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here is a brief history of CO2 changes in the last 50 years, a rise in CO2 levels is a good indicator of global warming via greenhouse gases. air samples are collected daily at the Mauna Loa observatory.
-1960-2006, Mauna Loa 22% increase from 313 – 381 ppm
-1997-98, increased 2.87 ppm previous annual average 1.4 ppm
-2004-06, went from 377 - 381 ppm
-Could reach 450-550 ppm by 2050
-prior to industrial revolution CO2 level 150-200 ppm, now 380 ppm

there are already changes in growing seasons in the US;
-1950-1990 the average daily min temp. increase 2-3°C in much of Western US
-Since 1990, 10 warmest years on record in US (National Academy of Science)
-New England's growing season is 8 days longer on average than in 1950
-1965, most of Massachusetts in Zone 5 now zone 6
-lilacs are blooming on an average 8 days sooner in the Eastern US they flower in response accumulated temperature - an indicator of warming trends