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Your story on space junk notes that the untrammelled and exponential growth of the number of satellites in Earth orbit, largely for private profit, stacks up problems, known and possibly unknown, for the future ( 30 October, p 42 ). When once useful artefacts, designed with a limited lifespan in mind, outlive their utility, they …

Models of consciousness that you detail don't explain why evolution of biological information processing should create conscious subjective experience ( 4 September, p 44 ). One possibility is that subjective experience is a mechanism for continuity of processing under a biological constraint – the timescale of transmission across synapses . This is substantially slower than …

The five graphs in "Who is tackling climate change best?" ( 30 October, p 10 ) should have stated emissions in millions of tonnes of CO 2 equivalent.

Ingrid Newkirk, People for the Ethical Treatment of Animals, Washington DC, US Hurrah that many nations have pledged to stop razing forests by 2030, but, at the rate they are being felled, will any be left by then? Reactions to the COP26 climate summit show that while many are unhappy with the failure to take …

COP26 focused on persuading all nations to aim for net-zero carbon emissions by 2050, in the hope this will prevent global temperature rising by any more than 1.5°C ( 23 October, p 36 ). The major polluters are unlikely to meet the first target, and so the second will probably be exceeded in the near …

The skull of a small child found on a ledge in a cave with no signs of carnivore activity or large-scale water movement doesn't indicate to me that it is firm evidence of the early human species Homo naledi burying its dead ( 13 November, p 9 ). Young children hide in nearly inaccessible places …

Regarding Albert Einstein's wish, amid conundrums on the nature of reality thrown up by quantum theory, that he could be sure the moon was still there if he wasn't looking at it ( 6 November, p 38 ). I wonder if a related question sheds any light on this: is the sun there if I …

Quantum mechanics doesn't predict that the moon isn't there when you aren't looking at it, but that you can't simultaneously measure its position and momentum (and other properties) with absolute certainty. However, the uncertainty is defined by Planck's constant, which is so small that the effect is insignificant for visible objects.

The increased demand for minerals to build renewable energy kit, such as wind turbines, solar panels and mega batteries, emphasises the need for a circular economy in which the necessary minerals, once extracted from the ground, are never reburied as waste, but reused again and again ( 13 November, p 38 ). It also highlights …

I have an idea regarding lifestyle approaches to try to reduce the chance of developing Alzheimer's disease ( 6 November, p 46 ). Older people aren't in a position to wait for a drug cure, so every beneficial lifestyle strategy should be practised. Two important ideas have been suggested: getting seven hours of sleep and …