Many second generation bioenergy crops are lauded for their contribution to climate change mitigation efforts, particularly those involving minimizing greenhouse gas emissions and enhancing carbon sequestration. As a bioenergy crop, switchgrass rates well in a number of climate mitigation metrics (see Vadas et al. 2008). It is a perennial crop with deep roots (Lemus and Lal 2005), which are often implicated in its carbon sequestration abilities. Switchgrass performs better than maize in its carbon sequestration rates (Searchinger et al. 2008; Davis et al. 2012; but see Follett et al. 2012), and when combined with human health costs associated with fine particulate matter emissions in biofuel feedstock growth and processing, switchgrass comes across as far superior to maize (Hill et al. 2009). Not only does switchgrass perform better than a number of other bioenergy crop alternatives with respect to CO2 emissions (Monti et al. 2009), but production of agronomic switchgrass has also been tied to low NO2 and CH4 emissions when compared to most alternatives (Monti et al. 2012). Surely, different crop management strategies will contribute to variability in greenhouse gas emissions metrics (Monti et al. 2012), but overall it would appear that switchgrass is a leading bioenergy crop candidate in this critical area.

The high regard bestowed on switchgrass for its production-associated greenhouse emissions metrics may be tempered, however, by repercussions of landscape and land-use change that would be necessary to provide mandated amounts of ethanol in the U. S. The estimated amount of current non-agricultural land that would need to shift to second generation bioenergy crops to reach government mandates in ethanol production is upwards of 200,000 km2 to possibly three times that number (McDonald et al. 2009). This shift is typically not integrated into life cycle analyses of climate mitigation aspects of switchgrass production, but its impact cannot be overlooked. The clearing of forests and the changeover of range-, hay-, and pasture-lands to accommodate dedicated bioenergy crops, like switchgrass, would immediately result in substantial net CO2 emissions, which some studies have estimated to amount to approximately 350 Mt/ converted ha (Searchinger et al. 2008). Future conversion of non-arable land worldwide for crop production could result in > 3 Gt/yr of greenhouse gas emissions by 2050 (Tilman et al. 2011). The difficult and multifaceted challenge here would be to have quick-establishing (e. g., rapid growth in the early growing season), highly productive perennial bioenergy crops capable of substantial carbon storage in their roots. Currently, no CO2- related cap-and-trade laws are in effect, and treatment of CO2 emissions as pollutants by the U. S. Environmental Protection Agency is in its infancy; how these issues may affect future landscape conversion to switchgrass crop fields is unknown.

Those involved with improvement efforts in switchgrass will also need to be cognizant of climate forecasts in the areas it will be grown. For one, increased atmospheric CO2 levels may not necessarily lead to increased productivity (i. e., no "CO2 fertilization" effect) in switchgrass (Fay et al.

2012) . In addition, habitat suitability and climate envelope models already have switchgrass incapable of growing in a number of regions where some of today’s highest-yielding lowland cultivars originated in the southcentral U. S. by as early as 2040 in some of the "best case" scenarios (Barney and DiTomaso 2010; Tulbure et al. 2012). The hotter conditions forecast for the southeastern U. S., which include current areas of highest predicted switchgrass biomass yields, will dictate that breeding improvements be dedicated towards better water-use efficiency (see Le et al. 2011) and related traits aimed at "climate proofing" (sensu Oliver et al. 2009) the crop. This is noteworthy and may seem perplexing, given that switchgrass currently is noted for exhibiting high water-use efficiencies, higher than some alternatives (VanLoocke et al. 2012).