APART from the dreaded coal-fired power stations, one of the greatest contributors to greenhouse gas production are our modes of transport - trains, planes and automobiles.
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Most city trains are electric these days and - if powered from renewables - are as green a mode of transport as you'll find.
More regional trains, like our own Bathurst Bullet, use diesel locomotives, but trains are one of the more energy-efficient ways of moving people and cargo around.
Automobiles are getting greener by the day.
Sales of electric and hybrid vehicles have shot up, and again, provided the charging energy is from renewables, they're also pretty green.
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That leaves planes.
Airplanes use fairly vast amount of hydrocarbon (fossil) fuels getting people around the planet.
But the energy requirements of planes are too large to consider using rechargeable batteries like electric cars.
Plug-in commercial aircraft are just not possible.
Hydrogen is a possible solution.
Hydrogen was used in the Saturn 5 rockets that took the Apollo missions to the moon, and when burned with oxygen it produces water, so zero greenhouse emissions.
It can power modified jet engines, and can also be used in fuel cells to produce electricity to drive motors.
On paper, it's the ideal fuel.
But it has a lot of disadvantages, too.
At atmospheric pressures it's around 3000 times less energy-dense than conventional fossil fuels, and even when liquefied it's four times less, and that brings on another problem.
Liquid hydrogen has to be kept at minus 235 degree Celsius, near absolute zero.
It's horrendous stuff to contain - it's the smallest atom there is and gets through containers and pipes that would otherwise be fine for other fuels.
It can't be stored in the wings, it needs tanks in the fuselage. And it's dangerous - remember the Hindenburg disaster?
But German company H2FLY has successfully flown an aircraft powered by an electric motor driven by a fuel cell using liquid hydrogen.
It doesn't have a huge range or carrying capacity, but it does demonstrate that the technology is viable.
There's a long way to go until you'll fly to London or even Sydney on a hydrogen powered plane, but the nay-sayers have already been proven wrong on some counts.
As early as 1988 a Tupolev TU-155 was modified to use hydrogen, and Airbus has plans for commercial hydrogen-powered aircraft by 2035.
So ... watch this space.
