I have heard that the Gaia theory means that the Earth is alive. What does that mean, exactly? Roger Middleton, Chester
The Earth system (Gaia) shares many attributes with a living cell; it metabolises, it responds to changes in its environment, it can die, and it reduces its internal entropy by taking in high quantum energy as sunlight and excreting infra-red radiation to space. It does not reproduce, but something that has lived about 3 billion years hardly needs to reproduce; selection theory asserts that organisms reproduce at a rate reciprocally related to their lifespan. Gaia's reproduction rate would therefore be expected to be less than one in three billion years.
Some scientists say that your suggestions for geoengineering sea algae will never work. Is it just pie in the sky? Guy Brewer, Nottingham
Those who claim that encouraging algal growth in the ocean will not reduce CO2 abundance in the air might be right, but they do not know for sure. Their arguments are based on calculations using theoretical models and not, as they should be in science, on observation and experiment. Evidence from the ice cores of Antarctica and from ocean sediments suggests that algal growth was more abundant in the ice ages. We also know from Antarctic ice core data that the low temperatures of the ice age were closely associated with low CO2. It reached as low as 180 parts per million, and this requires powerful biological pumps. What better than those of the abundant ocean algae?
Conventional farming methods produce higher yields at less expense. Is organic farming really good for the environment? Matthew Fell, York
All kinds of farming are less good for the environment than natural ecosystems, such as forests, scrub and deserts. Organic farming might be better than agribusiness per hectare, but if it produces less food, more land would be farmed and consequently there may be nothing to choose between organic and agribusiness farming so far as the environment is concerned.
Is it true that you think that Gaia has always worked in our favour but that with climate change Gaia will work against us? Helen Barrington, Winchester
Gaia works in her favour and tries to keep a habitable planet. Most certainly she does not work specifically in our favour. When we do environmental damage we may disable Gaia's ability to help. Indeed, Gaia may now be changing the Earth in a way unfavourable for us.
You have said that the UN's scientists are underestimating the speed of climate change in their official reports. Is that right? Chris Hastings, Dover
Yes, scientists who observe sea-level rise find it is happening about 1.6 times faster than the Intergovernmental Panel on Climate Change predicted. Other observers find the rate of melting of floating ice in the North polar ocean is happening faster than predicted. Just now we are long on theory and short on observations.
If we are past the point of no return, what should we do? Edward Farmer, Pickering
If we are beyond the point of no return, then our greatest efforts should go towards adapting and making sure that we survive as a species. Beyond this mythical tipping point it would be too late to try to stop global heating. If we have not yet reached the tipping point, I still think that we should concentrate on adapting and surviving. We are not good at working together to reduce land use and CO2 emissions, and there may be little time left in which to do it.
Why are you so much in favour of nuclear power as a solution? Doesn't it have lots of dangers? Hatty Hamilton, Exeter
I am in favour of nuclear energy for small, densely populated nations such as the UK, Germany, France, others in Europe, and Japan. Such nations need an abundant supply of electricity to continue civilised life, and there is no alternative to nuclear energy; we used to use coal, gas and oil but now know we cannot. Nuclear energy also happens to be the safest, the most economical and reliable of energy sources. It is foolish to reject it. Its safety record in the UK – and we are not the best – is a vast improvement over that of coal, gas or oil. Remember over 5,000 people died in one night in London in the 1950s from coal-smoke poisoning. Apart from water power and solar energy in desert nations, renewable energy is inefficient, expensive and unreliable, but with huge subsidies it makes a great deal of money for its developers. Most arguments against nuclear energy are propaganda and it is well worth asking who benefits from the flood of misinformation.
Why do you attack the green movement so much? Aren't you a green yourself? Howard James, Manchester
Yes, I am a green but, not surprisingly, an old-fashioned one. My difference with the modern greens is mainly over their failure to see that the countryside has intrinsic value for wildlife, for food supply and as park land for our ever more urban society. The countryside should not be regarded as an industrial site for wind or solar energy. I also dislike the rampant ignorance of science shown by modern greens, especially the idiotic way they class all chemicals as bad. We are all made from a mixture of chemicals and not from some mysterious spiritual brew.
Nuclear power is fantastically expensive – no nuclear power plant has ever survived without vast sums of public money subsidising it. Aren't your well-publicised positions just playing into the hands of the well-financed nuclear lobby? David Lowe, Whitehaven
Nuclear power is not more costly than coal power. It could be much less expensive than coal, but has been made expensive by the prolonged legal and political objections that confront all attempts to build a new nuclear power station. It is nonsense to suggest that there is a wealthy nuclear energy lobby. The nuclear fuel industry is tiny compared with the coal, oil and gas industries, and small compared with the renewable energy industry. The small size of the nuclear fuel industry is because one gram of uranium can deliver as much energy as a ton of coal or oil. If uranium were as costly as gold it would barely affect the price of electricity.
Why are you so hostile to renewable energy in general and wind power in particular? Norma Jones, Halifax
Mainly because they do not work under north European conditions and because they have become, through the abuse of subsidies by the greedy, a scam. I like the idea of solar thermal energy in nations with sunlit deserts, and wind energy in places where the wind blows constantly and few people live.
Where do you go to escape? Mary Plant, Plymouth
I would love to know where to go. Just now I am far too busy to escape. Our holidays are in the South West, mostly walking.
You are determined to leave the planet and "see the face of Gaia" on one of Richard Branson's strange-looking rockets. Isn't that environmentally damaging? David Christopher, Birmingham
Probably no more damaging than driving from Devon to London and back a few times or flying to Majorca every summer. If by seeing Gaia from Sir Richard's spacecraft I can give a better account of Gaia, it will have been worthwhile.
You once wrote that, as a result of global warming, "billions of us will die and the few breeding pairs of people that survive will be in the Arctic, where the climate remains tolerable". How should we choose the breeding pairs? Louise Smith, Hamilton
The Earth is already selecting the survivors. Those that move to a safer place when life gets tough are likely to survive. We are not clever enough to judge who should survive and should not try. But we may have to chose when faced by the awful question, who you allow aboard the lifeboat when there is room for only one more?
You're nearly 90. What ambitions do you have left? Henry Allen
To enjoy life to the full while I still can.