Showing papers in "Nature Climate Change in 2020"
TL;DR: In this paper, the authors compile government policies and activity data to estimate the decrease in CO2 emissions during forced confinements during the COVID-19 pandemic, which drastically altered patterns of energy demand around the world.
Abstract: Government policies during the COVID-19 pandemic have drastically altered patterns of energy demand around the world. Many international borders were closed and populations were confined to their homes, which reduced transport and changed consumption patterns. Here we compile government policies and activity data to estimate the decrease in CO2 emissions during forced confinements. Daily global CO2 emissions decreased by –17% (–11 to –25% for ±1σ) by early April 2020 compared with the mean 2019 levels, just under half from changes in surface transport. At their peak, emissions in individual countries decreased by –26% on average. The impact on 2020 annual emissions depends on the duration of the confinement, with a low estimate of –4% (–2 to –7%) if prepandemic conditions return by mid-June, and a high estimate of –7% (–3 to –13%) if some restrictions remain worldwide until the end of 2020. Government actions and economic incentives postcrisis will likely influence the global CO2 emissions path for decades.
1,461 citations
National Center for Atmospheric Research1, Pusan National University2, Cornell University3, Geophysical Fluid Dynamics Laboratory4, University of Texas at Austin5, Lamont–Doherty Earth Observatory6, University of Paris7, University of Victoria8, Northwestern University9, Cooperative Institute for Research in Environmental Sciences10, University of Colorado Boulder11, ETH Zurich12, Institute of Arctic and Alpine Research13, Max Planck Society14, University of California, Los Angeles15, Hokkaido University16, University of California, Irvine17, University of Exeter18
TL;DR: In this article, a collection of initial-condition large ensembles (LEs) generated with seven Earth system models under historical and future radiative forcing scenarios provides new insights into uncertainties due to internal variability versus model differences.
Abstract: Internal variability in the climate system confounds assessment of human-induced climate change and imposes irreducible limits on the accuracy of climate change projections, especially at regional and decadal scales. A new collection of initial-condition large ensembles (LEs) generated with seven Earth system models under historical and future radiative forcing scenarios provides new insights into uncertainties due to internal variability versus model differences. These data enhance the assessment of climate change risks, including extreme events, and offer a powerful testbed for new methodologies aimed at separating forced signals from internal variability in the observational record. Opportunities and challenges confronting the design and dissemination of future LEs, including increased spatial resolution and model complexity alongside emerging Earth system applications, are discussed. Climate change detection is confounded by internal variability, but recent initial-condition large ensembles (LEs) have begun addressing this issue. This Perspective discusses the value of multi-model LEs, the challenges of providing them and their role in future climate change research.
426 citations
Massachusetts Institute of Technology1, University of Alaska Fairbanks2, Woods Hole Oceanographic Institution3, University of Bremen4, Alfred Wegener Institute for Polar and Marine Research5, California Institute of Technology6, National Oceanic and Atmospheric Administration7, Texas State University8, Lund University9, Pennsylvania State University10, Potsdam Institute for Climate Impact Research11, VU University Amsterdam12, Utah State University13, United States Naval Academy14, Environment Canada15, University of Gothenburg16, University of Cambridge17, Naval Postgraduate School18, University of California, Irvine19, University of Washington20, University College London21, Langley Research Center22, University of Wisconsin-Madison23, Finnish Meteorological Institute24, Leipzig University25, Columbia University26, Gwangju Institute of Science and Technology27
TL;DR: The Arctic has warmed more than twice as fast as the global average since the late twentieth century, a phenomenon known as Arctic amplification (AA), and progress has been made in understanding the mechanisms that link it to midlatitude weather variability as discussed by the authors.
Abstract: The Arctic has warmed more than twice as fast as the global average since the late twentieth century, a phenomenon known as Arctic amplification (AA). Recently, there have been considerable advances in understanding the physical contributions to AA, and progress has been made in understanding the mechanisms that link it to midlatitude weather variability. Observational studies overwhelmingly support that AA is contributing to winter continental cooling. Although some model experiments support the observational evidence, most modelling results show little connection between AA and severe midlatitude weather or suggest the export of excess heating from the Arctic to lower latitudes. Divergent conclusions between model and observational studies, and even intramodel studies, continue to obfuscate a clear understanding of how AA is influencing midlatitude weather.
423 citations
TL;DR: In this paper, the authors estimate that the direct effect of the pandemic-driven response will be negligible, with a cooling of around 0.01 ± 0.005°C by 2030 compared to a baseline scenario that follows current national policies.
