The carbon dioxide concentration in the atmosphere is measured. It's uncontroversial. It's going up. We know that has a tendency to warm the atmosphere and we should be worried about that.

The 'clean energy' challenge deserves a commitment akin to the Manhattan project or the Apollo moon landing.

Scientists surely have a special responsibility. It is their ideas that form the basis of new technology. They should not be indifferent to the fruits of their ideas. They should forgo experiments that are risky or unethical.

It's becoming clear that in a sense the cosmos provides the only laboratory where sufficiently extreme conditions are ever achieved to test new ideas on particle physics. The energies in the Big Bang were far higher than we can ever achieve on Earth. So by looking at evidence for the Big Bang, and by studying things like neutron stars, we are in effect learning something about fundamental physics.

I'm a technological optimist in that I do believe that technology will provide solutions that will allow the world in 2050 to support 9 billion people at an acceptable standard of living. But I'm a political pessimist in that I am concerned about whether the science will be appropriately applied.

I recall a lecture by John Glenn, the first American to go into orbit. When asked what went through his mind while he was crouched in the rocket nose-cone, awaiting blast-off, he replied, "I was thinking that the rocket has 20,000 components, and each was made by the lowest bidder."

Collective human actions are transforming, even ravaging, the biosphere - perhaps irreversibly - through global warming and loss of biodiversity.