Pacific. This mechanism relies on the propagation of anomalies in the structure of the ventilated thermocline from the southeastern Pacific to the equatorial Pacific, modulating the strength of the EUC. The cycle is enhanced by atmospheric teleconnections between the equatorial Pacific and the southeast Pacific that periodically reverse the anomaly in thermocline thickness. If correct, our hypothesis predicts a return to a warm state of the equatorial Pacific when the Kelvin wave reaches the equator and the thermocline adjustment slows the EUC.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Oceans. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Radiocarbon in the Eastern Tropical Pacific: Implications for Changes in Equatorial Undercurrent Velocity and Decadal VariabilityAuthorsLaurenKuntziDDaniel P.SchragSee all authors Lauren KuntziDCorresponding Author• Submitting AuthorGaiascope, InciDhttps://orcid.org/0000-0002-2745-3435view email addressThe email was not providedcopy email addressDaniel P. SchragHarvard Universityview email addressThe email was not providedcopy email address

Carbon isotope values from shallow-marine carbonate rocks, including those from many dolomitized successions, are the primary lens through which we interpret the ancient carbon cycle. Carbon isotopes are typically regarded as being robust to alteration during dolomitization due to the high carbon content of the rock compared to the fluid. However, chemostratigraphic studies of the Neoproterozoic Tsagaan-Olom Group in southwestern Mongolia exhibit 3-12 parts per thousand differences in carbon isotopes between stratigraphically equivalent limestone and dolomitized successions. To understand the origins of this geochemical variation, we conducted detailed geological mapping, petrographic, isotopic (delta C-13, delta O-18, delta Ca-44/40, and delta Mg-26), and fluid inclusion analyses of carbonates in the Taishir, Ol, and Shuurgat formations of the Tsagaan-Olom Group. Stratigraphic and textural constraints distinguish fabric retentive early dolomitization (Dolomitization Event 1) that occurred in the Taishir and Ol formations during and soon after deposition of the ca. 635 Ma Marinoan cap dolomite of the Ol Formation and fabric destructive dolomitization that occurred after the deposition of the younger Shuurgat Formation (Dolomitization Event 2), either prior to or during early Cambrian foreland basin formation. The dolomitizing fluids moved through porous stratigraphic units bound by impermeable shale of the Khongor and Zuun-Arts formations. Dolomitization homogenized the isotopic records of all Neoproterozoic Tsagaan-Olom Group dolomites; undolomitized successions show more extreme negative excursions from more positive background values. The salinity and isotopic values of seawater, the dolomitizing fluids, and the original platform carbonate were estimated from fluid inclusion data and a numerical model of diagenetic dolomitization. These results demonstrate that both early and late dolomitization can have a profound effect on carbon isotopic records of carbonates; however, with a multi-isotope approach the original carbon isotopic composition of platform-top carbonate can be distinguished from that of seawater and other dolomitizing fluids.

Throughout most of the sedimentary record, the marine carbon cycle is interpreted as being in isotopic steady state. This is most commonly inferred via isotopic reconstructions, where two export fluxes (organic carbon and carbonate) are offset by a constant isotopic fractionation of ~25 (termed εorg-carb ). Sedimentary deposits immediately overlying the Marinoan snowball Earth diamictites, however, stray from this prediction. In stratigraphic sections from the Ol Formation (Mongolia) and Sheepbed Formation (Canada), we observe a temporary excursion where the organic matter has anomalously heavy δ13 C and is grossly decoupled from the carbonate δ13 C. This signal may reflect the unique biogeochemical conditions that persisted in the aftermath of snowball Earth. For example, physical oceanographic modeling suggests that a strong density gradient caused the ocean to remain stratified for about 50,000 years after termination of the Marinoan snowball event, during which time the surface ocean and continental weathering consumed the large atmospheric CO2 reservoir. Further, we now better understand how δ13 C records of carbonate can be post-depostionally altered and thus be misleading. In an attempt to explain the observed carbon isotope record, we developed a model that tracks the fluxes and isotopic values of carbon between the surface ocean, deep ocean, and atmosphere. By comparing the model output to the sedimentary data, stratification alone cannot generate the anomalous observed isotopic signal. Reproducing the heavy δ13 C in organic matter requires the progressively diminishing contribution of an additional anomalous source of organic matter. The exact source of this organic matter is unclear.

Sedimentary carbonates from the Neoproterozoic Era host some of the largest carbon isotope excursions in the geological record, in some cases as large as 12 parts per thousand to 15 parts per thousand. The origin of these signals remains a matter of debate - including whether they reflect local diagenetic effects, or record changes in the dissolved inorganic carbon (DIC) of global seawater that are indicative of shifts in the larger carbon cycle. Producing such changes through oxidation of organic carbon or methane requires unreasonably large fluxes that imply massive perturbations to carbon and oxygen budgets that are not observed. We present a plausible way to drive large changes in the delta C-13 of the global ocean via large-scale precipitation of isotopically fractionated authigenic carbonates distributed globally at modest concentrations in terrigenous sediments, particularly if the isotopic fractionation of authigenic carbonate shifts rapidly relative to average marine carbonate. We suggest that changes in the fractionation associated with authigenic carbonate precipitation could therefore be responsible for at least some of the variability in isotopic records drawn from primary marine carbonate. Using a diffusion-reaction model, we find that oscillations in sulfate concentrations are one mechanism for altering the isotopic composition of authigenic carbonates, thus forcing abrupt changes in seawater delta C-13. This mechanism is particularly effective under the geochemical conditions that likely prevailed during the Neoproterozoic. A sulfate-driven, authigenic origin for Neoproterozoic carbon isotope excursions makes testable predictions for the expression of the anomaly over depth transects, and for the evolution of delta S-34 through such events. (C) 2020 Elsevier B.V. All rights reserved.

The Equatorial Undercurrent (EUC) is a vital component of tropical Pacific circulation, helping to modulate the state of the equatorial Pacific Ocean. Here we compare the representation of the EUC in models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) with observations of the undercurrent. We find that the CMIP5 models consistently underestimate both the magnitude and variability of the EUC. Insufficient resolution as well as diffusivity parameterizations both contribute to a representation of the EUC that is too weak and too diffuse. Given the strong influence of the EUC on the evolution of tropical Pacific sea surface temperatures, model deficiencies in the EUC contribute to shortcomings in capturing ENSO dynamics and Pacific decadal variability. Further evaluation of the impact of EUC simulation on the climatology and variability in the tropical Pacific is necessary.