Abstract: The global response to the COVID-19 pandemic has led to a sudden reduction of both GHG emissions and air pollutants. Here, using national mobility data, we estimate global emission reductions for ten species during the period February to June 2020. We estimate that global NOx emissions declined by as much as 30% in April, contributing a short-term cooling since the start of the year. This cooling trend is offset by ~20% reduction in global SO2 emissions that weakens the aerosol cooling effect, causing short-term warming. As a result, we estimate that the direct effect of the pandemic-driven response will be negligible, with a cooling of around 0.01 ± 0.005 °C by 2030 compared to a baseline scenario that follows current national policies. In contrast, with an economic recovery tilted towards green stimulus and reductions in fossil fuel investments, it is possible to avoid future warming of 0.3 °C by 2050.
414 citations
University of Edinburgh1, University of California2, University of Sheffield3, University of Virginia4, Aarhus University5, University of California, Davis6, University of Barcelona7, Northern Arizona University8, University of Alaska Fairbanks9, Netherlands Organisation for Scientific Research10, University of Oslo11, University of Bergen12, VU University Amsterdam13, University of Exeter14, Institute of Arctic and Alpine Research15, University of Lapland16, Grand Valley State University17, University of Zurich18, Colgate University19, University of Oxford20, Open University21, Umeå University22, University of Stirling23, University of Tromsø24, Lund University25, University of Alaska Anchorage26, University of Texas at El Paso27, University of Greifswald28, University of Aberdeen29, Swiss Federal Institute for Forest, Snow and Landscape Research30
TL;DR: In this paper, a consensus is emerging that the underlying causes and future dynamics of so-called Arctic greening and browning trends are more complex, variable and inherently scale-dependent than previously thought.
Abstract: As the Arctic warms, vegetation is responding, and satellite measures indicate widespread greening at high latitudes. This ‘greening of the Arctic’ is among the world’s most important large-scale ecological responses to global climate change. However, a consensus is emerging that the underlying causes and future dynamics of so-called Arctic greening and browning trends are more complex, variable and inherently scale-dependent than previously thought. Here we summarize the complexities of observing and interpreting high-latitude greening to identify priorities for future research. Incorporating satellite and proximal remote sensing with in-situ data, while accounting for uncertainties and scale issues, will advance the study of past, present and future Arctic vegetation change.
407 citations
TL;DR: For example, the Gospers Mountain fire near Sydney has burned more than 510,000 ha since ignition by lightning on 26 October 2019; it is the largest forest fire recorded in Australia and continues to burn in late January 2020 as discussed by the authors.
Abstract: To the Editor — Between September 2019 and early January 2020, around 5.8 million hectares of mainly temperate broadleaf forest were burned in New South Wales (NSW) and Victoria (VIC) in eastern Australia by a series of mega fires. Many of the fires have exceeded 100,000 ha and continued to burn for weeks after their ignition1. The Gospers Mountain fire near Sydney has burned more than 510,000 ha since ignition by lightning on 26 October 2019; it is the largest forest fire recorded in Australia and continues to burn in late January 2020. Eastern Australia’s temperate broadleaf forests, dominated by eucalypts, are among the most fire-prone in the world, and major fires are relatively common2,3, yet annually only small percentages of this forest biome burn, typically below 2% even in the more extreme fire seasons4,5. Are the 2019/20 forest fires unprecedented in scale, and are they the result of unparalleled fuel conditions? To address these questions, we analysed global data for remotely sensed burned area at 500 m resolution6, covering all major global forest biomes7 for the past 20 years (November 2000 to June 2019). We computed the annual burned area as a percentage of the continental sections of each forest biome between November 2000 and June 2019, and for the NSW plus VIC sections of the Australian ‘temperate broadleaf and mixed’ (TBLM) forest biome resulting from the 2019/20 forest fires. The TBLM forest biome7 in eastern Australia covers about 27 Mha, close to the median size of other continental forest biomes (26 Mha), with about half of that forest area located in NSW and a fifth in VIC. Our analysis substantiates that the 2019/20 forest fires have burned a globally unprecedented percentage of any continental forest biome: 21% of the Australian TBLM forest biome has so far been burned in a single season (Fig. 1). This figure contrasts sharply with annual burned area percentages for all other continental forest biomes4, which have been well below 5%, except for the Asian and African sections of the Tropical and Subtropical Dry Broadleaf forests, for which medians of 8–9% have been recorded (Fig. 1). We note that the burned area of 21% for the Australian TBLM forest biome is an underestimate, as forest fires in Tasmania this season were not included in our analysis, and additional forest area may burn over the remainder of the fire season. Why have the fires been so devastating this year? Fires in eucalypt forests propagate primarily through the litter layer8, and its dryness effectively acts as the ‘on/off switch’ for forest fire activity. Naturally occurring firebreaks, such as moist gullies, swamps or south-facing slopes, that normally dissect the forest landscape have dried out during the current widespread drought, affecting the probability of mega forest fire events in a highly nonlinear way9. In 2019, predicted litter moisture content across the eastern Australian TBLM forest biome was at record low levels, and the total surface area of forest exceeding critical flammability thresholds9 was larger and more prolonged than ever recorded in the past 30 years (Fig. 2). Thus extreme drought provided the preconditions
383 citations
TL;DR: A failure to recognize the factors behind continued emissions growth could limit the world's ability to shift to a pathway consistent with 1.5 °C or 2 °C of global warming as mentioned in this paper.
