As polluters go, John Broadhurst is small potatoes. Every work day he drives alone from his home in Golden Valley, an inner-ring Twin Cities suburb, to his parking spot at the University of Minnesota in Minneapolis.
Broadhurst, born and raised in England, is a physicist. As well as anyone on the planet, he realizes the effect on the environment from burning fossil fuels.
The combustion is putting billions of tons of carbon dioxide into the atmosphere that's been sequestered underground for eons in oil and coal deposits.
Broadhurst's modest commute in his six-cylinder Taurus is a rather minor contributor to the problem. But he doesn't let himself off the hook.
"I started with a small automobile, and I've slowly grown to larger automobiles. And I feel guilty every time, but like most people, I don't do anything about it," Broadhurst says.
Not doing anything about it, collectively, is having a big impact. Scientists examining core samples find that carbon dioxide is at its highest level in 650,000 years. The United States is the largest contributor of carbon dioxide to the atmosphere.
There's been a flurry of scientific reports recently, asserting the carbon dioxide buildup is speeding global warming. Scientists think the warming is a reason polar ice is melting, and why weather patterns are more erratic.
But few of the changes are close enough or dramatic enough to spur changes in personal behavior.
Physicist Broadhurst can't offer much guidance as to when the effects of dramatically higher CO2 concentrations might cause us to sit up and take notice.
We have to alter human behavior, and altering human behavior is not done by one scientist writing a report.
"Whether it's over the next 10, 100 or 1,000 years, one can't really say," he says.
Broadhurst is leaning toward 1,000 years. But in the next breath, he hedges. He points to the "runaway effect," the sharp and sudden weather changes that might occur if global warming alters the ocean currents that influence temperature and precipitation.
Our personal transportation decisions rank fairly high among factors affecting the amount of carbon dioxide in the atmosphere.
Carbon dioxide emissions from cars, trucks and other vehicles are rising faster than from manufacturing or electrical generation, the other major sources of CO2.
Minnesota Pollution Control Agency spokesman Jeff Buss says the state's CO2 emissions from transportation are increasing.
"Cars today are kicking out as much CO2 as cars 20, 30 years ago," Buss says.
Part of the reason is our taste for vehicle power and size has outpaced improvements in engine efficiency.
Steve Winkelman, a policy analyst for the Washington, D.C.-based Center for Clean Air Policy, says overall national fuel economy has been flat or declining.
"Vehicle engines have actually gotten a lot more efficient. It's just that that efficiency has gone into moving more metal more quickly, so vehicles are heavier and have more power," Winkelman says.
There are also more drivers who own more vehicles, and drive more miles.
In Minnesota during the last decade, the Environmental Protection Agency reports CO2 emissions from transportation increased 16 percent, a rate faster than the state's population growth.
The rate at which vehicles gulp gas and emit CO2 breaks down this way.
Drivers in this country, operating everything from Hummers to hybrids, burn 400 million gallons of gasoline a day. The combustion creates four million tons of carbon dioxide.
There's a vast difference among vehicles.
A full-sized pickup truck with an eight-cylinder engine driven about 12,000 miles a year creates eight tons of carbon dioxide. A four-cylinder car driven the same distance creates four tons.
These days it's not uncommon for a family to own at least one of each. Twenty years ago Roseville resident Rob Murray and his wife owned one vehicle. Now the family of five owns three.
Murray's wife teaches at the same school two of his children attend, but they drive two vehicles.
"We do it basically for the freedom. Then my son can stay after and do the extra-curricular (activities) and then come home, and my wife isn't shackled to stay at school for a long period of time," Murray says.
St. Paul resident Liza Pryor laments the old days, when she and her husband could zip around in her high-mileage subcompact.
"We had to go to a bigger car because we had more kids, and car seats seemed to keep getting bigger and bigger," Pryor says.
In rural Minnesota, Carol Ford says she and others sacrifice fuel economy and exacerbate CO2 emissions by driving long distances to their jobs. Ford lives in Milan, in southwestern Minnesota, and drives 35 miles one way to Morris for work.
"I don't know what kind of adjustments you can make, because it's not like there's a lot of other places you can find a job if you're lucky enough to have one out here. You've got to be able to get to it," Ford says.
Twin Cities residents have more choices. However, low-density suburban development and a highly developed network of roads have a big impact on choices.
Most of the drivers choose to go it alone. Solo commutes are the rule rather than the exception. Metropolitan Council officials say nearly 80 percent of Twin Cities commuters drive to work alone.
All the personal choices aside, there are lots of fuel-saving advances being built into cars, and many more on the way.
