Even if you're a vegetarian and you line-dry all your laundry, it's still not a good idea to fly to Greece for a week. Even though you've always wanted to go.

You may ride everywhere on your bicycle, but you can wipe out more than half of the greenhouse gas emissions you save in a year of not driving with one round-trip flight to New York City--even though that conference on sustainable urban planning would enhance your professional growth.

Unfortunately, there is no technological fix for aircraft emissions anywhere in the near future. Moderate gains in fuel efficiency have been cancelled out by ever-increasing numbers of airplane passengers. (Every time you take a flight, you are counted as a passenger.) On US airlines, passenger numbers swelled from 438 million in 1990 to 769 million in 2007.

If we are going to reduce greenhouse gas emissions 80% from 1990 levels in time to save the lives of many people and countless creatures, most flying has to end quickly.

This is very upsetting. Airplanes have made travel much faster and more convenient. Perhaps most importantly, they have woven the world close together, and many of us have come to respect and love other cultures, people, and places because we were able to visit them easily.

One huge sticking point is that many of our families are spread across the US and the world, creating moral dilemmas about what one transportation expert calls "love miles." You miss your son who lives in Minnesota. Your best friend is getting married in Alaska. Your mother is battling breast cancer in Virginia. It hurts the atmosphere if you fly, and it hurts your family and friends if you don't.

I believe it is already time to reserve air travel for infrequent family trips and major necessities.

Here are the statistics. The US, with five percent of the world's population, contributes 25% of global greenhouse gas emissions. On average, each person in the US is responsible for 25.5 tons a year, a quarter of which comes from transportation. The average American travels 12,000 miles a year by car and 1,000 miles by airplane, according to carboncounter.org; this amount of travel adds six tons of greenhouse gases to the atmosphere, 5.5 tons for car travel and half a ton for air. (There is controversy about exact figures; this is a conservative estimate.)

Using these figures, if you fly roundtrip across the country once during a year, you're already 5,000 miles and 2.8 tons above the US average for air travel. And greenhouse gas emissions must be reduced 80% from 1990 levels by 2050 to prevent catastrophic climate change. So one very big problem with flying is simply the distances it covers.

That's not all. Jet fuel is made of kerosene, which releases not only carbon but nitrous oxide and other gases and particles high above the earth; this increases the impact of flights on global warming. According to the International Panel on Climate Change, the global warming effects of a flight are approximately 2.7 times those of its carbon emissions alone. Because of the long distances we fly and the cocktail of substances airplanes release, there are few ways of hurting the climate more quickly and easily than by flying.

Whenever possible, it is best to travel on buses, trains, and cars along with several other passengers. When you must fly, reducing the number of flight changes is helpful: Short flights burn on average 60% more fuel than longer ones, since planes use a lot of fuel taxiing, taking off, and landing. There is also some evidence that day flights hurt the atmosphere less than night flights.

Obviously, it helps to take fewer trips and stay in one place longer. On many websites, you can calculate the greenhouse gas impact of a trip and purchase carbon offsets to help with climate stabilization projects. An independent review by Clean Air Cool Planet gave the highest rating among tax-deductible US offset projects to Climate Trust, based in Portland (carboncounter.org), and Sustainable Travel (sustainabletravelinternational.org). You can also purchase carbon offsets through Delta, Northwest, American, Continental, and many European airlines.

The best solution is to stay home. We all need to limit our flights to those that are truly necessary, beginning now--unless we are willing to sacrifice the biosphere for our pleasure and convenience.