Abstract: A failure to recognize the factors behind continued emissions growth could limit the world’s ability to shift to a pathway consistent with 1.5 °C or 2 °C of global warming. Continued support for low-carbon technologies needs to be combined with policies directed at phasing out the use of fossil fuels.
317 citations
TL;DR: In this article, the authors show that ambient trends in shoreline dynamics, combined with coastal recession driven by sea level rise, could result in the near extinction of almost half of the world's sandy beaches by the end of the century.
Abstract: Sandy beaches occupy more than one-third of the global coastline1 and have high socioeconomic value related to recreation, tourism and ecosystem services2. Beaches are the interface between land and ocean, providing coastal protection from marine storms and cyclones3. However the presence of sandy beaches cannot be taken for granted, as they are under constant change, driven by meteorological4,5, geological6 and anthropogenic factors1,7. A substantial proportion of the world’s sandy coastline is already eroding1,7, a situation that could be exacerbated by climate change8,9. Here, we show that ambient trends in shoreline dynamics, combined with coastal recession driven by sea level rise, could result in the near extinction of almost half of the world’s sandy beaches by the end of the century. Moderate GHG emission mitigation could prevent 40% of shoreline retreat. Projected shoreline dynamics are dominated by sea level rise for the majority of sandy beaches, but in certain regions the erosive trend is counteracted by accretive ambient shoreline changes; for example, in the Amazon, East and Southeast Asia and the north tropical Pacific. A substantial proportion of the threatened sandy shorelines are in densely populated areas, underlining the need for the design and implementation of effective adaptive measures.
301 citations
California Institute of Technology1, Columbia University2, University of Bern3, University of California, Irvine4, University of Colorado Boulder5, University of Washington6, Princeton University7, Goddard Institute for Space Studies8, University of Central Florida9, United States Department of the Army10
TL;DR: In this paper, a multidisciplinary argument for the concept of connected extreme events is presented, and vantage points and approaches for producing climate information useful in guiding decisions about them are discussed.
Abstract: Extreme weather and climate events and their impacts can occur in complex combinations, an interaction shaped by physical drivers and societal forces. In these situations, governance, markets and other decision-making structures—together with population exposure and vulnerability—create nonphysical interconnections among events by linking their impacts, to positive or negative effect. Various anthropogenic actions can also directly affect the severity of events, further complicating these feedback loops. Such relationships are rarely characterized or considered in physical-sciences-based research contexts. Here, we present a multidisciplinary argument for the concept of connected extreme events, and we suggest vantage points and approaches for producing climate information useful in guiding decisions about them. The impacts of extreme weather and climate can be amplified by physical interactions among events and across a complex set of societal factors. This Perspective discusses the concept and challenge of connected extreme events, exploring research approaches and decision-making strategies.
255 citations
TL;DR: A new star has exploded back onto the climate scene: hydrogen as discussed by the authors, which offers possibilities to move away from fossil fuels, but it brings its own challenges, such as high energy consumption and high carbon footprint.
Abstract: A new star has exploded back onto the climate scene: hydrogen. It offers possibilities to move away from fossil fuels, but it brings its own challenges.
220 citations
TL;DR: In this paper, the role of these lakes as terrestrial storage for glacial meltwater is unknown and not accounted for in global sea level assessments, and they use scaling relations to estimate that global glacier lake volume increased by around 48%, to 156.5 km3, between 1990 and 2018.
Abstract: Glacial lakes are rapidly growing in response to climate change and glacier retreat. The role of these lakes as terrestrial storage for glacial meltwater is currently unknown and not accounted for in global sea level assessments. Here, we map glacier lakes around the world using 254,795 satellite images and use scaling relations to estimate that global glacier lake volume increased by around 48%, to 156.5 km3, between 1990 and 2018. This methodology provides a near-global database and analysis of glacial lake extent, volume and change. Over the study period, lake numbers and total area increased by 53 and 51%, respectively. Median lake size has increased 3%; however, the 95th percentile has increased by around 9%. Currently, glacial lakes hold about 0.43 mm of sea level equivalent. As glaciers continue to retreat and feed glacial lakes, the implications for glacial lake outburst floods and water resources are of considerable societal and ecological importance. Warming is increasing glacial lakes, and scaling relations show a 48% increase in volume for 1990 to 2018. All measures—area, volume, number—increased, providing water storage but also representing a potential hazard with the risk of outburst floods.
