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"Stevenson, S"
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Significant changes to ENSO strength and impacts in the twenty-first century: Results from CMIP5
Changes to the El Niño/Southern Oscillation (ENSO) and its atmospheric teleconnections under climate change are investigated using simulations conducted for the Coupled Model Intercomparison Project (CMIP5). The overall response to CO2increases is determined using 27 models, and the ENSO amplitude change based on the multi‐model mean is indistinguishable from zero. However, changes between ensembles run with a given model are sometimes significant: for four of the eleven models having ensemble sizes larger than three, the 21st century change to ENSO amplitude is statistically significant. In these four models, changes to SST and wind stress do not differ substantially from those in the models with no ENSO response, indicating that mean changes are not predictive of the ENSO sensitivity to climate change. Also, ocean vertical stratification is less (more) sensitive to CO2in models where ENSO strengthens (weakens), likely due to a regulation of the subsurface temperature structure by ENSO‐related poleward heat transport. Atmospheric teleconnections also show differences between models where ENSO amplitude does and does not respond to climate change; in the former case El Niño/La Niña‐related sea level pressure anomalies strengthen with CO2, and in the latter they weaken and shift polewards and eastwards. These results illustrate the need for large ensembles to isolate significant ENSO climate change responses, and for future work on diagnosing the dynamical causes of inter‐model teleconnection differences. Key Points ENSO amplitude is insignificant in the majority of IPCC‐class models ENSO amplitude change is not due to mean state or seasonal cycle changes The teleconnection response is sensitive to the ENSO amplitude change
Journal Article
Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer
Ozone holds a certain fascination in atmospheric science. It is ubiquitous in the atmosphere, central to tropospheric oxidation chemistry, yet harmful to human and ecosystem health as well as being an important greenhouse gas. It is not emitted into the atmosphere but is a byproduct of the very oxidation chemistry it largely initiates. Much effort is focused on the reduction of surface levels of ozone owing to its health and vegetation impacts, but recent efforts to achieve reductions in exposure at a country scale have proved difficult to achieve owing to increases in background ozone at the zonal hemispheric scale. There is also a growing realisation that the role of ozone as a short-lived climate pollutant could be important in integrated air quality climate change mitigation. This review examines current understanding of the processes regulating tropospheric ozone at global to local scales from both measurements and models. It takes the view that knowledge across the scales is important for dealing with air quality and climate change in a synergistic manner. The review shows that there remain a number of clear challenges for ozone such as explaining surface trends, incorporating new chemical understanding, ozone–climate coupling, and a better assessment of impacts. There is a clear and present need to treat ozone across the range of scales, a transboundary issue, but with an emphasis on the hemispheric scales. New observational opportunities are offered both by satellites and small sensors that bridge the scales.
Journal Article
Under a crimson sun : prospects for life in a red dwarf system
\"Gliese 581 is a red dwarf star some 20.3 light years from Earth. Red dwarfs are among the most numerous stars in the galaxy, and they sport diverse planetary systems. At magnitude 10, Gliese 581 is visible to amateur observers but does not stand out. So what makes this star so important? It is that professional observers have confirmed that it has at least four planets orbiting it, and in 2009, Planet d was described in the letters of The Astrophysical Journal as \"the first confirmed exoplanet that could support Earth-like life.\" Under a Crimson Sun looks at the nature of red dwarf systems such as Gliese as potential homes for life. Realistically, what are prospects for life on these distant worlds? Could life evolve and survive there? How do these planetary surfaces and geology evolve? How would life on a red dwarf planet differ from life on Earth? And what are the implications for finding further habitable worlds in our galaxy? Stevenson provides readers with insight into the habitability of planets and how this changes as time progresses and the central star evolves. Explore with him in this engaging, fascinating book the possibilities for finding life, from bacteria to more complex and even intelligent organisms, on red dwarf system planets\"-- Back cover.
Book
COVID-19 lockdown emission reductions have the potential to explain over half of the coincident increase in global atmospheric methane
Compared with 2019, measurements of the global growth rate of background (marine air) atmospheric methane rose by 5.3 ppb yr−1 in 2020, reaching 15.0 ppb yr−1. Global atmospheric chemistry models have previously shown that reductions in nitrogen oxide (NOx) emissions reduce levels of the hydroxyl radical (OH) and lengthen the methane lifetime. Acting in the opposite sense, reductions in carbon monoxide (CO) and non-methane volatile organic compound (NMVOC) emissions increase OH and shorten methane's lifetime. Using estimates of NOx, CO, and NMVOC emission reductions associated with COVID-19 lockdowns around the world in 2020 as well as model-derived regional and aviation sensitivities of methane to these emissions, we find that NOx emission reductions led to a 4.8 (3.8 to 5.8) ppb yr−1 increase in the global methane growth rate. Reductions in CO and NMVOC emissions partly counteracted this, changing (reducing) the methane growth rate by −1.4 (−1.1 to −1.7) ppb yr−1 (CO) and −0.5 (−0.1 to −0.9) ppb yr−1 (NMVOC), yielding a net increase of 2.9 (1.7 to 4.0) ppb yr−1. Uncertainties refer to ±1 standard deviation model ranges in sensitivities. Whilst changes in anthropogenic emissions related to COVID-19 lockdowns are probably not the only important factor that influenced methane during 2020, these results indicate that they have had a large impact and that the net effect of NOx, CO, and NMVOC emission changes can explain over half of the observed 2020 methane changes. Large uncertainties remain in both emission changes during the lockdowns and methane's response to them; nevertheless, this analysis suggests that further research into how the atmospheric composition changed over the lockdown periods will help us to interpret past methane changes and to constrain future methane projections.
