Thermometer on top of the world: At 78, John Wright returns to Iceland to check the glacier he first saw as a student

ONE of the problems facing researchers since concern arose over global warming is that they lack a thermometer, so to speak, with which to take the climate's temperature. Weather is naturally so changeable that it is difficult to sort out normal variation from any underlying signal that the climate may indeed be becoming warmer.

During the summer I revisited Lake Hagavatn, a glacial lake in Iceland, 58 years after I first went there as a Cambridge undergraduate. The lake's behaviour suggests that it could serve rather well as an indicator of climate change.

Early in this century, Hagavatn was dammed in by a glacier flowing down from the third largest of Iceland's ice caps, the Langjokull. In 1929, the waters burst out in a huge flood, which carved a gorge through the soft rock, creating a waterfall 200ft high. But within a decade, the glacier had retreated sufficiently to open up a lower exit, and the southern part of the lake began to dry out.

In 1934, along with two other Cambridge undergraduates, I surveyed the lake, waterfall, and glacier. Earlier this year, I described how, at the age of 78, I was preparing to return to Lake Hagavatn to survey any changes since my first visit (Independent, 15 June).

So, how did it work out? The short answer is that by luck and some cunning, and in spite of some bad weather, the expedition in which my wife and I took part was able to complete the whole field programme in the astonishingly short time of only five days.

We managed to photograph and film both the present exit of the lake and the dry gorge that the previous fall had created. We were able to visit the southern part of the 1934 lake, which had dried up in 1939 and which has some interesting possibilities for future research. The retreat of the glacier has halved the area of the lake.

My wife and I - without minders, as it was a relatively easy round trip of only 10 miles (16km) from our camp - visited the dry lake bed. This was the area where in 1934, I and my two companions had taken soundings from a boat. The shallow depth of a few metres that we had measured was confirmed when the 1939 flood, caused by the glacier uncovering the present lower exit, dropped the lake level by 10 metres, and dried out an area of five or so square kilometres. My wife and I wanted to see how fast this area had been colonised by plants, but there were only a few tufts of grass.

What struck us, however, was how flat this clay surface was; all around, rocks that had not been submerged were tortured into strange shapes by volcanic activity. Clearly this part of the lake must have been in existence for centuries or even millenia before the events of 1939 that dried it out. Clay beds like this are known as varves; like the rings of a tree, they provide a chronological record of the climate. In warm years the glaciers feeding them produce more meltwater and hence more clay, with thicker bands; and in cold years thinner ones.

If some enterprising scientists are looking for a useful task in Iceland, they could do worse than take deep boring equipment to this area. After trial bores to find the deepest part of the original rock floor, they could bring up a series of cores which, put in sequence, might provide a long record of the climate in the last century or so, supplementing the existing record of temperatures.

Such research would require considerable organisation and hard work because the area is accessible only either using pack ponies (as we did in 1934 from the south) or via a motorable track to the present exit - but then there is only a footbridge to cross with your equipment. In either case, the equipment would have to be disassembled into loads small enough for a pony or a man to carry. Here is a challenge that would be well worth while, preferably in conjunction with the University of Iceland. The agreement of the Iceland Research Council would be required, since it would involve removing specimens from the country.

Our expedition completed the observations to map the glacier in two days, using a camera mounted on top of a theodolite to speed the process. The hectic programme was made possible only by the younger members of our party. Led by Ted Grey, a teacher from Derby who had taken a group of pupils to Hagavatn in 1967, it included several of these, now in their forties, and one of their sons. They carried all the heavy gear, acted as 'minders' on difficult climbs, and were sufficiently interested to take over the survey observations when they realised that at 78 I would never get to the top of the mountain proposed for the second survey station.

They learnt how to do it on the first easier hill and, with great care and dedication, completed the angle observations and photography at the other. Preliminary calculations and visits by some of them to the glacier responsible for the remarkable behaviour of this lake make it clear that the glacier is at present retreating. Having lowered its surface by some 30 metres in the past six years, it is now very flat and decayed, although the area farther back is still badly crevassed, and may yet cause another temporary surge forward of the kind experienced in 1975 and 1980.

So, what next? Well, in 1937-38 I went on a wintering expedition to Thule, in north-west Greenland, in order to cross over on the sea ice and map (using photographs) 3,000 square miles of the Canadian Ellesmere Island. In the autumn, when the sea ice prevents the use of boats but is not yet safe for sledging, we mapped the area round Thule, including yet another glacier that has been in retreat; so we could investigate this again. Perhaps.

The author received the Polar Medal in 1942.

(Photographs omitted)