Integration of urban science and urban climate adaptation research: opportunities to advance climate action.

There is a growing recognition that responding to climate change necessitates urban adaptation. We sketch a transdisciplinary research effort, arguing that actionable research on urban adaptation needs to recognize the nature of cities as social networks embedded in physical space. Given the pace, scale and socioeconomic outcomes of urbanization in the Global South, the specificities and history of its cities must be central to the study of how well-known agglomeration effects can facilitate adaptation. The proposed effort calls for the co-creation of knowledge involving scientists and stakeholders, especially those historically excluded from the design and implementation of urban development policies.

A multi-city urban atmospheric greenhouse gas measurement data synthesis.

Urban regions emit a large fraction of anthropogenic emissions of greenhouse gases (GHG) such as carbon dioxide (CO2) and methane (CH4) that contribute to modern-day climate change. As such, a growing number of urban policymakers and stakeholders are adopting emission reduction targets and implementing policies to reach those targets. Over the past two decades research teams have established urban GHG monitoring networks to determine how much, where, and why a particular city emits GHGs, and to track changes in emissions over time. Coordination among these efforts has been limited, restricting the scope of analyses and insights. Here we present a harmonized data set synthesizing urban GHG observations from cities with monitoring networks across North America that will facilitate cross-city analyses and address scientific questions that are difficult to address in isolation.

Integrating solutions to adapt cities for climate change.

Record climate extremes are reducing urban liveability, compounding inequality, and threatening infrastructure. Adaptation measures that integrate technological, nature-based, and social solutions can provide multiple co-benefits to address complex socioecological issues in cities while increasing resilience to potential impacts. However, there remain many challenges to developing and implementing integrated solutions. In this Viewpoint, we consider the value of integrating across the three solution sets, the challenges and potential enablers for integrating solution sets, and present examples of challenges and adopted solutions in three cities with different urban contexts and climates (Freiburg, Germany; Durban, South Africa; and Singapore). We conclude with a discussion of research directions and provide a road map to identify the actions that enable successful implementation of integrated climate solutions. We highlight the need for more systematic research that targets enabling environments for integration; achieving integrated solutions in different contexts to avoid maladaptation; simultaneously improving liveability, sustainability, and equality; and replicating via transfer and scale-up of local solutions. Cities in systematically disadvantaged countries (sometimes referred to as the Global South) are central to future urban development and must be prioritised. Helping decision makers and communities understand the potential opportunities associated with integrated solutions for climate change will encourage urgent and deliberate strides towards adapting cities to the dynamic climate reality.

Climate and lawn management interact to control C4 plant distribution in residential lawns across seven U.S. cities.

In natural grasslands, C4 plant dominance increases with growing season temperatures and reflects distinct differences in plant growth rates and water use efficiencies of C3 vs. C4 photosynthetic pathways. However, in lawns, management decisions influence interactions between planted turfgrass and weed species, leading to some uncertainty about the degree of human vs. climatic controls on lawn species distributions. We measured herbaceous plant carbon isotope ratios (δ13 C, index of C3 /C4 relative abundance) and C4 cover in residential lawns across seven U.S. cities to determine how climate, lawn plant management, or interactions between climate and plant management influenced C4 lawn cover. We also calculated theoretical C4 carbon gain predicted by a plant physiological model as an index of expected C4 cover due to growing season climatic conditions in each city. Contrary to theoretical predictions, plant δ13 C and C4 cover in urban lawns were more strongly related to mean annual temperature than to growing season temperature. Wintertime temperatures influenced the distribution of C4 lawn turf plants, contrary to natural ecosystems where growing season temperatures primarily drive C4 distributions. C4 cover in lawns was greatest in the three warmest cities, due to an interaction between climate and homeowner plant management (e.g., planting C4 turf species) in these cities. The proportion of C4 lawn species was similar to the proportion of C4 species in the regional grass flora. However, the majority of C4 species were nonnative turf grasses, and not of regional origin. While temperature was a strong control on lawn species composition across the United States, cities differed as to whether these patterns were driven by cultivated lawn grasses vs. weedy species. In some cities, biotic interactions with weedy plants appeared to dominate, while in other cities, C4 plants were predominantly imported and cultivated. Elevated CO2 and temperature in cities can influence C3 /C4 competitive outcomes; however, this study provides evidence that climate and plant management dynamics influence biogeography and ecology of C3 /C4 plants in lawns. Their differing water and nutrient use efficiency may have substantial impacts on carbon, water, energy, and nutrient budgets across cities.

