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.