Q: Should we expect to see “plug-in” hybrid cars anytime soon? I’ve been hearing they are on the horizon but I wonder if that means in one year or 10.

– Bill A., Stratford, Conn.

A: Gasoline-electric hybrids now, like Toyota’s Prius, don’t need to plug in – you just fill their tanks with gas and the battery is charged by the internal combustion engine and by energy recaptured when braking. The battery runs the electric motor when idling, backing up, crawling in gridlock, maintaining speed while cruising, and for extra uphill power. The electric motor is a back-up; the hybrid relies mainly on the gas engine.

Plug-in hybrids let owners plug their cars into a standard electrical outlet to recharge. The vehicles will go 40 or 50 miles on a charge, ideal for commuters who drive short distances to work and back. Those drivers may be able to rely solely on electric power. The gasoline engine then becomes the supplemental one for when the car needs to go farther than the electric motor can take it.

According to researchers at the University of Cali­for­­nia, Davis, the electricity cost of powering a plug-in hybrid is about one-quarter the cost of the gas needed to run a like-sized, gas-only vehicle.

Toyota, by far the world’s largest producer of hybrid vehicles, says it expects to have a commercially viable plug-in hybrid as early as 2010. It is testing prototype versions of plug-in hybrids at two California universities.

US automakers are also jumping onto the plug-in bandwagon. General Motors says that it will have mass-market plug-in hybrids on the road by 2010. Ford has also developed a small fleet of plug-ins, but is not yet ready to offer them to the public. Chrysler’s Sprinter van was the first plug-in from a major US manufacturer, but it is currently available only to a limited number of institutions as a fleet vehicle.

Plug-ins have also caught on elsewhere. Chinese carmaker BYD plans to sell a plug-in hybrid sedan in the United States within five years. And Volkswagen hopes to have a plug-in hybrid Golf ready to roll by 2010.

Q: I was intrigued to hear that there were a number of ways one could modify or construct a roof on a house or office that would provide great environmental benefit. Can you enlighten?

– Bill Teague, Menlo Park, Calif.

A: Most roofs are designed to shed rain and so are hard and impermeable. As a result, rainwater runs off, collecting impurities on its way to municipal storm sewers, which eventually empty into local bodies of water.

Minimizing this runoff means that more impurities will remain in local soils where they can be broken down more easily into their constituent elements than if they are concentrated downstream. In order to achieve this goal, landscape architects have developed “green roofs,” which use living plants and soil on top of a building in order to absorb, collect, and reuse rainwater while also preventing runoff.

Buildings employing green roofs find many uses for the water collected, from watering exterior plantings at ground level to flushing toilets inside.

Steven Peck, of the Toronto-based nonprofit Green Roofs for Healthy Cities, says such roofs can play a big role in maintaining ecological integrity within paved-over areas. “The roofscapes of our cities are the last urban frontier – from 15 percent to 35 percent of the total land area – and the green roof industry can turn these wasted spaces into a force for cleaner air, cleaner water, energy savings, cooling, beauty, and recreation,” he says.

The US Environmental Protection Agency (EPA) encourages the creation of green roofs to mitigate the urban “heat island effect,” whereby temperatures in crowded cities can soar some 10 degrees F. higher than in less-developed areas nearby. Such roofs also provide amenity space for tenants, reduce heating and cooling costs, scrub carbon dioxide out of the air and heavy metals out of rainwater, and increase bird habitat.

Certain inorganic materials can also make an existing roof greener. The nonprofit Cool Roof Rating Council (CRRC), for instance, suggests roofing surfaces that reflect the sun’s heat so as to reduce the urban heat-island effect while improving residential energy efficiency. According to the group, “a cool roof reflects and emits the sun’s heat back to the sky.” Builders can check out CRRC’s website for a database of information on the radiative properties of various roofing surfaces.

Long-lasting roofs, like slate or metal ones, are also more ecologically appealing, though costly.

Got an environmental question? Write: EarthTalk, c/o E – The Environmental Magazine, Box 5098, Westport, CT 06881. Or: earthtalk@emagazine.com