The 50 most-cited articles in Geophysical Research Letters, 2016

Abstract New calculations of the radiative forcing (RF) are presented for the three main well‐mixed greenhouse gases, methane, nitrous oxide, and carbon dioxide. Methane's RF is particularly impacted because of the inclusion of the shortwave forcing; the 1750–2011 RF is about 25% higher (increasing from 0.48 W m −2 to 0.61 W m −2 ) compared to the value in the Intergovernmental Panel on Climate Change (IPCC) 2013 assessment; the 100 year global warming potential is 14% higher than the IPCC value. We present new simplified expressions to calculate RF. Unlike previous expressions used by IPCC, the new ones include the overlap between CO 2 and N 2 O; for N 2 O forcing, the CO 2 overlap can be as important as the CH 4 overlap. The 1750–2011 CO 2 RF is within 1% of IPCC's value but is about 10% higher when CO 2 amounts reach 2000 ppm, a value projected to be possible under the extended RCP8.5 scenario.

Abstract Aerosol‐planetary boundary layer (PBL) interactions have been found to enhance air pollution in megacities in China. We show that black carbon (BC) aerosols play the key role in modifying the PBL meteorology and hence enhancing the haze pollution. With model simulations and data analysis from various field observations in December 2013, we demonstrate that BC induces heating in the PBL, particularly in the upper PBL, and the resulting decreased surface heat flux substantially depresses the development of PBL and consequently enhances the occurrences of extreme haze pollution episodes. We define this process as the “dome effect” of BC and suggest an urgent need for reducing BC emissions as an efficient way to mitigate the extreme haze pollution in megacities of China.

Abstract A coastwide bloom of the toxigenic diatom Pseudo‐nitzschia in spring 2015 resulted in the largest recorded outbreak of the neurotoxin, domoic acid, along the North American west coast. Elevated toxins were measured in numerous stranded marine mammals and resulted in geographically extensive and prolonged closures of razor clam, rock crab, and Dungeness crab fisheries. We demonstrate that this outbreak was initiated by anomalously warm ocean conditions. Pseudo‐nitzschia australis thrived north of its typical range in the warm, nutrient‐poor water that spanned the northeast Pacific in early 2015. The seasonal transition to upwelling provided the nutrients necessary for a large‐scale bloom; a series of spring storms delivered the bloom to the coast. Laboratory and field experiments confirming maximum growth rates with elevated temperatures and enhanced toxin production with nutrient enrichment, together with a retrospective analysis of toxic events, demonstrate the potential for similarly devastating ecological and economic disruptions in the future.

Abstract River flooding can have severe societal, economic, and environmental consequences. However, limited understanding of the regional differences in flood‐generating mechanisms results in poorly understood historical flood trends and uncertain predictions of future flood conditions. Through systematic data analyses of 420 catchments we expose the primary drivers of flooding across the contiguous United States. This is achieved by exploring which flood‐generating processes control the seasonality and magnitude of maximum annual flows. The regional patterns of seasonality and interannual variabilities of maximum annual flows are, in general, poorly explained by rainfall characteristics alone. For most catchments soil moisture dependent precipitation excess, snowmelt, and rain‐on‐snow events are found to be much better predictors of the flooding responses. The continental‐scale classification of dominant flood‐generating processes we generate here emphasizes the disparity in timing and variability between extreme rainfall and flooding and can assist predictions of flooding and…

Abstract The observed decline in Arctic sea ice is projected to continue, opening shorter trade routes across the Arctic Ocean, with potentially global economic implications. Here we quantify, using Coupled Model Intercomparison Project Phase 5 global climate model simulations calibrated to remove spatial biases, how projected sea ice loss might increase opportunities for Arctic transit shipping. By midcentury for standard open water vessels, the frequency of navigable periods doubles, with routes across the central Arctic becoming available. A sea ice‐ship speed relationship is used to show that European routes to Asia typically become 10 days faster via the Arctic than alternatives by midcentury, and 13 days faster by late century, while North American routes become 4 days faster. Future greenhouse gas emissions have a larger impact by late century; the shipping season reaching 4–8 months in Representative Concentration Pathway (RCP)8.5 double that of RCP2.6, both with substantial interannual variability. Moderately, ice‐strengthened vessels likely enable Arctic transits for 10–12…

Abstract Declining mountain snowpack and earlier snowmelt across the western United States has implications for downstream communities. We present a possible mechanism linking snowmelt rate and streamflow generation using a gridded implementation of the Budyko framework. We computed an ensemble of Budyko streamflow anomalies (BSAs) using Variable Infiltration Capacity model‐simulated evapotranspiration, potential evapotranspiration, and estimated precipitation at 1/16° resolution from 1950 to 2013. BSA was correlated with simulated baseflow efficiency ( r 2 = 0.64) and simulated snowmelt rate ( r 2 = 0.42). The strong correlation between snowmelt rate and baseflow efficiency ( r 2 = 0.73) links these relationships and supports a possible streamflow generation mechanism wherein greater snowmelt rates increase subsurface flow. Rapid snowmelt may thus bring the soil to field capacity, facilitating below‐root zone percolation, streamflow, and a positive BSA. Previous works have shown that future increases in regional air temperature may lead to earlier, slower snowmelt and hence…

