(CNN) -- There are roughly 800 million cars in the world today. One day that number is going to mushroom -- but to what extent is anyone's guess. According to the Wall Street Journal, we could have 1 billion cars on the road by 2020. Forrester Research puts the number at 1.2 billion, according to Reuters. But it could be more than that.
A commuter train in Japan shows a hybrid car at a Tokyo station in 2003.
What we do know, however, is that more cars means more oil.
The world currently produces around 84 million barrels of oil a day, its biggest consumer being the United States, which consumes around 20.5 million barrels daily, 66 percent of it going toward powering America's transportation needs.
BP says the world has enough reserves to last another 40 years based on current consumption rates. But consumption rates are changing. According to the Institute for the Analysis of Global Security, world oil consumption will grow by 60 percent between now and 2020, with India and China leading the charge (China is predicted to overtake the United States as the world's biggest car market in 20 years; India could follow 10 years later).
The issue of demand versus supply and rising oil prices aside, one of the main issues surrounding oil consumption is its impact impact on the air we breathe. According to the Union of Concerned Scientists, oil-backed transportation is the single biggest air polluter in the United States, producing nearly two-thirds of the carbon monoxide, a third of the nitrogen oxides and a quarter of the hydrocarbons in the atmosphere.
According to the Sierra Club, just one car alone emits more than 63 tons of carbon dioxide (CO2) emissions into the air over its lifetime; with the sports utility vehicle, or SUV, emitting around 82 tons.
Transportation accounts for 50 percent of urban air pollution in the United States and 50 percent of all Americans now live in areas that fail to meet U.S. national air quality standards, according to the Electric Drive Transportation Association (EDTA).
But in China, a country named this year by some scientific organizations as the world's biggest polluter, the story is far worse. Car exhaust emissions now represent a staggering 80 percent of total air pollution.
There are some very real health issues connected with exhaust emissions. According to the EDTA, severe air pollution -- which vehicles have been acknowledged to have some role in -- is responsible for anywhere from 500,000 to 1 million deaths a year globally. Switching to cleaner cars seems an obvious move from many perspectives.
Different vehicle options
How this happens remains to be seen, but there are four distinct options on the table: hybrid cars, plug-in hybrid cars, battery electric cars, and fuel cell cars. For now, the auto industry appears to distinctly favor hybrid cars.
A hybrid car is essentially a vehicle powered by two different sources: an electric motor and an internal combustion engine which propels the car. The hybrid recaptures energy through a process known as regenerative braking -- where the energy normally lost through braking or coasting goes to power the electric motor.
According to Earth-Policy.org, if the United States replaced its entire fleet of passenger cars with "super-efficient" hybrids such as these over the next 10 years, gasoline use would be slashed in half.
Public hybrid buses have been tested in New York to some success, according to the EDTA, which says the buses emit 90 percent less particulate matter and 30 percent fewer greenhouse gases than regular diesel buses. Hybrid cars are also taking off in a big way in Sweden, which also is recording faster drops in CO2 emissions from new cars than any other European nation. (Europe's market share of global hybrid sales is still very small, estimated at less than 8 percent .The U.S. remains the world's biggest hybrid car market with 70 percent market share.)
However, hybrid technology has its critics. The main objections regard purchasing and maintenance costs; limited fuel economy on long-distance drives; faltering battery levels at high speeds; added car weight (hybrids tend to be 10 percent heavier than a similar sized car, according to NewCarPark.com); and conversely, negative environmental impacts. More energy is required to build a hybrid as it is a more complicated vehicle; it requires more copper wire than a regular car; and its battery which weighs more than 100 pounds is a potential environmental hazard, critics say.
But mainly, the questions many people have with hybrids is that they still rely on gasoline and they still pollute the atmosphere. While Earth-Policy says that hybrids could slash U.S. gasoline use in half, it also points out that the same usage of plug-in hybrids over the same period of time could result in gasoline usage being cut by 70 percent.
The advantage plug-in hybrids have over regular hybrids, its proponents say, is you can power them by electricity alone. They carry larger batteries than conventional hybrids, which, when recharged (by plugging them into the national grid) will give drivers up to 60 miles without any emissions at all.
A U.S. Department of Energy (DoE) study found that 73 percent of the nearly 217 million vehicles on America's roads could be charged with existing power plants to generate the electricity to charge the cars -- and greenhouse gases would fall by 27 percent as a result.
The new challenge would then be using existing power plants using coal or nuclear power. The DoE study also found -- not surprisingly - that particulate emissions would increase with the power grid having to charge all these vehicles.
The panacea is a car which produces no emissions at all -- some will say that it is an electric car, others say it is the fuel cell vehicle. For now, though, hybrid vehicles are possibly the best environmental option on the road. E-mail to a friend
Sources: Wall Street Journal, Reuters, Earth-Policy.org, University of Michigan, Goldman Sachs, Treehugger.com, Electric Drive Transportation Association, CNN.com, Sierra Club, Newcarpark.com, Plug-In America, Union of Concerned Scientists, EVWorld.com