Dr James Hansen is a grandfather and he worries about the kind of world his grandchildren will inherit.

He is also an eminent climate scientist - director of Nasa's Goddard Institute for Space Studies in New York and one of the first scientists to sound the alarm on global warming back in the 1980s - so he worries in particular about the kind of climate they will have to live their lives in.

"It is an intergenerational issue," he says. "Our parents didn't know they were causing a problem. Now we can only pretend we don't know."

It is an issue bedevilled by two gaps, chasms really.

One is about the science.

"There is a huge gap between what is understood by the relevant scientific community and what is known by the people who need to know - the public. That gap has actually increased in the last several years, partly because our knowledge of the science has changed and it has become clear the matter is more urgent than we realised even five years ago."

The other gap is between governments' rhetoric on the issue and what they are actually doing about it.

In an attempt to narrow the first gap, Hansen and 14 colleagues have written a paper, "The case for young people and nature". Google it. It is a sobering read.

The most fundamental thing to measure, he says, is the Earth's energy balance - how much energy it receives from the sun minus how much it radiates back out into space. A stable climate requires them to match.

In the past few years it has become possible to calculate that with a precision not previously attainable.

"The way we can measure that is by measuring the heat content of the oceans because that is where the energy has to go. The atmosphere is very thin. It doesn't hold much heat. But the ocean is 4km deep and holds a tremendous amount, and we are now measuring that very accurately," he said.

"What we see is the planet is out of balance by at least half a watt [per square metre of surface area per annum]. It doesn't sound like much but it is equivalent to every man, woman and child having 40 hair dryers and running them day and night all year long. That's enough to melt a lot of ice."

There is a lot of inertia in the climate system because of those deep oceans and ice sheets 2km thick.

As a result the global climate responds only slowly, at least initially, to natural and man-made pressures upon it.

That inertia, Hansen says, is not our friend.

"We have only seen about half of the climate effect of gases that are already in the atmosphere. And it means that once changes are under way it is very hard to stop them."

Another source of lags in the system is how long greenhouse gases persist in the atmosphere. Add 100 carbon dioxide molecules to the atmosphere today and 50 will still be up there in 25 years' time, and about 20 will still be there in 500 years' time.

"Consequently, today's changes of atmospheric composition will be felt most by today's young people and the unborn - in other words by people who have no possibility of protecting their own rights and their future wellbeing."

Hansen says knowing the earth's energy imbalance enables them to specify accurately what the concentration of CO2 in the atmosphere must be reduced to, in order to restore energy balance and stabilise the climate.

It is 350 parts per million.

Problem is, it is already 390ppm and rising fast when, Hansen argues, emissions need to be reducing by 6 per cent a year.

Which leads to the issue of what to do about it and the second big gap, that between official words and deeds.

Nearly 20 years ago governments formally acknowledged the need to avoid dangerous anthropogenic climate change.

Yet the resulting Kyoto protocol in 1997 was so ineffective that global emissions from fossil fuels have subsequently accelerated.

The United States did not ratify its undertakings at Kyoto. Attempts to enact at the federal level measures to put a price on carbon emissions have foundered, and the outlook on that score is "relatively bleak".

"We are all in the same boat and we are going to have to phase out fossil fuels. Those countries which decide they will move towards clean energies and energy efficiency will be the ones which benefit economically the most," Hansen said.

"China is investing a lot in renewable energies and nuclear power because they want to move away from fossil fuels. If the US continues to drag its feet and deny that there is a problem or that we should do anything about it, then we are going to become a second-rate economic power. That's the thing that might wake the US up."

Hansen advocates a straightforward, transparent carbon tax, the proceeds of which would be disbursed to the people equally.

That would allow those who reduced their carbon footprints to benefit financially, while those who preferred a high-carbon lifestyle would be free to have one - provided they paid.

The carbon price would need to keep rising, but not necessarily in a straight line; there would have to be some allowance for the economic cycle.

It would be up to the market to determine which technologies won the day.

Hansen is not a fan of the alternative model for pricing carbon, emission-trading schemes.

Companies in the smokestack sector prefer them because they know they can get governments to write the rules to favour them, he says. The Waxman-Markey Bill - ETS legislation passed by the US House of Representatives but rejected by the Senate - ran to more than 2000 pages.

And big banks love them because whenever trillions of dollars are changing hands you have to have banks involved and they know they will be able to make lots of money from trading. "But what value do they add?"

The New Zealand experience shows that a carbon tax is far from proof against special pleading, however.

When it was the Labour government's policy to have a carbon tax, trade-exposed emitters lobbied just as vociferously for exemptions. Some negotiated greenhouse agreements, which agreed the terms of such exemptions, had already been concluded by the time the government abandoned the tax and started work on an ETS. As with the ETS, agriculture was to be exempt for years.

And while it is true that China is investing heavily in green technologies, it is also building coal-fired power stations at a rate of knots.

But if Hansen is a little naive on the policy front, he is authoritative on the science and compelling on the ethics.