Climate change presents many threats to cities, compromising their ability to protect public health, or deliver critical services like sewage disposal or adequate protection against storm surges and flooding. Many cities are acutely aware of these threats, and are developing climate adaptation plans with strategies to mitigate, and adapt to, climate change impacts.
Cleveland, local climate change, and Urban Heat Islands
The city of Cleveland, OH has developed one such climate action plan—Sustainable Cleveland—as a blueprint to reduce the city’s greenhouse gases footprint and increase its citizens’ quality of life by fighting against the impacts of climate threats. One of the ways in which the city envisions doing that is by increasing vegetation cover in order to, among other things, reduce the city’s Urban Heat Island (UHI).
UHIs are microclimate patterns characteristic of cities, where the abundance of impervious surfaces (i.e., heat-trapping materials like concrete, pavement, asphalt, or glass) makes the thermal signature of cities look like a “heat island” surrounded by a “cooler sea” made up of natural land covers like forests, grasslands, agricultural fields, or mountains. UHIs are a local consequence of anthropogenic climate change (as opposed to global climate change brought on by carbon pollution).
Heat island mitigation can provide many benefits to cities. For example, UHI mitigation can result in lower summertime temperatures, contributing to reducing heat-related deaths and illnesses. Lower temperatures can also reduce summertime demand for air conditioning, which can reduce greenhouse gas emissions from energy production. Decreased production of lower-atmosphere ozone is also a known benefit of lower urban temperatures, and can help avoid asthma attacks triggered by ozone.
But the strength of the UHI, like many other urban features, is not distributed evenly across the city and does not affect everyone equally. People living in poverty, the elderly, people with pre-existing conditions like cardiovascular disease or diabetes, or without AC (or the means to pay for its use), are among the populations most vulnerable to becoming ill or perishing during hot weather and heat waves. Too often, poverty is also a marker for belonging to a racial or ethnic minority, providing a racial dimension to extreme heat vulnerability. UHIs are stronger in neighborhoods with little tree canopy cover because there is no shade to block incoming solar rays, and these neighborhoods are usually the poorest in a city.
After reading Cleveland’s climate action plan, I got interested in developing UHI research that could help get the best bang for the buck in planting temperature-lowering vegetation. So my colleagues and I (thanks to the support of the Natural Resources Defense Council’s Science Center) combined temperature, census data, and Cleveland’s recent urban tree canopy assessment to identify neighborhoods to target for increasing tree canopy cover. We focused on vegetation because trees are well known to regulate temperatures and have been listed by the EPA as one of four mitigation strategies that cities can deploy to lower UHI-related heat exposure.
Equitable solutions, not just technical ones
In our analysis, we gave equal consideration to both the social and biophysical environment dimensions of vegetation-based temperature reductions, looking for solutions that address temperature mitigation potentials in an equitable way—that is, in a manner that considers the most vulnerable populations together with the vegetation potential for UHI mitigation.
Why is this important? Well, equity-blind criteria for optimizing the locations of potential tree plantings may just prioritize the areas with the highest potentials. The areas with highest potential are typically those that already have trees planted in them, as opposed to lots of heat-absorbing, hard, and hard-to-replace surfaces like parking lots or pavement. The higher potential is often found in neighborhoods that already have lots of vegetation and typically house populations with higher incomes and general socioeconomic status that may not be as vulnerable to extreme heat as lower-income populations in ecologically deprived neighborhoods.
With these social and biophysical scientific criteria in mind, we explored how it all works out in Cleveland by looking at the relationship between summertime temperatures and populations that consistently are shown to be at higher risks for heat-related illness or death. Cleveland has a long history of racial segregation, and is today one of the most segregated cities in the United States. The city is split racially by the Cuyahoga River, with the bulk of African-Americans in the east, whites in the west, and the Hispanic population in clustered enclaves in the west side of town.
Who’s at highest heat risks in Cleveland?
Who is most exposed to high temperatures in Cleveland? In our research, we found that neighborhoods with high rates of Hispanics and without a high school diploma or GED equivalent were also the hottest areas of the city. But surprisingly, other high-risk populations like African Americans, the elderly, those who live alone, and those without air conditioning (AC) at home were not found to live in the hottest neighborhoods.
We also found—similar to many other UHI studies—that under fully-vegetated conditions, the potential for mitigating high temperatures was very large. But rarely do we find areas in neighborhoods that are fully vegetated (or fully bare). After all, neighborhoods have homes and multi-family dwellings, alleys, dirt lots, sidewalks, pavement, gardens, and backyards, and interspersed among those we also find grass and trees.
This is where the science and policy applicability of our work comes in. We tempered (pun intended) our temperature-reduction estimates to more realistic ones based on the actual potential for tree planting in Cleveland. Fortunately, Cuyahoga County just updated its urban tree canopy assessment, and created neighborhood-scale maps that show areas with grass or shrubs available for the establishment of tree canopy. We integrated these maps into our analysis.
When we looked at temperature-reduction potentials and demographics, we found that the neighborhoods with the highest and lowest vegetation-based cooling potentials had very clear racial differences (emphasis added):
“Among the three neighborhoods with highest cooling potential, there are clear racial differences: while Euclid-Green in the eastern fringe of the city is 92 percent Black, Edgewater and Kamm’s, on the western end, are 65 and 79 percent White. In the neighborhoods with the lowest…[temperature]… reduction potential, Buckeye-Shaker Square is overwhelmingly Black at 80 percent, Goodrich-Kirtland Park has high rates of social isolation (67 percent are one-person households), and Downtown has a fairly even split between Blacks and Whites (44 and 43 percent, respectively). Clark-Fulton, another neighborhood with some of the lowest… [temperature] …reduction potentials, registers high rates of Hispanics (48.3 percent), poverty (nearly 40 percent), and low educational achievement (38.2 percent).”
These findings confirm the clear differences in extreme heat exposure along racial and economic status, and reinforce the notion that in Cleveland and elsewhere we need interventions to fend off climate-related health threats that consider environmental and climate justice. If we were to consider the efficacy of reducing temperatures using only vegetation potential criteria, the most vulnerable neighborhoods in Cleveland (largely Hispanic, low-income, and with low educational achievement), would be left out of urban planning interventions. That would hardly lead us to expect equitable outcomes.
As we celebrate this year’s Hispanic Heritage Month, we must keep in mind that Hispanics and other minority communities on the front lines of climate change impacts actively struggle for the right to “equal opportunities and safe, healthy communities”. In our research, we have shown that when we combine science and social/environmental equity considerations, we can advocate for more equitable solutions to our pressing climate change problems.
Our research paper is published as:
Declet-Barreto, J., K. Knowlton, D.G. Jenerette, & A. Buyantuev. (2016). Effects of urban vegetation on mitigating exposure of vulnerable populations to excessive heat in Cleveland, Ohio. Weather, Climate, & Society. http://journals.ametsoc.org/doi/10.1175/WCAS-D-15-0026.1