Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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March/April 2008
heat-trapping or "greenhouse" gases such as carbon dioxide (CO
) in the Earth's atmosphere naturally absorb or trap some of it and radiate it back toward Earth, raising the temperature the way a radiator warms a room. This "natural greenhouse effect" is a good thing for life on Earth. Without the atmosphere, the average temperature of the planet would be a frigid -18°C (0°F), instead of the actual average of 15°C (59°F).
WHAT CAUSES CLIMATE CHANGE? Over the billions of years of Earth's history, it has experienced a wide range of natural climate changes. Over the lastmillion years, a dominant driver has been subtle fluctuations in the shape of Earth's orbit, which have resulted in a cyclical progression of ice ages. The last ice age ended about 18,000 years ago.
Since the Industrial Revolution, however, human activity has become a global-scale "force of nature," especially by adding greenhouse gases to the atmosphere faster than they can be removed by natural processes. This "enhanced greenhouse effect" is increasing the temperature of the atmosphere, much as pulling on a second blanket on a cool night absorbs more of the heat from your body and radiates it back to you.

The role of greenhouse gases in affecting the earth's climate has been understood for a long time. A Swedish scientist, Svante Arrhenius, published a paper in 1896 demonstrating that CO
from burning coal could warm the atmosphere.
He used very simple calculations to show that increasing the concentration of atmospheric CO
by 2 to 2.5 times could cause a warming of 3.4°C (6.1°F), a number close to the middle of the current range of projections (see "What are the projections for the future?"). The accuracy of this calculation is partly a result of lucky assumptions, but it is also partly a demonstration of the relatively simple nature of the basic physics of the greenhouse effect.

We have records of the amount of CO
in the atmosphere over the last 600,000 years. Atmospheric CO
has been carefully and continuously measured at Mauna Loa, Hawaii since 1957. Before 1957, we can infer atmospheric CO
from bubbles of air trapped in continental ice sheets on Greenland and Antarctica.

Before the Industrial Revolution, CO2 levels in the atmosphere were around 285 parts per million (ppm). When the Mauna Loa measurements began, the levels were around 315 ppm. Today they are over 380 ppm, a 33% increase over pre-industrial levels, and they are increasing about 2 ppm per year.16 There is no doubt that the increase in atmospheric CO
since the start of the Industrial Revolution is due to human activities.

Where does this human-induced CO
come from? In the early decades of the 20th century, most of the CO
from human activities came from the clearing of forests.

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Figure I. Relative contributions of greenhouse gases in the atmosphere from nine regions (EU = European Union; D1 = other developed countries such as Canada; FSU = Former Soviet Union; D2 = developing countries such as Brazil and Mexico; D3 = least developed countries, such as most of sub-Saharan Africa). Cumul = cumulative emissions since the start of the industrial revolution to 2004; flux = average emissions from 2000-2004; growth = five-year growth rate in emissions from 2000 to 2004; Pop = current population.