Models Reveal Climatic Impacts of Urban Expansion

by Dan McCabe

Greenhouse gases have earned a bad name for their impacts on global climate, but in modern cities, the built environment itself can contribute to climate change just as much. In order to quantify and analyze the impacts of urbanization on local and regional temperature and hydroclimate, Georgescu et. al. (2014) modeled the impacts of urban expansion in the contiguous United States in a variety of scenarios. The authors considered a range of different predicted population levels in the United States for the year 2100. Using advanced atmospheric models, they found that if no urban climate change mitigation measures were put into place by then, summertime urban-induced warming of 1–3 °C can be expected in cities, with exact values varying by location. These increased temperatures are due solely to the effects of the built environment, as simulations were run using climate data from 2001-2008 without any assumptions about future warming due to increased greenhouse gas emissions. Continue reading

Cellulosic Ethanol Technically Comes of Age

by Emil Morhardt

Making liquid fuel out of crop waste is, in principal, an extremely good idea. But the ethyl alcohol (ethanol) we add to gasoline—and also drink in vodka, wine and beer—has been made out of edible fruits and grains. Corn ethanol, the main gasoline substitute is made out of the corn kernels which otherwise might become corn meal or tortillas, cutting into the food supply and encouraging conversion of more land into cropland. There have been many studies on the effect of this land conversion on atmospheric CO2 levels, and it appears that it will often be a decade or more before the CO2 released from land conversion will be offset by the substitution of ethanol for fossil fuels. Continue reading

Factors Influencing CO2 Emissions in South Africa

by Monkgogi Bonolo Otlhogile

With an average growth rate of 4.3% between 2001 and 2007, South Africa joined Brazil, Russia, India and China as the fifth member of BRICS, an association for the five emerging economies of the world in 2010. However, South Africa also joined these countries as one of the major carbon dioxide emitters, producing 1% of the world’s emissions. The environmental Kuznets curve (EKC) hypothesis states that early economic development will result in an increase in environmental degradation. This includes pollutants such as carbon dioxide and sulfur, which are considered by-products of economic activity. Eighty-seven percent of the carbon dioxide emitted by South Africa is a by-product of coal-fueled Continue reading

Unexpectedly High Methane Concentrations over Shale Gas Fields

by Emil Morhardt

Methane, the main constituent of natural gas (both that from gas wells and from farm operations) is a powerful greenhouse gas, around 30 times more potent than CO2 over the hundred years after it is emitted. It is on the rise, and the culprit might be shale gas development, which utilizes hydraulic fracturing (fracking). Caulton et al. (2014) used an airplane to sample the air above a 2,800-square-kilometer area of the Marcellus shale formation gas fields in Pennsylvania. It was rich in methane, with between 2 and 15 grams heading skyward over each square kilometer every second, the upper limit of which is quite a lot higher than the 5 grams estimated from what was previously known about wellhead methane emissions; the authors suspected that the transient nature of gas leakage might be the reason, making very difficult to come up with an average over time from ground-level measurements. Since they were in an airplane, however, they could circle around areas of high concentrations and pinpoint the source. It turns out that…

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The importance of gas-aerosol interactions in policy making

In current emission comparisons, the most cost effective method is one which employs multicomponent climate change mitigation strategies. These strategies involve analyzing both the direct and indirect effects of various linked emissions. However, the information of indirect effects between most gaseous pollutants and aerosols is absent. Shindell et al. (2009) took two approaches to calculate the impact of emissions on aerosols and the influence of these on radiative forcing. They found that the global warming potential (GWP) is substantially larger when the direct effects of aerosol interactions are considered and increases further when aerosol-cloud interactions are taken into account. Therefore, as atmospheric chemistry links methane, ozone, and aerosols, the policies regarding multigas mitigation should take gas-aerosol interactions into consideration.  Alyson Stark
Shindell, D., Faluvegi, G., Koch, D., Schmidt, G., Unger, N., Bauer, S., 2009. Improved Attribution of Climate Forcing to Emissions. Science 326: 716-718.

Shindell et al. relied heavily on averaged radiative forcing (RF) to compare various emissions and to estimate GWP. To find the response of atmospheric composition to both the collective and individual effects of emissions, the researchers calculated abundance based RF, which measured the effects of all emissions changing concurrently, and emissions based forcing, which instead attributed the response to a specific pollutant. Using these two techniques, Shindell et al. estimated several 100-year GWPs. These computations demonstrated that, with the absence of aerosol responses, the results were similar to previous studies. However, once the radiative effects are regarded, the GWP becomes larger in both methane and carbon monoxide (CO). Compared to the initial situation with no aerosol, the possible methane emission range increases, from approximately 25 to a range of 25-40 over a 100 year horizon. Similarly, the CO emissions initially begin at a 1-3 range and increases to a 3-8 range.  However,  the accuracy of these values is essential, and it there still remains much uncertainty in the predictions. For example, increases in these values also increases carbon dioxide, which perpetuates higher GWPs, and leads to more uncertain policymaking. 

