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Hot and Getting Hotter: Heat Islands Cooking U.S. Cities

Research Report by Climate Central

Cities are almost always hotter than the surrounding rural area but global warming takes that heat and makes it worse. In the future, this combination of urbanization and climate change could raise urban temperatures to levels that threaten human health, strain energy resources, and compromise economic productivity.

Summers in the U.S. have been warming since 1970. But on average across the country cities are even hotter, and have been getting hotter faster than adjacent rural areas. (report continues below interactive)

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RELATED CONTENT
Full Report
PDF


Appendix B:
Satellite Images

PDF

Appendix C:
Urban Ozone Charts

PDF

Press Release
PDF

 

With more than 80 percent of Americans living in cities, these urban heat islands — combined with rising temperatures caused by increasing heat-trapping greenhouse gas emissions — can have serious health effects for hundreds of millions of people during the hottest months of the year.  Heat is the No.1 weather-related killer in the U.S., and the hottest days, particularly days over 90°F, are associated with dangerous ozone pollution levels that can trigger asthma attacks, heart attacks, and other serious health impacts.

Our 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.

Urban heat measured by satellite in Louisville, Ky. Click image to enlarge. 

Several independent studies have shown that urban heat islands (in the U.S., and around the world) do not bias global warming measurements, ruling out the possibility that rising global temperatures have been caused by urbanization alone.

Research suggests that urban planning and design that incorporates more trees and parks, white roofs, and alternative materials for urban infrastructure can help reduce the effects of urban heat islands.

But rising greenhouse gas emissions are projected to drive average U.S summer temperatures even higher in the coming decades, exacerbating urban heat islands and their associated health risks. 

Research report written by Alyson Kenward, Senior Scientist and Research Director for Climate Central; Dan Yawitz, Research Analyst and Multimedia Fellow; Todd Sanford, Climate Scientist; and Regina Wang, Research Fellow.

Comments

By Fernandez (Calgary)
on August 20th, 2014

So, questions:

a) If many of the weather stations are within the developed cities, thus registering this exacerbated UHI effect on the temperature. How can we confidently say that the globe is warming and not that the anthropogenic warming effect is merely localized?

b) And, how can global warming, although it seems to have been stagnant for some time now in spite of the UHI effect on temperature measurements, can take it away and increase it?

I am struggling to comprehend this, but i am no scientist. Please help

Reply to this comment

By John Ward (Gainesville/FL/32605)
on August 24th, 2014

Fernandez, the first place I check for controverial claims concerning global warming is Sketical Science. Currently they have responses to 176 claims by climate change deniers in which the science is discussed. Here’s the one on the urban heat island effect: http://www.skepticalscience.com/urban-heat-island-effect-intermediate.htm

Here is a link to another trusted source. I don’t visit it as much, because it is aimed at other climate scientists, so it can get technical and deal with issues that have less general interest. And it doesn’t post new material as often as the other blogs I chack daily. But when any controversy is in the air, I look to see what it says. Anyway, here is a report on a research paper by its author on the subject in question that gives you an inside look at how the study was done. I think it’s easy enough to follow and should give you an even better understanding of the urban heat island effect in relation to global warming than the discussion on Skeptical Science. And the commentary below, mostly by climate scientists, is better informed.

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By Dan (Imperial, Ca.)
on September 11th, 2014

If you want to talk about hottest cities in the United States It start and ends with the deserts of southern California. Back in my college days I was a county life guard and I can’t remember the exact month, but I had read an outside sign that had the highest temperature I had ever saw it was 133 degrees. The next day it was a little cooler 131 degrees. So I would put a little more testing of The Imperial county. I grew up in Ohio and it was rough at first. When I moved to the desert of California I didn’t know it got that hot. Good thing though, or I would of never moved out here. But to show that I must a little crazy because I am still here. Its been about 33 years now that I lived here, we hate to see summer coming but we have some of the best weather in the winter. Its does not snow and for the most part the temp. is in the 70’s. We do have people who come and spend the winter months from all over world. I live about 100 miles away from San Diego so that helps, but I live only less then 20 miles from the Mexico boarder which can be cool to check it out especial if you are with people who know there way around Mexico and know the language. Well if you don’t add a city from the Imperial county the top 10 list will not ever be true. Thanks Dan

Reply to this comment

By Kelly (Austin, TX)
on September 12th, 2014

This article is not about the hottest cities in the United States, it is about urban heat island effect. Imperial County is undoubtably very hot, but the difference between urban and rural temperatures is not as extreme as the highlighted cities. Read carefully before you comment.

Reply to this comment

By Tula (Louisville)
on August 21st, 2014

Fernandez, I just noticed this in the full report. It seems to answer your question:
“Several independent studies have shown that urban heat islands (in the U.S., and around the
world) do not bias global warming measurements, ruling out the possibility that rising global
temperatures have been caused by urbanization alone.”

