Nonlinear threshold behavior during the loss of Arctic sea ice

In light of the rapid recent retreat of Arctic sea ice, a number of studies have discussed the possibility of a critical threshold (or “tipping point”) beyond which the ice–albedo feedback causes the ice cover to melt away in an irreversible process. The focus has typically been centered on the annual minimum (September) ice cover, which is often seen as particularly susceptible to destabilization by the ice–albedo feedback. Here, we examine the central physical processes associated with the transition from ice-covered to ice-free Arctic Ocean conditions. We show that although the ice–albedo feedback promotes the existence of multiple ice-cover states, the stabilizing thermodynamic effects of sea ice mitigate this when the Arctic Ocean is ice covered during a sufficiently large fraction of the year. These results suggest that critical threshold behavior is unlikely during the approach from current perennial sea-ice conditions to seasonally ice-free conditions. In a further warmed climate, however, we find that a critical threshold associated with the sudden loss of the remaining wintertime-only sea ice cover may be likely

Urban heat stress vulnerability in the U.S. Southwest: The role of sociotechnical systems

Heat vulnerability of urban populations is becoming a major issue of concern with climate change, particularly in the cities of the Southwest United States. In this article we discuss the importance of understanding coupled social and technical systems, how they constitute one another, and how they form the conditions and circumstances in which people experience heat. We discuss the particular situation of Los Angeles and Maricopa Counties, their urban form and the electric grid. We show how vulnerable populations are created by virtue of the age and construction of buildings, the morphology of roads and distribution of buildings on the landscape. Further, the regulatory infrastructure of electricity generation and distribution also contributes to creating differential vulnerability. We contribute to a better understanding of the importance of sociotechnical systems. Social infrastructure includes codes, conventions, rules and regulations; technical systems are the hard systems of pipes, wires, buildings, roads, and power plants. These interact to create lock-in that is an obstacle to addressing issues such as urban heat stress in a novel and equitable manner

Surface constraints on the depth of the Atlantic meridional overturning circulation: Southern Ocean versus North Atlantic

Paleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean

The Importance of Ice Vertical Resolution for Snowball Climate and Deglaciation

Sea ice schemes with a few vertical levels are typically used to simulate the thermodynamic evolution of sea ice in global climate models. Here it is shown that these schemes overestimate the magnitude of the diurnal surface temperature cycle by a factor of 2–3 when they are used to simulate tropical ice in a Snowball earth event. This could strongly influence our understanding of Snowball termination, which occurs in global climate models when the midday surface temperature in the tropics reaches the melting point. A hierarchy of models is used to show that accurate simulation of surface temperature variation on a given time scale requires that a sea ice model resolve the e-folding depth to which a periodic signal on that time scale penetrates. This is used to suggest modifications to the sea ice schemes used in global climate models that would allow more accurate simulation of Snowball deglaciation

Greening Cities in an Urbanizing Age: The Human Health Bases in the Nineteenth and Early Twenty-first Centuries

Defined here as the introduction or conservation of outdoor vegetation in cities, urban greening has bloomed during periods of intensive urbanization. This was true in the nineteenth century and it seems to be the case again today, as a range of greening practices is co-arising during a third, and perhaps final, period of global urbanization. Human health has been a recurring theme underlying the enduring aspiration to integrate nature with city. Using change over time as a conceptual frame, this paper offers a comparative assessment of municipal greening in the nineteenth and early twenty-first centuries, focusing on the potential implications upon, and the relationship between, such activity, urban design, and public health. In so doing, the narrative bridges theory, science, and practice, and dovetails with discourse on urban ecosystem services. Part one assesses prominent drivers and types of greening in nineteenth-century industrial cities, a pioneering period in this evolving narrative. Part two reviews contemporary literature on the human health benefits of urban green spaces, and draws comparisons to the Industrial Era. Part three explores potential links between contemporary greening practice and scholarship on related health benefits, wherein proximal greening emerges as a distinct form, and possible norm, for twenty-first-century urban design

Hawthorne\u27s Transcendental Ambivalence in Mosses from an Old Manse

Nathaniel Hawthorne’s collection of short stories, Mosses from an Old Manse, serves as his contribution to the philosophical discussions on Transcendentalism in Concord, MA in the early 1840s. While Ralph Waldo Emerson, Henry David Thoreau, and the other individuals involved in the Transcendental club often seem to readily accept the positions presented in Emerson’s work, it is never so simple for Hawthorne. Repeatedly, Hawthorne’s stories demonstrate his difficulty in trying to identify his own opinion on the subject. Though Hawthorne seems to want to believe in the optimistic potential of the spiritual and intellectual ideal presented in Emersonian Transcendentalism, he consistently dwells on the evil and blackness that may be contained in the human heart. The collection of short stories written while Hawthorne lived in Concord and surrounded himself with those dominant literary figures represents the clearest articulation of his ambivalent position on Transcendentalism

