Urban Heat and Urban Landscapes

Local heat island phe­nom­e­na, not always direct­ly cor­re­lat­ed with glob­al warm­ing-relat­ed events, are com­mon end­points for a host of inter­lock­ing process­es. Impor­tant vari­ables for assess­ing these local process­es include the lev­el of urban foresta­tion, cit­ed by Emi­ly Nobel Maxwell as crit­i­cal to evap­o­tran­spi­ra­tion and cool­ing; the aggre­gate effects of site-spe­cif­ic micro­cli­mates, dis­cussed with case stud­ies by Pip­pa Bras­hear; and the reper­toire of diverse plant species that have adapt­ed to a city’s par­tic­u­lar con­di­tions, a spe­cial inter­est of Ed Toth. Cities need to keep all of these vari­ables, and more, in mind when assess­ing the con­tri­bu­tion of their land­scapes to heat island effects.

Bri­an Stone’s com­ments at the out­set of this pan­el includ­ed a key def­i­n­i­tion: “urban heat man­age­ment lessens the risk of heat-relat­ed ill­ness through land cov­er change and reduced waste heat emis­sions.” The sequence of terms may sub­tly indi­cate pri­or­i­ties: the pri­ma­ry con­cern, pro­tect­ing human health, requires holis­tic inter­ven­tion in land­scapes as well as imple­men­ta­tion of heat-reduc­ing mea­sures. Health depends on com­plex sys­tems and feed­back loops with­in entire envi­ron­ments, not just tem­per­a­ture-spe­cif­ic improve­ments. Rein­forc­ing the impor­tance of human health impacts, Regi­nald Blake cit­ed obser­va­tions by the New York City Pan­el on Cli­mate Change that heat and humid­i­ty will com­bine to ele­vate mor­tal­i­ty and mor­bid­i­ty over the com­ing decades. Though these bur­dens are sys­temic, they fall uneven­ly on dif­fer­ent pop­u­la­tion seg­ments. Con­sid­er­a­tions of envi­ron­men­tal jus­tice call for increased atten­tion and research, he said, ide­al­ly cul­mi­nat­ing in high-res­o­lu­tion map­ping of neigh­bor­hood-lev­el vul­ner­a­bil­i­ties, includ­ing air qual­i­ty and mold. Nobel Maxwell also point­ed out that while flood­ing caus­es greater dam­age to infra­struc­ture, “extreme heat is the num­ber one weath­er- and nat­ur­al dis­as­ter-relat­ed threat to human health in the coun­try.”

The "Urban Heat and Urban Landscapes" panelists, with moderator James Russell.

The “Urban Heat and Urban Land­scapes” pan­elists, with mod­er­a­tor James Rus­sell.

After hear­ing sev­er­al exam­ples of man­u­fac­tured replace­ments of nat­ur­al sys­tems lead­ing to unsus­tain­able urban heat island effects, mod­er­a­tor James Rus­sell provoca­tive­ly con­jec­tured that per­haps “the sub­urbs have got it fig­ured out: less of a heat island effect, lots more green cov­er.” Rus­sell fol­lowed up by ask­ing, “Should we actu­al­ly not be think­ing so hard about den­si­fi­ca­tion, and think of de-den­si­fi­ca­tion?” Stone emphat­i­cal­ly reject­ed this infer­ence on the grounds that “most envi­ron­men­tal prob­lems tend to be min­i­mized with increased den­si­ty,” includ­ing stormwa­ter gen­er­a­tion, ener­gy con­sump­tion, rel­a­tive emis­sion lev­els, and the loss­es of species habi­tat and migra­to­ry cor­ri­dors asso­ci­at­ed with sprawl. “The heat island,” he con­tin­ued, “is the one excep­tion.” In oth­er words, the rel­a­tive ther­mal com­fort asso­ci­at­ed with low-den­si­ty devel­op­ment comes at the cost of too many exter­nal­i­ties in oth­er areas.

Most envi­ron­men­tal prob­lems tend to be min­i­mized with increased den­si­ty. The heat island is the one excep­tion, but the rel­a­tive ther­mal com­fort asso­ci­at­ed with low-den­si­ty devel­op­ment comes at too high a cost in oth­er areas.

While the pan­elists agreed that den­si­ty is gen­er­al­ly a pos­i­tive step toward achiev­ing a sym­bi­ot­ic rela­tion­ship with nature, they also acknowl­edged that we are far from achiev­ing that sym­bio­sis. View­ing cities as com­po­nents of wider nat­ur­al sys­tems rather than excep­tions to them, Toth advo­cat­ed a man­age­ment approach root­ed in the recog­ni­tion “that nature is here, that func­tion­ing ecosys­tems are here, that bio­log­i­cal process­es are here, and that they don’t just stop exist­ing because it’s high­ly urban­ized.”

Cities are com­po­nents of wider nat­ur­al sys­tems, rather than excep­tions to them. The ecosys­tems that are here don’t stop exist­ing because the region is now high­ly urban­ized.

In keep­ing with the sen­ti­ment that even the most focused objec­tives and out­comes exist with­in a larg­er con­text, Bras­hear not­ed that dis­crete dis­as­ters pro­vide oppor­tu­ni­ties to study long-range aspects of change and resilience. For exam­ple, the rela­tion between acute events and chron­ic process­es was par­tic­u­lar­ly salient in the August 2003 north­east­ern pow­er black­out, which was trig­gered, as Stone not­ed, by a heat wave.

Click here to read addi­tion­al insights by Stone, Bras­hear, Toth, Nobel Maxwell, and Blake.