Model calculations indicate that to avoid a temperature increase of 3.6oF, we must stabilize CO2 concentrations at about 450 parts per million (ppm) or less.
This turns out to be a tall order. Today, CO2 concentration is 380 ppm. The rate of increase is about 2 ppm per year, and is expected to accelerate. If we follow a "business as usual" course, we could cross the 450 ppm tipping point well before 2050.
Naturally, the more CO2 we produce, the higher CO2 concentration in the atmosphere. Four gigatons of carbon emitted into the atmosphere will raise CO2 concentrations by 1 ppm (see CO2 Arithmetic, Science Magazine). Worldwide, 7 to 8 gigatons of carbon are emitted into the atmosphere each year.
To avoid the tipping point, global CO2 emissions should peak no later than 15 years from now, and then begin to decrease. By 2050, emissions must be about 50 percent less than today, and by the end of the century 75 percent less. (Note that this is a reduction in total emissions, not the reduction relative to projected business-as-usual emissions that President Bush referred to in his 2007 State of the Union address.)
Reducing CO2 emissions by 75 percent will require a profound change in the way we produce and use energy, but there is no need for panic or despair. If we get started now, we can make this transition slowly, a percent or two each year.
Comment: Every bit helps. Changes in habit can be especially useful if they become permanent. The U.S. is incredibly wasteful. Many people pay no attention to their energy usage, running their air-conditioners round the clock, idling their cars for no reason, leaving lights on when they leave the house for the day. Changing these bad habits will yield a big dividend immediately, buying us time to put in place more complex changes such as improving efficiency standards for appliances and gas mileage for cars, and building up our renewable energy capacity.
The Sierra Club has created the 2% solution campaign to help us get started. It helps people set a reasonable goal for themselves and encourages them to become aware of their actual energy usage. See my series on Understand Your Electricity to figure out which electricity cuts will have the most impact.