University of Denver1, Joint Global Change Research Institute2, Finnish Environment Institute3, University of Washington4, Stockholm Environment Institute5, Wageningen University and Research Centre6, Potsdam Institute for Climate Impact Research7, RAND Corporation8, International Institute for Applied Systems Analysis9, Netherlands Environmental Assessment Agency10, Utrecht University11, National Autonomous University of Mexico12, Ritsumeikan University13, German Development Institute14, Calcutta Institute of Engineering and Management15
TL;DR: It is concluded that the SSP–RCP scenario framework has been widely adopted across research communities and is largely meeting immediate needs, however, some mixed successes and a changing policy and research landscape present key challenges.
Abstract: Long-term global scenarios have underpinned research and assessment of global environmental change for four decades. Over the past ten years, the climate change research community has developed a scenario framework combining alternative futures of climate and society to facilitate integrated research and consistent assessment to inform policy. Here we assess how well this framework is working and what challenges it faces. We synthesize insights from scenario-based literature, community discussions and recent experience in assessments, concluding that the framework has been widely adopted across research communities and is largely meeting immediate needs. However, some mixed successes and a changing policy and research landscape present key challenges, and we recommend several new directions for the development and use of this framework.
TL;DR: The COVID-19 pandemic will be an unprecedented test of governments' ability to manage compound risks, as climate hazards disrupt outbreak response around the world as discussed by the authors, but climate adaptation also needs a long-term strategy for pandemic preparedness.
Abstract: The COVID-19 pandemic will be an unprecedented test of governments’ ability to manage compound risks, as climate hazards disrupt outbreak response around the world. Immediate steps can be taken to minimize climate-attributable loss of life, but climate adaptation also needs a long-term strategy for pandemic preparedness.
TL;DR: In this article, the authors used data from a global field survey and a nine-year field experiment to show that warmer temperatures increase the relative abundance of soil-borne potential fungal plant pathogens, and provided a global atlas of these organisms along with future distribution projections under different climate change and land-use scenarios.
Abstract: Understanding the present and future distribution of soil-borne plant pathogens is critical to supporting food and fibre production in a warmer world. Using data from a global field survey and a nine-year field experiment, we show that warmer temperatures increase the relative abundance of soil-borne potential fungal plant pathogens. Moreover, we provide a global atlas of these organisms along with future distribution projections under different climate change and land-use scenarios. These projections show an overall increase in the relative abundance of potential plant pathogens worldwide. This work advances our understanding of the global distribution of potential fungal plant pathogens and their sensitivity to ongoing climate and land-use changes, which is fundamental to reduce their incidence and impacts on terrestrial ecosystems globally. Plant pathogens threaten food security and ecosystem health. Projections of potential fungal plant pathogens under different warming and land-use scenarios indicate that warming temperatures under climate change will lead to increases in the relative abundance of such pathogens in most soils worldwide.
National Center for Atmospheric Research1, Earth System Research Laboratory2, Cooperative Institute for Research in Environmental Sciences3, University of California, Irvine4, Lawrence Livermore National Laboratory5, Monash University6, United States Bureau of Reclamation7, Oregon State University8, University of Michigan9, University of California, San Diego10, University of Freiburg11, University of Nebraska–Lincoln12, Bureau of Meteorology13, University of Arizona14
TL;DR: Flash droughts are a recently recognized type of extreme event distinguished by sudden onset and rapid intensification of drought conditions with severe impacts as discussed by the authors, and they unfold on subseasonal-to-seasonal timescales (weeks to months).
Abstract: Flash droughts are a recently recognized type of extreme event distinguished by sudden onset and rapid intensification of drought conditions with severe impacts. They unfold on subseasonal-to-seasonal timescales (weeks to months), presenting a new challenge for the surge of interest in improving subseasonal-to-seasonal prediction. Here we discuss existing prediction capability for flash droughts and what is needed to establish their predictability. We place them in the context of synoptic to centennial phenomena, consider how they could be incorporated into early warning systems and risk management, and propose two definitions. The growing awareness that flash droughts involve particular processes and severe impacts, and probably a climate change dimension, makes them a compelling frontier for research, monitoring and prediction. Flash droughts, which develop over the course of weeks, are difficult to forecast given the current state of subseasonal-to-seasonal prediction. This Perspective offers operational and research definitions, places them in the broader context of climate and suggests avenues for future research.
TL;DR: In this article, the authors quantify changes in ocean stratification down to depths of 2,000m using the squared buoyancy frequency N2 and newly available ocean temperature/salinity observations.
