Everyone knows commercial airliners are little more than flying CO2 machines, and the airline industry is taking the first steps toward making them cleaner. But far less attention has been paid to the environmental impacts of the airports where they land. They are, in a word, a mess.

Officials from many of North Americas airports made the case this week at the Eco-Aviation Conference in Washington that they're trying to clean up their acts. To be fair, the task is daunting and there are some examples of innovation.

But overall, the report card is underwhelming.

It's easy to see the size of the problem when you realize many airports are, essentially, small cities. Dallas-Forth Worth International Airport, one of the nation's largest, is a good example. It has five terminals, six parking garages, a physical plant and a rental car complex. The airport covers 18,000 acres - that's about 28 square miles - and serves 60 million people a year. The airport has enormous fleets of gas-powered tugs to move planes, tractors to move luggage and shuttles to move people. All of that internal combustion creates a lot of C02 and a lot of ground-level ozone. Run-off from the myriad chemicals that keep everything running smoothly sometimes find their way into groundwater supplies.

Airports aren't the only ones to blame. Tens of millions of people pass through them, and every one of them leaves something behind. The National Natural Resources Defense Council estimates that the average departing passenger leaves behind almost 1 1/2 pounds of trash - and that doesn't include the stuff they leave behind on the planes.

So what are airports doing about it? An Airports Council International (ACI) survey of 78 airports that represent 60 percent of North American passenger traffic (a skimpy sample, if you ask us) praises them for "impressive work."

Really?

The survey ticks off a lot of stats that sound good but show much tangible progress. Twenty-three of the airports surveyed have "environmental management systems" and 15 percent publish regular environmental reports (gasp!). But many of them don't have any benchmarks for measuring their eco-impact and fewer than half have conducted greenhouse gas inventories or developed metrics for measuring their ground pollution.

That said, there are some good things happening. Nineteen of the airports sampled are buying renewable energy and another nine are generating it on their own. Fifty-one have implemented solid waste management programs and 33 have the infrastructure to support clean vehicles.

There are some standouts: the new Terminal A at Boston Logan, with heat-reflecting roof and windows, low-flow faucets, waterless urinals, self-dimming lights and storm water filtration, is the nation's first terminal to be a certified green building.

Kris Russel, the Senior Environmentalist at Dallas Fort Worth International, points out that while green certification is an admirable goal there are less extensive but highly effective ways to make airports greener. Beyond reworking its lighting and HVAC systems, DFW's central plant uses electric chillers and a thermal energy storage tank. Airport buses now burn compressed natural gas obtained from an on-site CNG station. John F. Kennedy Airport in New York has eliminated 90 percent of its chemical de-icers in favor of infrared technology that melts snow and ice off planes.

Airports also are partnering with airlines to green up their operations. Continental, for example, is working with George Bush Interncontinental Airport in Houston to adopt electric ground equipment like airplane tugs. The airline says it will cut emissions by such equipment as much as 75 percent.

Other solutions are surprisingly low-tech. By consolidating rental car operations in a single facility, DFW was able to cut the size of its shuttle fleet in half. LED signs in the parking garages direct people to open spots, reducing the time they spend circling the lot. Airports like JFK and Dallas are taking the lead. Others will follow eventually, and it's then that real progress will be seen.

Photo by Flickr user El Fotopakismo.