There is plenty of evidence that, over the medium term of the last few decades, average temperatures have been edging up. Not dramatically, but enough to influence the behaviour of the plants and animals around us. Thirty-five years ago, when I returned from working in Kenya, I was an enthusiastic exhibitor of plants at the annual show of the Alpine Garden Society held on the second Saturday in May in Southport. I exhibited particularly Rhododendrons, Primulas and their close relatives Dodecatheons and Dionysias. Now, many of the very same plants flower in mid-April, in the open, not in the shelter of a greenhouse. This is a consequence of milder winters and earlier springs.

Animals too have changed their behaviour - birds and insects - particularly butterflies and moths - have extended their natural ranges further north where there are suitable habitats available. One example of change due to global warming has already produce evolutionary change in the Blackcap. Blackcaps are warblers, which breed in Europe and migrate to Africa for the winter. However, milder winters have meant that over the past thirty years more and more Blackcaps are wintering in Britain. Ringing recoveries show that these are not British breeding birds opting to stay for the winter, but breeding birds from central Europe which have adopted a new migration route. In the autumn they fly north-west to Britain rather than south to Africa. Aviary experiments show that the migration direction is genetically controlled with offspring from parents that migrate north-west inheriting the preference for the new route.

This warming has always been part of a process of fluctuating temperatures over past ages. Vineyards flourished in the North Yorkshire Wolds in late mediaeval times but there were Ice Fairs when the Thames froze solid in the 16^th and 17^th centuries. In Napoleonic times, cereal cultivation in the Pennines extended much higher than would ever be thought feasible now because of a milder spell in the climate. Even I can remember the water freezing on the edge of Cardigan bay when I was a post-graduate research worker at Aberystwyth in the mid 1950s.

The problem remains - how are we to disentangle short term global warming lasting a few decades from long term climatic change lasting many centuries or even millennia? There is at present a great deal of concern about global warming created as a consequence of atmospheric pollution from our industrial processes and our affluent lifestyles. There is little doubt that large scale production of carbon dioxide, methane and other gases is creating a "blanket" in the upper atmosphere, preventing the radiation of heat out from the earth into the outer reaches of interplanetary space and so cooling our climate. But it is equally true to say that there have been much longer term variations in our climate by major expansions and contractions of our polar ice caps and our glacial fields.

The causes of these changes in the past, and probably in the future, are not fully understood - they may be related to changes in the activity of the sun and the amount of solar heat radiated. There is plenty of evidence that our polar ice caps and our glacier fields are shrinking at the present but whether this is due solely to pollution induced global warming and what is the influence of longer term climatic changes cannot as yet be determined.

In Chorley, we are fortunate to have on our doorstep at Red Moss, Horwich, one of the best studied sites of peat deposition with its record of preserved pollen in the whole of the British Isles. Pollen analysis from Red Moss gives us a picture of the vegetational history of the area and therefore an understanding of past climatic changes. Perhaps they can give us a clue to future developments.

Since the end of the Pliocene, some 2 million years ago, we have had four Ice Ages in the Northern Hemisphere, known as the Gunz, Mindel, Riss and Wurm, separated by three interglacial periods. Each glacial period was interrupted a number of times by a period of ameliorated conditions. The Wurm Ice Age had two such periods of slight improvement in the climate. We are now in the Flandrian - either the fourth interglacial or a post glacial period - only time will tell. This post glacial period began about ten thousand years ago as the ice caps retreated.

The Red Moss shows six major assemblages of tree pollen after the improvement from the first tundra type vegetation following the slow change in the climate and the gradual movement of the ice sheet north. These are:-

1. A birch, pine, juniper period. About 10,000 to 9,500 before the present. Birch is the dominant pollen type with considerable poplar and willow. Grasses and Cyperaceae are the dominant herb vegetation.
2. A birch, pine, hazel period. About 10,000 to 8,800 before present. Juniper and willow decline and grasses and Cyperaceae remain dominant herbs.
3. Hazel, pine period. About 8,800 to 8,200 before present. This period is marked by the arrival of the first elm and oak pollen.
4. Pine, hazel and elm period. About 8,200 to 7,100 before present. Birch declines and pine becomes dominant with elm exceeding oak.
5. Oak, elm alder period. About 7,100 to 5,000 before present. Birch and pine decline and alder increase dramatically. Lime and ash become common and ericaceous pollen is present in large amounts.
6. Oak, alder period. About 5,000 before present to present day. Elm, lime and ash decline and oak becomes dominant. There is a marked increase in pollen from plants of open and disturbed land such as Nettles, Dock, Ribwort Plantain, and compositae such as Artemesia and Wild Chamomile.

Note that the above dates are derived from radiocarbon dating and are necessarily approximations.

Man, 10,000 years before the present, was still in the Palaeolithic or Old Stone Age and had little impact, as a hunter-gatherer, on his environment. It was only about 5,000 years before the present that man entered his Neolithic stage and slowly introduced farming and stock rearing. The Bronze Age began roughly 4,000 years before the present and so, with improved axes, did the felling of trees and the slow clearance of the forests. This accelerated with the coming of the Iron Age, about 2,500 years before the present, and the combination of a slowly increasing human population with increased tree felling and increased grazing intensity from domestic stock, reduced tree seedling regeneration, and led to the treeless moors now so familiar to us.

What this brief survey of our post-glacial vegetational history does demonstrate is how very recent it all is. Previous interglacial periods have lasted for as long as 100,000 years and as short as 20,000 years. It is impossible to forecast when, or if, we will enter another glacial period. If we do it will probably swamp the effects of global warming due to current atmospheric pollution, and our present spell of milder winters and damp summers will be swept away in an icy grip.

In the meantime, if global warming continues, we can look forward to a more Mediterranean type climate and to growing olive trees in Chorley and harvesting our winter wheat in spring. Let us hope it is a north Mediterranean climate and not a south Mediterranean climate with its droughts and deserts.