Evidence of humans' adverse environmental impact is everywhere, writes Philip Temple.

The number of people in New Zealand who continue to deny climate change, from government ministers down, continues to astonish.

To these people, a consensus of leading scientists and the world's political leaders are all wrong, engaged in some gigantic global scam to line their own pockets at the expense of ours.

Like latter-day Canutes, the deniers sit on the beach declaring, "No, the water level is not rising. No it's not getting any hotter. Because me and my mates know better."

Well, let me describe to all the deniers what I have been seeing with my own eyes over the past 50 years: the irrefutable evidence for global warming.

When I was undertaking research as curator for the displays at the Department of Conservation's Aoraki/Mount Cook Visitor Centre, I came across a Timaru Herald clipping from the mid-1930s.

The chief guide at Mount Cook, Vic Williams, was reported as being concerned about the warming that was taking place. He had seen significant changes in the glaciers since he first went to Mount Cook, at the end of World War I.

Half a century ago, when I started climbing and writing about mountains, I soon came across photographic evidence of what had happened to the Mount Cook glaciers since the first photographs were taken in the 1860s.

At that time, people climbed up to reach the surface of the Tasman Glacier.

The terminal ice of the Mueller Glacier butted into the slopes of Mount Wakefield, forcing the Hooker River to go beneath.

A hundred years later, the Mueller Glacier had withdrawn and shrunk to moraine slag heaps.

And when I first set foot on the Tasman Glacier, in March 1960, the first small melt lake had appeared at its terminal.

Today, the Mueller Glacier has all but disappeared except at higher altitudes, leaving behind lakes and collapsing moraines. A popular tourist activity at Mount Cook is taking a boat tour around the great Tasman Glacier Lake to observe the melting bergs.

In 1962, I undertook the first climbs in the Carstensz Mountains of West Papua with Heinrich Harrer.

Up to 5000m high, but lying not far below the Equator, the Carstensz were remarkable for having tropical glaciers to compare with the African Ruwenzori and Kilimanjaro.

But when we discovered the cairns left by the first Dutch explorers to reach these glaciers in 1936, we could already measure a dramatic retreat.

One of our first ascents was of Idenburg Top, to the west of the main range. Its summit was covered with ice like a skull cap. But it was so thin that it was possible to plunge an ice axe through to the rock beneath.

This climb was not only a first ascent but also the first and only time anyone stood on the Idenburg Top ice cap.

The ice had gone within a decade. The most recent satellite photos of the Carstensz Mountains show that the extensive glaciers we traversed in 1962 are reduced to three or four lumps of ice atop the highest points.These, too, will be gone within a decade.

Whether the melting glaciers are at Mount Cook, the Carstensz or anywhere else, they all graphically illustrate that mean temperatures have been steadily rising for about 150 years. Glacier melting and retreat has been relentless here since about 1880.

The signature of global warming is writ large and clear across the face of the Southern Alps.

Global warming and its attendant climate changes are beyond dispute and the only area for discussion is the extent, rather than the fact, of human contribution in the way of "greenhouse gases".

I first witnessed the effect of atmospheric pollution on a grand scale when I climbed to the 3000m summit of Greece's Mount Olympus with my then 10-year-old son in 1979. There was no clear view. The landscape of northern Greece and the waters of the Thermaic Gulf were shrouded in a purplish-grey industrial haze.

Flying into Athens was like descending into soup.

This experience, and witnessing the death of forests in northern Europe from acid rain, were clear proof that we were poisoning the very air we breathe.

If you believe that human emissions are having little effect then you must also believe, for example, that clearing vast areas of the world's forests has not caused desertification and flooding.

That entire ocean species, such as North Atlantic cod, have not been destroyed by overfishing.

That diverting the waters of the great Aral Sea did not turn it into a salt pan.

The list of ecological disasters caused by humans would need more space than is available in today's newspaper. So to believe that our release of vast quantities of carbon dioxide and methane into the atmosphere is having no noticeable effect beggars belief.

You may think efforts to reduce emissions will hit your pocket too hard. But if we do not do so, our pockets will disappear as fast as the glaciers.

Philip Temple is a Dunedin writer.