Abstract Studies of coastal vulnerability due to climate change tend to focus on the consequences of sea level rise, rather than the complex coastal responses resulting from changes to the extreme wave climate. Here we investigate the 2013/2014 winter wave conditions that severely impacted the Atlantic coast of Europe and demonstrate that this winter was the most energetic along most of the Atlantic coast of Europe since at least 1948. Along exposed open‐coast sites, extensive beach and dune erosion occurred due to offshore sediment transport. More sheltered sites experienced less erosion and one of the sites even experienced accretion due to beach rotation induced by alongshore sediment transport. Storm wave conditions such as were encountered during the 2013/2014 winter have the potential to dramatically change the equilibrium state (beach gradient, coastal alignment, and nearshore bar position) of beaches along the Atlantic coast of Europe.

Abstract From January 2014 to August 2016, sea surface temperatures (SSTs) along the Washington, Oregon, and California coasts were significantly warmer than usual, reaching a maximum SST anomaly of 6.2°C off Southern California. This marine heat wave occurred alongside the Gulf of Alaska marine heat wave and resulted in major disturbances in the California Current ecosystem and massive economic impacts. Here we use satellite and blended reanalysis products to report the magnitude, extent, duration, and evolution of SSTs and wind stress anomalies along the West Coast of the continental United States during this event. Nearshore SST anomalies along the entire coast were persistent during the marine heat wave, and only abated seasonally, during spring upwelling‐favorable wind stress. The coastal marine heat wave weakened in July 2016 and disappeared by September 2016.

Abstract The 2015–2016 El Niño is by some measures one of the strongest on record, comparable to the 1982–1983 and 1997–1998 events that triggered widespread ecosystem change in the northeast Pacific. Here we describe impacts of the 2015–2016 El Niño on the California Current System (CCS) and place them in historical context using a regional ocean model and underwater glider observations. Impacts on the physical state of the CCS are weaker than expected based on tropical sea surface temperature anomalies; temperature and density fields reflect persistence of multiyear anomalies more than El Niño. While we anticipate El Niño‐related impacts on spring/summer 2016 productivity to be similarly weak, their combination with preexisting anomalous conditions likely means continued low phytoplankton biomass. This study highlights the need for regional metrics of El Niño's effects and demonstrates the potential to assess these effects before the upwelling season, when altered ecosystem functioning is most apparent.

Abstract In the atmosphere nighttime removal of volatile organic compounds is initiated to a large extent by reaction with the nitrate radical (NO 3 ) forming organic nitrates which partition between gas and particulate phase. Here we show based on particle phase measurements performed at a suburban site in the Netherlands that organic nitrates contribute substantially to particulate nitrate and organic mass. Comparisons with a chemistry transport model indicate that most of the measured particulate organic nitrates are formed by NO 3 oxidation. Using aerosol composition data from three intensive observation periods at numerous measurement sites across Europe, we conclude that organic nitrates are a considerable fraction of fine particulate matter (PM 1 ) at the continental scale. Organic nitrates represent 34% to 44% of measured submicron aerosol nitrate and are found at all urban and rural sites, implying a substantial potential of PM reduction by NO x emission control.

Present-day Venus is an inhospitable place with surface temperatures approaching 750K and an atmosphere 90 times as thick as Earth's. Billions of years ago the picture may have been very different. We have created a suite of 3-D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 Gya, present-day Venus orbital parameters, an ocean volume consistent with current theory, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a rotation period slower than ~16 Earth days, despite an incident solar flux 46-70% higher than Earth receives. At its current rotation period, Venus's climate could have remained habitable until at least 715 million years ago. These results demonstrate the role rotation and topography play in understanding the climatic history of Venus-like exoplanets discovered in the present epoch.

Abstract Asia's high plateaus are sensitive to climate change and have been experiencing rapid warming over the past few decades. We found 99 new lakes and extensive lake expansion on the Tibetan Plateau during the last four decades, 1970–2013, due to increased precipitation and cryospheric contributions to its water balance. This contrasts with disappearing lakes and drastic shrinkage of lake areas on the adjacent Mongolian Plateau: 208 lakes disappeared, and 75% of the remaining lakes have shrunk. We detected a statistically significant coincidental timing of lake area changes in both plateaus, associated with the climate regime shift that occurred during 1997/1998. This distinct change in 1997/1998 is thought to be driven by large‐scale atmospheric circulation changes in response to climate warming. Our findings reveal that these two adjacent plateaus have been changing in opposite directions in response to climate change. These findings shed light on the complex role of the regional climate and water cycles and provide useful information for ecological and water resource…

Abstract Land aridity has been projected to increase with global warming. Such projections are mostly based on off‐line aridity and drought metrics applied to climate model outputs but also are supported by climate‐model projections of decreased surface soil moisture. Here we comprehensively analyze soil moisture projections from the Coupled Model Intercomparison Project phase 5, including surface, total, and layer‐by‐layer soil moisture. We identify a robust vertical gradient of projected mean soil moisture changes, with more negative changes near the surface. Some regions of the northern middle to high latitudes exhibit negative annual surface changes but positive total changes. We interpret this behavior in the context of seasonal changes in the surface water budget. This vertical pattern implies that the extensive drying predicted by off‐line drought metrics, while consistent with the projected decline in surface soil moisture, will tend to overestimate (negatively) changes in total soil water availability.

