Re "Fracking boom in U.S. at core of energy revolution" (Page A1, Nov. 28): I greatly enjoyed this and other recent in-depth articles on the current U.S. energy boom due to hydraulic fracturing. However, I was disappointed that environmental concerns were sequestered in their own article; they are critical to understanding the phenomenon in full context. We cannot, as a world, burn more than 20 percent of our proven fossil fuel reserves, or we will push the climate system into a radically different regime, one extremely harsh for our species. We are already on track to burn up that allotment within 15 years or so. We must immediately, wrenchingly shift our energy system, or we will face severe hardship and death in my lifetime. Either way, we won't be mining natural gas at current rates for 100 years as Obama claims.
-- Martin MacKerel, San Francisco