Adapting Urban Water Systems to Manage Scarcity in the 21st Century: The Case of Los Angeles.

Acute water shortages for large metropolitan regions are likely to become more frequent as climate changes impact historic precipitation levels and urban population grows. California and Los Angeles County have just experienced a severe four year drought followed by a year of high precipitation, and likely drought conditions again in Southern California. We show how the embedded preferences for distant sources, and their local manifestations, have created and/or exacerbated fluctuations in local water availability and suboptimal management. As a socio technical system, water management in the Los Angeles metropolitan region has created a kind of scarcity lock-in in years of low rainfall. We come to this through a decade of coupled research examining landscapes and water use, the development of the complex institutional water management infrastructure, hydrology and a systems network model. Such integrated research is a model for other regions to unpack and understand the actual water resources of a metropolitan region, how it is managed and potential ability to become more water self reliant if the institutions collaborate and manage the resource both parsimoniously, but also in an integrated and conjunctive manner. The Los Angeles County metropolitan region, we find, could transition to a nearly water self sufficient system.

Soil carbon and nitrogen accumulation in residential lawns of the Salt Lake Valley, Utah.

Urban lawn ecosystems are widespread across the United States, with fertilization rates commonly exceeding plant nitrogen (N) uptake rates. While urban soils have been shown to accumulate C and N over time, the long-term balance of N inputs and losses from lawn soils remains largely uncertain. We sampled residential lawn soils aged 7-100 years in the Salt Lake City metropolitan area as a means of inferring changes in total nitrogen (TN) content, organic carbon (OC) content, C:N ratio, and δ15N of bulk soil over time. Core-integrated (0-40 cm) TN and OC stocks increased linearly by 2.39 g N m-2 year-1 and 29.8 g OC m-2 year-1 over the 100-year chronosequence. TN and OC percent were also negatively correlated with elevation. Multiple linear regression models including housing age and elevation as covariates, explained 68 and 62% of variability in TN and OC stocks respectively. δ15N increased with housing age, soil depth, and clay content, suggesting N removal over time, especially in poorly drained soils. We quantified potential hydrologic and gaseous N losses over time by comparing observed N accumulation to different historic fertilization scenarios. Modeling and isotopic results suggest that, while soil N has accumulated over time, the majority of N added to lawns in the Salt Lake Valley over 50 years of fertilization was likely lost from surface soils via denitrification or leaching.

Evapotranspiration and water yield of a pine-broadleaf forest are not altered by long-term atmospheric [CO2 ] enrichment under native or enhanced soil fertility.

Changes in evapotranspiration (ET) from terrestrial ecosystems affect their water yield (WY), with considerable ecological and economic consequences. Increases in surface runoff observed over the past century have been attributed to increasing atmospheric CO2 concentrations resulting in reduced ET by terrestrial ecosystems. Here, we evaluate the water balance of a Pinus taeda (L.) forest with a broadleaf component that was exposed to atmospheric [CO2 ] enrichment (ECO2 ; +200 ppm) for over 17 years and fertilization for 6 years, monitored with hundreds of environmental and sap flux sensors on a half-hourly basis. These measurements were synthesized using a one-dimensional Richard's equation model to evaluate treatment differences in transpiration (T), evaporation (E), ET, and WY. We found that ECO2 did not create significant differences in stand T, ET, or WY under either native or enhanced soil fertility, despite a 20% and 13% increase in leaf area index, respectively. While T, ET, and WY responded to fertilization, this response was weak (<3% of mean annual precipitation). Likewise, while E responded to ECO2 in the first 7 years of the study, this effect was of negligible magnitude (<1% mean annual precipitation). Given the global range of conifers similar to P. taeda, our results imply that recent observations of increased global streamflow cannot be attributed to decreases in ET across all ecosystems, demonstrating a great need for model-data synthesis activities to incorporate our current understanding of terrestrial vegetation in global water cycle models.

