Proper management of Mattawoman’s watershed would reduce contributions to climate change
Introduction
Sprawl development and its associated loss of forest are primary threats to the health of Mattawoman Creek.[1] These threats also contribute to global warming by increasing the emission of the greenhouse gas CO2 (carbon dioxide). A “greenhouse gas,” when distributed in our atmosphere, traps heat much as a pane of glass does in a greenhouse: it is transparent to sunlight but blocks the earth’s radiant heat from escaping into outer space. A report by the National Academy of Sciences (NAS), an independent body of the nation’s top scientists tasked to evaluate technical issues for the U.S. government, states [2]:
“In the judgment of most climate scientists, Earth’s warming in recent decades has been caused primarily by human activities that have increased the amount of greenhouse gases in the atmosphere.”
In addition to the documented increase in global surface temperatures, the price of disturbing our atmosphere with excess greenhouse gases is predicted to include altered patterns of storm frequency and intensity, new rainfall distributions with attendant drought and desertification, and more. In a phrase: climate change.
The three most significant greenhouse gases, in order of importance, are water vapor, carbon dioxide, and methane. [2] Earth is hospitable to life only because these gases assist sunlight in warming the globe. But the balance between heat input and the loss mediated by greenhouse gases is not immutable. Human activity is pouring prodigious quantities of CO2 into the atmosphere (see Fig. 1), which is leading to global warming. The NAS reports [2]:
“The recent rapid rise in both surface temperature and CO2 is one of the indications that humans are responsible for some of th
is unusual warmth.”
Urban sprawl-development is increasingly recognized as a significant contributor to excess CO2 emissions because it depends so highly on burning fossil fuels for extended vehicle trips and for heating and air conditioning of large houses. For the case of a mostly forested watershed like that of Mattawoman, sprawl is a double-edged sword: we not only lose the natural CO2 uptake, or sink, afforded by forest, but replace it with the CO2 source of sprawl.
Deforestation adds CO2 to the atmosphere
Forest is the best land use for water quality. [3, 4, 5] This is justification enough for the preservation of woodlands that protect a Chesapeake Bay resource as stellar as Mattawoman Creek. Protection of water quality is one of the many “ecological services” that nature provides us, such as soil creation, oxygen and food production, and climate regulation. In fact, per acre, the ecological services provided by forest have the highest value of all dry-land habitat types. [6]
With respect to climate change, a growing forest converts CO2 in the atmosphere to wood through the chemistry of photosynthesis, and hence acts as a “sink” for CO2. Thus, the biomass of a forest sequesters CO2 from the atmosphere. The vast majority of Mattawoman forests, as in the rest of Maryland, are second growth, meaning they have a century or more of growth before CO2 removal from the atmosphere may be balanced by CO2 emissions (due to decaying tree fall, for example). [7] Estimates for northern hardwood forests arrive at sequestration rates in excess of two tons of CO2 per acre per year. [7] Our mid-Atlantic forests, with larger trees and a longer growing season, would sequester even more. Hence preservation of our forested land helps decelerate the rapidly increasing CO2 levels and is a worthy endeavor.
The practice in our locale of clear cutting for subdivisions and other development exacerbates CO2 emissions because a substantial fraction of our woodlands are simply burned, which converts the carbon bound in wood and roots back into CO2. In addition, vast quantities of carbon bound in forest soils begins a conversion to CO2 once the overlying forest is cleared, [7] and is further degraded during grading operations.
Finally, there is the question of “albedo,” which refers to the fraction of incident sunlight that is directly scattered and reflected from the earth back to space. For example snow is very reflective (has high albedo), and therefore has a cooling influence. In contrast, asphalt is highly absorptive, and thus contributes a warming influence. The effects of albedo for various landscapes are presently a subject of scientific research, but the planet’s forests appear to play a significant role. Compared to bare ground or many croplands, for example, a forest is more absorptive. [8] However, compared to the urban environment, broadleaf forests, such as characterize our region, appear to be more reflective (have higher albedo). [9, 10] Therefore, the replacement of forest with more absorptive urbanization potentially represents yet another contribution to global warming, in addition to loss of forest sequestration, to CO2 emitted by burning, and conversion to CO2 of soil carbon.
