Hey Mr. Green,
I've heard that humans account for only 3 percent of global carbon emissions, with the rest coming from volcanoes, forest fires, and plant decay. Is this true, and if so, how can reducing our carbon footprint make an appreciable difference to the global situation? –Cary in Atlanta, Georgia
Three percent may not sound like much, but if that small percent isn't reabsorbed each year, it keeps accumulating until you have a much bigger percent and a real problem. Think of it this way: If you only gain 3 percent in weight a year, it's not much. But if you never shed that weight, you'll balloon from a 135-pound lightweight-boxer to a full-size heavyweight (albeit a rather flabby one) in 20 years.
The natural world is estimated to emit a grand total of 770.3 billion tons of CO2 annually. Millions of life forms on land, from bacteria to elephants, pump out 440 billion tons of CO2, while the oceans release another 330 billion tons, and volcanoes add 300 million each year. Human activity, primarily from burning fossil fuel, but also from agriculture and forest burning and clearing, accounts for "only" 32.3 billion tons, New Scientist reports.
But remember, those hundreds of billions of tons emitted by nature are also absorbed by nature. As a result, the CO2 content in the atmosphere has remained fairly steady at 280 to 300 parts per million, or ppm, for the past half million years (We know this from examining the air trapped in icecaps and other evidence.) But when we started burning large amounts of fossil fuels two centuries ago, the concentration of CO2 in the air began to rise. In short, we started pumping out more CO2 than the natural world could absorb. In the blink of a geological eye we've been releasing carbon that had taken millions of years to build up in oil, coal, and natural gas. Over the past 50 years, the ppm of CO2 has jumped from 315 to 380.
The oceans and other carbon sinks can reabsorb only around 40 percent of human-caused, or "anthropogenic," emissions. The rest stays in the atmosphere. Therefore, we have to drastically reduce our carbon footprint, or the CO2 content will continue rise, causing the world to heat up even more than it already has.