Development of brownfield sites can help significantly in controlling greenhouse gases and the impact that they have on global warming. Reducing the impacts of greenhouse gases can be accomplished through reducing the amount of gases created or by sequestering the gases once they have been released into the environment. The process of sequestering greenhouse gases may result in carbon offsets. Greenhouse gas technology is anything but "light as air." In fact it can be quite heavy; as a result it's instructive to provide a primer on greenhouse gases before delving into carbon offsets.

Methane flare is used to burn off excess gas in a more environmentally friendly way. Photo © http://www.flickr.com/photos /greatvalleycenter/

Common greenhouse gases include carbon dioxide, methane, nitrous oxide (N₂O), various fluorocarbons, and a few others. Collectively these gases trap heat and energy in the atmosphere like the warm, moist condition in a greenhouse; hence the name "greenhouse gases" are commonly referred to by their acronym, GHGs, and are frequently lumped together by assessing their global warming potentials (GWPs) and converting into carbon dioxide (CO₂) equivalent emissions. The CO₂ equivalent emissions are referred to as carbon equivalents (abbreviated as CO_2e), or sometimes just simply called carbon.

Collectively the creation of greenhouse gases is referred to as a carbon footprint. The more gases an activity creates, the larger the carbon footprint. When I drive my car to work there is a footprint; when I ride my bicycle to the train station, there is a smaller footprint. Once a greenhouse gas is created it will remain in the atmosphere until it is naturally destroyed or purposely removed. For example, the natural absorption of carbon dioxide into plants destroys the carbon dioxide structure, resulting in different non-greenhouse gas compounds, including the oxygen we breathe.

Carbon Offsets
In some cases greenhouse gases are purposefully taken out of the environment through trapping, emissions avoidance or conversion, making them unavailable to add to the greenhouse gas effect. An example of trapping is carbon sequestration, where GHG emissions are pumped into underground wells, stored in the ocean, or taken up by plants. An example of avoidance is replacing high-bleed valves in natural gas distribution systems with no-bleed valves. An example of conversion is flaring methane. Although flaring methane still results in GHG emissions, the methane is converted into CO₂ and the carbon equivalents are reduced because the global warming potential of methane is 21 times that of CO₂.

The purposeful removal of greenhouse gases from the environment is sometimes referred to as creating a carbon offset, which are also considered a type of commodity that can be purchased or sold, but are frequently based on the total carbon equivalents generated by a specific project compared to a baseline scenario. When consistently defined on a viable financial exchange, carbon offsets are considered in the same way as carbon credits.

Management of carbon offsets is a fast growing financial model based on a set of middleman companies that support the sequestration or removal of carbon at sites and projects that do not have the financial support to reduce their carbon footprint impacts on the environment. Carbon offset companies solicit investment funds (from the general public or carbon neutral-sensitive companies) and then invest the funds into the activity that reduces the carbon footprint.

The flaring of methane gas generated at a municipal landfill, rather than leaving it seep through the landfill cover into the atmosphere, is a good example. Many municipal landfills do not have the funds to capture the methane gas for energy or to flare (burn) the methane gas, and therefore it seeps into the atmosphere as a greenhouse gas. The offset company funds the flaring or capture for energy of the methane and receives the offset, which it logs as a "permanent reduction" of carbon dioxide equivalents. Although GHGs are still emitted through flaring, the net result is fewer carbon dioxide equivalents, which results in the permanent reduction. Other offset projects include the sequestration of carbon by burying it deep underground in existing mines or caverns where it can't seep back into the atmosphere.

The pushback about carbon offsets is that many projects, like the landfills cited, initiate the same activities without applying the concept of carbon reduction. For example, I worked on a project at a large municipal landfill where we evaluated the potential of reclaiming the methane for energy production and found that it was financially feasible to recover the methane and generate energy. The cost of the methane recovery and energy production was amortized over a period of a few years and the municipality quickly earned the return from their investment. Offset credits were not considered.

Carbon Credits, Offsets at Brownfields
Brownfield site development can have significant impacts on a regional basis regarding the generation of greenhouse gases and consequent reduction in carbon footprint. Examples of carbon footprint reduction activities during design include:

• Reuse of existing infrastructure (e.g., connection to existing sewers, power and water lines eliminate the carbon footprint created by extending these services to greenfield development sites);
• Low impact development design (e.g., bioswale vegetation will sequester carbon and treat stormwater);
• Sustainable property cleanups (e.g. recycling of building materials and local remediation suppliers, will reduce the carbon footprint), and;
• Landscaping with perennial plantings will eliminate long-term operation and maintenance of carbon footprints (e.g., eliminate weekly grass cutting).

The largest impact on regional greenhouse gas creation, however, is in the long-term carbon footprint reduction during the "use phase" of the brownfield development. This is especially true for urban locations and transit oriented developments (TODs). Redevelopment of urban brownfields, where people walk to work, or travel short distances to work; or where they live a short distance to a transportation connection will reduce vehicle transportation and impacts when compared to an outer edge site. The carbon footprint impacts of distant travel include more fuel use, time of travel, construction of new roadways and installation of new infrastructure; all of which have a large impact on carbon footprints.

Kyoto Protocols and Cap-and-Trade
The international Kyoto Protocol mandated a preferred global approach for setting carbon emission limits for nations signing the Protocol, which spawned a cap-and-trade system for companies that produce more (or less) carbon than allowed. A good definition of how carbon trading works was given on the CBS Interactive networking site BNET: "Under a basic cap-and-trade scheme, if a company's carbon emissions fall below a set allowance, that company can sell the difference—in the form of credits—to other companies that exceed their limits." Carbon credits are a financial commodity, like corn or oil, and are defined and regulated. A carbon credit in Oklahoma is the same as a carbon credit in Florida.

The U.S. did not sign the Kyoto Protocol, and therefore American corporate participation in cap-and-trade is voluntary. Some U.S. companies, especially global nationals, set "voluntary" allowance targets based on Kyoto. Companies that over-generate carbon purchase credits from a viable financial market and thereby achieve their carbon emission target. For instance, the Chicago Climate Exchange (CCX) is the leading financial exchange where these transactions occur, and, similar to any financial market, the value (price) is subject to market fluctuations. Current prices are about $2 per metric ton. One benefit of buying carbon credits is that companies can buy time to adjust their approach to managing their carbon footprint and yet still advance their assumed responsibility to respond to global warming issues.

Ken Kastman is P.E., URS Corp., Chicago office