Global Warming

The gasoline-powered automobile was invented just over 100 years ago, when the industrial revolution was still young. Streams had long been dammed to turn mills, and coal was on its way to widespread use—it was already powering steamships and locomotives. But most energy used by humans still came from traditional fuels such as wood. In 1890, the world population was about 1.5 billion but growing rapidly. The amount of carbon dioxide (co2) in the atmosphere was just over 290 parts per million, not yet noticeably over its level throughout pre-industrial civilization.

The world population has now topped six billion and is still growing rapidly. During the past century, the amount of fossil fuel we consume has risen nearly five times faster than population. As a result, the amount of co2 in the atmosphere is now over 360 parts per million and climbing. This rapid increase in co2 concentration represents the enormous impact of our energy-consumptive lifestyle on the planet, and it is causing dangerous changes to the earth's climate. The past decade has already seen many years with above-normal temperatures. The changes in weather patterns and increases in severe events are consistent with climate disruption. Recent years have been among the warmest ever recorded.

Carbon dioxide is the most important of what are known as greenhouse gases, compounds that enable the earth's atmosphere to trap heat, like a greenhouse, but on a global scale. Too much greenhouse gas in the atmosphere causes global warming, an increase in global average temperatures above what they normally would be.

The risks of global warming are many. Human health is threatened by more frequent and severe heat waves and the spread of tropical diseases. Lives can be lost because of rising sea levels and more severe storms, which can also damage regional and national economies. The disruptions to climate are unpredictable but certainly risky. While some areas may see greater coastal flooding and inundating rains, other regions may experience droughts. Both agriculture and natural habitats can be harmed. Future generations will bear the brunt of these risks, but the effects of global warming have already been detected. Although we cannot attribute any given event to climate change, the increased risks have created a call for action to curtail co2 emissions around the world.

Oil is now the world's dominant fuel. There are over 600 million cars and trucks in the world. Both here and abroad, transportation accounts for most oil use. In the United States, we now have more motor vehicles than licensed drivers, and we travel over 2 trillion miles per year, burning 120 billion gallons of gasoline. Not counting the "upstream" emissions from producing the fuel, the result is over a billion tons of co2 pollution each year.

US cars and light trucks alone account for more energy-related co2 than the nationwide emissions of all but four other countries in the world (china, Russia, japan, and India). Our vehicles produce nearly as much co2 as all of India, which has more than triple our population. US cars and trucks emit more than twice as much fossil-fuel co2 as the economies of either south Korea or Mexico, and over three times as much as the whole of brazil. Although some of these countries are growing and industrializing rapidly, it will be decades before their level of co2 pollution per person approaches ours.

Fuel Economy And Air Pollution

The amount of co2 emitted by a vehicle is essentially proportional to the amount of fuel burned. Thus, fuel-efficient vehicles are the best choice for helping to stop global warming. And gas-guzzlers are global polluters.

For other forms of air pollution, the relation between fuel economy and emissions is more complex. Automobile emissions are regulated to a given number of grams per mile, independently of a vehicle's fuel economy, but standards are weaker for many gas-guzzling light trucks. Moreover, several factors cause NOx, HC, CO, and PM pollution to be higher when a vehicle's fuel economy is lower.

In real-world use, most vehicles' emissions are much higher than the standards levels. The reasons include the fact that automakers' and EPA emissions tests fail to fully represent real-world driving, malfunction of emissions control systems, deterioration of components, inadequate or incorrect maintenance, and sometimes tampering. A portion of this excess pollution is proportional to a vehicle's rate of fuel consumption. Automobiles that meet a more stringent emissions standard are generally cleaner than those that meet a less stringent standard. However, among vehicles that meet the same standard, those with higher fuel economy generally produce less air pollution.

A significant amount of pollution also occurs in supplying vehicles with fuel. These so-called upstream emissions occur everywhere from the oil well and refinery to the filling station and gas tank, before the fuel gets to the engine. Upstream emissions associated with a vehicle are proportional to its fuel consumption. For an average car, upstream hydrocarbon and pm10 emissions are about twice the tailpipe emissions of those pollutants. Lesser but still-significant amounts of other pollutants are also related to the amount of fuel burned. Examples include NOx from tanker trucks delivering gasoline and a whole soup of pollutants from oil refineries. Thus, higher fuel consumption implies greater pollution.

Efficiency And Safety

Other things being equal, a smaller, lighter vehicle is more fuel efficient and less polluting than a larger, heavier vehicle. But are smaller, lighter vehicles less safe? The answer is more complex than one might think. In a two-car collision, occupants of the heavier vehicle are typically subjected to lower crash forces. However, the heavier vehicle generally inflicts higher forces on the occupants of the lighter vehicle. Thus, while individuals may gain a measure of protection by driving heavier vehicles, they do so at others' expense. In and of itself, weight does not improve overall safety. Any type of vehicle, including a small car, can be safe when it has well-designed features that improve occupant protection (such as stable, energy-absorbing structures), and properly used occupant restraints (like seat belts, air bags, and child safety seats).

Many of these safety-enhancing features are reflected in crash-test scores and the related safety ratings published by national highway traffic safety administration (nhtsa) and several consumer publications which we list at the end of this section. These safety ratings are particularly helpful for identifying vehicles that have superior structures, since such features are engineered into a vehicle and are not obvious based on size, shape, or body style.

Better vehicle structures absorb crash energy, cushioning occupants from the severity of an impact. Well-designed structures also act as a "safety cage" that protects occupants from being crushed or otherwise injured by intruding parts of a crashing vehicle. Seat belts protect occupants from striking the inside parts of a vehicle or being thrown out of it. Using seat belts doubles the chance of surviving a serious crash in any vehicle, and air bags further enhance that protection. Improved stability lowers the risk of rollover, a form of accident associated with high fatalities and serious injuries. All of these design features improve safety without adverse trade-offs.

Large and heavy personal vehicles are a mixed bag when it comes to safety. For one thing, heavier vehicles are a greater menace to others on the road, including pedestrians, bicyclists, motorcyclists and occupants of smaller vehicles. Sport utility vehicles (SUVs) illustrate the fallacy of "larger is safer" simplistic thinking. While SUVs are heavier on average than passenger cars, they don't necessarily have lower fatality rates. Not only are SUVs more hazardous to others on the road, they threaten their own occupants with higher rollover risks. Many SUVs are less stable than passenger cars and provide poor occupant protection in rollovers. Pickup trucks have the same safety deficiencies as SUVs and many vans are only moderately better. Newer "crossover" body styles, such as all-wheel-drive wagons and sport wagons, reduce some of the safety liabilities of traditional SUVs. In general, because they are detrimental to both safety and the environment, SUVs should be avoided by consumers without an ongoing need for their larger power, capacity, or off-road ability.

The best guidance is to check the safety ratings as well as the green scores of models that you are considering, so that you can find the safest and greenest vehicle that meets your needs. The government safety ratings measure how well a car or truck protects its occupants in various crash tests; recently added rollover ratings indicate how stable a vehicle is. The ratings do not tell the whole story about vehicle safety, since they fail to account for how harmful a vehicle is to others on the road and how well it protects its own occupants in a rollover crash. Nevertheless, crash-test scores are a good comparison of the relative safety of vehicles within a given size class. Look for the "buying a safer car" feature on NHTSA’s website ( Additional advice, information on crashworthiness, and descriptions of safety features by make and model are provided by the ultimate car book and consumer reports.