With epic smog in the Los Angeles Basin, the state of California has led the nation's charge to reduce emissions from road vehicles for more than half a century.
And it is now state policy to reduce emissions of climate-change gases, primarily carbon dioxide, from its 30 million-plus road vehicles by a whopping 80 percent by 2050.
How the state will get there, however, depends on what kinds of zero-emission vehicles the public actually wants to buy.
Earlier models from the powerful California Air Resources Board had assumed that by 2050, a majority of those vehicles would be powered by hydrogen fuel cells.
Now, CARB has partnered with other state agencies to model a range of options for pathways in 2030 that would keep the state on the road to that goal.
The document is worth reading in some detail, but one of its core questions has to do with the powertrain technology.
Put simply: Will most zero-emission vehicles sold in the state in 2030 be powered by hydrogen fuel cells, or not?
The alternative powertrain, of course, uses similar electric-drive technology to turn the wheels, but powered by electricity from a storage battery rather than that generated by a hydrogen fuel cell.
While all automakers are at least researching the use of hydrogen fuel cells for vehicle propulsion, the three global makers who are most committed to fuel cells are Toyota, Honda, and Hyundai.
2016 Toyota Mirai hydrogen fuel-cell car, Newport Beach, CA, Nov 2014
Hyundai offered leases on its Tucson Fuel Cell utlity vehicle starting last June; it delivered 54 of the hydrogen vehicles through December 2014.
The 2016 Toyota Mirai will go on sale within several months, initially in the same limited areas of California where Hyundai sells--due to the scarcity of hydrogen fueling sites.
And Honda plans to launch a production version of its FCV Concept sedan within a couple of years, although the final vehicle hasn't yet been shown.
As of today, the U.S. is approaching 300,000 plug-in vehicles on the road--out of a population of about 250 million vehicles. In other words, it's early days yet.
But the emergence of electric-car maker Tesla Motors, let alone plug-in vehicles selling in volume from Nissan, Chevrolet, and BMW, may be causing CARB to rethink its assumptions.
The study looking at a variety of options for the next 15 years should provide fodder for more discussions about how to achieve the state's ambitious goal with vehicles that consumers have proven willing to buy.