By Sheri A. Shiflett, Ph.D. (presently a Biological Sciences Environmental Manager at the U.S. Army Corps of Engineers)
One of the most satisfying events of a career in science is in getting the results of your hard work, sweat (and tears) published in a scientific publication – vetted by your (critical) peers. Will all the data collected lead to some interesting insights that others can use to make the world a better place? This is what I hope happens with all of my research, which was recently published in Science of the Total Environment. This is also important to share back with the funder of our research program – the NASA Earth Science program (as part of the HyspIRI preparatory mission) and Earthwatch. Funders including governmental agencies such as NASA are under increasing scrutiny to demonstrate the relevance of public funding for science and it is valuable for them to see that their funding results in making the planet a better place for all.
The research focused on characterizing how vegetation contributes to cooling of urban environments and cooling effectiveness of vegetation throughout the day and across a dramatic climate gradient spanning mild coastal to extreme desert conditions in Southern California (SoCal). This research was part of my Postdoctoral Research project from the University California-Riverside in the Jenerette Lab, where I worked from 2014 – 2015. I chose to study interactions between vegetation cover, land surface and air temperatures across SoCal because not only does the Greater Los Angeles Region include cities with vastly different climates (i.e., temperature and precipitation patterns), but the area includes over 18 million residents who are subjected to increasing frequency of high temperature periods.
Over the past 40 years, many studies have shown that urban environments are hotter than the surrounding countryside – a phenomenon termed the urban heat island effect. Cities have a high amount of built (i.e. buildings, pavement, etc.) cover relative to natural or green space, and the materials used to build cities are associated with an increased capacity to hold heat leading to warmer nighttime urban air temperatures. In urban areas, thermal characteristics of various surfaces show a high degree of variation from place to place and as a result, there may be large differences in land surface and air temperatures within a city that change throughout the day, and from season to season. These two parameters, land surface temperature and air temperature both have important implications for human well-being. High air temperature influences energy demands and can lead to excessive air conditioning use. In addition, surface temperature and air temperature are both related to human health and are linked to heat-related illnesses.
In many cities across the United States, increasing vegetation cover is being widely used as a tool to reduce both land surface temperature and air temperature. Vegetation has a particularly important role in cooling land surface temperature (LST) during daytime because it provides physical shading of surfaces and reduces the amount of heat absorbed by a surface compared to if it was fully exposed to full sunlight. The vegetation canopy itself also reflects back some of the sun’s rays to the atmosphere. Vegetation can also cool air temperatures via evapotranspiration (i.e., release of water vapor to the atmosphere from plants, soils, land surfaces, and water bodies). When liquid water (in plants and soils) is converted to water vapor, heat is absorbed from the atmosphere, causing a reduction in air temperature. Evapotranspiration can be also be thought of similarly to sweating. When a surface “sweats” and loses water to the atmosphere in the form of water vapor, both the evapotranspiring surface (e.g. leaf, skin) and the surrounding air are cooled in the process.
I wanted to find out the degree to which trees provided cooling benefits in Southern California cities. In our study, we placed temperature sensors in 100 trees in each of three cities: Los Angeles, Riverside, and Palm Springs in order to help us distinguish between nighttime and daytime changes in temperature associated with increasing vegetation cover. Each city represented a different climate. At one end of the spectrum, Los Angeles has a coastal climate, whereas at the other end, Palm Springs is characterized by an arid desert climate. In addition to placing temperature sensors across this large regional network, we also placed approximately 24 temperature sensors at the University of California Riverside’s Agricultural Station to measure vegetation cooling effectiveness at a smaller scale and across different land cover types including bare soil, grass cover, and tree cover of two different heights and canopy volume (short and tall trees which had canopies that reached 2 m/~6.5 ft and 4 m/~13 ft height). For our microscale work, we situated sensors immediately above the ground’s surface and also slightly above human standing height (2 m or ~6.5 ft). During our study period, NASA flew an aircraft above the SoCal region on June 13, 2014 and collected remotely sensed data of land surface temperature and additional aerial imagery of the area.
These images below show where temperature sensors were placed in three major cities throughout SoCal and a map of land surface temperature overlaying satellite imagery of SoCal.
This images below shows sensors being deployed in the field! We had a lot of fun driving around and finding the nearest tree to our pre-selected, randomized locations within each city.
The series of images below show the different types of plots at the University of California’s Agricultural Research Station.