Abstract: Seawater generally forms stratified layers with lighter waters near the surface and denser waters at greater depth. This stable configuration acts as a barrier to water mixing that impacts the efficiency of vertical exchanges of heat, carbon, oxygen and other constituents. Previous quantification of stratification change has been limited to simple differencing of surface and 200-m depth changes and has neglected the spatial complexity of ocean density change. Here, we quantify changes in ocean stratification down to depths of 2,000 m using the squared buoyancy frequency N2 and newly available ocean temperature/salinity observations. We find that stratification globally has increased by a substantial 5.3% [5.0%, 5.8%] in recent decades (1960–2018) (the confidence interval is 5–95%); a rate of 0.90% per decade. Most of the increase (~71%) occurred in the upper 200 m of the ocean and resulted largely (>90%) from temperature changes, although salinity changes play an important role locally. Seawater properties—temperature, salinity and density—cause stratification of the water column, limiting vertical exchange. Considering down to 2,000 m, ocean stratification is shown to have increased ~5.3% since 1960, with ~71% of the change occurring in the upper 200 m primarily from warming.
TL;DR: In this paper, the authors identify the basins most at risk from changing snowmelt patterns, where up to 40% of irrigation demand must be met by new alternative water supplies under a 4'°C warming scenario.
Abstract: Snowpack stores cold-season precipitation to meet warm-season water demand. Climate change threatens to disturb this balance by altering the fraction of precipitation falling as snow and the timing of snowmelt, which may have profound effects on food production in basins where irrigated agriculture relies heavily on snowmelt runoff. Here, we analyse global patterns of snowmelt and agricultural water uses to identify regions and crops that are most dependent on snowmelt water resources. We find hotspots primarily in high-mountain Asia (the Tibetan Plateau), Central Asia, western Russia, western US and the southern Andes. Using projections of sub-annual runoff under warming scenarios, we identify the basins most at risk from changing snowmelt patterns, where up to 40% of irrigation demand must be met by new alternative water supplies under a 4 °C warming scenario. Our results highlight basins and crops where adaptation of water management and agricultural systems may be especially critical in a changing climate. Snowmelt runoff is an important source of water for irrigating agricultural crops in high-mountain Asia, Central Asia, western Russia, western US and the southern Andes. Climate change places water resources in these basins at risk, indicating the need to adapt water management.
TL;DR: In this article, the authors report observations of key physical drivers, biological responses and consequences for humans, including subsistence hunting, commercial fishing and industrial shipping in the northern Bering and Chukchi marine shelf ecosystem.
Abstract: The highly productive northern Bering and Chukchi marine shelf ecosystem has long been dominated by strong seasonality in sea-ice and water temperatures. Extremely warm conditions from 2017 into 2019—including loss of ice cover across portions of the region in all three winters—were a marked change even from other recent warm years. Biological indicators suggest that this change of state could alter ecosystem structure and function. Here, we report observations of key physical drivers, biological responses and consequences for humans, including subsistence hunting, commercial fishing and industrial shipping. We consider whether observed state changes are indicative of future norms, whether an ecosystem transformation is already underway and, if so, whether shifts are synchronously functional and system wide or reveal a slower cascade of changes from the physical environment through the food web to human society. Understanding of this observed process of ecosystem reorganization may shed light on transformations occurring elsewhere. Exceptionally warm years in 2017–2019 have caused changes in the physical and biological characteristics of the Pacific Arctic Ocean. What these changes mean for the ecosystem and societal consequences will depend on if they are evidence of a transformation or anomalies in the system.
TL;DR: In this paper, a single day of globally observed temperature and moisture was used to detect the fingerprint of externally driven climate change, and conclude that Earth as a whole is warming, even when ignoring the long-term global warming trend.
Abstract: For generations, climate scientists have educated the public that ‘weather is not climate’, and climate change has been framed as the change in the distribution of weather that slowly emerges from large variability over decades1–7. However, weather when considered globally is now in uncharted territory. Here we show that on the basis of a single day of globally observed temperature and moisture, we detect the fingerprint of externally driven climate change, and conclude that Earth as a whole is warming. Our detection approach invokes statistical learning and climate model simulations to encapsulate the relationship between spatial patterns of daily temperature and humidity, and key climate change metrics such as annual global mean temperature or Earth’s energy imbalance. Observations are projected onto this relationship to detect climate change. The fingerprint of climate change is detected from any single day in the observed global record since early 2012, and since 1999 on the basis of a year of data. Detection is robust even when ignoring the long-term global warming trend. This complements traditional climate change detection, but also opens broader perspectives for the communication of regional weather events, modifying the climate change narrative: while changes in weather locally are emerging over decades, global climate change is now detected instantaneously. Detection and attribution typically aims to find long-term climate signals in internal, often short-term variability. Here, common methods are extended to high-frequency temperature and humidity data, detecting instantaneous, global-scale climate change since 1999 for any year and 2012 for any day.
