Urban Heat and Urban Landscapes (Extended Coverage)
Brian Stone, Jr., PhD, Associate Professor and PhD Program Director, Georgia Tech University School of City & Regional Planning
Understanding cities as not just containers for lots of people exposed to global-scale climate change, but as containers that are amplifying climate change, is important. We want to be as specific as we can be in the terminology we’re using and the trends we’re measuring. I work with a group that has been measuring warming trends in cities for years, and this is important because cities are different from the planet as a whole. The difference between curves for urban and rural temperature observations is the amount of additional warming happening in the cities, not driven by greenhouse gases; it’s driven by land-use change and waste-heat emissions. The rate of warming in cities is more rapid than in rural areas, and in the aggregate we see cities warming at about twice the rate of rural areas and twice the rate of the planet as a whole.
The other four drivers of climate change in cities have been mentioned: loss of vegetation, impervious materials, waste heat, and urban morphology. If we’re only focused on greenhouse gases, we’re missing a big piece of the puzzle. So we’ll be hearing about strategies focused on vegetation enhancement; increasing reflectivity, albedo; morphology, channeling wind, moisture, and energy; and then this piece that we probably don’t focus on enough, particularly in a place like Manhattan, waste heat. We’re thinking about various strategies for the emerging field of urban heat management, but with an end point that’s really focused on public health, and this is the way we conceptualize it. It’s not just enough to think about cooling down cities; we need to think about where we’re cooling down cities and how we’re deploying these strategies.
Emily Nobel Maxwell, NYC Program Director for Urban Sustainability, The Nature Conservancy
At the New York City program at the Nature Conservancy we think a lot about the role of natural infrastructure in community and climate resilience. We’re asked to participate with the Mayor’s Office of Recovery and Resiliency to help co-lead an urban heat-island working group. The notion of scale is important: while every building or landscape does contribute, it is the whole that will drive the city’s actual temperature profile. We can disaggregate it by neighborhoods, which is critical because there are areas that are more vulnerable than others, but we need planning that relates between the individual site level and the whole.
We looked in the literature at the relative strength of our understanding of different kinds of cover types and the research correlating them to different aspects of urban heat (outdoor temperature, indoor temperature, air versus surface, energy use). A lot of research tells us that urban forests have an impact on outdoor air temperature, but where you don’t see dot lines that are strong, it doesn’t mean that there’s not a connection; it means that there’s not yet an evidence base. This is also a call to designers on how you can monitor your buildings to help increase our understanding of the relationship of the features you’re installing.
If there’s nothing else that you can leave with today, the correlation between more vegetative cover and lower temperatures is really critical. Our landscape is 50% to 94% hardscape, depending on where you are: in midtown Manhattan you’re looking at about 94% impervious surface; in suburban-feeling, residential Brooklyn, Queens, or Staten Island, you’re looking at about 50% cover. Recently, NASA released a study indicating that as soon as you hit the level of 35% impervious surface, you reach a tipping point where you start that warming effect that is different than regular climate change. That window between 35% impervious and 65% is a rapidly warming window that we actually can impact. We want to get our impervious surface closer to below 35%.
One of the most opportune areas for increasing our green surface is our rooftop real estate, and that can be in the form of green roofs or cool roofs. In New York City I’ve seen estimates from about 30,000 to about 40,000 acres of roof space; that’s to say that we have more roof space than the entire area of Manhattan. If we look to our roofs as areas of opportunity for retrofitting or building out new or better, we have a huge amount of opportunity. There is recent research that’s coming out, as we learn more about the economics of green roofs versus cool roofs, suggesting that green roofs perform extremely well, even financially, and that it’s worth considering them as an alternative to cool roofs, even though they both have cooling effects.
Pippa Brashear, Director of Planning and Resilience, SCAPE Landscape Architecture
What are the scale and context in which landscape architects usually have the opportunity to intervene in this issue and where we most often work? The reality of the urban context is that microclimate matters in addition to climate; you have regional climate conditions, physical conditions, and ecological conditions, but it’s different at the urban scale. The other reality of design at the urban scale, and particularly the urban public realm, is that it is usually a retrofit. You’re not designing new cities.
A small space outside a public library in Queens has a lot of access issues, but there’s something about comfort, being in a green environment. Addressing issues of access and and mobility, creating spaces and systems even at this very small scale (a thin sliver of space between the building and the sidewalk), we can create lighter-colored surfaces, pervious surfaces, and a variety of vegetation that cools. The water collection funnels water from underneath all that permeable paving into a tank, because this is entirely on structure. A lot of the landscapes that we have to build, especially in these small, constrained places, are very much like buildings, needing tight collaboration between architects and engineers to create thin sections and introduce natural landscapes for cooling environments.
Using what we have, the mature trees and canopy, is a way to be efficient with our resources. (As an arborist friend tells me, “It takes 100 years to grow a 100-year-old tree.”) Landscape architects are often asked to work at a corridor scale on streets and streetscapes; there’s a layered section of vegetation and planting, and we need to think about the health of the whole environment to support those ecosystems. We’re often asked to plant a tree, but you have to create a space below it, looking at soil volume, healthy soils, and other healthy plants. Highly compacted turf grass isn’t doing much for us; in many places, it’s not much better than paving for both water and cooling.