Long-term urban carbon dioxide observations reveal spatial and temporal dynamics related to urban characteristics and growth.

Cities are concentrated areas of CO2 emissions and have become the foci of policies for mitigation actions. However, atmospheric measurement networks suitable for evaluating urban emissions over time are scarce. Here we present a unique long-term (decadal) record of CO2 mole fractions from five sites across Utah's metropolitan Salt Lake Valley. We examine "excess" CO2 above background conditions resulting from local emissions and meteorological conditions. We ascribe CO2 trends to changes in emissions, since we did not find long-term trends in atmospheric mixing proxies. Three contrasting CO2 trends emerged across urban types: negative trends at a residential-industrial site, positive trends at a site surrounded by rapid suburban growth, and relatively constant CO2 over time at multiple sites in the established, residential, and commercial urban core. Analysis of population within the atmospheric footprints of the different sites reveals approximately equal increases in population influencing the observed CO2, implying a nonlinear relationship with CO2 emissions: Population growth in rural areas that experienced suburban development was associated with increasing emissions while population growth in the developed urban core was associated with stable emissions. Four state-of-the-art global-scale emission inventories also have a nonlinear relationship with population density across the city; however, in contrast to our observations, they all have nearly constant emissions over time. Our results indicate that decadal scale changes in urban CO2 emissions are detectable through monitoring networks and constitute a valuable approach to evaluate emission inventories and studies of urban carbon cycles.

Trees grow on money: urban tree canopy cover and environmental justice.

This study examines the distributional equity of urban tree canopy (UTC) cover for Baltimore, MD, Los Angeles, CA, New York, NY, Philadelphia, PA, Raleigh, NC, Sacramento, CA, and Washington, D.C. using high spatial resolution land cover data and census data. Data are analyzed at the Census Block Group levels using Spearman's correlation, ordinary least squares regression (OLS), and a spatial autoregressive model (SAR). Across all cities there is a strong positive correlation between UTC cover and median household income. Negative correlations between race and UTC cover exist in bivariate models for some cities, but they are generally not observed using multivariate regressions that include additional variables on income, education, and housing age. SAR models result in higher r-square values compared to the OLS models across all cities, suggesting that spatial autocorrelation is an important feature of our data. Similarities among cities can be found based on shared characteristics of climate, race/ethnicity, and size. Our findings suggest that a suite of variables, including income, contribute to the distribution of UTC cover. These findings can help target simultaneous strategies for UTC goals and environmental justice concerns.

Assessing the homogenization of urban land management with an application to US residential lawn care.

Changes in land use, land cover, and land management present some of the greatest potential global environmental challenges of the 21st century. Urbanization, one of the principal drivers of these transformations, is commonly thought to be generating land changes that are increasingly similar. An implication of this multiscale homogenization hypothesis is that the ecosystem structure and function and human behaviors associated with urbanization should be more similar in certain kinds of urbanized locations across biogeophysical gradients than across urbanization gradients in places with similar biogeophysical characteristics. This paper introduces an analytical framework for testing this hypothesis, and applies the framework to the case of residential lawn care. This set of land management behaviors are often assumed--not demonstrated--to exhibit homogeneity. Multivariate analyses are conducted on telephone survey responses from a geographically stratified random sample of homeowners (n = 9,480), equally distributed across six US metropolitan areas. Two behaviors are examined: lawn fertilizing and irrigating. Limited support for strong homogenization is found at two scales (i.e., multi- and single-city; 2 of 36 cases), but significant support is found for homogenization at only one scale (22 cases) or at neither scale (12 cases). These results suggest that US lawn care behaviors are more differentiated in practice than in theory. Thus, even if the biophysical outcomes of urbanization are homogenizing, managing the associated sustainability implications may require a multiscale, differentiated approach because the underlying social practices appear relatively varied. The analytical approach introduced here should also be productive for other facets of urban-ecological homogenization.

Transpiration sensitivity of urban trees in a semi-arid climate is constrained by xylem vulnerability to cavitation.