Pedestrianizing Streets as a Strategy for Urban Greenways

As adequate space to implement new greenways in dense urban centers is difficult to come by, it is vital that planners begin to increasingly tap into the vehicular landscape as a means of creating new space for the public benefit. Pedestrianization of streets helps to reclaim the significant amount of underutilized road space in cities and adjust it to better fit the human scale. In doing so, these pedestrianization efforts unlock copious amounts of space which can become home to new greenways.(Lee 2015). Not only do pedestrian zones offer an opportunity for greenway implementation, the aims of these areas are serendipitous and thus result in co-benefits when planned together. In planning pedestrian zones, greenways have been utilized towards improving the livability of the area through the provision of shade, noise reduction, air filtration, and their associated mental health benefits.(Maritz 2019). Further, the space freed up by removing automobile traffic from a street is often too wide to be comfortable for pedestrians, therefore greenways are being used to create narrower pedestrian areas with a green edge or median.(Dostal 2021).For greenways, pedestrian zones can help improve accessibility and thus the use of these areas while also creating calmer car-free environments for people and local wildlife alike. Additionally, such integrated developments have similarly been envisioned alongside urban waterways which are often dominated by vehicular traffic, experience higher volumes of foot traffic, and hold great potential for greenways as a means to manage flooding while keeping the land usable for recreational purposes.(Horte 2020). Through a systematic review and analysis of separate pedestrian zone and greenway literature, the commonalities and co-benefits of planning these interventions in an integrated manner are derived to form a clear breakdown of best practices. In doing so, the recommendations formed argue why pedestrianizing streets should be pursued as a genuine strategy and opportunity to create new greenways in higher density urban areas

Urban Greenways: A Systematic Review and Typology

Greenways are multifunctional linear landscapes that provide a range of socio-ecological benefits. As a domain of landscape planning research, greenways gained traction in the late 20th century and today, there is substantial interest in greenway planning and design. This is especially true in urban areas, as noted at the sixth Fábos Conference on Landscape and Greenway Planning. Yet, cities encompass biophysical flows, sociopolitical relationships, and formal structures that are distinct from non-urban areas and urban greenways may reflect an evolving type of landscape planning and design that is related to but distinct from greenways writ large. To the best of our knowledge, there has been no previous review of scholarship on greenways in an urban context. We address the aforementioned gaps by reporting on a systematic assessment of peer-reviewed literature. The review encompasses 52 refereed articles using the term “urban greenway” or “urban greenways” in the title, abstract, or keywords drawn from three prominent academic databases. Our analysis covers seven research categories, and this undergirds a typology and definition of urban greenways. In so doing, we seek to illuminate typical traits of urban greenways to inform future landscape planning scholarship and practice

Building Connections to the Minute Man National Historic Park: Greenway Planning and Cultural Landscape Design

The Minute Man National Historic Park (NHP) in Massachusetts commemorates the Battle of Lexington and Concord (1775) that began the American Revolution. The National Park created in 1959 seeks to interpret the battle and restore the agricultural landscapes of the revolutionary period. The Park is situated within the larger Freedom’s Way National Heritage Area (FWNHA) that was designated in 2009 to preserve the extensive historic cultural resources of the region, including such gems as Thoreau’s Walden Pond. Unfortunately, the Minute Man NHP is divided into four units and bisected by a busy state highway that makes wayfinding challenging for visitors. Moreover, the National Trust for Historic Preservation listed the Minute Man NHP “as one of America’s most endangered Places” (NTHP, 2003) due to the impacts of surrounding traffic, noise, and incompatible developments. There are several existing and proposed projects including the Battle Road Trail, Minute Man Bike Trail and Scenic Byway that have the potential to link the Park’s resources, but key connections are missing to create a coherent network. To address these challenges, this project, a partnership between the US National Park Service, FWNHA, and the University of Massachusetts, Amherst uses greenways as an organizing element to improve pedestrian and bike connections from the Park to the many nearby historic, cultural, and recreation resources, while providing visitors an alternative to touring the park by car. An undergraduate landscape architecture studio under the direction of the authors developed a greenway plan for the surrounding area with regional connections, as well as designed key sites and corridors along this greenway network that act as gateways and nodes for cultural and historical interpretation. This project exemplifies the challenges of historic and cultural planning within a developed suburban setting where local and regional recreation demands put pressure on historic landscapes. In addition, the fact that several key sites are outside the jurisdiction of the National Park in municipal, non-profit, or private ownership exemplifies the need for collaborative planning efforts. Finally, the project shows the management issues that continue after designation of historic corridors