Sprawl development contributes disproportionately to CO2 emission
Exurban development sprawls over the landscape at a rate that exceeds population growth [11], especially in our region of the country.[12] Consider, for example, that in the twenty-five years preceding 2020, more land will be converted to housing in the Chesapeake Bay region than in the past three and one-half centuries, according to the Maryland Office of Planning. [11]
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Fig. 2 U.S. CO2 emissions by sector
(U.S. Environmental Protection Agency, Ref. 13) |
New highways through lightly populated areas use taxpayer funds to subsidize development. The ensuing sprawl constitutes a recognized “growth inducing impact” of a highway. And sprawl is recognized as a significant contributor to global warming because of its large demands for fossil fuels. The burning of fossil fuels, such as oil and coal, is the primary source of CO2 emissions. [13] The residential sector and personal vehicle use are not minor players in the U.S. inventory of CO2 emissions (see Fig. 2), and it would be wise to reduce these contributions, especially when it is recognized that the U.S., with 5% of the world’s population, accounts for 23% of the global fossil-fuel emission of CO2. [13]
Consider the workday commute of about 25 miles (one-way) to Washington DC of a vehicle from the area that would be opened to development by Charles County’s Cross County Connector extension. Using government data [14], one can compute that a single average vehicle (with a consumption of 20 miles per gallon) would contribute 6 tons of CO2 to the atmosphere per year. If transit-oriented housing opportunities were made available in attractive, pedestrian friendly centers, the transportation emissions of CO2 engendered by the sprawl model would be significantly curtailed.
Like transportation emissions, the CO2 emissions to heat and cool exurbia are also disproportionately large. Nationally, average house size has increased continuously for decades. From 1970 to 1990, house size increased from fewer than 1500 square feet to over 2000 [15], and continues to climb [16]. In Montgomery Co., MD, average house size now exceeds 2500 square feet. [16] Comparing energy usage between two similar but differently sized domiciles of 1700 and 2200 square feet, one can compute that the larger house emits more than two additional tons of CO2 annually. [17, 18] While quantitative results vary depending on many details, it is clear that larger houses emit more greenhouse gases.
Because electricity is provided primarily from coal-fired power plants, additional benefits of reducing electricity usage would be less mercury emissions and preservation of cherished landscapes like the Appalachian Mountains, which are presently being shoved into stream valleys to reveal coal seams in a process called “mountain top removal.”
Summary: a well managed watershed would reduce emission of global-warming gases and would enhance the quality of life
Because a watershed managed for water quality would necessarily preserve forest cover, CO2 would continue to be sequestered in wood. In a forested watershed, curbing sprawl must be considered an important tool for forest preservation, and would yield very significant reductions in CO2 emissions because of reduced transportation levels and reduced heating and air conditioning of large houses. Preserving forest would also improve the quality of life because of decreased traffic congestion and air pollution, increased recreational opportunities, and the desirability of green space.
To realize such a vision requires the foresight to provide attractive housing opportunities in transit-oriented and pedestrian-friendly communities. The planning community has long known the advantages of this approach, and the excess costs to the taxpayer of sprawl development.[19] Hence enlightened watershed management would reduce costs to people, enhance quality of life, and as current scientific understanding shows, would at the same time help abate global warming while protecting the living resources and promoting the health of Mattawoman Creek, the Potomac River, and the Chesapeake Bay.
REFERENCES
[1] U.S. Army Corps of Engineers, Baltimore District, “Mattawoman Creek Watershed Management Plan,” Charles County, MD, August, 2003. Available at
[6] R. Costanza et al., "The Value of the World's Ecosystem Services and Natural Capital," Nature Vol. 387 (1997), p. 256, Table 2.
[8] S. Gibbard et al., “Climate Effects of Global Land Cover Change,” Geophysical Research Letters, vol. 32, p. L23705 (2005).
[11] F.K. Benfield, M.D. Raimi, and D. T. Chen, Once There Were Greenfields: How Urban Sprawl Is Undermining America's Environment, Economy and Social Fabric, (New York: Natural Resources Defense Council, 1999), as excerpted at www.nrdc.org/cities/smartgrowth/rpave.asp.
[15] “Better Not Bigger,” Eben Fodor, New Society Publishers, 2nd Ed., 2001, p. 23. Average house size based on 1995 statistics from the U.S. Census Bureau.
[17] The amount of CO2 emitted per kilowatt hour (kWh) of electricity consumption depends on locale. In Maryland, the figure is 1.4 pounds of CO2 per kWh, according to the U.S. Dept. of energy. See http://www.eia.doe.gov/oiaf/1605/e-factor.html
[18] Data from an energy-savings calculator developed by the Lawrence Berkeley National Laboratory. See http://hes.lbl.gov/