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For almost four decades, climate scientists have been extracting cores of ice from polar regions. From the chemistry of ancient ice, they have constructed theories about how these cold wastelands drive the world’s climate system. But 29 years ago, a lowly American graduate in coal geology decided that the high ice fields of the tropics might tell a different story. After 50 expeditions in the Himalayas, the Andes, East Africa and Tibet, Lonnie Thompson has probably spent more time at high altitude than any other researcher. And with thousands of metres of ice cores stored away in his deep freeze, he believes he will prove the tropics are the true source of much of the Earth’s climate variability. Fred Pearce went to meet the man and his ice

From coal to ice – that’s a long journey. How did it happen?

I was brought up outside Gassaway, a railroad town in the coalfields of West Virginia. We were poor and life was tough. I had to take jobs just to get lunch money for school. But I joined the Boy Scouts, and that provided an opportunity to go camping in the mountains all over West Virginia. I was also fascinated by the weather. I ran a weather station in our barn and had daily charts mailed to me by the weather bureau. From those, I did amateur forecasts and won bets at school as to whether it would rain or not. Often that got me my lunch money.

I received a scholarship that allowed me to attend university, where I started in physics but ended up studying coal geology. I thought this was a good way to a job in West Virginia. But then I became involved with the Institute of Polar Studies at The Ohio State University in Columbus and went to Antarctica. I became hooked on ice and my PhD involved analysing the dust in ice cores drilled at the US’s Byrd Station in the Antarctic and Camp Century in Greenland. I began to realise what a wonderful record these cores contained and, as you can see, I never left this field of study.

What made you switch from polar to tropical ice?

There was a British fellow called John Mercer at the institute. He was a real maverick who had produced atlases of the glaciers of the world.

He had two big boxes of pictures of tropical glaciers and I started wondering why no one was taking cores from them. I showed some pictures of Quelccaya, a big ice cap in the Peruvian Andes, to the National Science Foundation and we received $7000 funding, which was about enough to get there.

But while we were there, John got robbed and when we got back to Lima he said he had no money to get me home. So he left me, a young student, in a developing country for the first time, stranded with all our equipment.

I scrounged some money and food and got back to Columbus with the equipment and about 50 cents. I was really mad. For two years, I wouldn’t talk to John. But later we became good friends, and when he died I went down with his family to spread his ashes.

John was a guy with great originality of thought. He was the first person to suggest that the west Antarctic ice sheet could collapse with global warming.

What do the tropical cores show?

Quelccaya, my first glacier, is the one I keep returning to, as it continues to provide unique information. You can see the 20th-century warming very clearly in the record of oxygen isotopes preserved in the ice.

It also provided the first tropical record of the little ice age between the 14th and 19th centuries, and it records the three decades of drought around AD 600 that probably finished off the pre-Colombian Moche empire. Quelccaya also contains a record of El Niño climate variations.

Another big discovery was a huge spike in the amount of dust in many tropical ice cores 4200 years ago. The dust seems to signify widespread drought and desertification. It coincided with the collapse of several civilisations, including the Akkadian empire in Mesopotamia and a crisis in Egypt, where writing on ancient tombs records drought, mass migrations and sand dunes crossing the Nile.

What is life like, working high in the mountains? It can’t be easy…

Doing heavy work at 6000 metres is tough. On one trip we were up on Quelccaya for three months. We had to cut the ice cores by hand into 6000 samples, take them downhill on our backs, then melt them and put the water in bottles sealed with wax. By the end, the place looked like a mining camp from the 1800s.

In more recent years, we rented ice cream trucks from freezer companies to transport our cores from the bottom of the mountain. We also pioneered many new techniques.

Early on, the Peruvian military offered us a helicopter to take our drills and generators to the summit of Quelccaya. But the air was too thin for the helicopter and it became unstable and impossible to control, so we had to abandon that approach.

Without drills and power, we improvised by finding a crevasse where we could take samples by hand from exposed ice. From those samples we published a paper on the history of El Niño.

We developed a lightweight drill and used photovoltaic cells to provide power. You can take the solar panels into the field on horses and to the summit by porter.

Where is this research taking you now? Any big breakthroughs?

One big question that we are trying to answer is why glaciers across the tropics started growing at different times.

So for instance, Dasuopu in the southern Himalayas is the highest and coldest glacier we have ever drilled. But the ice there only started to accumulate after the end of the last ice age, about 8000 years ago.