Abstract When stressed by low soil water content (SWC) or high vapor pressure deficit (VPD), plants close stomata, reducing transpiration and photosynthesis. However, it has historically been difficult to disentangle the magnitudes of VPD compared to SWC limitations on ecosystem‐scale fluxes. We used a 13 year record of eddy covariance measurements from a forest in south central Indiana, USA, to quantify how transpiration and photosynthesis respond to fluctuations in VPD versus SWC. High VPD and low SWC both explained reductions in photosynthesis relative to its long‐term mean, as well as reductions in transpiration relative to potential transpiration estimated with the Penman‐Monteith equation. Flux responses to typical fluctuations in SWC and VPD had similar magnitudes. Integrated over the year, VPD fluctuations accounted for significant reductions of GPP in both nondrought and drought years. Our results suggest that increasing VPD under climatic warming could reduce forest CO 2 uptake regardless of changes in SWC.

Abstract The emergence of rapid Arctic warming in recent decades has coincided with unusually cold winters over Northern Hemisphere continents. It has been speculated that this “Warm Arctic, Cold Continents” trend pattern is due to sea ice loss. Here we use multiple models to examine whether such a pattern is indeed forced by sea ice loss specifically and by anthropogenic forcing in general. While we show much of Arctic amplification in surface warming to result from sea ice loss, we find that neither sea ice loss nor anthropogenic forcing overall yield trends toward colder continental temperatures. An alternate explanation of the cooling is that it represents a strong articulation of internal atmospheric variability, evidence for which is derived from model data, and physical considerations. Sea ice loss impact on weather variability over the high‐latitude continents is found, however, to be characterized by reduced daily temperature variability and fewer cold extremes.

Abstract This bottom‐up modeling study, supported by new population census 2011 data, simulates ozone (O3) and fine particulate matter (PM2.5) exposure on local to regional scales. It quantifies, present‐day premature mortalities associated with the exposure to near‐surface PM2.5 and O3 concentrations in India using a regional chemistry model. We estimate that PM2.5 exposure leads to about 570,000 (CI95: 320,000–730,000) premature mortalities in 2011. On a national scale, our estimate of mortality by chronic obstructive pulmonary disease (COPD) due to O3 exposure is about 12,000 people. The Indo‐Gangetic region accounts for a large part (~42%) of the estimated mortalities. The associated lost life expectancy is calculated as 3.4 ± 1.1 years for all of India with highest values found for Delhi (6.3 ± 2.2 years). The economic cost of estimated premature mortalities associated with PM2.5 and O3 exposure is about 640 (350–800) billion USD in 2011, which is a factor of 10 higher than total expenditure on health by public and private expenditure.

Abstract The Antarctic Circumpolar Current is an important component of the global climate system connecting the major ocean basins as it flows eastward around Antarctica, yet due to the paucity of data, it remains unclear how much water is transported by the current. Between 2007 and 2011 flow through Drake Passage was continuously monitored with a line of moored instrumentation with unprecedented horizontal and temporal resolution. Annual mean near‐bottom currents are remarkably stable from year to year. The mean depth‐independent or barotropic transport, determined from the near‐bottom current meter records, was 45.6 sverdrup (Sv) with an uncertainty of 8.9 Sv. Summing the mean barotropic transport with the mean baroclinic transport relative to zero at the seafloor of 127.7 Sv gives a total transport through Drake Passage of 173.3 Sv. This new measurement is 30% larger than the canonical value often used as the benchmark for global circulation and climate models.

Abstract Particulate matter is a major concern for public health, causing cancer and cardiopulmonary mortality. Therefore, governments in most industrialized countries monitor and set limits for particulate matter. To assist policy makers, it is important to connect the chemical composition and severity of particulate pollution to its sources. Here we show how agricultural practices, livestock production, and the use of nitrogen fertilizers impact near‐surface air quality. In many densely populated areas, aerosols formed from gases that are released by fertilizer application and animal husbandry dominate over the combined contributions from all other anthropogenic pollution. Here we test reduction scenarios of combustion‐based and agricultural emissions that could lower air pollution. For a future scenario, we find opposite trends, decreasing nitrate aerosol formation near the surface while total tropospheric loads increase. This suggests that food production could be increased to match the growing global population without sacrificing air quality if combustion emission is decreased.