Reply to this comment

By Gaabi (Portland, OR 97219)
on August 21st, 2014

I live in Portland, Oregon, which I assumed is the Portland that’s No. 4 on the list.  My city has a “green” reputation that used to be more true than it is today.  For years we’ve had smog, something I never say in my life before the early 90’s.  We still have many, many more trees than any other city I’ve been in and we’re surrounded by forested suburbs and actual wilderness and we’re still No. 4 on this list.

I’ve noticed for a long time that there was a pronounced drop in temperature between the east side of the Willamette river, where there are many houses and apartments packed together, and the west side south of downtown, where there are more trees, more hills and less developed sidewalks, less structured city blocks.  Within just a mile south of downtown during the summer, the temperature will plummet as soon as you get under the trees. 

I’ve also noticed that our conifers are being replaced by deciduous trees, mainly maples, and I wonder what difference that makes.

Thank you very much for this article, my only request would be with reporting problems, couldn’t you also investigate and report on suggested solutions, if there are any? It would be good to hear something of what we could try to do to correct problems or try to make things better, rather than just the bad news.

Reply to this comment

By eugene (Bemidji, MN 56601)
on August 21st, 2014

I think if a person wants to understand climate change, that person will have to put in some personal time studying.  For example, climate is a system with many parts interacting and impacting the global climate.  If a person is relying on what others say, that person can become very confused.  Recently I emailed an old acquaintance and his response was he is just tired of the arguing which led me to assume he simply watches the news and expects to be informed.  If a person reads actual research, there isn’t much of a debate.  Out of approximately 15,000 climatologists only 450 don’t agree that climate change is due to human activity. 

I am not a scientist but, on a daily basis, read research and over time, I have built up a decent knowledge base.  Unfortunately, not many are willing to do so.  Survival and Monday night football are much more exciting.  My neighbor reads nothing claiming he is a slow reader.  I’ve mentioned to him there are plenty of documentaries, on several sites, that he could view.  He rattles on about being a slow reader. My read is this, by far the most incredible event of the past several million yrs is happening and will destroy his carefully laid plans of retirement.  The real answer is he simply doesn’t want to put forth the effort.

The help Fernandez needs is some web sites but he will have to make the effort to follow up.

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By Mark (Chalfont St Giles)
on September 14th, 2014

As somebody who understands statistics and takes the time to look into the facts behind man-made global warming I can tell you that the argument is very far from settled.  As for climate scientists their views should be seen as turkeys offering an opinion on Christmas—the fact that so many support man-made global warming is very revealing.  In other sciences, such as physics, it’s seen as healthy for scientists to oppose theories because this is the way that truth emerges.  In climate science anybody who opposes the mantra is seen as a pariah.

History will look back on this period in disbelief at how easily people—especially politicians—have been taken in by scaremongerers.  The climate will change: fact.  How much impact does man have on this?  Very little.  It would be far better for money to be spent on helping people to address natural forces about which we can do nothing than to pretend reducing carbon dioxide will make any difference.

There are plenty of things for parents to worry about for their children’s future.  Man-made global warming should be very low on that list—I speak as a parent and it’s not even on mine.

Reply to this comment

By Steven Blisdell
on September 19th, 2014

“...the argument is very far from settled. .” No. Wrong. The physics, chemistry, geophysics, hydrology, glaciology, atmospheric sciences, flow dynamics, and yes, climate science are settled. Humans are causing extremely rapid, accelerating, and potentially catastrophic global warming,

“...the fact that so many [scientists] support man-made global warming is very revealing.” Yes! Revealing of two hundred years of theory and decades of research, with thousands of scientists publishing increasingly accurate investigations and assessments of Earth’s rapidly changing climate. Excellent observation.

“...In climate science anybody who opposes the mantra is seen as a pariah.” No. Wrong. Climate science proceeds like any other field - peer reviewed research stands as the best available information until more accurate or novel research adds to and/or supplants earlier research. Wrong again.

“History will look back on this period in disbelief at how easily people—especially politicians—have been taken in by scaremongerers.” Wow! Wrong again! You’re batting .000, a perfect average! Congratulations! On second thought, you’re right. Seeing as how 11 of the hottest 12 years on record (counting an almost certain 2014) have occurred this century, within ten years “history will look back on this period in disbelief” at folks such as yourself, and all those who have lied to, misinformed, and criminally misled the American people, and people will wonder at how easily they were taken in by the lies, manipulations, and criminal disregard for the Earth and its inhabitants shown by the forces of greed, ignorance, and malevolence. 

“There are plenty of things for parents to worry about for their children’s future.  Man-made global warming should be very low on that list—I speak as a parent and it’s not even on mine.”
I’m very sorry to hear you care so little for your children’s future. No worries - there are enough thoughtful, competent adults in the world to make up for it.

