Historical Climatic Changes

The existence of coal seams in Antarctica and of dinosaur fossils in Spitsbergen (which is within the Arctic Circle) demonstrates that climates have changed radically during the millions of years of the Earth's history.

We also know that the positions of the continents have changed, and are still changing, as a result of movements of the Earth's crustal plates.

Hence we can postulate, for ex­ample, that during the Cretaceous period (which lasted from about 140 to 65 million years ago), when fossil evidence shows that breadfruit, fig trees and luxurious ferns grew on Disko Island in Baffin Bay, Greenland, and so this island must have been much closer to the equator than it is today.

But plate movements are slow, averaging little more than a centimeter a year. Hence the advances and retreats of the huge ice sheets dur­ing the recent Pleistocene Ice Age (from about 1,800,000 to 11,000 years ago) and the even more recent climatic fluctuations experienced in the last 1,000 years cannot be explained by plate tectonics.

Evidence has been accumulating of frequent cli­matic cycles, with alternating warm or wet peri­ods and cold or dry ones.

During the Pleistocene Ice Age, for example, there were five major periods in Europe when the ice advanced, and these glacial ages were punctuated by interglacial phases (also called interstadials). Some scientists believe that we are now in a fifth interstadial, although they have not been able to predict the date of onset of the sixth glacial age.

The evidence comes from several sources, including rock cores drilled from the ocean floors. In these core samples the abundance of fossils of cer­tain marine organisms that proliferate during warm conditions and become scarcer in cold peri­ods shows cyclic variations, which indicates that the climate also varied periodically.

Further evi­dence has been obtained from analyses of cores of ice from the ice sheets, soil samples and tree rings.

According to recent findings it appears that the Northern Hemisphere had a warmer climate between ad900 and 1300 than it does today. It was in the tenth century that Norsemen founded a settlement in Greenland, where average temperatures were estimated to be 1-4°C higher than they are today, but this settlement had disap­peared by the end of the fifteenth century, probably because of the gradually worsening climate.

In Europe the period 1450-1850 is often called the Little Ice Age. Although no precise figures exist before the invention of meteorological instruments, there is much evidence for the Little Ice Age from historical documents (including records of crop failures and paintings of frozen rivers which never freeze today), and from mod­ern analyses of such factors as seed and pollen counts in soils and deposits dating from that period. From 1850 the climate became warmer, although a few decades ago there seems to have been a certain amount of cooling - as evidenced by the fact that in 1968 Arctic ice reached as far south as north-eastern Iceland, the first time this had occurred for 40 years.