Expert Commentary

Urban adaptation can roll back warming of emerging megapolitan regions

2014 study in the Proceedings of the National Academy of Sciences (PNAS) exploring the potential of urban adaptation strategies to counteract the effects of long-term global climate change.


As the world’s population grows, so do cities. By 2030 more than 60% of all people on Earth — 5 billion out of the 8 billion — will live in urbanized areas. While some of the growth will be accommodated through increased density, nearly 6 million square kilometers of land could be urbanized, much of it with low-density sprawl that spreads outward from established city centers.

When development is compact and walkable, urbanization can have a restraining effect on climate change — on average, denser cities have significantly lower per-capita C02 emissions than suburban and rural areas. However, urbanized environments are subject to what is known as the “heat island effect”: Dark-colored buildings and especially pavement — omnipresent in automobile-dominated suburbs — absorb heat from the sun during the day and release it at night. A 2013 study found that asphalt can be 20 to 30 degrees Celsius hotter than ambient air temperatures during the day.

Research has explored the potential of reducing the heat-island effect through simple changes in the built environment. A 2009 study by the Lawrence Berkeley National Laboratory, “Global Cooling: Increasing World-Wide Urban Albedos to Offset CO2,” demonstrated that increasing the reflectivity of pavement and roofs can be highly effective: Every 100 square feet of dark roof area that is painted white can offset the emission of one ton of atmospheric CO2. Consequently, the 20 tons of CO2 that the average American generates annually could be offset by 2,000 square feet of white roof.

Many cities are beginning to plan in these ways. MIT’s Department of Urban Studies and Planning issued an overview: “Progress and Challenges in Urban Climate Adaptation Planning: A Global Survey.” That 2012 report, which examined data from 468 cities worldwide, finds:

Sixty-eight percent of cities worldwide report that they are pursuing adaptation planning, with Latin American and Canadian cities having the highest rates of engagement (95% and 92% respectively) and the U.S. having the lowest (59%). Cities are engaged in different activities associated with adaptation planning. At the extremes, 37% report being in the preparatory stages, while 18% are working on implementation, although many in this latter group may be linking implementation to ongoing work activities, such as land use and coastal zone planning, rather than pursuing dedicated adaptation activities.

A 2014 study published in the Proceedings of the National Academy of Sciences (PNAS), Urban Adaptation Can Roll Back Warming of Emerging Megapolitan Regions,” looks at the potential of large-scale urban adaptation strategies to counteract the effects of long-term global climate change. The researchers — Matei Georgescu of Arizona State University and Philip E. Morefield, Britta G. Bierwagen, and Christopher P. Weaver of the Environmental Protection Agency — note that approximately 25% of the expected increase in urbanized areas will occur in China and India, and Africa is projected to have a 600% increase: “Urban expansion within these areas will almost certainly be highly concentrated, potentially exposing highly vulnerable populations to land-use-driven climate change.”

In the study, the researchers modeled a range of possible scenarios for population growth, urban expansion and climate change within the contiguous United States by the year 2100. The minimum projected population for that year (referred to as scenario B1) was 380 million inhabitants — a figure expected to be surpassed by 2050 — and the maximum scenario (called A2) corresponded to 690 million. Compared to the year 2000, by 2100 an additional 208,000 to 261,000 square kilometers of U.S. land could be urbanized. Strategies to reduce the induced warming included both green and cool roofs as well as hybrid approaches.

The study’s findings include:

  • Without any adaptive urban design, by 2100 the expansion of existing U.S. cities into regional megalopolises could raise near-surface temperatures between 1 and 2 degrees Celsius over large regions.
  • In the scenario assuming the greatest increase in population, urbanization and greenhouse-gas emissions, urban areas will significantly contribute to regional warming. Their projected contribution to regional warming include 15% in Chicago/Detroit and 27% in California, and could reach 50% in some areas. “This warming is a significant fraction of the 21st century greenhouse gas-induced climate change simulated by global climate models.”
  • Careful planning and urban design as well as retrofitting of existing buildings can offset a significant percentage of regional warming over large scales, and even reverse it. For example, in California the maximum scenario (A2) would increase temperatures by 1.31 degrees Celsius. At the same time, 100% deployment of cool roofs would result in a temperature drop of 1.47 degrees Celsius — more than the increase.
  • While cool roofs reduce energy demand during hot weather, they can increase it during the winter when buildings must be heated more. However, the energy gain from reduced air-conditioning use in summer more than makes up for the need for increased winter heating: “For every 1 degree Celsius of environmental warming, cooling energy demand increases by 5% to 20%. For every 1 degree Celsius of environmental cooling, demand for heating energy increases by 3 to 15%.”
  • There is no one-size-fits-all solution, as different regions have different needs. Solutions have to be tailored to regions’ specific geographic and climate demands.

“Our results quantify how judicious choices in urban planning and design cannot only counteract the climatological impacts of the urban expansion itself but also, can, in fact, even offset a significant percentage of future greenhouse warming over large scales,” the researchers conclude. “Additional approaches requiring investigation include incorporation of permeable surfaces, which can reduce peak storm water runoff” and seasonally adjustable cool roofs, which would allow energy savings in both hot and cool weather.

Keywords: greenhouse gases, global warming, climate change, @leightonwalter, @journoresource

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