Screenshot of Directflight.com&39's new tool for finding carbon-friendly flights shows the &39'greenest&39' and &39'cheapest&39' options from its selection of airlines.

Directflights.com is now offering a search engine for finding the cheapest carbon-friendly flights between two destinations, the company announced today.

The tool was developed in conjunction with The Carbon Consultancy, a British company that offers reports on emissions data and offset procurement information for travel industry companies.

It offers to show international flights, as well as those contained within the U.K. and Europe, or contained within the U.S., among other places. Flights can be viewed by cheapest only, or by most carbon-friendly with a choice to see flights in different currency options.

The tool ranks flights on a carbon-friendly scale of 1 to 10, with 1 being the most carbon-friendly.

The carbon ratings are not the exact data for a given flight, but based on the carbon footprint of the plane model used by a particular airline for that particular route, using official EU aircraft fuel consumption data, and UK government fuel to emissions conversion values, according to Directflights.com.

The poster-flight for the site is a Virgin Atlantic flight between London and New York that costs 376 pounds ($597), only 41 pounds ($65) more than the least carbon-friendly option, according to a Directflights.com press release.

The tool is limited to certain airlines, and testing of several flights found that applying the carbon-friendly option is usually a lot more than the 41-pound difference example given by Directflights.com.

The Web site may serve more to show to people just how expensive it is to fly mixed-fuel flights compared to conventional jet fuel flights.

Looking at U.S. flights with the tool, one could choose the cheap roundtrip New York City (JFK) to San Francisco (SFO) AirTran option for $257, or a carbon-friendly Delta flight for $624. The Delta link led to a Virgin America flight priced at $625 before taxes.

Virgin America, as well as Virgin Atlantic, have both been adding biofuels to its fleet fuel mix, as well as offering consumer carbon offsets.

The most egregious difference was in a search for a roundtrip flight from Bristol airport in England (BRS) to Berlin Schoenfield airport in Germany (SXF). It rendered only one option, a KLM flight for a whopping 1,369 pounds ($2,174). Since it was the only flight the tool found, it was also labeled greenest. That's an insane price to pay for conscientious flying compared to EasyJet's 111-pound ($176) budget option for the same dates. As the site only includes certain airlines, the EasyJet flight was obviously not even included in its grid. One would have to know of the famous European budget airline's common routes.