We are conducting the biggest experiment with nature in human history by pumping billions of tonnes of carbon dioxide into the atmosphere. Early results show the world is heating up dangerously. The obvious answer is to put an end to the experiment as soon as possible.

But many scientists and engineers are tempted to fight fire with fire – or pollution with pollution – and counteract one risky experiment with another. We have heard much this week about cooling the Earth through technical fixes, labelled “geo-engineering”.