Im Rahmen unserer Serie zum städtischen Wärmeinseleffekt schauen wir heute über den Teich nach Amerika. Bereits im Jahr 1996 erschien im Bulletin of the American Meteorological Society eine Abbildung von Goodridge, die die Erwärmung des 20. Jahrhunderts ab 1910 in verschiedenen Regionen Kaliforniens unterschieden nach Einwohnerzahl zeigt (Abbildung 1). Interessanterweise weisen die bevölkerungsreichsten Regionen die stärkste Erwärmung auf.
Abbildung 1: Erwärmung des 20. Jahrhunderts ab 1910 in verschiedenen Regionen Kaliforniens unterschieden nach Einwohnerzahl. Quelle: Goodridge 1996.
Das Petition Projekt zeigt eine ähnliche Graphik aus Kalifornien für die Zeit von 1940-1996 (Abbildung 2).
Abbildung 2: Erwärmungsraten für kalifornische Counties, aufgeschlüsselt nach Einwohnerzahl. Betrachteter Zeitraum: 1940-1996. Berücksichtigt wurden 107 Wetterstationen in 49 Counties. Quelle: Petition Projekt
In einem Artikel vom 3. August 2014 dokumentierte WUWT eindrucksvolle UHI-Effekte für die Innenstädte von Detroit, Atlanta und Phoenix. Kurz darauf, am 20. August 2014, erschien auf Climate Central eine systematische Übersicht über die Wärmeinseln US-amerikanischer Großstädte (leider sind alle Temperaturangaben in Fahrenheit):
In a new report, Climate Central analyzes how UHI and climate change have affected 60 of the biggest American cities since 1970. The study examines the difference between average summer temperatures in urban areas and nearby rural areas. Some cities had much higher temperature differences: 23 different cities experienced single days that were an astonishing 20°F warmer than the rural areas around them. […] The study ranks the 60 cities by the intensity of their heat island effect, shows how heat drives air pollution (ozone levels) in nearly every city analyzed, lists cities that have far more days over 90°F than adjacent rural areas, and shows how most cities are warming faster than the surrounding rural areas. Analysis of summer temperatures in 60 of the largest U.S. cities found that:
- 57 cities had measurable urban heat island effects over the past 10 years. Single-day urban temperatures in some metro areas were as much as 27°F higher than the surrounding rural areas, and on average across all 60 cities, the maximum single-day temperature difference was 17.5°F.
- Cities have many more searing hot days each year. Since 2004, 12 cities averaged at least 20 more days a year above 90°F than nearby rural areas. The 60 cities analyzed averaged at least 8 more days over 90°F each summer compared to adjacent rural areas.
- More heat can increase ozone air pollution. All 51 cities with adequate data showed a statistically significant correlation between higher daily summer temperatures and bad air quality (as measured by ground-level ozone concentrations). Temperatures are being forced higher by increasing urbanization and manmade global warming, which could undermine the hard-won improvements in air quality and public health made over the past few decades.
- In two thirds of the cities analyzed (41 of 60), urbanization and climate change appear to be combining to increase summer heat faster than climate change alone is raising regional temperatures. In three quarters (45 of 60) of cities examined, urbanized areas are warming faster than adjacent rural locations.
- The top 10 cities with the most intense summer urban heat islands (average daily urban-rural temperature differences) over the past 10 years are: Las Vegas (7.3°F), Albuquerque (5.9°F), Denver (4.9°F), Portland (4.8°F), Louisville (4.8°F), Washington, D.C. (4.7°F), Kansas City (4.6°F), Columbus (4.4°F), Minneapolis (4.3°F), Seattle (4.1°F)
- On average across all 60 cities, urban summer temperatures were 2.4°F hotter than rural temperatures.
Urban heat islands are even more intense at night. Over the past 10 years, average summer overnight temperatures were more than 4°F hotter in cities than surrounding rural areas.
Natürlich hat auch New York seine eigene Wärmeinsel. Rosenzweig et al. machten sich im Jahr 2009 Gedanken, wie man die Hitze in der Stadt bekämpfen könnte. Der NIPCC berichtete über die Arbeit:
Rosenzweig et al. (2009) compared „the possible effectiveness of heat island mitigation strategies to increase urban vegetation, such as planting trees or incorporating vegetation into rooftops, with strategies to increase the albedo of impervious surfaces.“ With respect to the magnitude of the problem they were seeking to address, they report that „surface air temperatures elevated by at least 1°C have been observed in New York City for more than a century (Rosenthal et al., 2003; Gaffin et al., 2008), and the heat island signal, measured as the difference between the urban core and the surrounding rural surface air temperature readings taken at National Weather Service stations, averages ~4°C on summer nights (Kirkpatrick and Shulman, 1987; Gedzelman et al., 2003; Gaffin et al., 2008),“ with the greatest temperature differences typically being sustained „between midnight and 0500 Eastern Standard Time (EST; Gaffin et al., 2008).“ And on a day that they studied quite intensively (14 August 2002), they report that at 0600 EST, „the city was several degrees warmer than the suburbs, and up to 8°C warmer than rural areas within 100 km of the city.“
With respect to mitigation strategies, the twelve researchers determined that „the most effective way to reduce urban air temperature is to maximize the amount of vegetation in the city with a combination of tree planting and green roofs.“ Based on modeling studies of these approaches, for example, they estimated that this strategy could reduce simulated citywide urban air temperature by 0.4°C on average, and 0.7°C at 1500 EST, while simulated reductions of up to 1.1°C at 1500 EST could be expected in some Manhattan and Brooklyn neighborhoods, „primarily because there is more available area in which to plant trees and install vegetated roofs.“
Das Rezept gegen die Hitze: Mehr Bäume pflanzen! Im April 2012 gab es dann die Pointe: Terra Daily berichtete über einen andere Studie, die fand, dass die Bäume gerade aufgrund der Wärmeinsel in der Innenstadt New Yorks besser wachsen als im ländlichen Umland.
City streets can be mean, but somewhere near Brooklyn, a tree grows far better than its country cousins, due to chronically elevated city heat levels, says a new study. The study, just published in the journal Tree Physiology, shows that common native red oak seedlings grow as much as eight times faster in New York’s Central Park than in more rural, cooler settings in the Hudson Valley and Catskill Mountains.
Nun wurden in den letzten Jahren im “Kampf gegen die Klimaerwärmung” unzählige Windkraftanlagen in die Landschaft gesetzt. Das Klima dankt, sollte man meinen. Nun fanden jedoch Forscher, dass das ganze Gegenteil der Fall ist. Die Windräder produzieren durch ihre Bewegung eine spürbare Wärmeinsel, die man als Wind Engine Heat Island (WEHI) bezeichnen könnte. Der Deutschlandfunk berichtete bereits am 30. April 2012 über das Phänomen: