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Carbon and the city: Tracking emissions from megacities

The first megacity was Rome, but today, Europe’s urban population growth rate is slowing. Many of the fastest-growing cities are now in Africa and Asia.



London, a founding member of the C40 Cities Climate Leadership Group, is currently one of the 30 largest urban agglomerations, but is expected to drop off the list by 2025.


© Kowalczyk

In 2004, Denver built its FasTracks light rail system as part of an attempt to reduce carbon emissions.


© Liu

A monitoring station perched atop the Eiffel Tower is part of the CO2-MEGAPARIS project trying to identify urban emissions sources in Paris, the only European city monitoring carbon dioxide.


©Benh LIEU SONG, Creative Commons Attribution-ShareAlike 3.0 Unported

Sometime in the first century A.D., Rome’s population passed 1 million. It took more than 18 centuries for a city to surpass the 10 million mark, which both New York City and Tokyo did by 1950. Just six decades later, the world now has about 20 such “megacities” with populations of 10 million or more, including the largest, Tokyo, with a population of 35 million.

Worldwide, the urban population now exceeds the rural population and, according to United Nations’ estimates, urban populations are expected to double by 2050, with most of the growth occurring in cities in the developing world. Such population growth will bring about myriad social, environmental and economic issues, including increased demand for energy, transportation, sanitation services, water and food — and each of those will be affected by climate.

Large cities are especially susceptible to the ravages of climate change, including heat waves and storm surges, given that many are built on coasts or rivers. Moreover, recent research finds that urban areas also contribute to local climate by influencing their own weather, for example, via the urban heat island effect and particulate emissions, which can increase precipitation. The influence of cities on climate is not just local, however. Their greenhouse gas emissions are having a global impact. Seventy-five percent of global carbon dioxide emissions from fossil fuel use currently comes from cities, which cover just 3 percent of Earth’s land surface.

Urban populations are rising and, in some regions, emissions are expected to increase even faster than population. According to a 2010 World Bank study, while the populations of megacities located in the developing world are expected to grow by 4 percent a year on average in the coming years, their greenhouse gas emissions are anticipated to rise by 10 percent or more a year.

In order to cut emissions effectively, experts say, decision-makers will need to know whether specific carbon mitigation policies are having the intended impact on greenhouse gas concentrations in the atmosphere.

Riley Duren, architect for the Megacities Carbon Project at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., sees large cities as a good place to start. “The large and rapidly changing carbon emissions of megacities make them critically important and their intense atmospheric signatures and trends are easier to detect than those of entire countries,” Duren says.

Now, an international pilot project involving universities, as well as state, federal and international agencies is working to improve local carbon monitoring and modeling to find out, in detail, where the carbon is coming from. The Megacities project is gearing up to monitor local carbon emissions in Paris and Los Angeles, and the team is exploring options for including a third city, in Asia or South America, to diversify the pilot network. Ultimately, the effort could be expanded to provide detailed local and regional emissions data to the growing number of cities that are adopting plans that limit carbon emissions.

Global Carbon Monitoring

Carbon dioxide is currently monitored around the planet by a number of different observing systems using primarily ground-based instruments. The task of ensuring the “calibration, comparability and compatibility” of the global data collected by this network falls to NOAA’s Earth System Research Laboratory (ESRL) in Boulder, Colo., where the standards are maintained.

ESRL runs its own observatories around the world, including stations in Alaska, Hawaii, the South Pole and Greenland, to collect long-term measurements of carbon dioxide, carbon monoxide, methane, nitrous oxide, surface and stratospheric ozone, halogenated compounds, hydrocarbons, sulfur gases, aerosols, and solar and infrared radiation. Many of these atmospheric constituents are affected or produced by human activity, and some of them are produced in concert with carbon dioxide, which gives researchers ways to track carbon dioxide that arises from human activity.

In recent years, observations from instruments on aircraft and satellites have increased the density of measurements of carbon dioxide and other gases with an eye toward improving confidence in estimates of emissions sources. Researchers at NOAA, NASA, multiple Department of Energy labs and other U.S. agencies, along with their international counterparts, and others in academia are working to improve the myriad ground-, air- and space-based observing systems and to develop robust methods for interpreting the data. In particular, efforts under the auspices of the interagency U.S. Carbon Cycle Science Program are focusing on resolving the emissions of individual regions. The National Institute of Standards and Technology is also sponsoring a study of these methods in Indianapolis, Ind.

Being better able to identify and quantify sources and sinks in cities will ultimately help determine the progress of mitigation efforts, says Jim Butler, director of the Global Monitoring Division at ESRL. “There are thousands of different groups trying to reduce carbon dioxide emissions,” he says, “and they are going to need to know: Is it working?”

To be useful to decision-makers, this information will need to identify and attribute trends in human emissions of a given gas to a specific area, city or activity. Although challenging, the task is possible, experts say, but it will require cooperation and the coordination of different technologies and techniques.

“We’re entering an era when it’s possible for research teams to pool resources and apply different measurement systems, models and methods to focus on specific carbon management questions for individual megacities,” Duren says.

