"The greenhouse gas footprint of consuming ruminant meat is, on average, 19–48 times higher than that of high-protein foods obtained from plants (Fig. 2), when full life cycle analysis including both direct and indirect environmental effects from ‘farm to fork’ for enteric fermentation, manure, feed, fertilizer, processing, transportation and land-use changes are considered." (page 3)
The above quote refers to Figure 2 (page 4): It breaks down the full life-cycle ghg footprint of ruminants per kg of product vs other forms of protein; furthermore it breaks down the ruminant categories into beef (extensive), sheep, beef meadow systems, and beef (intensive).
"). In terms of short-term climate change mitigation during the next few decades, if all the land used for ruminant
livestock production were instead converted to grow natural vegetation, increased CO2 sequestration on the order of 30–470% of the greenhouse gas emissions associated with food production could be expected5,11."
[Ashley's words] The point is that on a life cycle - per kg of product basis, sheep and beef are both major contributors to climate change. However, the Science Daily article highlights that beef are responsible for the majority of livestock ghg emissions.]
Also see: http://www.ewg.org/meateatersguide/a-meat-eaters-guide-to-climate-change-health-what-you-eat-matters/climate-and-environmental-impacts/
From: Dario Caro, Steven J. Davis, Simone Bastianoni, Ken Caldeira, “Global and regional trends in greenhouse gas emissions from livestock,” Climatic Change, 2014: 8, accessed July 23, 2014, doi: 10.1007/s10584-014-1197-x
William J. Ripple et al., "Ruminants, climate change and climate policy," Nature Climate Change, 4, (January 2014): 3, accessed July 23, 2014, http://www.health.gov/dietaryguidelines/dga2015/comments/uploads/CID230_Ripple__2014_NatureClimateChange-Ruminants.pdf