TL;DR: In this paper, a range of ecosystems contain irrecoverable carbon that is vulnerable to release upon land use conversion and, once lost, is not recoverable on timescales relevant to avoiding dangerous climate impacts.
Abstract: Avoiding catastrophic climate change requires rapid decarbonization and improved ecosystem stewardship. To achieve the latter, ecosystems should be prioritized by responsiveness to direct, localized action and the magnitude and recoverability of their carbon stores. Here, we show that a range of ecosystems contain ‘irrecoverable carbon’ that is vulnerable to release upon land use conversion and, once lost, is not recoverable on timescales relevant to avoiding dangerous climate impacts. Globally, ecosystems highly affected by human land-use decisions contain at least 260 Gt of irrecoverable carbon, with particularly high densities in peatlands, mangroves, old-growth forests and marshes. To achieve climate goals, we must safeguard these irrecoverable carbon pools through an expanded set of policy and finance strategies. In order to limit warming and the most severe consequences of climate change, net global carbon emissions must reach zero by 2050. Many ecosystems contain carbon that would be irrecoverable on this timescale if lost and must be protected to meet climate goals.
TL;DR: This article showed that Rossby waves with wavenumbers 5 and 7 have a preferred phase position and constitute recurrent atmospheric circulation patterns in summer, which can induce simultaneous heat extremes in specific regions.
Abstract: In an interconnected world, simultaneous extreme weather events in distant regions could potentially impose high-end risks for societies1,2. In the mid-latitudes, circumglobal Rossby waves are associated with a strongly meandering jet stream and might cause simultaneous heatwaves and floods across the northern hemisphere3–6. For example, in the summer of 2018, several heat and rainfall extremes occurred near-simultaneously7. Here we show that Rossby waves with wavenumbers 5 and 7 have a preferred phase position and constitute recurrent atmospheric circulation patterns in summer. Those patterns can induce simultaneous heat extremes in specific regions: Central North America, Eastern Europe and Eastern Asia for wave 5, and Western Central North America, Western Europe and Western Asia for wave 7. The probability of simultaneous heat extremes in these regions increases by a factor of up to 20 for the most severe heat events when either of these two waves dominate the circulation. Two or more weeks per summer spent in the wave-5 or wave-7 regime are associated with 4% reductions in crop production when averaged across the affected regions, with regional decreases of up to 11%. As these regions are important for global food production, the identified teleconnections have the potential to fuel multiple harvest failures, posing risks to global food security8. A large-scale meandering in the jet stream can cause simultaneous heat extremes in distant regions. When Rossby waves with wavenumbers 5 and 7 dominate circulation, there is an increased risk of heat extremes across major food-producing regions, raising the potential of multiple crop failures.
TL;DR: Many recently updated climate models show greater future warming than previously as mentioned in this paper, but the risk of such eventualities only emphasizes the need for rapid and deep reductions in emissions, which is not the case in this paper.
Abstract: Many recently updated climate models show greater future warming than previously. Separate lines of evidence suggest that their warming rates may be unrealistically high, but the risk of such eventualities only emphasizes the need for rapid and deep reductions in emissions.
Ghent University1, Centre national de la recherche scientifique2, Seoul National University3, Hohai University4, Met Office5, Forest Research Institute6, Alfred Wegener Institute for Polar and Marine Research7, Tsinghua University8, National Center for Atmospheric Research9, Max Planck Society10, Ludwig Maximilian University of Munich11, Peking University12, University of Exeter13, University of Augsburg14, University of Victoria15, Oeschger Centre for Climate Change Research16, Auburn University17
TL;DR: In this paper, the authors estimate that a unit change in the leaf area index (LAI) leads to a 3.66% increase in latent and sensible fluxes, respectively, over the 1982-2016 period.
Abstract: Changes in vegetation structure are expected to influence the redistribution of heat and moisture; however, how variations in the leaf area index (LAI) affect this global energy partitioning is not yet quantified. Here, we estimate that a unit change in LAI leads to 3.66 ± 0.45 and −3.26 ± 0.41 W m−2 in latent (LE) and sensible (H) fluxes, respectively, over the 1982–2016 period. Analysis of an ensemble of data-driven products shows that these sensitivities increase by about 20% over the observational period, prominently in regions with a limited water supply, probably because of an increased transpiration/evaporation ratio. Global greening has caused a decrease in the Bowen ratio (B = H/LE) of −0.010 ± 0.002 per decade, which is attributable to the increased evaporative surface. Such a direct LAI effect on energy fluxes is largely modulated by plant functional types (PFTs) and background climate conditions. Land surface models (LSMs) misrepresent this vegetation control, possibly due to underestimation of the biophysical responses to changes in the water availability and poor representation of LAI dynamics. Changes in the leaf area index alter the distribution of heat and moisture. The change in energy partitioning related to leaf area, increasing latent and decreasing sensible fluxes over the observational period 1982–2016, is moderated by plant functional type and background climate.