Abstract Precipitation is expected to respond differently to various drivers of anthropogenic climate change. We present the first results from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), where nine global climate models have perturbed CO 2 , CH 4 , black carbon, sulfate, and solar insolation. We divide the resulting changes to global mean and regional precipitation into fast responses that scale with changes in atmospheric absorption and slow responses scaling with surface temperature change. While the overall features are broadly similar between models, we find significant regional intermodel variability, especially over land. Black carbon stands out as a component that may cause significant model diversity in predicted precipitation change. Processes linked to atmospheric absorption are less consistently modeled than those linked to top‐of‐atmosphere radiative forcing. We identify a number of land regions where the model ensemble consistently predicts that fast precipitation responses to climate perturbations dominate over the slow,…

Abstract Madden‐Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical troposphere, has a significant impact on global weather and climate. Here we present that the year‐to‐year variation of the MJO activity shows significant changes with the quasi‐biennial oscillation (QBO) in the tropical stratosphere. Specifically, the boreal winter MJO amplitude, evaluated by various metrics, is typically stronger than normal during the QBO easterly phase at 50 hPa and weaker than normal during the QBO westerly phase at 50 hPa. This relationship, which is possibly mediated by the QBO‐related static stability and/or vertical wind shear changes in the tropical upper troposphere and lower stratosphere, is robust whether or not the activeness of the MJO or QBO is taken into account. This result suggests a new potential route from the stratosphere that regulates the organized tropical convection, helping to improve the prediction skill of the boreal winter MJO.

Abstract Large earthquakes within stable continental regions (SCR) show that significant amounts of elastic strain can be released on geological structures far from plate boundary faults, where the vast majority of the Earth's seismic activity takes place. SCR earthquakes show spatial and temporal patterns that differ from those at plate boundaries and occur in regions where tectonic loading rates are negligible. However, in the absence of a more appropriate model, they are traditionally viewed as analogous to their plate boundary counterparts, occurring when the accrual of tectonic stress localized at long‐lived active faults reaches failure threshold. Here we argue that SCR earthquakes are better explained by transient perturbations of local stress or fault strength that release elastic energy from a prestressed lithosphere. As a result, SCR earthquakes can occur in regions with no previous seismicity and no surface evidence for strain accumulation. They need not repeat, since the tectonic loading rate is close to zero. Therefore, concepts of recurrence time or fault slip rate do…

Abstract The severe 2010 heat wave in western Russia was found to be influenced by anthropogenic climate change. Additionally, soil moisture‐temperature feedbacks were deemed important for the buildup of the exceptionally high temperatures. We quantify the relative role of both factors by applying the probabilistic event attribution framework and analyze ensemble simulations to distinguish the effect of climate change and the 2010 soil moisture conditions for annual maximum temperatures. The dry 2010 soil moisture alone has increased the risk of a severe heat wave in western Russia sixfold, while climate change from 1960 to 2000 has approximately tripled it. The combined effect of climate change and 2010 soil moisture yields a 13 times higher heat wave risk. We conclude that internal climate variability causing the dry 2010 soil moisture conditions formed a necessary basis for the extreme heat wave.

Abstract The most recent Intergovernmental Panel on Climate Change assessment report concludes that the Atlantic Meridional Overturning Circulation (AMOC) could weaken substantially but is very unlikely to collapse in the 21st century. However, the assessment largely neglected Greenland Ice Sheet (GrIS) mass loss, lacked a comprehensive uncertainty analysis, and was limited to the 21st century. Here in a community effort, improved estimates of GrIS mass loss are included in multicentennial projections using eight state‐of‐the‐science climate models, and an AMOC emulator is used to provide a probabilistic uncertainty assessment. We find that GrIS melting affects AMOC projections, even though it is of secondary importance. By years 2090–2100, the AMOC weakens by 18% [−3%, −34%; 90% probability] in an intermediate greenhouse‐gas mitigation scenario and by 37% [−15%, −65%] under continued high emissions. Afterward, it stabilizes in the former but continues to decline in the latter to −74% [+4%, −100%] by 2290–2300, with a 44% likelihood of an AMOC collapse. This result suggests that an…

Abstract The occurrence of large, high‐intensity wildfires requires plant biomass, or fuel, that is sufficiently dry to burn. This poses the question, what is “sufficiently dry”? Until recently, the ability to address this question has been constrained by the spatiotemporal scale of available methods to monitor the moisture contents of both dead and live fuels. Here we take advantage of recent developments in macroscale monitoring of fuel moisture through a combination of remote sensing and climatic modeling. We show there are clear thresholds of fuel moisture content associated with the occurrence of wildfires in forests and woodlands. Furthermore, we show that transformations in fuel moisture conditions across these thresholds can occur rapidly, within a month. Both the approach presented here, and our findings, can be immediately applied and may greatly improve fire risk assessments in forests and woodlands globally.

Abstract The ability of spaceborne Global Navigation Satellite System (GNSS) bistatic radar receivers to sense changes in soil moisture is investigated using observations from the low Earth orbiting UK TechDemoSat‐1 satellite (TDS‐1). Previous studies using receivers on aircraft or towers have shown that ground‐reflected GNSS signals are sensitive to changes in soil moisture, though the ability to sense this variable from space has yet to be quantified. Data from TDS‐1 show a 7 dB sensitivity of reflected signals to temporal changes in soil moisture. If the effects of surface roughness and vegetation on the reflected signals can be quantified, spaceborne GNSS bistatic radar receivers could provide soil moisture on relatively small spatial and temporal scales.

