Read about his remarkable career here - a rather bare-bones summary, it has to be said. Here's an extract:
MacKay's contributions in machine learning and information theory include the development of Bayesian methods for neural networks, the rediscovery ... of low-density parity-check codes, and the invention of Dasher, a software application for communication especially popular with those who cannot use a traditional keyboard... In 2008 he completed a book on energy consumption and energy production without fossil fuels called Sustainable Energy – Without the Hot Air. MacKay used £10,000 of his own money to publish the book, and the initial print run of 5,000 sold within days. Bill Gates ... called it "one of the best books on energy that has been written." (Source: Wiki)And it was from there that he went on to be the Chief Scientist at DECC 2009-14. Now you might reasonably imagine that doesn't speak too well for him: but quite the reverse - he was a lone voice of rationality in a morass of green-blob activist civil servants and half-brained politicians, and did his level best to show them the light, even breaking through sometimes with truths they just couldn't suppress. Even though they proceeded to ignore them. His work on showing that burning wood pellets for electricity generation can result in greater net CO2 emissions than coal-burning (the calculations are very complex) was revolutionary, and informs the debate that still has a long way to go before we finally stop the farce of pretending that industrial-scale biomass-burning is 'renewable'.
A great public man, as well as a very nice chap.