The market explosion in China played a major role in overall vehicle population growth in 2010, with registrations jumping 27.5%. Total vehicles in operation in the country climbed by more than 16.8 million units, to slightly more than 78 million, accounting for nearly half the year’s global increase.

The leap in registrations gave China the world’s second-largest vehicle population, pushing it ahead of Japan, with 73.9 million units, for the first time.

India’s vehicle population underwent the second-largest growth rate, up 8.9% to 20.8 million units, compared with 19.1 million in 2009.

What’s stunning is how far countries like China and India still have to go. Right now, there’s one car in China for every 17.2 people, compared with one car for every 1.3 people in the United States. If China caught up to the U.S. ownership rate, the country would field a billion vehicles all by itself. And car ownership in China would have surged even faster had the country not recently scrapped a series of auto subsidies last year.

So where is this all heading? According to the International Transport Forum, the global vehicle population could reach 2.5 billion by 2050. Daniel Sperling, a professor at UC Davis’s Institute of Transportation Studies, put out a useful presentation last year running through what that two billion cars would mean. Take energy: Right now, the world produces about 87 million barrels of oil per day, and most of that comes from conventional sources. A world with two billion—or more—cars will likely require boosting that to 120 million barrels per day or beyond, Sperling argues. And, given that production from conventional wells is expected to flatten in the coming decades, getting to that level will mean relying on unconventional sources like the tar sands in Alberta.

That, in turn, poses problems for climate change. Transportation, after all, currently accounts for 23 percent of the world’s greenhouse-gas emissions. And most of that unconventional oil is significantly dirtier, from a CO2 perspective, than the traditional stuff. If people in the developing world keep buying vehicles—a good thing, to be clear, since it’s a sign that the world is getting wealthier—then simple upgrades in fuel-efficiency alone aren’t going to be enough to stop a steady uptick in global temperatures. Sperling expanded on the argument in this Yale Environment360 piece, arguing that this shows the need for alternatives like electric vehicles and noting that “sprawling land use and vehicle use must be managed and restrained.” Not an easy task.