Abstract Coastal wetlands are among the most valuable ecosystems on Earth, where ecosystem services such as flood protection depend nonlinearly on wetland size and are threatened by sea level rise and coastal development. Here we propose a simple model of marsh migration into adjacent uplands and couple it with existing models of seaward edge erosion and vertical soil accretion to explore how ecosystem connectivity influences marsh size and response to sea level rise. We find that marsh loss is nearly inevitable where topographic and anthropogenic barriers limit migration. Where unconstrained by barriers, however, rates of marsh migration are much more sensitive to accelerated sea level rise than rates of edge erosion. This behavior suggests a counterintuitive, natural tendency for marsh expansion with sea level rise and emphasizes the disparity between coastal response to climate change with and without human intervention.

Abstract Anthropogenic CO2 emissions from fossil fuel combustion have large impacts on climate. In order to monitor the increasing CO2 concentrations in the atmosphere, accurate spaceborne observations—as available from the Orbiting Carbon Observatory‐2 (OCO‐2)—are needed. This work provides the first direct observation of anthropogenic CO2 from OCO‐2 over the main pollution regions: eastern USA, central Europe, and East Asia. This is achieved by deseasonalizing and detrending OCO‐2 CO2 observations to derive CO2 anomalies. Several small isolated emission areas (such as large cities) are detectable from the anomaly maps. The spatial distribution of the CO2 anomaly matches the features observed in the maps of the Ozone Monitoring Instrument NO2 tropospheric columns, used as an indicator of atmospheric pollution. The results of a cluster analysis confirm the spatial correlation between CO2 and NO2 data over areas with different amounts of pollution. We found positive correlation between CO2 anomalies and emission inventories. The results demonstrate the power of spaceborne data for…

Abstract Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub‐ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O‐lines) and the separatrices. Inside each current filament, kinetic‐scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X‐lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O‐lines but not X‐lines.

Abstract The concomitant occurrence of extreme precipitation and winds can have severe impacts. Here this concomitant occurrence is quantified globally using ERA‐Interim reanalysis data. A logistic regression model is used to determine significant changes in the odds of precipitation extremes given a wind extreme that occurs on the same day, the day before, or the day after. High percentages of cooccurring wind and precipitation extremes are found in coastal regions and in areas with frequent tropical cyclones, with maxima of more than 50% of concomitant events. Strong regional‐scale variations in this percentage are related to the interaction of weather systems with topography resulting in Föhn winds, gap winds, and orographic drying and the structure and tracks of extratropical and tropical cyclones. The percentage of concomitant events increases substantially if spatial shifts by one grid point are taken into account. Such spatially shifted but cooccurring events are important in insurance applications.

Abstract Eddy‐resolving regional ocean model results in conjunction with synthetic float trajectories and observations provide new insights into the recirculation of the Atlantic Water (AW) in Fram Strait that significantly impacts the redistribution of oceanic heat between the Nordic Seas and the Arctic Ocean. The simulations confirm the existence of a cyclonic gyre around the Molloy Hole near 80°N, suggesting that most of the AW within the West Spitsbergen Current recirculates there, while colder AW recirculates in a westward mean flow south of 79°N that primarily relates to the eastern rim of the Greenland Sea Gyre. The fraction of waters recirculating in the northern branch roughly doubles during winter, coinciding with a seasonal increase of eddy activity along the Yermak Plateau slope that also facilitates subduction of AW beneath the ice edge in this area.

Abstract We report on laboratory experiments to shed light on dust charging and transport that have been suggested to explain a variety of unusual phenomena on the surfaces of airless planetary bodies. We have recorded micron‐sized insulating dust particles jumping to several centimeters high with an initial speed of ~0.6 m/s under ultraviolet illumination or exposure to plasmas, resulting in an equivalent height of ~0.11 m on the lunar surface that is comparable to the height of the so‐called lunar horizon glow. Lofted large aggregates and surface mobilization are related to many space observations. We experimentally show that the emission and re‐absorption of photoelectron and/or secondary electron at the walls of microcavities formed between neighboring dust particles below the surface are responsible for generating unexpectedly large negative charges and intense particle‐particle repulsive forces to mobilize and lift off dust particles.

Abstract The 2015 Paris Agreement aims to limit warming to 2 or 1.5°C above preindustrial level, although combined Intended Nationally Determined Contributions (INDCs) are likely insufficient to achieve these targets. We propose a set of idealized emission pathways consistent with the targets. If countries reduce emissions in line with their INDCs, the 2°C threshold could be avoided only if net zero greenhouse gas emissions (GHGEs) are achieved by 2085 and late century negative emissions are considerably in excess of those assumed in Representative Concentration Pathway (RCP) 2.6 (net −5 Gt CO2/yr, compared with −1.5 Gt CO2/yr in RCP2.6). More aggressive near‐term reductions would allow 2°C to be avoided with less end‐of‐century carbon removal capacity. A 10% cut in GHGEs by 2030 (relative to 2015) could likely achieve 2°C with RCP2.6 level negative emissions. The 1.5°C target requires GHGEs to be reduced by almost a third by 2030 and net zero by 2050, while a 50 year overshoot of 1.5°C allows net zero GHGEs by 2060.

