For decades we’ve been told that mankind has only about 10 years to stop pumping out “greenhouses gases” or we will do irreparable harm to Earth’s climate. But the science behind this claim is extremely shaky, and good policy never comes from bad thinking.

Earth’s climate has always been unstable. It has repeatedly changed dramatically, including in ways that would be problematic for humans if they happened today. But people certainly didn’t cause any of these changes before the glaciers last retreated 12,000 years ago, and indeed almost nobody claims we caused any before the 20th century. And it’s bad science to assert that the same processes have been going on for 500 million years or more, but the cause suddenly and inexplicably changed 75 years ago.

Many journalists and politicians say, or shout, that there is a scientific consensus around anthropogenic climate change. In fact there is lively debate about what is going on, and broad awareness of the limits of our knowledge and especially of our powers of prediction. Climate is not merely unstable, it is “chaotic” in the technical sense of not being susceptible to reliable mathematical modeling using linear algebra because it is “sensitively dependent on initial conditions” or in the once-familiar acronym, SDIC.

What scientists do know, what no informed person disputes, is that the Earth has been considerably warmer than it is today for most of its history, with intermittent cold periods. It has also had far more carbon dioxide in its atmosphere than it does today. Yet it was not essentially uninhabitable in, say, the time of the dinosaurs. And these higher concentrations of CO2 did not cause a “greenhouse effect” then. So the theory that rising concentrations of CO2 will render it both far hotter and far less hospitable fail the elementary test of correspondence with known facts.

For the past 2.5 million years the Earth has been unusually cold, with repeated glaciations whose periodic advances and retreats science cannot model or predict. In fact we are still in an “ice age” today, with significant polar ice, though in a relatively warmer “interglacial”. Fortunately. Civilization would be impossible without the warming that started suddenly 12,000 years ago, and would become very difficult if the glaciers began another sudden advance. If the Earth actually is now warming, relative to 15,000 years ago or indeed the “Little Ice Age” from the end of the Middle Ages into the mid-19th century, it would be neither surprising nor man-made.

Nor would it necessarily be alarming if it continued. But in any case, if we are not causing and cannot alter patterns of climate change, the intelligent thing to do is prepare to adapt rather than wasting vast resources on doomed ecological engineering projects on a planetary scale whose impact we cannot possibly predict. And examining the past history of the Earth, and measuring the claims of global warming alarmists against that history, we find that man-made global warming is a theory without proper scientific support, and a lousy basis for making policy choices.