Trees Defend Cities, Citizens From Extreme Weather

By Linh Anh Cat, Forbes

new study demonstrates that urban trees saves money and lives. Trees reduce deaths, injuries and electricity consumption for air-conditioning. Trees also promote wellness.

High air temperatures are a significant public health threat, killing an estimated 12,000 people annually worldwide in a typical year, with one heat wave in Europe in 2003 estimated to have killed more than 70,000 people. Higher air temperatures increase mortality and morbidity by causing heat stroke and exhaustion, as well as by exacerbating existing cardiovascular, pulmonary, and renal diseases. Higher air temperatures also increase the need for indoor cooling, with temperature spikes being associated with a significant rise in electricity use. Climate change is projected to increase average air temperatures, as well as the frequency and severity of heat waves, thus potentially leading to large increases in mortality and electricity demand. Cities are increasingly focused on strategies that can minimize the impact of high air temperature on both their residents’ health and the demands on electric utilities.

High air temperatures in the summer as well as heat waves threaten those with existing health conditions, especially those with cardiovascular, pulmonary, and renal conditions. Spikes in temperature cause electricity demand to rise in tandem, which puts increasing loads on the electrical grid during the day. One way to reduce heat-related injury and death, as well as smooth out electricity usage, is urban tree cover. Trees provide shade and cool down the air around them through evapotranspiration, a process trees use to move water through their branches.

Researchers from the Nature Conservancy, NASA, and Stanford University examined urban tree cover in 97 cities across the U.S. and applied their findings across the entire U.S. urban population.

Their study is one of the most comprehensive studies on the impact of heat in cities, especially since they quantified the connection to human health and electrical demand across a large dataset.

In the past, urban tree cover prevented more heat-related mortality, when air-conditioning was not common in U.S. households. However, as we rely more on air-conditioning, electricity costs have gone up. The researchers decided to capture costs saved by urban tree cover from reduced electricity consumption as well as mortality and morbidity from heat-related health problems.

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Within the 97 cities that were studied, each person saved about $21 to 49 each year from the presence of tree cover. When the researchers applied their results across urban populations in the U.S., the total amount saved in “heat-reduction services” by urban trees is estimated at $5.3 to 12.1 billion each year.

The temperature of buildings and pavement can be up to 10 to 20 degrees Celsius cooler on a summer day thanks to shade from trees. Changes in air temperature are more modest at 0.5 to 2.0 degrees Celsius, but this is still enough to significantly reduce impacts on people and the electrical grid. Cities with forests in their larger parks have seen air temperatures reduced by up to 5 degrees Celsius. This cooling effect is found downwind up to several hundred meters away (a few city blocks).

Unfortunately, the value of tree cover for reducing heat-related mortality seems to have declined significantly in recent decades.

It’s worth noting that urban tree cover likely benefits those with a higher socioeconomic status. Urban areas that are poor have less tree cover are suffering more from heat waves. Among other changes, increasing urban tree cover in these areas could save many more lives and prevent expensive hospital visits for those who cannot afford it.

Read the original article in Forbes about urban forests and public health.

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