Biomass09-1007-EditThe new IPCC report, the Fifth Assessment tells us once again that climate change is not a future threat but a present peril. We have added sufficient greenhouse gases to the atmosphere that we are committed to substantial warming even if we dramatically reduce our fossil fuel emissions within a few decades.

Most of us now live in cities. In 2014, 54% of us live in cities, and that will rise to 66% by 2050 (United Nations). Increasing urbanization is generally a good thing for the planet – city dwellers are more energy efficient, for example. However, cities tend to amplify the effects of global warming because of the urban heat island effect and greater air pollution than in rural areas. A healthy urban forest can mitigate the urban heat island and reduce the impacts of global warming.

You may not think of your city as an urban forest, but it probably is.  Most urban areas try to plant trees along streets, in parks and many property owners plant trees.  The total of all these trees is the urban forest, and it is a lot more than a collection of trees. Urban forests create many of the same environmental services as wild forests.  The urban forest decreases erosion, reduces stormwater runoff, improves water quality, and improves the physical and spiritual well-being of urban dwellers.

One important function of a healthy urban forest is to reduce urban heat.  Trees cool cities two ways: 1)  shade reduces the heat load on urban surfaces, especially on dark pavement; and 2) the evaporation of water from leaves has a cooling effect.  The cooling effect of trees extends beyond the shaded area, as this figure shows (scroll down to continue reading; click image for larger version):

Map of urban heat island.

Two parking lots on the same day, ambient temperature was 87F. Left- few trees, temperatures are 7-10 degrees above ambient. Right – moderate number of trees, temperatures are 2-4 degrees above ambient.  The effect shown is due to evaporation of water from the trees. If there was more shade over the impervious asphalt surface, the effect would be larger. On the same day, temperature under a nearby park canopy was 84.  Map and data by the author.

If a modest number of trees can mitigate the urban heat island effect, a dense canopy cover can do even more.  American Forests recommends a canopy cover target of 40% – meaning that for every acre of urban forest land, 0.4 acres are shaded by trees.  While some American cities are doing even better than that – Nashville, for example, has 47% canopy cover – the urban canopy cover of most cities is declining.

Street trees

Street trees require adequate space if they are to survive and contribute to a healthy urban forest.

In 2012, David Nowak and Eric Greenfield of the USDA Forest Service reported that urban forest canopy cover is declining across the US, with a loss rate of about 4 million trees per year (summary of the article; original article, pdf).  In spite of the huge benefits of a healthy urban forest, and substantial amounts of money invested in urban forestry efforts, cities are falling behind in maintaining an adequate canopy cover.

What is replacing those trees?  The short answer is impervious cover  – pavement, mostly.  Impervious cover has exactly the opposite effects of a tree canopy – the dark surface heats the city, and water immediately runs off into drainage basins and creeks without soaking into the ground.  We’ve all had the experience of stepping out of a car into a parking lot on a hot day – the parking lot is roasting hot, and we flee for the shelter of the air-conditioned mall or office building.

To counter this problem, many cities have made commitments to increasing their urban forest canopy.  New York launched an ambitious plan to plant 1 million trees to increase its canopy cover, but has fallen behind because so many of the newly-planted trees have died, and because their is no budget for maintenance of the new trees or older trees.

Tree canopy in Louisville

Healthy tree canopy in Louisville, Kentucky

Louisville, Kentucky, has the dubious honor of the fastest growing urban heat problem in the nation, according to research by Brian Stone and his Urban Climate Lab (see Stone’s book, or visit the Climate Lab web site).  Between 1961 and 2000, the temperature difference between urban Louisville and surrounding rural areas increased by 1.67F per decade.  Louisville is nestled in a valley next to the Ohio River, and therefore suffers from the low elevation lack of wind, and the mugginess of the nearby river. Little can be done about that.  Louisville also has the lowest urban tree canopy cover of any city in Stone’s study. Although the city as a whole had 30% cover, downtown was less than 10%.   Louisville’s canopy cover is declining steeply. The preliminary results of a recent canopy cover study showed a 9% decline in canopy cover in one area of town in only 12 years.  The combination of weather, including ice and wind storms, and the lack of any real urban forest policy caused the loss, but it is now getting much worse because of the depredations of the emerald ash borer.

A properly planted and maintained tree reduces the impacts of climate change on cities

A properly planted and maintained tree reduces the impacts of climate change on cities

Planting more trees is not necessarily the solution to making cities more resilient in the face of climate change.  It is the fate of those trees that is important.  There is little point in planting more urban trees if they die after a few years.  The cost of replacing trees every five or ten years is far greater than the benefit of those trees.  On the other hand, the benefits of a tree that lives 100 years or more are many times greater than the cost of their planting and maintenance.

There is a huge difference between an urban forestry program and planting a bunch of trees. Urban forestry is a management approach to maintaining a healthy, diverse forest canopy in our cities.  Tree planting programs tend to be short-sighted and lack a comprehensive plan.

The next articles in this series will examine best practices in urban forestry, worst practices in urban forestry, and the challenge of tropical urban forestry.