Reply to this comment

By Rudy Haugeneder (Victoria, BC, Canada)
on August 22nd, 2014

Touch a light bulb and it is hot. At night big and small cities are light bulbs—light pollution—and that doesn’t include the heat buildings naturally and release when the sun disappears. Time to shrink the human population dramatically in order to reduce future urban growth that, as well as resulting in many existing buildings being demolished and trees and other vegetation grown on the empty lots, will turn night into night again while also allowing rivers to run free again. And no, science isn’t going to fix the heat/nightlight problem. Any solution mankind comes up with other than shrinking our population, will, like always, just result in currently unimaginable worse problems. It always has and always will.

Reply to this comment

By Bob (charlotte nc 28210)
on September 18th, 2014

You go first.

Reply to this comment

By Lindsay Harmon
on August 22nd, 2014

Fernandez,

Several studies of large global datasets have examined how much urban heat could be contributed to total global warming. While there a lot of urban temperature stations, more than 70% of the stations in the Global Historical Climatology Network are in towns or locations with fewer than 50,000 people (not very urban). And when you look at these rural stations, which represent good spatial coverage of global land surface, temperatures are still rising, which isn’t due to the urban heat island effect. A good (albeit technical) explanation about this can be found here: http://www.realclimate.org/index.php/archives/2013/02/urban-heat-islands-and-u-s-temperature-trends/?wpmp_tp=1

While our analysis can’t identify how much of the warming in each city is due to urbanization and how much is from overall rising temperatures, it does show that cities are hotter than their nearby rural areas. If global temperatures continue to rise as projected (if greenhouse gas emissions continue unabated) then most areas are going to get hotter, and cities in particular will feel the combination of the climatic warming and added heat from urbanization.

Reply to this comment

By Maddy Diedrichsen (Malcolm, Nebraska)
on August 24th, 2014

I’m junior in Malcolm, Ne and I’m doing an informative speech on microclimatology. I was wondering if you (climate central) had a satellite image of urban heat in Lincoln, NE? I would like to use the picture as a visual aide to show people about the different microclimates in Lincoln like Memorial Stadium and pioneers park. Thanks!

Reply to this comment

By Ted Rice (Eagle Bridge, NY 12057)
on August 24th, 2014

We do little in the US to reduce city heating. White or reflective roofs and buildings, such as are used in the Middle East, would help, as would light colored pavement as opposed to black.Spacing buildings farther apart and filling in with trees and grass would also help. In some places, buildings could be underground, reducing heating and cooling costs, while the surface is planted over.

Reply to this comment

By Dale Hall (CEDAR CREST, NM 87008)
on August 24th, 2014

The elevation change from the center of Albuquerque at 5000 feet to the subburb area is about 2000 feet.  This would amount to 8 degrees temperature difference.  Hopefully this was accounted for, if not so much for the number 2 spot in the “heat island” list.

Reply to this comment

By Craig Hildreth
on August 25th, 2014

This was my first thought as a resident here as well.  Our mountains have a huge effect compared to a place like Kansas City.

Reply to this comment

By Lindsay Harmon
on August 25th, 2014

Dear Maddy from Malcolm,

Sadly, we do not have a satellite image for Lincoln, NE on hand. The nearest major cities we looked at were Kansas City, MO and Des Moines, IA. 

If you’d like to see them, they can be found on page 14 & 11, respectively, here: http://assets.climatecentral.org/pdfs/UHI-AppendixB.pdf

Good luck on your speech and let us know if there is anything else we can do to help!

Reply to this comment

By Craig Hildreth
on August 25th, 2014

Most of the surrounding rural areas are at higher elevation than ABQ, NM, in our case.  Also, the rural area to the south are lower and colder in winter at night due to drainage.  I’m curious how geography is controlled here.

Reply to this comment

By Lydia Bezou-Hojnacki (New Orleans, LA 70115)
on August 25th, 2014

Black pavement (macadam) absorbs and retains heat; so do the composite, flat roofs which are finished with similar materials as street surfaces are.  And then, there are the intensely hot sodium lights that line our streets.  Tall buildings trap the heat at street level. 

We do this to ourselves….  and the air-conditioning exhaust fans add more heat to the yard or to the rooftop…...urban areas are blanketed and trapped by both man-made heat and solar heat.

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By Wayne Johnson (30309)
on August 26th, 2014

I may have hit a wrong button somewhere but for nearly all the cities the report says for me that the city is cooler than the surrounding rural areas. Is this correct? Also for Atlanta, first time through it said hotter than rural area, now it says average 2.0 degrees *cooler.* Maybe something’s messed up.

Reply to this comment

By Wayne Johnson (30309)
on August 26th, 2014

After I click on San Diego, the word hotter changes to cooler and stays cooler when I click on other cities that previously said hotter. Problem in software.

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By Don Kovac (HEMET, Ca., 92544)
on September 18th, 2014

Has anybody examined scientifically the results to expect here in southern Cal. because of what many people here are doing because of the drought?  Example: reducing or eliminating lawns, cutting down and or not planting trees and cutting down or not watering existing lawns- as required by the local politicians?  Cities are also permitting severe “cutting back” of existing trees -in many cases killing the trees.  All of these can only only add to the climate change phenomenon.

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