The United States may not have as much economically viable underground CO2 storage space as previously thought

A new model to predict the economic viability of CO2 geosequestration in sandstone saline aquifers indicates that previous estimates for storage potential in the U. S. may be overly optimistic (Eccles et al., 2009).  The model identifies an estimated minima for storage costs in a typical basin in the range of $2–7 per ton CO2 sequestered, based on estimates of a maximum CO2 storage potential and a maximum CO2 injection rate.    Eccles et al. use data from carbon capture and storage pilot projects to explain that many assumptions in their model lead to artificially high estimates for the maximum storage potential and the maximum injection rate, and as a result, they conclude that geosequestration will be even more expensive than their model conservatively indicates. However, Eccles et al. proceed to apply the model to identifying economically optimal storage basins in the United States.— Shanna Hoversten
Eccles, J., Pratson, L., Newell, R., Jackson, R., 2009. Physical and Economic Potential of Geological CO2 Storage in Saline Aquifers. Environmental Science & Technology 43, 1962–1969.


J. K. Eccles and colleagues at the Nicholas School of the Environment, Duke University, begin building their model by estimating maximum storage potential as a function of the optimal injection depth and the available void space in the formation.  However, this estimate does not account for the reality of most pilot projects, during which the CO2 has bypassed the majority of the available pore space.  The maximum injection rate is calculated based on a determination of the injection-induced pressure that would cause hydraulic fracturing beyond the perforated zone around the well. However, comparison of the modelled results with the pilot project at Nagaoka, Japan indicates that lower injection rates are probably more realistic due to engineering constraints and actual reservoir conditions.  The cost per ton of CO2 sequestered is generated based on the total cost of drilling, injection, equipment, and operation and maintenance, notably excluding the costs that would arise from capture and transport of the CO2.  Finally, the cost for storage in a typical basin in the United States was computed using estimates for storage potential and the cost per ton of CO2 stored. 

Results from the modelling indicate that although depth is an important determinant of storage potential, it is not the most important factor in storage cost.  While increased depth can increase the cost by a factor of two, layer thickness and permeability of the storage reservoir can increase cost by a factor of fifty.  This hints at the myriad of basin characteristics that need to be assessed before arriving at a viable cost estimate.  Additionally, costs within a single basin are likely to differ considerably due to the extreme variability in aquifer characteristics.  The most important conclusion that can be drawn from this analysis is that the amount of CO2 storage provided by low-cost regions within saline aquifers in the United States is considerably lower than the estimates reported by previous studies.  The study by Eccles et al. suggests that there are only perhaps ten storage reservoirs in the United States that would have an average storage cost of below $10 per ton CO2.  If more basins are to become economically viable for CO2 storage, then policymakers will need to devise a regime that imposes a rather significant cost on carbon.—Shanna Hoversten

Dissolved CO2 in Oceans Lowers pH and Decreases Aragonite Concentrations

The consumption of fossil fuels since the industrial revolution has dramatically increased the amount of CO2 dissolved in the oceans, the effects of which have only recently fallen under the lens of scientific research. The Earth’s oceanic ecosystems are dependent on the balance between pH levels and dissolved carbonic compounds such as CO32-, aragonite and calcite, all of which are drastically effected by CO2 dissolution. Studies indicate that pH levels have dropped by about 0.1, (Feely et al., 2009) since the industrial era and will continue to drop in conjunction with a decrease in carbonic compounds in the decades to come. This change may eventually disable marine organisms’ ability to produce calcium carbonate shells.— Julia Levy

Feely R., Doney S., Cooley S. (2009). Present Conditions and Future Changes in a High-CO2 World. Oceanography 22, 36-47

Richard A. Feely and associates at the World Ocean Circulation Experiment/Joint Global Ocean Flux Study spent nine years conducting cruises to measure different carbon parameters in oceans throughout the world. Carbonate ion, aragonite, calcite, and pH are among the parameters that the researchers measured. They then used the National Center for Atmospheric Research to predict future changes in the aforementioned parameters.
Their findings indicate that the current pH of the oceans is 8.1 but will drop to 7.8 by the end of this century. Concentrations of CO32- are also projected to decrease drastically before the turn of the century. They are projected to drop low enough that the concentrations of aragonite and calcite in the oceans will become undersaturated (anticipated to occur 2020 and 2050 respectively). It is this decrease in carbon compounds that will make it difficult if not completely impossible for aquatic creatures to form calcium carbonate shells.
The mechanism by which the inability to form carbonate shells occurs through a series of chemical reactions. After CO2 in the atmosphere is dissolved into the ocean, it reacts with water to produces more H+ ions (decreasing the pH of the ocean). The newly available H+ ions react with CO32-, decreasing the concentration of CO32-.  As mentioned before, this decreases aragonite and calcite concentrations, inhibiting the formation of calcium carbonate shells. The projections made by Feely and his colleagues indicate that this century is a pivotal time for humankind to decrease their CO2 emissions, or else marine ecosystems will be in danger as early as the year 2020.— Julia Levy