Our Living Breakwaters project is large-scale infrastructure more related to coastal resilience than heat island; it’s designed to attenuate waves, build beaches, and reverse erosion off the coast of Staten Island. It’s an integrated project not only to reduce risk but to build social resiliency and create a robust, functioning ecology with juvenile fish and historic oyster beds. Reintroducing and engaging those natural systems helps us create a sustainable landscape that can keep functioning through time.
Ed Toth, Director, Greenbelt Native Plant Center Seed Collection & Banking Program, NYC Department of Parks and Recreation
I run a center in which we propagate native plants on a large scale, working from collections of seeds from native populations. I work mostly in natural systems, although our materials make their way into situations from green roofs to green infrastructure to whatnot, and in doing that work over the last 30 years we’ve come to know the flora of the city intimately. Even today, in an urbanized center such as New York, we can recognize 28 distinct habitat types in the city – remnant habitat types, some quite degraded, but nonetheless there. Somewhere in the neighborhood of one third of the flora that existed prior to settlement of New York City are still here: some 750 to 1,000 species. I would propose to you that those are 750 to 1,000 tools to do all the work you are talking about, drawn from 28 habitat types, 28 distinct situations in which these plants have evolved through evolutionary time.
All it took was Sandy to make apparent how much we needed the coastal systems. These are communities that meet many of the challenges that we’re facing, whether it’s salt inundation or erosion or coastal conditions; these are intact and functioning ecosystems. They need to be understood in their biological context, worked with, and managed so that their integrity remains. These plants exist not only in the ecosystems but in populations, and the population level is where adaptation occurs. All the things that have been going on with these plants for millennia have been happening within these populations. What is really critical in my world is to maintain the genetic variability that exists distinctly in populations.
There is a great deal of talk about bringing southern species up into the north and using them because of increased heat. However, we have to be careful in moving populations around, because there are significant chances and instances of maladaptation when we take those actions. There is a methodology and a science behind where we source our materials from, not just the species but the populations we sample. One of the critical tools that can help us is seed banking. This is something that we do at my facility as well, not only for the city but now on a regional basis throughout the Mid-Atlantic. In fact, we are part of a network to create a national seed bank for the United States. This is the infrastructure we need to make sure that in dealing with climate change we are sourcing material correctly and we have the materials we need from the proper locations.
The center I run for the city, with its ability to produce hundreds of thousands of plants and to bank seed, is probably the only such municipal facility in the United States, and unfortunately what we need is to have something like that in virtually every municipality. The effort is tremendously underfunded, but the attention is moving in this direction. Most encouragingly, in June 2015, 13 federal agencies drafted and released a national seed strategy for the next five years. This is something that needs to be fully adapted by all the federal agencies and then fully funded.
Reginald Blake, PhD, Assistant Professor, Biological Sciences, CUNY – New York City College of Technology
I think the consensus is in place that the climate has changed, is changing, and will continue to change. COP21 (the 21st Conference of Parties, the Paris Climate Conference) is around the corner, and hopefully we’ll get an agreement out of it. Globally last year was the warmest year on record; that’s since records have been kept from the 1880s. The last below-average global temperature for any month was February of ’85, and the large-scale processes we know are driving much of regional climate. Studies just prepared and presented by Jim Hansen show that indeed the climate is on a fast track for change.
This past summer, temperatures here in New York City averaged about 2° F above normal, and August 2015 was the third warmest August on record at Central Park. We had the longest streak of about 80° F 62 days in a row. We had three heat waves. Precipitation was below average. These are stats perhaps you didn’t know, but here’s what we’re seeing as we look at the city scale. The New York City Panel on Climate Change (NPCC) is a panel of natural and social scientists that Mayor Bloomberg convened in 2008 to help mayors decide on and apply climate risk information; we have just completed a new report (NPCC2). We make projections based on GCMs (global climate model simulations) and do a downscaling of 35 GCMs; we use the same setup that’s given in the IPCC (Intergovernmental Panel on Climate Change) Fifth Assessment Report.
If you look at the temperature projections for the 2050s, looking at the 25th percentile, most of the models are predicting a change of somewhere between 4° and 6° F; here’s a mean temperature change of 4.1° to 5.7°. Mean precipitation is projected to increase, the frequency of heat waves is expected to increase, and sea levels are expected to rise. The take-home is that our heat index (a way of coupling temperature with humidity, an index as to how the temperature really feels) is going to increase in New York City. In a public health study looking at 18 years of data, centered around the 1980s and projecting that through the 2080s, looking at heat-related deaths with two different climate-change/emission scenarios (high end and low end), the mortality rates from increased heat will be increasing throughout this coming century.
An adaptation pathway is the basic fundamental principle that we use in our studies. For NPCC3, reconvened June 30, 2015, we’re focusing on humidity along with extreme events. We’re looking at what’s going on in the neighborhood scale; we’ve never done this before. We’re looking at critical infrastructure, and we’re trying to build a beta-version operational indicators and monitoring system. It has not been done anywhere in the world. Finally, we want to do an enhanced mapping protocol. We’re trying to move the science forward and continue so that New York City remains the leading city as far as climate change is concerned globally.