Cities Taking the Initiative

In the absence of substantive action by the largest emitting countries, many cities have forged ahead with their own carbon emission management initiatives. The C40 Cities Climate Leadership Group is an international coalition formed to reduce greenhouse gas emissions and increase energy efficiency in large cities around the world. Founded under the leadership of the mayor of London in 2005, with assistance from the Clinton Climate Initiative, it is now chaired by New York City Mayor Michael Bloomberg. The initiative includes some of the largest cities in the world, including Berlin, Hong Kong, Jakarta, Johannesburg, London, Los Angeles, New York City, São Paulo, Seoul and Tokyo.

Megacities feature a diversity of emissions sources: manufacturing, lighting, heating and cooling, cooking, various modes of transportation, wastewater treatment systems and landfills. In addition to carbon dioxide, some of these sources can release methane, a greenhouse gas that is 25 times more potent than carbon dioxide on 100-year timescales.

Although all cities tend to be large carbon emitters, their emissions footprints vary significantly. Some cities in the developed world, including Los Angeles, San Francisco and Boston, have very large natural gas infrastructures, which leak methane. Other cities like Mumbai and Delhi have a large amount of emissions from wood-burning cookstoves. The locations of power plants, which are influenced by zoning and environmental laws, also vary from city to city.

“In Los Angeles, electricity is produced by gas turbine plants within the Los Angeles Basin, whereas cities like Beijing have coal-fired power plants within the city limits,” Duren says. “So we expect Los Angeles’ carbon dioxide emissions to be dominated by its freeway traffic and Beijing’s by its power plants.”

Although the mix of emissions varies by city, the mitigation policy options are generally broadly applicable, he adds.

Many of the city-scale climate action plans capitalize on the compact geography and high population densities of cities to develop sustainable practices for low-carbon living, such as bike paths or mass transit. For example, in Denver, Colo., a new FasTracks light rail system was built in 2004 to serve the city’s population of 2.2 million.

Other U.S. cities also have developed plans to invest in renewable energy and conservation measures. According to the U.S. Conference of Mayors, more than 1,000 U.S. cities, including many with populations of less than 100,000, have agreed to meet or exceed Kyoto Protocol targets in reducing greenhouse gas emissions, an effort that starts with determining their sources.

Under the Comprehensive Renewable Energy Program, the city of Houston, Texas, has restructured its energy contracts to allow the purchase of up to 50 percent of the municipal government’s annual energy needs from wind energy providers. As of January 2012, the city had purchased more than 430 million kilowatt-hours of wind power, supplying 35 percent of its total electricity and making it the top-ranked municipality in green power usage, according to the U.S. Environmental Protection Agency.

As early as 2003, the city of Stamford, Conn., instituted a plan to reduce greenhouse gas emissions to 20 percent below 1998 levels by 2018 by making schools more energy efficient, installing a solar energy system at the recycling center, and installing LED traffic lights and energy-efficient streetlights.

San Francisco has adopted a program to recycle restaurant grease into biodiesel for the city’s fleet of 1,500 diesel buses and trucks. It is estimated that processing 50,000 gallons of grease per month reduces the city’s carbon dioxide emissions by more than 6,000 metric tons per year.

When it comes to assessing the success of these programs, however, the amount of carbon emissions reduced can currently only be estimated using inventories derived from activity data.

For example, with FasTracks, Denver estimates a reduction in carbon dioxide emissions of 60,249 metric tons per year. This figure is calculated based on ridership levels (from which fewer auto trips are inferred) and the average amount of carbon a passenger vehicle emits annually (approximately 5.23 metric tons). Although it is a reasonable estimate of the reduction in carbon emissions, it isn’t an empirical measurement of the carbon dioxide levels in the atmosphere over Denver.

This is why more widespread and robust carbon monitoring systems are needed, Duren says. “How else can decision-makers determine whether specific climate policies are meeting the ultimate objective?” he asks. “How can local citizens, businesses and other stakeholders know if their efforts are making a difference?”

The Challenges of Tracking Local Emissions

Attributing anthropogenic carbon concentrations to a specific source is challenging because emissions become dispersed and comingled with carbon emissions from the land and ocean. With sparse data from actual atmospheric measurements, the exact source of urban carbon emissions can be hard to pinpoint, even more so if monitoring stations are far from the sources. Additionally, because monitoring stations cannot be located everywhere, researchers also need detailed information about large-scale wind and convection patterns to model the transport of emissions between the source and the monitoring station.

Currently the only city in Europe to tackle the problem of monitoring carbon dioxide concentrations is Paris, a megacity with a population of 12 million. The CO2-MEGAPARIS pilot project, a research project of the French National Research Agency, began in 2009 and is scheduled to run through March next year.

“We need to monitor these greenhouse gases if we want to take proper action in reducing them,” says project director Irene Xueref-Remy, an atmospheric scientist at the Laboratory of Climate and Environmental Sciences in Gif-sur-Yvette, about 30 kilometers southwest of Paris.

