Global warming just a natural cycle?

Global warming comes and goes in 1,500 year cycles which may have more to do with cosmic rays than fossil fuel emissions, according to a new book, reports The Telegraph.

According to Dennis Avery, one of the authors, “If this were a CO2 driven warming it should have started in 1940 and risen strongly from there. In fact warming started in 1850 and rose sharply until 1940 then decreased for 35 years.”

He claims that only half the warming that has happened since 1940 - 0.2 degrees according to his measurements - can be ascribed to man made emissions. The rest he says is natural variability.

“If you factor in the warming from the cyclical trends, it is not very frightening,” he said.

The authors say that history, ice core studies and stalagmites all agree on a natural cycle at roughly that interval that is superimposed on the longer, stronger ice ages and interglacial phases.

They point as evidence of this natural cycle to the “Climate Optimum” - a period of warmer and wetter weather than the present Earth’s climate, which took place 9,000 years ago to 5,000 years ago, and a cooling event 2,600 years ago.

During the Roman warming period from 200 BC to around AD 600 North Africa and the Sahara were wetter and supported crops. In more recent times they point to the Medieval warming of 900 to 1300, when Eric the Red’s descendants colonised Greenland and the Little Ice Age of 1300 to 1850 which saw the Norse dairy farmers on Greenland grow short from malnutrition and eventually die out.

Avery suggests that the natural cycle of warming and cooling may come from variations in cosmic rays which have been linked to cloud formation.

This theory was validated in a recent paper in a Royal Society journal (noted on this blog, for instance here) by scientists from the Danish National Space Centre who showed that "sub-atomic particles - cosmic rays from exploding stars - play a major role in making clouds. During the past century cosmic rays became scarcer as vigorous activity by the sun forced them away. So there was less cloud cover to reflect away sunlight and a warmer world, according to the Danish scientists".

You don't have to believe in the cosmic rays. All you need to accept is that climate wasn't constant before industrialisation happened along.

That must raise questions about spending trillions of dollars to make the world poorer.