Establishing quantitative links between plant hydraulic properties and the response of transpiration to environmental factors such as atmospheric vapor pressure deficit (D) is essential for improving our ability to understand plant water relations across a wide range of species and environmental conditions. We studied stomatal responses to D in irrigated trees in the urban landscape of Los Angeles, California. We found a strong linear relationship between the sensitivity of tree-level transpiration estimated from sap flux (m(T); slope of the relationship between tree transpiration and ln D) and transpiration at D=1 kPa (E(Tref)) that was similar to previous surveys of stomatal behavior in natural environments. In addition, m(T) was significantly related to vulnerability to cavitation of branches (P(50)). While m(T) did not appear to differ between ring- and diffuse-porous species, the relationship between m(T) and P(50) was distinct by wood anatomy. Therefore, our study confirms systematic differences in water relations in ring- versus diffuse-porous species, but these differences appear to be more strongly related to the relationship between stomatal sensitivity to D and vulnerability to cavitation rather than to stomatal sensitivity per se.

Nitrous oxide emissions from wastewater treatment and water reclamation plants in southern California.

Nitrous oxide (N2O) is a long-lived and potent greenhouse gas produced during microbial nitrification and denitrification. In developed countries, centralized water reclamation plants often use these processes for N removal before effluent is used for irrigation or discharged to surface water, thus making this treatment a potentially large source of N2O in urban areas. In the arid but densely populated southwestern United States, water reclamation for irrigation is an important alternative to long-distance water importation. We measured N2O concentrations and fluxes from several wastewater treatment processes in urban southern California. We found that N removal during water reclamation may lead to in situ N2O emission rates that are three or more times greater than traditional treatment processes (C oxidation only). In the water reclamation plants tested, N2O production was a greater percentage of total N removed (1.2%) than traditional treatment processes (C oxidation only) (0.4%). We also measured stable isotope ratios (δN and δO) of emitted N2O and found distinct δN signatures of N2O from denitrification (0.0 ± 4.0 ‰) and nitrification reactors (-24.5 ± 2.2 ‰), respectively. These isotope data confirm that both nitrification and denitrification contribute to N2O emissions within the same treatment plant. Our estimates indicate that N2O emissions from biological N removal for water reclamation may be several orders of magnitude greater than N2O emissions from agricultural activities in highly urbanized southern California. Our results suggest that wastewater treatment that includes biological nitrogen removal can significantly increase urban N2O emissions.

Transpiration of urban forests in the Los Angeles metropolitan area.

Despite its importance for urban planning, landscape management, and water management, there are very few in situ estimates of urban-forest transpiration. Because urban forests contain an unusual and diverse mix of species from many regions worldwide, we hypothesized that species composition would be a more important driver of spatial variability in urban-forest transpiration than meteorological variables in the Los Angeles (California, USA) region. We used constant-heat sap-flow sensors to monitor urban tree water use for 15 species at six locations throughout the Los Angeles metropolitan area. For many of these species no previous data on sap flux, water use, or water relations were available in the literature. To scale sap-flux measurements to whole trees we conducted a literature survey of radial trends in sap flux across multiple species and found consistent relationships for angiosperms vs. gymnosperms. We applied this relationship to our measurements and estimated whole-tree and plot-level transpiration at our sites. The results supported very large species differences in transpiration, with estimates ranging from 3.2 +/- 2.3 kg x tree(-1) x d(-1) in unirrigated Pinus canariensis (Canary Island pine) to 176.9 +/- 75.2 kg x tree(-1) x d(-1) in Platanus hybrida (London planetree) in the month of August. Other species with high daily transpiration rates included Ficus microcarpa (laurel fig), Gleditsia triacanthos (honeylocust), and Platanus racemosa (California sycamore). Despite irrigation and relatively large tree size, Brachychiton populneas (kurrajong), B. discolor (lacebark), Sequoia sempervirens (redwood), and Eucalyptus grandis (grand Eucalyptus) showed relatively low rates of transpiration, with values < 45 kg x tree(-1) x d(-1). When scaled to the plot level, transpiration rates were as high as 2 mm/d for sites that contained both species with high transpiration rates and high densities of planted trees. Because plot-level transpiration is highly dependent on tree density, we modeled transpiration as a function of both species and density to evaluate a likely range of values in irrigated urban forests. The results show that urban forests in irrigated, semi-arid regions can constitute a significant use of water, but water use can be mitigated by appropriate selection of site, management method, and species.