TL;DR: In this article, the authors use five Coupled Model Intercomparison Project 5 class Earth system model large ensembles to show how the Arctic is transitioning from a dominantly frozen state and to quantify the nature and timing of an emerging new Arctic climate in sea ice, air temperatures and precipitation phase (rain versus snow).
Abstract: The Arctic is rapidly warming and experiencing tremendous changes in sea ice, ocean and terrestrial regions. Lack of long-term scientific observations makes it difficult to assess whether Arctic changes statistically represent a ‘new Arctic’ climate. Here we use five Coupled Model Intercomparison Project 5 class Earth system model large ensembles to show how the Arctic is transitioning from a dominantly frozen state and to quantify the nature and timing of an emerging new Arctic climate in sea ice, air temperatures and precipitation phase (rain versus snow). Our results suggest that Arctic climate has already emerged in sea ice. Air temperatures will emerge under the representative concentration pathway 8.5 scenario in the early- to mid-twenty-first century, followed by precipitation-phase changes. Despite differences in mean state and forced response, these models show striking similarities in their anthropogenically forced emergence from internal variability in Arctic sea ice, surface temperatures and precipitation-phase changes. The short observational record makes it difficult to gauge how unprecedented recent Arctic warming is. A multi-model large ensemble estimates a new Arctic climate has emerged for sea-ice extent. As the Arctic shifts from a primarily frozen state, temperature and precipitation follow within decades.
TL;DR: In this paper, the authors used panel data on legislative activity in 133 countries over the period 1999-2016 to identify statistically the short-term and long-term impact of climate legislation.
Abstract: The international response to climate change has been inadequate, but not zero. There are 1,800 climate change laws worldwide. We use panel data on legislative activity in 133 countries over the period 1999–2016 to identify statistically the short-term and long-term impact of climate legislation. Each new law reduces annual carbon dioxide (CO2) emissions per unit of gross domestic product by 0.78% nationally in the short term (during the first three years) and by 1.79% in the long term (beyond three years). The results are driven by parliamentary acts and by countries with a strong rule of law. In 2016, current climate laws were associated with an annual reduction in global CO2 emissions of 5.9 GtCO2, more than the US CO2 output that year. Cumulative CO2 emissions savings from 1999 to 2016 amount to 38 GtCO2, or one year’s worth of global CO2 output. The impact on other greenhouse gases is much lower. Climate change laws are shown to reduce national CO2 emissions by 0.78% in their first three years and 1.79% in the longer term. These reductions add up to 38 GtCO2 of avoided emissions for 1999–2016—equal to a year of CO2 emissions.
TL;DR: In this article, the authors highlight trends in primary production over the last two decades and consider changes to Arctic phenology by borealization and hidden under-ice blooms, and how the diversity of phytoplankton assemblages might evolve in a novel Arctic biogeochemical landscape.
Abstract: Changes in the Arctic atmosphere, cryosphere and Ocean are drastically altering the dynamics of phytoplankton, the base of marine ecosystems. This Review addresses four major complementary questions of ongoing Arctic Ocean changes and associated impacts on phytoplankton productivity, phenology and assemblage composition. We highlight trends in primary production over the last two decades while considering how multiple environmental drivers shape Arctic biogeography. Further, we consider changes to Arctic phenology by borealization and hidden under-ice blooms, and how the diversity of phytoplankton assemblages might evolve in a novel Arctic ‘biogeochemical landscape’. It is critical to understand these aspects of changing Arctic phytoplankton dynamics as they exert pressure on marine Arctic ecosystems in addition to direct effects from rapid environmental changes. Ongoing Arctic changes are impacting phytoplankton. This Review considers recent primary productivity trends and the environmental drivers, as well as how these are changing, that drive phytoplankton diversity in the region.
TL;DR: In this article, the effects of multiple droughts at a range of scales from individual trees to the globe from 1900 to 2018 were quantified by combining cross-biome datasets of tree growth, tree mortality and ecosystem water content.