Abstract Flooding is projected to become more frequent as warming temperatures amplify the atmosphere's water holding capacity and increase the occurrence of extreme precipitation events. However, there is still little evidence of regional changes in flood risk across the USA. Here we present a novel approach assessing the trends in inundation frequency above the National Weather Service's four flood level categories in 2042 catchments. Results reveal stark regional patterns of changing flood risk that are broadly consistent above the four flood categories. We show that these patterns are dependent on the overall wetness and potential water storage, with fundamental implications for water resources management, agriculture, insurance, navigation, ecology, and populations living in flood‐affected areas. Our findings may assist in a better communication of changing flood patterns to a wider audience compared with the more traditional approach of stating trends in terms of discharge magnitudes and frequencies.

Abstract Recent studies show that the vigorous seasonal cycle of the mixed layer modulates upper ocean submesoscale turbulence. Here we provide model‐based evidence that the seasonally changing upper ocean stratification in the Kuroshio Extension also modulates submesoscale (here 10–100 km) inertia‐gravity waves. Summertime restratification weakens submesoscale turbulence but enhances inertia‐gravity waves near the surface. Thus, submesoscale turbulence and inertia‐gravity waves undergo vigorous out‐of‐phase seasonal cycles. These results imply a strong seasonal modulation of the accuracy of geostrophic velocity diagnosed from submesoscale sea surface height delivered by the Surface Water and Ocean Topography satellite mission.

Abstract We find that during the dusty season on Mars (southern spring and summer) of years without a global dust storm there are three large regional‐scale dust storms. The storms are labeled A, B, and C in seasonal order. This classification is based on examining the zonal mean 50 Pa (∼25 km) daytime temperature retrievals from TES/MGS and MCS/MRO over 6 Mars Years. Regional‐scale storms are defined as events where the temperature exceeds 200 K. Examining the MCS dust field at 50 Pa indicates that warming in the Southern Hemisphere is dominated by direct heating, while northern high latitude warming is a dynamical response. A storms are springtime planet encircling Southern Hemisphere events. B storms are southern polar events that begin near perihelion and last through the solstice. C storms are southern summertime events starting well after the end of the B storm. C storms show the most interannual variability.

Abstract Decomposing cloud feedback into components due to changes in several gross cloud properties provides valuable insights into its physical causes. Here we present a refined decomposition that separately considers changes in free tropospheric and low cloud properties, better connecting feedbacks to individual governing processes and avoiding ambiguities present in a commonly used decomposition. It reveals that three net cloud feedback components are robustly nonzero: positive feedbacks from increasing free tropospheric cloud altitude and decreasing low cloud cover and a negative feedback from increasing low cloud optical depth. Low cloud amount feedback is the dominant contributor to spread in net cloud feedback but its anticorrelation with other components damps overall spread. The ensemble mean free tropospheric cloud altitude feedback is roughly 60% as large as the standard cloud altitude feedback because it avoids aliasing in low cloud reductions. Implications for the “null hypothesis” climate sensitivity from well‐understood and robustly simulated feedbacks are discussed.

Abstract We map recent Greenland Ice Sheet elevation change at high spatial (5 km) and temporal (monthly) resolution using CryoSat‐2 altimetry. After correcting for the impact of changing snowpack properties associated with unprecedented surface melting in 2012, we find good agreement (3 cm/yr bias) with airborne measurements. With the aid of regional climate and firn modeling, we compute high spatial and temporal resolution records of Greenland mass evolution, which correlate (R = 0.96) with monthly satellite gravimetry and reveal glacier dynamic imbalance. During 2011–2014, Greenland mass loss averaged 269 ± 51 Gt/yr. Atmospherically driven losses were widespread, with surface melt variability driving large fluctuations in the annual mass deficit. Terminus regions of five dynamically thinning glaciers, which constitute less than 1% of Greenland's area, contributed more than 12% of the net ice loss. This high‐resolution record demonstrates that mass deficits extending over small spatial and temporal scales have made a relatively large contribution to recent ice sheet imbalance.

Abstract The quasi‐biennial oscillation (QBO) is a tropical lower stratospheric, downward propagating zonal wind variation, with an average period of ~28 months. The QBO has been constantly documented since 1953. Here we describe the evolution of the QBO during the Northern Hemisphere winter of 2015–2016 using radiosonde observations and meteorological reanalyses. Normally, the QBO would show a steady downward propagation of the westerly phase. In 2015–2016, there was an anomalous upward displacement of this westerly phase from ~30 hPa to 15 hPa. These westerlies impinge on or “cutoff” the normal downward propagation of the easterly phase. In addition, easterly winds develop at 40 hPa. Comparisons to tropical wind statistics for the 1953 to present record demonstrate that this 2015–2016 QBO disruption is unprecedented.

