Some environmentalists still think of automakers as unrepentant defilers of the Earth.
But increasingly, people dedicated to protecting the planet acknowledge that car companies must be free to produce products that people want - and must be able to make a profit - while they try to minimize the impact of cars and trucks on the natural world.
Fortunately for automakers, that kind of realism is common among leaders of environmentally oriented organizations pressing for action to limit hazards posed by greenhouse gases.
Here are three leaders who approach the issue from the perspectives of investment, government experience and science.
Mindy Lubber: Warming as a business opportunity
LUBBER: The business case
Mindy Lubber is a leading voice for looking at climate change as a business proposition.
She heads a Boston-based organization called Ceres, formerly known as the Coalition for Environmentally Responsible Economies.
Its members are environmental and public interest groups and investment funds, including some pension funds, with total assets of about $400 billion. Ceres also represents the Investor Network on Climate Risk, institutional investors with about $3 trillion in assets.
Besides trying to protect the planet and shareholder investments, Ceres seeks to show companies and the investment community that dealing with climate change should be viewed as an opportunity to make money.
A 2005 Ceres report on the auto industry made a simple but arresting point: The companies whose products produce the most greenhouse gases could be in the most financial jeopardy when society grapples with climate change.
No surprise: The Detroit 3 are big emitters.
Lubber acknowledges that the business case for addressing climate change is a tough one to make to struggling companies such as the Detroit 3. And it's not an easy sell to any corporate executive whose main focus is the next quarterly earnings report.
But she thinks the message is getting through, slowly.
"We are certainly seeing radical change in rhetoric, and I think action will follow," Lubber says. "They can't afford not to act at this point."
Eileen Claussen: Come to the table
CLAUSSEN: Govt. must act
Eileen Claussen has been in the fight almost from the beginning.
Her style is insistent, not shrill. And she believes the marketplace will sort out which technologies are best for reducing vehicle emissions of greenhouse gases. In her view, the leading contenders are battery-electric vehicles, hydrogen and cellulosic ethanol - made from plant waste rather than grain.
But she also believes some government controls are necessary.
Claussen was an administrator of atmospheric programs at the EPA from 1987 to 1993 and was an assistant secretary of state when the global community completed the Kyoto climate-change treaty in 1997.
Even before the treaty was final, the U.S. Senate - in a symbolic 1997 pre-emptory resolution - turned thumbs down by a vote of 95-0. Soon after taking office, President Bush said he would not submit the treaty for a formal Senate vote. About 170 countries have ratified the treaty.
For eight years Claussen has led the Pew Center on Global Change. It favors a comprehensive approach to the problem, including mandatory controls and voluntary actions by business.
Among the 41 companies that work with the Pew Center, Toyota is the only automaker.
Member companies don't give money but are expected to reduce their own emissions, make technology available to others and support good public policies.
Toyota has yet to take a strong stand on U.S. policy, Pew officials noted.
Claussen says overseas-based automakers have been more willing than the Detroit 3 to discuss climate policy at international conferences. And she believes that when legislation begins to percolate in the United States, some car companies will reach this conclusion:
"At some point they (will) decide something is going to happen, and they better come to the table and at least talk about what they like and what they don't like."
Stephen Pacala: 60 mpg by midcentury
PACALA: Car guy, optimist
Stephen Pacala, a self-described car lover, heads the Carbon Mitigation Initiative at Princeton University. Ford Motor Co. and BP PLC, the London-based energy company, are the major corporate contributors.
As an environmentalist, Pacala worries that worst-case predictions about climate change could come true - that the polar ice caps will melt and vast coastal areas will be flooded.
But as a car-loving optimist, he also believes humans can prevent disaster without giving up the vehicles they enjoy so much. Pacala, who drives an aging Ford Ranger pickup, cites the torque curve of electric cars as evidence that environmentally friendly vehicles can be fun to drive.
"The good news is that most of the problem is not coming out of the tailpipes of people's cars," Pacala says. "It's simply a misconception that people have."
But it's also true, he says, that cars and trucks are part of the problem and therefore should contribute to the solution.
The Princeton program is not working directly on vehicle technology to reduce greenhouse-gas emissions. But one of its big accomplishments, Pacala says, has been to develop a way to look at climate change as a manageable problem.
The scheme has attracted a lot of attention.
It divides expected greenhouse-gas emission increases over the next half century into wedges, each one representing a reasonable step that could be taken to avoid an increase in emissions. In effect, each economic sector can see what it would have to do to meet the goal of stabilizing greenhouse gases.
Viewing cars and trucks as one wedge, Pacala says the global car and truck population will triple to 2 billion vehicles in the next 50 years. So vehicles will have to average about 60 mpg by midcentury to keep emissions near today's levels.
It sounds daunting, but Pacala insists the industry need not try the hardest thing - hydrogen fuel cells - first. Instead, he says, it should focus on known and available technology, such as plug-in hybrids.
The advantage of relying more on electric power in vehicles, such as plug-in hybrids, is this: Emissions from dozens of utility plants are easier to control than those from millions of motor vehicles. Plants can use fuels with lower emissions, such as nuclear or biomass, or the carbon dioxide can be captured and stored, scientists say.
Arguing that automakers can reach the goal of dramatically improved fuel economy while still satisfying customers and making money, Pacala says, "We are a clever species."
You may e-mail Harry Stoffer at firstname.lastname@example.org