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People don’t always produce more CO2 from one year to the next. When the global economy weakens, emissions from human activities can actually drop slightly for a year or two, as they did in 2009. When the economy rebounds, so can emissions. Yet in either case, the accumulation of CO2 in the atmosphere continues to rise over time, as shown in the graph to the right. It’s a bit like a savings account: even if your contributions get smaller in a tight budget year, the total in your account still goes up.
Vegetation also makes a difference, because growing plants absorb CO2. Large-scale atmospheric patterns such as El Niño and La Niña bring varying amounts of flooding, drought, and fires to different regions at different times, which affects global plant growth. Thus, the amount of human-produced CO2 emissions absorbed by plants varies from as little as 30% to as much as 80% from year to year. Over the long term, just over half of the CO2 we add to the atmosphere remains there for as long as a century or more. About 25% is absorbed by oceans, and the rest by plants. This "balance sheet" is known as the global carbon budget.
It’s not yet clear which forests absorb the most CO2. Because the answer will influence global planning and diplomatic agreements on climate, scientists are working hard to measure how CO2 varies by latitude, altitude, and season. One such study is HIPPO, a field project led by NCAR and colleagues from 2009 to 2011 to take pole-to-pole measurements aboard an airborne laboratory, the NSF/NCAR Gulfstream V jet. Satellites such as Japan's GOSAT and others on the drawing board at NASA will help fill in more carbon-budget details.