Further north, in central Tibet, the ice on the Puruogangri ice field only dates back 6400 years. The pattern in the Himalayas seems to be that the start date for glacier formation gets more recent as you go north.

That pattern seems to hold across the tropics. The glacier on Mount Kilimanjaro, Tanzania, which is very close to the equator in east Africa, started to grow 11,700 years ago. And south of the equator in the Andes, the Huascaran glacier in Peru started forming about 19,000 years ago, while further south in Bolivia, Sajama began to grow 25,000 years ago. So it is not distance from the equator that matters, nor is it anything to do with glaciation at the poles. The big question is: what drives it?

What’s your theory?

We have a paper coming out soon that will argue that the answer is a wobble in the Earth’s orbit known as the precession, which has a well-known 21,000-year cycle. It determines where the zone of maximum solar heating occurs in the tropics.

That zone progressed from south of the equator to north of it during this period, and many tropical glaciers seem to have begun soon after incoming solar radiation at their latitude was at its maximum.

Doesn’t the sun melt glaciers, not create them?

It can. But up where the glaciers first formed, it has always been extremely cold, and often very dry. In practice, glaciers are dependent on precipitation.

Maximum solar radiation brings maximum snowfall, and that is when the glaciers start to form and can grow. Once growth begins, it creates a momentum of its own.

The conventional view is that global climate has been driven by the polar regions, where the great ice caps form and where the ocean currents begin. But you have a different view…

I believe global climate is driven from the tropics. Half the Earth’s surface is in the tropics. It is where the majority of the heat reaches the Earth and from there is distributed round the globe. It is where the great climate systems like the monsoon and El Niño are based, and it is the hub of the hydrological cycle.

“Very abrupt climate change could happen in little more than a year”

There have always been global climate changes going on, and many of our tropical ice cores show signatures in isotopes and dust and so on that are identical to cores in Greenland and Antarctica. In climate models you can only make that happen when you include large tropical events, and I am convinced that is what is happening in reality.

Look at El Niño today: it has an impact on the whole globe by redistributing moisture in the tropical hydrological system.

Is global warming melting your glaciers?

Undoubtedly. Of course there will always be local circumstances, but the complete pattern of melting that we can see happening across the world right now can only be due to a change that is truly global.

What can glaciers tell us about the impact of climate change?

Most tropical glaciers are now smaller than they have been for about 5000 years, when there was a sudden cooling. In 2002 on Quelccaya, we found a plant exposed at the base of a glacier. Two months ago we went back and there were thousands of them, at 20 sites. They had all been frozen suddenly 5200 years ago.

This was the time when Ötzi the ice man met his fate in the European Alps [Ötzi’s body was found in 1991 and has since been feted as the oldest mummy preserved by ice]. And this was also the time when the annual growth rings of trees in the British Isles were the narrowest they have been in 7000 years of records. Intriguingly, it was also when the Sahara dried out very suddenly and turned from a land of lakes to desert.

I believe these events were a dramatic and sudden global response to probably a small change in the solar radiation hitting the Earth. You could say this shows that natural climate change can be dramatic without human interference. That’s true, but it also raises important questions about the susceptibility of climate, including today’s, to abrupt change.

We should be concerned that CO₂ emissions might send us above a natural threshold. Instead of the gradual climate change that is predicted, we could get very abrupt change. If the evidence of the ice cores is anything to go by, it could all happen in little more than a year.

Where next?

I may be 57, but I’m not finished yet. I would like to go to the Arctic islands off eastern Siberia, which are some of the coldest places on Earth. My wish list also includes Heard Island in the southern Indian Ocean, a terrible windswept place which has an ice field, and New Guinea, the one place in the middle of the El Niño system that still has ice. It has got to be cored soon or the ice will all have melted.

Many of these tropical glaciers are disappearing now. In 20 years, probably the only bit of Kilimanjaro ice left in the world will be in our freezer. Keeping the ice archive for the future is now part of our mission.

I also want to look at our existing ice cores to see what else we can discover. The fire history of the Amazon for instance is there in the chemistry of ice from the Andes. And we could look for micro-organisms, which may have their own stories to tell. We might develop methods that will one day help us discover life in ice cores drilled on Mars. But I admit I may be a bit old to make that trip.