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"Levy, Peter"
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Evidence for a weakening relationship between interannual temperature variability and northern vegetation activity
Satellite-derived Normalized Difference Vegetation Index (NDVI), a proxy of vegetation productivity, is known to be correlated with temperature in northern ecosystems. This relationship, however, may change over time following alternations in other environmental factors. Here we show that above 30°N, the strength of the relationship between the interannual variability of growing season NDVI and temperature (partial correlation coefficient
R
NDVI-GT
) declined substantially between 1982 and 2011. This decrease in
R
NDVI-GT
is mainly observed in temperate and arctic ecosystems, and is also partly reproduced by process-based ecosystem model results. In the temperate ecosystem, the decrease in
R
NDVI-GT
coincides with an increase in drought. In the arctic ecosystem, it may be related to a nonlinear response of photosynthesis to temperature, increase of hot extreme days and shrub expansion over grass-dominated tundra. Our results caution the use of results from interannual time scales to constrain the decadal response of plants to ongoing warming.
Northern Hemisphere photosynthesis is thought to respond positively to temperature variations, yet the strength of this relationship may change over time. Here, using a combination of satellite data and models, the authors assess the temporal change of this relationship over the past three decades.
Journal Article
The great uprising : race riots in urban America during the 1960s
Between 1963 and 1972 America experienced over 750 urban revolts. Considered collectively, they comprise what Peter Levy terms a 'Great Uprising'. Levy examines these uprisings over the arc of the entire decade, in various cities across America. He challenges both conservative and liberal interpretations, emphasizing that these riots must be placed within historical context to be properly understood. By focusing on three specific cities as case studies - Cambridge and Baltimore, Maryland, and York, Pennsylvania - Levy demonstrates the impact which these uprisings had on millions of ordinary Americans. He shows how conservatives profited politically by constructing a misleading narrative of their causes, and also suggests that the riots did not represent a sharp break or rupture from the civil rights movement. Finally, Levy presents a cautionary tale by challenging us to consider if the conditions that produced this 'Great Uprising' are still predominant in American culture today.
Book
Infilled Ditches are Hotspots of Landscape Methane Flux Following Peatland Re-wetting
Peatlands are large terrestrial stores of carbon, and sustained CO₂ sinks, but over the last century large areas have been drained for agriculture and forestry, potentially converting them into net carbon sources. More recently, some peatlands have been re-wetted by blocking drainage ditches, with the aims of enhancing biodiversity, mitigating flooding, and promoting carbon storage. One potential detrimental consequence of peatland re-wetting is an increase in methane (CH₄) emissions, offsetting the benefits of increased CO₂ sequestration. We examined differences in CH₄ emissions between an area of ditch-drained blanket bog, and an adjacent area where drainage ditches were recently infilled. Results showed that Eriophorum vaginatum colonization led to a “hotspot” of CH₄ emissions from the infilled ditches themselves, with smaller increases in CH₄ from other re-wetted areas. Extrapolated to the area of blanket bog surrounding the study site, we estimated that CH₄ emissions were around 60 kg CH₄ ha⁻¹ y⁻¹ prior to drainage, reducing to 44 kg CH₄ ha⁻¹ y⁻¹ after drainage. We calculated that fully re-wetting this area would initially increase emissions to a peak of around 120 kg CH₄ ha⁻¹ y⁻¹, with around two-thirds of the increase (and 90% of the increase over pre-drainage conditions) attributable to CH₄ emissions from E. vaginatum-colonized infilled ditches, despite these areas only occupying 7% of the landscape. We predicted that emissions should eventually decline toward pre-drainage values as the ecosystem recovers, but only if Sphagnum mosses displace E. vaginatum from the infilled ditches. These results have implications for peatland management for climate change mitigation, suggesting that restoration methods should aim, if possible, to avoid the colonization of infilled ditches by aerenchymatous species such as E. vaginatum, and to encourage Sphagnum establishment.