The application of δ¹8O and δD for understanding water pools and fluxes in a Typha marsh.

The δ¹8O and δD composition of water pools (leaf, root, standing water and soil water) and fluxes [transpiration (T), evaporation (E)] were used to understand ecohydrological processes in a managed Typha latifolia L. freshwater marsh. We observed isotopic steady-state T and deep rooting in Typha. The isotopic mass balance of marsh standing water showed that E accounted for 3% of the total water loss, T accounted for 17% and subsurface drainage (D) accounted for the majority (80%). There was a vertical gradient in water vapour content and isotopic composition within and above the canopy sufficient for constructing an isotopic mass balance of water vapour during some sampling periods. During these periods, the proportion of T in evapotranspiration (T/ET) was between 56 ± 17% and 96 ± 67%, and the estimated error was relatively high (>37%) because of non-local, background sources in vapour. Independent estimates of T/ET using eddy covariance measurements yielded similar mean values during the Typha growing season. The various T/ET estimates agreed that T was the dominant source of marsh vapour loss in the growing season. The isotopic mass balance of water vapour yielded reasonable results, but the mass balance of standing water provided more definitive estimates of water losses.

Water relations of coast redwood planted in the semi-arid climate of southern California.

Trees planted in urban landscapes in southern California are often exposed to an unusual combination of high atmospheric evaporative demand and moist soil conditions caused by irrigation. The water relations of species transplanted into these conditions are uncertain. We investigated the water relations of coast redwood (Sequoia sempervirens) planted in the urbanized semi-arid Los Angeles Basin, where it often experiences leaf chlorosis and senescence. We measured the sap flux (J(O)) and hydraulic properties of irrigated trees at three sites in the Los Angeles region. We observed relatively strong stomatal regulation in response to atmospheric vapour pressure deficit (D; J(O) saturated at D < 1 kPa), and a linear response of J(O) to photosynthetically active radiation. Total tree water use by coast redwood was relatively low, with plot-level transpiration rates below 1 mm d(-1) . There was some evidence of xylem cavitation during the summer, which appeared to be reversed in fall and early winter. We conclude that water stress was not a direct factor in causing leaf chlorosis and senescence as has been proposed. Instead, the relatively strong stomatal control that is adaptive in the native habitat of coast redwood may lead to carbon limitation and other stresses in semi-arid, irrigated habitats.

Greenhouse gas emissions from global cities.

The world's population is now over 50% urban, and cities make an important contribution to national greenhouse gas (GHG) emissions. Many cities are developing strategies to reduce their emissions. Here we ask how and why emissions differ between cities. Our study often global cities shows how a balance of geophysical factors (climate, access to resources, and gateway status) and technical factors (power generation, urban design, and waste processing) determine the GHGs attributable to cities. Within the overall trends, however, there are differences between cities with more or less public transit while personal income also impacts heating and industrial fuel use. By including upstream emissions from fuels, GHG emissions attributable to cities exceed those from direct end use by upto 25%. Our findings should help foster intercity learning on reducing GHG emissions.

Wood anatomy constrains stomatal responses to atmospheric vapor pressure deficit in irrigated, urban trees.

Plant transpiration is strongly constrained by hydraulic architecture, which determines the critical threshold for cavitation. Because species vary greatly in vulnerability to cavitation, hydraulic limits to transpiration and stomatal conductance have not generally been incorporated into ecological and climate models. We measured sap flow, leaf transpiration, and vulnerability to cavitation of a variety of tree species in a well-irrigated but semi-arid urban environment in order to evaluate the generality of stomatal responses to high atmospheric vapor pressure deficit (D). We found evidence of broad patterns of stomatal responses to humidity based on systematic differences in vulnerability to cavitation. Ring-porous taxa consistently had vulnerable xylem and showed strong regulation of transpiration in response to D, while diffuse-porous taxa were less vulnerable and transpiration increased nearly linearly with D. These results correspond well to patterns in the distribution of the taxa, such as the prevalence of diffuse-porous species in riparian ecosystems, and also provide a means of representing maximum transpiration rates at varying D in broad categories of trees.