Abstract The ocean in Jupiter's moon Europa may have redox balance similar to Earth's. On Earth, low‐temperature hydration of crustal olivine produces substantial hydrogen, comparable to any potential flux from volcanic activity. Here we compare hydrogen and oxygen production rates of the Earth system with fluxes to Europa's ocean. Even without volcanic hydrothermal activity, water‐rock alteration in Europa causes hydrogen fluxes 10 times smaller than Earth's. Europa's ocean may have become reducing for a brief epoch, for example, after a thermal‐orbital resonance ∼2 Gyr after accretion. Estimated oxidant flux to Europa's ocean is comparable to estimated hydrogen fluxes. Europa's ice delivers oxidants to its ocean at the upper end of these estimates if its ice is geologically active, as evidence of geologic activity and subduction implies.

Abstract Regional evapotranspiration (ET) can be enhanced by human activities such as irrigation or reservoir impoundment. Here the potential of using Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage data in water budget calculations to detect human‐induced ET change is investigated over the Haihe River basin of China. Comparison between GRACE‐based monthly ET estimate (2005–2012) and Global Land Data Assimilation System (GLDAS)‐modeled ET indicates that human‐induced ET due to intensive groundwater irrigation from March to May can only be detected by GRACE. GRACE‐based ET (521.7 ± 21.1 mm/yr), considerably higher than GLDAS ET (461.7 ± 29.8 mm/yr), agrees well with existing estimates found in the literature and indicates that human activities contribute to a 12% increase in ET. The double‐peak seasonal pattern of ET (in May and August) as reported in published studies is well reproduced by GRACE‐based ET estimate. This study highlights the unique capability of GRACE in detecting anthropogenic signals over regions with large groundwater consumption.

Abstract The deformation of slow‐moving landslides developed in clays induces endogenous seismicity of mostly low‐magnitude events ( M L <1). Long seismic records and complete catalogs are needed to identify the type of seismic sources and understand their mechanisms. Manual classification of long records is time‐consuming and may be highly subjective. We propose an automatic classification method based on the computation of 71 seismic attributes and the use of a supervised classifier. No attribute was selected a priori in order to create a generic multi‐class classification method applicable to many landslide contexts. The method can be applied directly on the results of a simple detector. We developed the approach on the seismic network of eight sensors of the Super‐Sauze clay‐rich landslide (South French Alps) for the detection of four types of seismic sources. The automatic algorithm retrieves 93% of sensitivity in comparison to a manually interpreted catalog considered as reference.

Abstract Skillful sea ice forecasts from days to years ahead are becoming increasingly important for the operation and planning of human activities in the Arctic. Here we analyze the potential predictability of the Arctic sea ice edge in six climate models. We introduce the integrated ice‐edge error (IIEE), a user‐relevant verification metric defined as the area where the forecast and the “truth” disagree on the ice concentration being above or below 15%. The IIEE lends itself to decomposition into an absolute extent error, corresponding to the common sea ice extent error, and a misplacement error. We find that the often‐neglected misplacement error makes up more than half of the climatological IIEE. In idealized forecast ensembles initialized on 1 July, the IIEE grows faster than the absolute extent error. This means that the Arctic sea ice edge is less predictable than sea ice extent, particularly in September, with implications for the potential skill of end‐user relevant forecasts.

Abstract Methane emissions from lakes are widely thought to be highly irregular and difficult to quantify with anything other than numerous distributed measurement stations and long‐term sampling campaigns. In spite of this, a large majority of the study sites north of 50°N have been measured over surprisingly short time periods of only one to a few days. Using long‐term data from three intensively studied small subarctic lakes, we recommend that measurements of diffusive methane flux and ebullition should be made over at least 11 and 39 days scattered throughout the ice‐free season using depth‐stratified sampling at 3 and 11 or more locations, respectively. We further show that low temporal and spatial resolutions are unlikely to cause overestimates. Therefore, we argue that most sites measured previously are likely underestimated in terms of emission potential. Avoiding these biases seen in much of the contemporary data is crucial to further constrain large‐scale methane emissions from northern lakes and ponds.

Abstract We investigate the climate feedback parameter α (W m −2 K −1 ) during the historical period (since 1871) in experiments using the HadGEM2 and HadCM3 atmosphere general circulation models (AGCMs) with constant preindustrial atmospheric composition and time‐dependent observational sea surface temperature (SST) and sea ice boundary conditions. In both AGCMs, for the historical period as a whole, the effective climate sensitivity is ∼2 K ( α ≃1.7 W m −2 K −1 ), and α shows substantial decadal variation caused by the patterns of SST change. Both models agree with the AGCMs of the latest Coupled Model Intercomparison Project in showing a considerably smaller effective climate sensitivity of ∼1.5 K ( α = 2.3 ± 0.7 W m −2 K −1 ), given the time‐dependent changes in sea surface conditions observed during 1979–2008, than the corresponding coupled atmosphere‐ocean general circulation models (AOGCMs) give under constant quadrupled CO 2 concentration. These findings help to relieve the apparent contradiction between the larger values of effective climate sensitivity diagnosed from…