Journal Article
Mapping soil moisture across the UK: assimilating cosmic-ray neutron sensors, remotely sensed indices, rainfall radar and catchment water balance data in a Bayesian hierarchical model
Soil moisture is important in many hydrological and ecological processes. However, data sets which are currently available have issues with accuracy and resolution. To translate remotely sensed data to an absolute measure of soil moisture requires mapped estimates of soil hydrological properties and estimates of vegetation properties, and this introduces considerable uncertainty. We present an alternative methodology for producing daily maps of soil moisture over the UK at 2 km resolution (“SMUK”). The method is based on a simple linear statistical model, calibrated with 5 years of daily data from cosmic-ray neutron sensors at ∼ 40 sites across the country. The model is driven by precipitation, humidity, a remotely sensed “soil water index” satellite product and soil porosity. The spatial variation in the parameter describing the soil water retention (and thereby the response to precipitation) was estimated using daily water balance data from ∼ 1200 catchments with good coverage across the country. The model parameters were estimated by Bayesian calibration using a Markov chain–Monte Carlo method, so as to characterise the posterior uncertainty in the parameters and predictions. The approach reduces uncertainty by integrating multiple data sources, all of which have weaknesses but together act as a better constraint on the true soil moisture. The model explains around 70 % of the variance in the daily observations with a root-mean-square error of 0.05 m3 m−3, better than results from more complex process-based models. Given the high resolution of the inputs in time and space, the model can predict the very detailed variation in soil moisture which arises from the sporadic nature of precipitation events, including the small-scale and short-term variations associated with orographic and convective rainfall. Predictions over the period 2016 to 2023 demonstrated realistic patterns following the passage of weather fronts and prolonged droughts. The model has negligible computation time, and inputs and predictions are updated daily, lagging approximately 1 week behind real time.
Journal Article
Multiple mechanisms of Amazonian forest biomass losses in three dynamic global vegetation models under climate change
The large-scale loss of Amazonian rainforest under some future climate scenarios has generally been considered to be driven by increased drying over Amazonia predicted by some general circulation models (GCMs). However, the importance of rainfall relative to other drivers has never been formally examined. Here, we conducted factorial simulations to ascertain the contributions of four environmental drivers (precipitation, temperature, humidity and CO₂) to simulated changes in Amazonian vegetation carbon (Cveg), in three dynamic global vegetation models (DGVMs) forced with climate data based on HadCM3 for four SRES scenarios. Increased temperature was found to be more important than precipitation reduction in causing losses of Amazonian Cveg in two DGVMs (Hyland and TRIFFID), and as important as precipitation reduction in a third DGVM (LPJ). Increases in plant respiration, direct declines in photosynthesis and increases in vapour pressure deficit (VPD) all contributed to reduce Cveg under high temperature, but the contribution of each mechanism varied greatly across models. Rising CO₂ mitigated much of the climate-driven biomass losses in the models. Additional work is required to constrain model behaviour with experimental data under conditions of high temperature and drought. Current models may be overly sensitive to long-term elevated temperatures as they do not account for physiological acclimation.
Journal Article
Measurements of methane and nitrous oxide in human breath and the development of UK scale emissions
Exhaled human breath can contain small, elevated concentrations of methane (CH 4 ) and nitrous oxide (N 2 O), both of which contribute to global warming. These emissions from humans are not well understood and are rarely quantified in global greenhouse gas inventories. This study investigated emissions of CH 4 and N 2 O in human breath from 104 volunteers in the UK population, to better understand what drives these emissions and to quantify national-scale estimates. A total of 328 breath samples were collected, and age, sex, dietary preference, and smoking habits were recorded for every participant. The percentage of methane producers (MPs) identified in this study was 31%. The percentage of MPs was higher in older age groups with 25% of people under the age of 30 classified as MPs compared to 40% in the 30+ age group. Females (38%) were more likely to be MPs than males (25%), though overall concentrations emitted from both MP groups were similar. All participants were found to emit N 2 O in breath, though none of the factors investigated explained the differences in emissions. Dietary preference was not found to affect CH 4 or N 2 O emissions from breath in this study. We estimate a total emission of 1.04 (0.86–1.40) Gg of CH 4 and 0.069 (0.066–0.072) Gg of N 2 O in human breath annually in the UK, the equivalent of 53.9 (47.8–60.0) Gg of CO 2 . In terms of magnitude, these values are approximately 0.05% and 0.1% of the total emissions of CH 4 and N 2 O reported in the UK national greenhouse gas inventories.
Journal Article