Xueref-Remy is studying how best to measure and inventory the carbon that comes out of the city and separate it from the natural background levels around Paris, which in 2007 adopted a local action plan to cut greenhouse gas emissions by 25 percent relative to 2004 levels by 2020.

“The main source of emissions in Paris is traffic and heating [and cooling from the residential sector], but this is superimposed on biospheric activity, photosynthesis and respiration,” Xueref-Remy says. The question, she says, is, “How can we separate the contribution of plants [which both photosynthesize and respire] from anthropogenic activity?”

Working in conjunction with AIRPARIF, Paris’ regional air quality monitoring agency, CO2-MEGAPARIS has three main monitoring stations, including one atop the Eiffel Tower, 317 meters above the city. The stations are sited to take advantage of local wind patterns, and capture, alternatively, rural or urban signals, depending on which way the winds blow with the changing weather. Thus, the urban signal can be discerned by subtracting out the background levels from rural areas.

Although the project is not yet complete, Xueref-Remy says some results are emerging. “So far, what we can see is that there is a clear emissions plume of carbon dioxide from the city. We suspected that, but we were not sure we would be able to measure it so clearly,” she says. They have also learned, she adds, that three stations are not enough, and that researchers need to measure more species of carbon.

The next step, through the Megacities project, is to monitor carbon monoxide and carbon dioxide simultaneously, Xueref-Remy says. Both are emitted by combustion, but the ratio of carbon monoxide to carbon dioxide varies depending on the source, which will help researchers tease apart the relative contributions of traffic versus heating and cooling of buildings, for example.

In addition, to put the carbon data in context, Xueref-Remy is working with a team that monitored air quality over Paris over the same time span, collecting data on non-greenhouse gas air pollutants, like nitrous oxides and sulfur oxides, as part of the European Union’s MEGAPOLI project (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation). CO2-MEGAPARIS and AIRPARIF also have plans to deploy more joint monitoring stations in the future.

“Traditionally, the two communities [those that monitor air pollutants and those that monitor greenhouse gases] have not often worked together, but I’m convinced of the value of collaborating,” Xueref-Remy says.

The Megacities Project: Spotlight on Los Angeles

In Los Angeles, a city with a population of 18 million that is infamous for its traffic and smog, researchers trying to monitor carbon dioxide face different challenges.

Like Paris, Los Angeles adopted a climate action plan in 2007, called the Green LA Plan, with the goal of cutting emissions by 35 percent relative to 1990 levels by 2030. Unlike Paris, where emissions form a plume that tracks with wind patterns, Los Angeles is a coastal city lying in a bowl-shaped basin, where slow-moving air causes carbon emissions to form a dome over the city. A few carbon dioxide monitoring stations are scattered around the city, some of which have been collecting data for a decade, but the detectors are few and far between.

“We’re studying it, but the current data are incomplete,” says Eric Kort, a W. M. Keck Institute of Space Studies postdoctoral fellow at JPL. Kort is working with Duren on some of the technical details and instrumentation to be used in the Megacities project. Kort says the project will draw on a variety of monitoring techniques, including carbon dioxide detectors placed on tall towers and buildings, as well as both ground- and space-based remote sensing. Ground-based sensors can provide a high-resolution measure of the amount of carbon dioxide in a column between Earth and the upper atmosphere at a single location. Satellites can provide more widespread coverage than individual monitoring stations, but to use them locally, the satellites need to be able to focus in on a city-sized area and be ground-truthed, or calibrated against carbon levels measured in air samples taken at the same time. Kort is currently doing some preliminary studies on the Japanese GOSAT satellite, which can hone in on a city-sized area.

“We haven’t done this before [monitored emissions on the city-scale], so we don’t know what exactly is needed to do it,” he says. “The idea behind the Megacities pilot project is to go out and sample megacities using a number of different methods to really capture and understand the problem [of sourcing carbon emissions] and extend it to other cities.”

Kort says the very fact that megacities have high anthropogenic emissions makes the task of ascribing them to a specific source somewhat easier. Such attribution is usually very difficult because human emissions are dispersed and mixed with carbon coming from the land and ocean, and the two signals must be disentangled.

“If you look at a megacity, you have very strong anthropogenic emissions in a small area, and that signal starts to dwarf the biosphere signal,” Kort says. “So that’s actually an upside when it comes to attribution.”

Kort also emphasizes that the project’s goal is to monitor carbon, not just carbon dioxide. The project will track multiple carbon species, including carbon dioxide, carbon monoxide and methane. Carbon monoxide is produced by vehicles, but not by the biosphere, so it can be a valuable marker, he adds. Methane comes from agriculture, landfills and leaks in natural gas distribution systems. These all must be teased apart. “Some sources are more well known than others,” he says.

The goal of the Megacities project is to develop monitoring systems that will help cities identify their own emissions sources to create mitigation plans that address each city’s unique emissions profile.

“We’re doubling the rate at which we put carbon dioxide into the atmosphere every 35 years,” says Butler of ESRL. “That’s why we have to be able to determine where it’s coming from.”

Sara E. Pratt
Tuesday, May 22, 2012 - 16:00