Abstract Following strong westerly wind bursts in boreal winter and spring of 2014, both the scientific community and the popular press were abuzz with the possibility of a major El Niño developing. However, during the boreal summer of 2014, the Bjerknes feedback failed to kick in, aided and abetted by a strong easterly wind burst. The widely anticipated major 2014–2015 El Niño event failed to materialize and even failed to qualify as an El Niño by conventional definitions. However, the boreal summer easterly wind burst had the effect of not only inhibiting the growth of the El Niño event but also preventing and then reversing the discharge of the equatorial heat content that typically occurs during the course of an El Niño event. This head start of equatorial heat content helped push the 2015–2016 El Niño event to extreme magnitude.

Abstract The timing of melt onset affects the surface energy uptake throughout the melt season. Yet the processes triggering melt and causing its large interannual variability are not well understood. Here we show that melt onset over Arctic sea ice is initiated by positive anomalies of water vapor, clouds, and air temperatures that increase the downwelling longwave radiation (LWD) to the surface. The earlier melt onset occurs; the stronger are these anomalies. Downwelling shortwave radiation (SWD) is smaller than usual at melt onset, indicating that melt is not triggered by SWD. When melt occurs early, an anomalously opaque atmosphere with positive LWD anomalies preconditions the surface for weeks preceding melt. In contrast, when melt begins late, clearer than usual conditions are evident prior to melt. Hence, atmospheric processes are imperative for melt onset. It is also found that spring LWD increased during recent decades, consistent with trends toward an earlier melt onset.

Abstract Extreme and large‐scale warming events in the ocean have been dubbed marine heatwaves, and these have been documented in both the Northern and Southern Hemispheres. This paper examines the intensity, duration, and frequency of positive sea surface temperature anomalies in the North Atlantic and North Pacific Oceans over the period 1950–2014 using an objective definition for marine heatwaves based on their probability of occurrence. Small‐area anomalies occur more frequently than large‐area anomalies, and this relationship can be characterized by a power law distribution. The relative frequency of large‐ versus small‐area anomalies, represented by the power law slope parameter, is modulated by basin‐scale modes of natural climate variability and anthropogenic warming. Findings suggest that the probability of marine heatwaves is a trade‐off between size, intensity, and duration and that region specific variability modulates the frequency of these events.

Abstract The 2015 drought in the Indo‐Gangetic Plain posed new challenges related to food and water security and affected the lives of millions. All‐India monsoon rainfall in 2015 was the tenth driest year on record (1906–2015) with a deficit of 14.5%, and the Indo‐Gangetic Plain witnessed a rainfall deficit of 25.8% (third rank event). Drought severity was amplified by deficits from the previous years in the Indo‐Gangetic Plain and other parts of India. The Indo‐Gangetic Plain faced a 2 year cumulative deficit of 51%, and the drought of 2014–2015 was unprecedented with a return period of 542 years. The GRACE data showed the occurrence of consecutive negative terrestrial water storage and groundwater anomalies in 2014 and 2015, mainly centered over the Indo‐Gangetic Plain region. Notwithstanding uncertainty in future projections, the multiyear droughts in the same regions can pose challenges for water resources and agriculture.

Abstract The intense plume activity at the South Pole of Enceladus together with the recent detection of libration hints at an internal water ocean underneath the outer ice shell. However, the interpretation of gravity, shape, and libration data leads to contradicting results regarding the depth of ocean/ice interface and the total volume of the ocean. Here we develop an interior structure model consisting of a rocky core, an internal ocean, and an ice shell, which satisfies simultaneously the gravity, shape, and libration data. We show that the data can be reconciled by considering isostatic compensation including the effect of a few hundred meter thick elastic lithosphere. Our model predicts that the core radius is 180–185 km, the ocean density is at least 1030 kg/m3, and the ice shell is 18–22 km thick on average. The ice thicknesses are reduced at poles decreasing to less than 5 km in the south polar region.

Abstract On 24 August 2016 a magnitude ML 6.0 occurred in the Central Apennines (Italy) between Amatrice and Norcia causing nearly 300 fatalities. The main shock ruptured a NNW‐SSE striking, WSW dipping normal fault. We invert waveforms from 26 three‐component strong motion accelerometers, filtered between 0.02 and 0.5 Hz, within 45 km from the fault. The inferred slip distribution is heterogeneous and characterized by two shallow slip patches updip and NW from the hypocenter, respectively. The rupture history shows bilateral propagation and a relatively high rupture velocity (3.1 km/s). The imaged rupture history produced evident directivity effects both N‐NW and SE of the hypocenter, explaining near‐source peak ground motions. Fault dimensions and peak slip values are large for a moderate‐magnitude earthquake. The retrieved rupture model fits the recorded ground velocities up to 1 Hz, corroborating the effects of rupture directivity and slip heterogeneity on ground shaking and damage pattern.