Journal Article
The nature of life and its potential to survive
This book looks at the persistence of life and how difficult it would be to annihilate life, especially a species as successful as humanity. The idea that life in general is fragile is challenged by the hardiness of microbes, which shows that astrobiology on exoplanets and other satellites must be robust and plentiful. Microbes have adapted to virtually every niche on the planet, from the deep, hot biosphere, to the frigid heights of the upper troposphere. Life, it seems, is almost indestructible. The chapters in this work examine the various scenarios that might lead to the extermination of life, and why they will almost always fail. Life's highly adaptive nature ensures that it will cling on no matter how difficult the circumstances. Scientists are increasingly probing and questioning life's true limits in, on and above the Earth, and how these limits could be pushed elsewhere in the universe. This investigation puts life in its true astronomical context, with the reader taken on a journey to illustrate life's potential and perseverance. -- Provided by publisher.
Book
The role of anthropogenic aerosols in future precipitation extremes over the Asian Monsoon Region
The role of anthropogenic aerosols in future projections (up to 2100) of summertime precipitation and precipitation extremes over the Asian monsoon region is investigated, by comparing two sets of the Community Earth System Model (CESM1) large ensemble simulations under the Representative Concentration Pathway 8.5 scenario (RCP8.5) and the corresponding scenario with aerosol fixed at 2005 levels (RCP8.5_FixA). The model is verified to be performing well in capturing present-day (1986–2005) climate and precipitation extremes. Our results suggest that the Asian monsoon region would become progressively warmer and wetter in the future under RCP8.5, while precipitation extremes will be significantly aggravated due to anthropogenic aerosol mitigation, particularly over East Asia. Specifically, aerosol reductions are found to shift the distribution of precipitation mean and extremes to larger values. For example, aerosol reductions would result in an increased likelihood of extreme precipitation (e.g. the maximum consecutive 5-day precipitation amount) and related disasters. Sensitivities of changes in precipitation mean and extremes to local warming from aerosol reductions are much larger than that from greenhouse gas increases. This is particularly important over East Asia in accordance with larger magnitudes of aerosol reductions compared to South Asia. Finally, by investigating the response of the climate system to aerosol changes, our findings demonstrate that aerosol induced precipitation changes would be dominated by aerosol–radiation–cloud forcing over northern East Asia and aerosol forcing induced large-scale circulation anomalies over southern East and South Asia.
Journal Article
Granite skyscrapers : how rock shaped earth and other worlds
In this book, David Stevenson offers us a look at the evolution of planets as they move from balls of mixed molten rock to vibrant worlds capable of hosting life. Embedded in our everyday architecture and in the literal ground beneath our feet, granite and its kin lie at the heart of many features of the Earth that we take for granted. From volcanism and mountain building to shifting water levels and local weather patterns, these rocks are closely intertwined with the complex processes that continue to shape and reshape our world. This book serves as a wonderful primer for anybody interested in our planet's geological past and that of other planets in our Solar System and beyond. It illustrates not only how our planet's surface evolved, but also how granite played a pivotal role in the creation of complex, intelligent life on Earth. There has long been a missing element in popular astronomy literature, which Stevenson now aims to fill: how geological and biological evolution work in a complex partnership, and what our planet's own diversity can teach us about other rocky worlds.
Book
Trends in global tropospheric hydroxyl radical and methane lifetime since 1850 from AerChemMIP
We analyse historical (1850–2014) atmospheric hydroxyl (OH) and methane lifetime data from Coupled Model Intercomparison Project Phase 6 (CMIP6)/Aerosols and Chemistry Model Intercomparison Project (AerChemMIP) simulations. Tropospheric OH changed little from 1850 up to around 1980, then increased by around 9 % up to 2014, with an associated reduction in methane lifetime. The model-derived OH trends from 1980 to 2005 are broadly consistent with trends estimated by several studies that infer OH from inversions of methyl chloroform and associated measurements; most inversion studies indicate decreases in OH since 2005. However, the model results fall within observational uncertainty ranges. The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic near-term climate forcer emissions (increases in anthropogenic nitrogen oxides and decreases in CO). Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH, whilst increases in aerosol-related emissions have slightly reduced OH. Halocarbon emissions have dramatically reduced the stratospheric methane lifetime by about 15 %–40 %; most previous studies assumed a fixed stratospheric lifetime. Whilst the main driver of atmospheric methane increases since 1850 is emissions of methane itself, increased ozone precursor emissions have significantly modulated (in general reduced) methane trends. Halocarbon and aerosol emissions are found to have relatively small contributions to methane trends. These experiments do not isolate the effects of climate change on OH and methane evolution; however, we calculate residual terms that are due to the combined effects of climate change and non-linear interactions between drivers. These residual terms indicate that non-linear interactions are important and differ between the two methodologies we use for quantifying OH and methane drivers. All these factors need to be considered in order to fully explain OH and methane trends since 1850; these factors will also be important for future trends.
Journal Article