He’s only been in office a few days, but President Obama has already triggered an earthquake under the global car industry.

His green light for new fuel economy rules was expected. Obama has ordered the Transportation Department to come up with new guidelines by this March.

These will probably demand that the average 2016 model-year car sold in the US is good for nearly 40 (imperial) mpg. Demanding, perhaps, but not onerous.

However, the real shock is his decision to allow the Environmental Protection Agency to consider California’s request to set its own fuel economy standards.

California has long led the world in demanding cleaner engines. Over 40 years ago, it established the Clean Air Act, which forced car makers selling in the state to produce engines that emitted progressively lower levels of the pollutants that contribute to health-damaging smog.

Indeed, California effectively banned diesel engines in 1990 and today all the buses in LA run on clean-burning compressed natural gas.

However, because CO2 emissions do not directly affect human health, the gas was not considered under the Clean Air Act. In 2003, however, California wanted to change this, and bring CO2 emissions under the remit of the Clean Air Act. The US government prevented the move.

So, when it gets the green light from the EPA, governor Schwarzenegger, will probably set fuel economy regulations that are even stricter than the US national regulations.

Alarmingly for the global car industry, 18 other US states are set to adopt California’s new fuel economy laws once they have been established.

It’s not clear yet whether California will simply stick with forcing car makers to sell vehicles that average more than 40mpg, or whether it will also demand even lower levels of tailpipe pollution as well.

California has only recently, somewhat reluctantly, allowed diesel cars to be sold again in the state, under pollution regulations so stringent they won’t be introduced in the EU until 2013.

Given a free hand, will they kill diesel power off again? If they do, meeting the 40mpg+ target will be very difficult and expensive for car makers.

In any case, it seems that President Obama’s willingness to set California free to set its own environmental standards could trigger profound changes in future cars and could even kill off some bigger vehicles altogether.

If you never thought you’d see a four-cylinder Porsche again, President Obama has just made that much, much more likely.