Two striking facts about the Hay festival this weekend have been (a) the weather – everyone has doubtless commented on the sunshine blazing down on what is usually a Somme-like vista of mud seen through slanting rain – and (b) the huge audiences for the science talks, with both of the big tents filled to capacity for two talks by Martin Rees and one by Steve Jones.

Whereas the weather might in fact be a cause for concern rather than celebration – is this another bit of proof that our basking comes at the expense of the ice caps – the other fact is genuinely a matter of celebration. The interest shown in science, the informed questions that followed the talks, and the queues at the signings afterwards, were indicators of the healthy fact that there is a lively constituency of interest in science, which in this 50th anniversary year of CP Snow's Two Cultures essay is a positive sign.

In the first of his two lectures Martin Rees talked about what we might expect, and what we should be concerned about, in our world in 2050. The points he made are to be found set out at greater length in his book Our Final Century (he had first entitled this Our Final Century? his publishers removed the question mark; the American edition, in line with the greater interest in immediate gratification over there, was entitled Our Final Hour) It's a good thing Rees keeps iterating his views about the risks we face from "error and terror", given the immense harm that would accrue from very small probability events: some we might avoid, some we might rethink, some we would at least understand as they happen to us.

His second lecture was on the cosmos, an infinitely fascinating topic whoever annotates the slides – but when it is Rees's eloquence, dry sense of humour and prodigious expertise doing it, the combination is unbeatable. That is why a thousand people paid for tickets to come and listen. From the discussion afterwards it was clear that at least many in the audience were at home with talk of parsecs and the Crab Nebula.

Steve Jones's lecture was equally packed. Celebrations of Darwin and discussions of evolutionary theory are everywhere in this anniversary year, constituting one of the biggest episodes of public education in science yet attempted. Darwin and Darwinism were therefore Jones's theme. One of the high points was an anecdote illustrating the way creationists can reconcile the contradiction of fact with what they would like to believe: Jones told us that he had spent a year teaching in Botswana some years ago, where a fundamentalist form of Calvinism has long subsisted. He asked one of his students there how he reconciled the study of biology with his faith. The student replied, "it's easy; I simply accept that you evolved, but we were created."

Hay is a place where grounds for optimism are to be found, in the survival of reading, in the flourishing of intelligent interest in science and ideas, in the liveliness of the public conversation. Great stuff.