In a final part of our study, we measured air temperatures during and after a substantial heatwave in the Greater Los Angeles area to determine whether or not vegetation was also effective at reducing air temperature during heatwaves. The heatwave that occurred from September 14 – 16, 2014 set several records throughout the region, including breaking a century old air temperature record for the date on September 15, 2014 and was associated with the highest electricity demand recorded in Los Angeles County! The energy demand on September 16, 2016 at 6235 MW was nearly double the amount on a typical day in Los Angeles. Heat alerts, advisories, and warnings were released by many local agencies and cooling stations were opened throughout the region to help provide relief.
This figure below shows air temperature patterns throughout the region in Los Angeles, Riverside, and Palm Springs. The heat wave, which occurred from September 14 – 16, 2014 (DOY 257-259) is notable on the figure and can be observed via increased orange/red tones across all three urban areas (Los Angeles (L) is on top, Riverside (R) is in the middle and Palm Springs (P) is along the bottom.
Our major finding was that increased vegetation cover does indeed reduce air and surface temperatures at various times of day and within each city. When we measured air and surface temperatures at regional scales (i.e., in the three cities), we found that the cooling effect by vegetation on surface temperature was most pronounced during the day and the cooling effects on air temperature were was most pronounced at night. When we measured air temperature at a very local scale (i.e., at the agricultural orchard), we found that vegetation not only cools air temperature at night but also during the day, especially immediately above the land surface. Two figures below show some of our primary findings:
This figure above shows the Time of Day from Midnight until 11 p.m. (or 0 – 23 hours) on the x-axis and the y-axis is the degree to which the amount of vegetation relates to the amount of green vegetation cover at that site to air temperature. Each dot on this figure represents a different day and the study was conducted over a 105 day period, meaning there are 105 dots for each hour of the day. As numbers decrease below zero, it indicates that as the amount of vegetation increases, air temperature is decreasing, and vice versa. The black dots are days/times where the relationship between vegetation cover and air temperature was statistically significant, while the gray dots are days/times where the relationship was not significant. The farther a dot is from zero, the greater the effect of vegetation cover on air temperature. In general, this figure shows that in Los Angeles and Riverside, increasing vegetation cover had a stronger effect on reduction in air temperature at night and no significant effect during the middle of the day, whereas in Palm Springs, increased vegetation cover reduced air temperature throughout the day – at least for many of the days sampled (there are a few grey dots for those days for which there was no effect):
This second figure above shows the Time of Day from Midnight until 11 p.m. (or 0 – 23 hours) on the x-axis and air temperature on the y-axis. The top graph shows how air temperatures near the ground surface (0.1 m) varies across a 24 hour period, and the bottom graph shows air temperature at standing human height (2m/~6.5 ft) also across a day. The black line represents air temperature with no trees above, while the gray line represents air temperature where the tall vegetation canopy is present. The evidence from these data illustrate how much cooler the air temperature is with a tree above at all times of the day and night. The cooling benefits of trees are greatest near the ground’s surface, during the middle of the day, providing as much as a 10oC (or 18oF) cooling benefit. Cooling at 6.5 feet can be up to 4oC (or 7oF).
Our data suggest initiatives to increase green cover in urban areas can mitigate urban warming and potentially reduce negative health and energy consumption consequences of high urban temperatures. Our work also showed that vegetation cooling effectiveness was increased during periods of extreme heat. That is to say that during heat waves, we found that vegetation contributed to more cooling than when each region was not experiencing a heatwave. Therefore, efforts to increase vegetation in high heat risk regions may have valuable impacts on heat vulnerability. In looking to retrofit current urban environments and build new urban infrastructure for the projected large increases in urban residents over the coming decades, using vegetation to reduce potential heat impacts from combined global and regional warming is a crucial sustainability opportunity.
This study answered some important questions – but brought up the next step that we are interested in. For example, what are the optimal arrangements of vegetation that provide maximal cooling benefits? What are the tradeoffs between water usage of trees and the cooling benefits they provide. Finally, there is a lot of variation among different tree species in terms of the size of leaves, the density of canopy coverage, the amount of water they require to grow and in turn transpire from their leaves! More research is needed to determine differences among species and to help city planners and residents decide which trees to plant in their cities and urbanized neighborhoods. I look forward to the work by others including Peter Ibsen and Julie Ripplinger from the Jenerette lab as well as the citizen scientists who are helping them to start to answer these questions.