Abstract: Climate change-driven increases in drought frequency and severity could compromise forest ecosystems and the terrestrial carbon sink1–3. While the impacts of single droughts on forests have been widely studied4–6, understanding whether forests acclimate to or become more vulnerable to sequential droughts remains largely unknown and is crucial for predicting future forest health. We combine cross-biome datasets of tree growth, tree mortality and ecosystem water content to quantify the effects of multiple droughts at a range of scales from individual trees to the globe from 1900 to 2018. We find that subsequent droughts generally have a more deleterious impact than initial droughts, but this effect differs enormously by clade and ecosystem, with gymnosperms and conifer-dominated ecosystems more often exhibiting increased vulnerability to multiple droughts. The differential impacts of multiple droughts across clades and biomes indicate that drought frequency changes may have fundamentally different ecological and carbon-cycle consequences across ecosystems. Drought frequency will probably increase under climate change, posing a potential risk to forests. Forest response is variable, but subsequent droughts generally have a negative impact at the tree and ecosystem scales, with systems dominated by conifers particularly vulnerable.
TL;DR: The authors employ a meta-analysis approach to synthesize the evidence from 30 country-level studies that estimate the effect of slow-and rapid-onset events on migration worldwide, finding that environmental hazards affect migration, although with contextual variation.
Abstract: The impact of climate change on migration has gained both academic and public interest in recent years. Here we employ a meta-analysis approach to synthesize the evidence from 30 country-level studies that estimate the effect of slow- and rapid-onset events on migration worldwide. Most studies find that environmental hazards affect migration, although with contextual variation. Migration is primarily internal or to low- and middle-income countries. The strongest relationship is found in studies with a large share of countries outside the Organisation for Economic Co-operation and Development, particularly from Latin America and the Caribbean and sub-Saharan Africa, and in studies of middle-income and agriculturally dependent countries. Income and conflict moderate and partly explain the relationship between environmental change and migration. Combining our estimates for differential migration responses with the observed environmental change in these countries in recent decades illustrates how the meta-analytic results can provide useful insights for the identification of potential hotspots of environmental migration. Using a meta-analysis approach, the authors find robust evidence that environmental factors play a role in explaining migration patterns across countries and over time, but the size of the effects depend on the economic and sociopolitical context, and the environmental factors considered.
TL;DR: In this article, the authors investigated the "drought paradox" for the European Alps using a 1,212-station database and hyper-resolution ecohydrological simulations to quantify blue (runoff) and green (evapotranspiration) water fluxes.
Abstract: Climate change can reduce surface-water supply by enhancing evapotranspiration in forested mountains, especially during heatwaves. We investigate this ‘drought paradox’ for the European Alps using a 1,212-station database and hyper-resolution ecohydrological simulations to quantify blue (runoff) and green (evapotranspiration) water fluxes. During the 2003 heatwave, evapotranspiration in large areas over the Alps was above average despite low precipitation, amplifying the runoff deficit by 32% in the most runoff-productive areas (1,300–3,000 m above sea level). A 3 °C air temperature increase could enhance annual evapotranspiration by up to 100 mm (45 mm on average), which would reduce annual runoff at a rate similar to a 3% precipitation decrease. This suggests that green-water feedbacks—which are often poorly represented in large-scale model simulations—pose an additional threat to water resources, especially in dry summers. Despite uncertainty in the validation of the hyper-resolution ecohydrological modelling with observations, this approach permits more realistic predictions of mountain region water availability. Mountain forest drought can paradoxically increase evapotranspiration (green water), helping vegetation at the expense of runoff (blue water). This is quantified for the 2003 event in the European Alps, highlighting underappreciated vulnerability of blue-water resources to future warmer summers.
TL;DR: In this article, a global spatially explicit analysis of life cycle GHG emissions for lignocellulosic crop-based bioenergy with carbon capture and storage (BECCS) is performed.
Abstract: Bioenergy with carbon capture and storage (BECCS) can act as a negative emission technology and is considered crucial in many climate change mitigation pathways that limit global warming to 1.5–2 °C; however, the negative emission potential of BECCS has not been rigorously assessed. Here we perform a global spatially explicit analysis of life-cycle GHG emissions for lignocellulosic crop-based BECCS. We show that negative emissions greatly depend on biomass cultivation location, treatment of original vegetation, the final energy carrier produced and the evaluation period considered. We find a global potential of 28 EJ per year for electricity with negative emissions, sequestering 2.5 GtCO2 per year when accounting emissions over 30 years, which increases to 220 EJ per year and 40 GtCO2 per year over 80 years. We show that BECCS sequestration projected in IPCC SR1.5 °C pathways can be approached biophysically; however, considering its potentially very large land requirements, we suggest substantially limited and earlier deployment. Negative emissions technologies are a cornerstone of many mitigation scenarios that limit global warming under 2 °C. Depending on the conditions, bioenergy with carbon capture and storage can provide negative emissions but requires large amounts of land and should be deployed early and with limits.