CO2 output of industries may change due to improved operational efficiency or changes in industry size.
The U.S. EPA recently published a report “U.S. CARBON DIOXIDE EMISSIONS AND INTENSITIES OVER TIME: A DETAILED ACCOUNTING OF INDUSTRIES, GOVERNMENT AND HOUSEHOLDS” (April 2010) shedding some light on these issues. It focuses on energy-related C02 emissions, which comprises the bulk of overall emissions for most industries.
The following graph illustrates that when controlling for industry output (changes in industry size), manufacturing and “other services” are experiencing solid and steady declines, households emit more each year but at a decreasing rate in recent years, and agriculture, transportation, and government, are holding steady (no big increases or declines, though government has shown some signs of intensity reduction in earlier time frames).