Planet is warming but nobody can pick the point of no return, expert says

To ask if the science of climate change is "settled" is a stupid question, says Stanford University climatologist Professor Stephen Schneider.

It never will be, he insists.

Appearing before the emissions trading scheme review select committee yesterday, he said climate science dealt with a very complex system.

Some things were well established, in other areas there were competing explanations, and in yet others only speculation.

"The fact that there are speculative components does not eliminate the fact that there are well-established bits," Schneider said.

"System science works on the preponderance of evidence and you have to make risk management assessments."

In the well-established basket are the so-called "hockey stick" graphs showing steep rises in levels of carbon dioxide, methane and nitrous oxide since the dawn of the industrial era.

"For the past 150 years there is a trend of rising global mean temperatures, rising sea levels and falling snow cover in the Northern Hemisphere."

The key word is "trend". There was often considerable variability around those trend lines, Schneider argued.

Both sceptics and alarmists often went wrong by extrapolating from too short a period or too localised an effect. Or from the views of too few experts.

It could take time to resolve apparent inconsistencies in the data, he said - 20 years for reconciling temperature records from the surface, weather balloons and satellites. But it had been done.

And when the Intergovernmental Panel on Climate Change (IPCC) concluded that "the warming of the climate system is unequivocal" it chose the word with care. When it came to the future, there was a "cascade of uncertainties", Schneider said.

The IPCC uses a range of projections for emissions growth which make different assumptions about population, economic growth and technological change. Compounding that is a range of uncertainty about how sensitive elements of the climate are to emissions.

And there is a further layer of uncertainty about regional impacts of climate change on the things people care about.

But even the low end of the resulting range of outcomes (1.1C of warming by 2100) is twice what we have experienced. The high end (6.4C) would be catastrophic.

On the question of where the thresholds for irreversible change lie, nobody knows.

"In the light of that I'm always asked: 'What would you do?' Well, that's a value judgment," Schneider said.

"I think that if there is a problem created by a smaller percentage of the world's population, and it's potentially irreversible, they have a special responsibility to reverse it."