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5 Responses

  1. planetexperts says:

    Dan- What this argument fails to account for is the different regions of warming on the Earth. While it is certainly true that atmospheric temperatures have increased more slowly since 2001, ocean surface temperatures have been rising much more quickly. Because the ocean absorbs about 90% of the heat associated with greenhouse gases, that is where the majority of the excess heat is going. Critics of climate change often point to slower atmospheric warming plus the growth of sea ice in the eastern Antarctic as proof that it doesn't exist, but as NASA points out ( http://www.planetexperts.com/amount-antarctic-sea… ), while Antarctic ice is growing in that region, it does not make up for the difference in net ice melt from the Arctic and Greenland. Ocean warming, coupled with its rapid acidification ( http://www.planetexperts.com/oceans-acidifying-fa… ), is making it extremely difficult for shellfish to thrive and has begun to decimate fisheries on both east and west coasts. Most troubling, NASA reports that the ocean's ability to absorb heat is slowing down, meaning that at some point the ocean will have reached its peak absorption rate. At that point we may see some very intense atmospheric warming.

  2. Dan Pangburn says:

    Trying to account for different regions is a far more complex problem, and, at least until they fix the GCMs, impossible to do. This statement is in the first paragraph of the paper: "It [the paper] does not even address local climate . . .". The paper addresses GLOBAL climate change [only]. Climate change includes both uptrends and down trends of average global temperature.

    The uptrend in the last quarter of the 20th century, which got called Global warming, happened because a surface temperature uptrend in the net of ocean cycles got combined with a fairly steady global uptrend which went on 1700-2005.

    The idea that increasing the amount of CO2 in the atmosphere from 3 parts per 10,000 to 4 parts per 10,000 would have a significant effect on how the ocean absorbs heat is not credible.

    The analysis at http://agwunveiled.blogspot.com results in R^2 = 0.9049 when considering only the two drivers, 1) the time-integral of sunspot number anomalies and 2) an approximation of net ocean cycles. That means that all factors not explicitly considered (such as the 0.09 K s.d. random uncertainty in reported annual measured temperature anomalies, aerosols, CO2, other non-condensing ghg, volcanoes, ice change, etc.) must find room in that unexplained 9.51%. Note that a coefficient of determination, R2 = 0.9049 means a correlation coefficient of 0.95.

    The ocean acidification thing is bogus. The oceans warmed, on average, 1700-2005. Warmer water has lower soluability for CO2. Besides, PH is about 8, which is basic.

    NASA has gone completely off the rails. They have even changed historical measurements to make it look more like it is still warming. Statements like "the ocean's ability to absorb heat is slowing down" makes it appear that no one with a voice there knows anything about thermodynamics/heat transfer.

  3. Dan Pangburn says:

    You don't get it. It's not what I say, it's what the analysis says.

    Apparently there is far more in the paper at http://agwunveiled.blogspot.com than you have yet comprehended.

    NASA's narratives e.g."The current warming trend . . " contridict even their own reported temperature anomalies. Average global temperature anomalies are reported by 5 agencies. I graph them all (which includes NASA GISS) and their average at http://endofgw.blogspot.com. The trend is flat since before 2001. Others say that there has been no statistically significant increase in 18 years.

    Oceans cover 71% of the planet. Do you actually expect it to be true that PH is the same everywhere and that they checked it to 3 significant figures everywhere 250 years ago? Are yu aware that any PH above 7 is basic?

  4. Dan Pangburn says:

    The solid sources (well, maybe semi-solid since they are mostly government agencies) are provided in the reference list on my blog(s). I merely 'connect the dots' and do a little arithmetic. All of the data used to discover that CO2 change has no significant effect on climate is government agency sourced.

    The equation allows prediction of temperature trends using data up to any date. The predicted temperature anomaly trend in 2013 calculated using data to 1990 and actual sunspot numbers through 2013 is within 0.012 K of the trend calculated using data through 2013. The predictions depend on sunspot predictions which are not available past 2020. The predictions to 2037 for two assumptions of SS numbers are graphed in Figure 1 of the "AGWunveiled" paper.

    There is a lot of random uncertainty in the measured data (S.D. approx 0.09 K wrt the calculated up-and-down trend) which make the limit range fairly wide. Current measured data are about 1.5 sigma above the trend determined using data since before 1900. All previous data are within 2.5 sigma.

  5. Tom Kimmerer says:

    Dan, your comments are entirely inconsistent with the available science. I encourage you to look at the current IPCC 5th assessment. The evidence for global warming caused by industrial greenhouse gas emissions is overwhelming.

    There is no pause in global temperatures, but there certainly has been a pause in increases in eastern US temperatures. However, that does not argue against the thesis of this series of articles. We need to reduce urban air pollution, reduce the impact of the urban heat island effect and prepare for a warmer world. All of the steps I propose in the three articles that make up this series will increase the quality of urban life.

    You may choose not to believe that climate change is real, but that is not a rational reason to argue against improving the quality of our lives.

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