Bruce Jones, an automotive technology professor at Minnesota State University, Mankato, says new transmissions and more efficient fuel combustion technology are adding miles to each gallon of gas burned.
Jones says the result is reduced CO2, because of the one-to-one relationship between driving and carbon dioxide emissions.
A 5 percent increase in fuel efficiency, he says, means a corresponding decline in CO2.
"Just that small 5 percent increase gets us moving in a direction of reducing the total CO2 load 5 percent," he says.
Jones says there are plenty of gasoline-powered cars in showrooms that can cut carbon dioxide emissions nearly in half -- if a driver is willing to buy a 40-mile-per-gallon vehicle instead of one that gets 22, the national average.
"They (manufacturers) all make them that are in the upper 30s to low 40s in fuel economy," Jones says.
Engines powered with diesel fuel get even better mileage, and with more power.
University of Minnesota mechanical engineering professor David Kittelson says in Europe, cars burning diesel account for nearly half the vehicles on the road.
"If we went to a dieselized passenger car fleet, we could reduce CO2 emissions, global warming gases very significantly," Kittleson says.
Diesel-powered vehicles get much better mileage than ones using gasoline. But diesel is the ugly duckling of automotive fuels, powering what many regard as clanky, smelly engines that belch clouds of black smoke.
However, there's no telltale fuel smell in the University of Minnesota's diesel lab, as Kittelson leads a tour. Nearly a dozen diesel engines of various sizes sit on benches or in metal cradles.
Kittelson shows off redesigned engines for passenger cars and heavy equipment that use much less fuel.
"The performance level is the same, but the CO2 emissions are 30 percent lower," Kittleson says.
On a trip to France in the summer of 2005, Doug Toavs and his wife rented a diesel-powered Ford Focus. Toavs says the car was peppy, quiet and comfortable, and got well over 40 miles per gallon.
Fuel efficiency is a big priority for his family. Toavs and his wife live in Chisago City north of the Twin Cities, and work for the same company in St. Paul, but on different schedules. They commute in two vehicles, driving more than 150 miles a day.
He's ready to buy the diesel-powered Ford Focus he rented in France.
"I went on the Web to see if I could find one of those because it seemed to be a logical next car for us, and it's not available in this country," Toavs says.
Plenty of Americans appear ready to consider vehicles that get better mileage. Car makers are responding with additional choices.
However, nearly all the manufacturers cater to buyers who want lots of horsepower, hauling power and room, even if the capacity is rarely used.
Toyota, famous for its fuel sippers including hybrids, has a new version of its full-size Tundra pickup that weighs more than two and a half tons. It hasn't been rated for fuel consumption, but comparable models get 16 miles per gallon.
The choices illustrate what Doug Toavs says is Americans' attitudes toward what we drive.
"We have a mindset here that's so independent, that's 'My way or forget about it,'" Toavs says.
Attempts by lawmakers to step in and regulate that mindset have been slow in coming. Eight states are trying to nudge drivers toward more fuel efficient vehicles by adopting California's tougher air emission standards.
The standards don't directly cap carbon dioxide. But the rules cause car manufacturers to build more efficient vehicles that burn less gasoline, thus creating less CO2.
Sen. John Marty, DFL-Roseville, who chairs the Senate Environment and Natural Resources Committee, says it's time to encourage Minnesotans to buy more fuel-efficient vehicles. This session he'll propose Minnesota adopt California's vehicle emission standards.
"If we change the standards ... then regardless of people's habits, we'll have a very significant reduction in carbon dioxide," Marty says.
However, Rep. Tom Hackbarth, R-Anoka, the chair of the House Environment and Natural Resources Committee, says his constituents don't want tighter vehicle emission standards.
Hackbarth agrees global warming is happening, but says there's not enough known about the causes.
"I need to hear both sides of the story before we actually initiate some laws, that put some burdens on some folks that may not be necessary," Hackbarth says.
The U of M's John Broadhurst says attention to how our behavior contributes to CO2 emissions is overdue, given the risks posed by global warming.
"We should be alarmed because we have to alter human behavior, and altering human behavior is not done by one scientist writing a report," Broadhurst says. "It's done by people wishing to alter their behavior as a mass, rather than just an individual."
But drivers appear reluctant to alter habits. There's no evidence we're willing to drive less, car pool or downsize the family fleet.
In the public policy realm there's little political taste at the moment for higher gasoline taxes, tighter emissions standards and higher fuel efficiency requirements that would reduce emissions.
The only exception occurs in April, when federal hearings begin on proposed modest increases to fuel efficiency standards for light trucks -- standards which haven't been increased in 15 years.