Global Warming – Back to the Future

Dr Duncan L Copp

September 2002
Updated December 2005

No-one should doubt the importance of global warming. It's arguably the most significant environmental issue facing the population of the planet today. While little doubt exists among the scientific community that human activity is changing the climate, the real issue is trying to predict the degree and timing of that change. Current estimates suggest a warming of 3-5°C by 2100. But to what extent can the climate change without human interference?

There is considerable evidence to suggest that it can and that presents quite an alarming picture. Earth's climate has already undergone many dramatic swings and this was long before we created technologies capable of altering the atmosphere. We need to fully appreciate this picture in order to put global warming (or cooling) predictions into perspective and to understand the degree to which we humans may be the cause.

Climates past

With all the talk of warming, it may seem surprising to learn that we're living in a period that geologists call The Great Ice Age, a period that spans the last 2.6 million years. Ice ages have occurred throughout Earth's history and the last 2.6 million years is a mere blink of an eye in the great scheme of things. Yet this period is significant because scientists know more about climate change during this ice age than any other. This is purely because the events are recent enough to have left clues that are yet to be erased by the natural processes of erosion.

The term 'ice age' is actually rather misleading. During an ice age the planet is not always gripped by ice, as one might imagine. Instead there are marked warmer and cooler episodes known as glacial (cool) and interglacial (warmer) periods. We currently live in an interglacial period, with the last glacial period ending a mere 18,000 years ago. Travel back some 20,000 years and the world was a very different place. Then, many parts of the Northern Hemisphere were frozen wastelands. Ice sheets over 3 kilometres thick covered much of North America and northern Europe. Huge glaciers flowed as far south as Berlin and New York, and icebergs were common off the coast of Portugal. Global sea levels were 100 metres lower than they are today. We could have walked from England to France.

Travel back a further 100,000 years and you'd encounter the last warm interglacial period. Now elephants, rhinoceroses, hippopotamuses and cave lions roam subtropical southern England, the bones of which have been unearthed in Trafalgar Square, London, amongst other places. In fact, 49 cold-warm swings have been identified within The Great Ice Age.

The climate detectives

Piecing together Earth's past climate is one of science's greatest detective stories. Fortunately for us, Earth didn't commit the perfect crime; it's left numerous clues. The extent and duration of the last glacial period (approximately 110,000 to 18,000 years ago) is known mainly from direct evidence. Geologists study such clues as scars in the landscape left by moving ice sheets or glaciers, and strange distributions of rocks and sediments dumped by the ice when it melted. However, reconstructing the climate over the past few 100,000 years requires a unique type of geological time machine.

At a bleak outpost called Vostok in east Antarctica, scientists endure six months of darkness and bitter cold to extract cores of ice from over 2.5 kilometres below the surface. Locked within the ice are priceless samples of ancient air - microscopic bubbles that became trapped in snow hundreds of thousands of years ago. By studying the composition and comparing the different amounts of gases contained in the bubbles, it's possible to reconstruct past climate conditions.

Ice-core samples can be used to construct an incredibly detailed picture of climatic temperatures as far back as about 400,000 years. On average, global temperatures during glacial periods are about 5°C cooler than today's global average (15°C), but in interglacial periods global temperatures are 2-3°C higher. What's more, the detective work has been detailed enough to show that some climatic swings have occurred in very short periods of time. For example, the globe warmed by 7°C in just a few decades at the end of a 1000-year cold snap that happened around 12,000 years ago.

It seems then that natural climate changes are just as significant as those predicted for human-induced global warming. So which should we fear the most? Understanding what causes natural swings in climate temperature may help answer the question.

Cause of climate wobbles

The timing of glacial and interglacial events relates to cycles in Earth's rotation and its journey around the Sun. Neither Earth's orbit around the Sun nor its rotation are perfect. And when Earth 'wobbles' it influences its own climate over periods of tens or hundreds of thousands of years. This observation was made early in the 20th century by a Yugoslav astronomer, Milutin Milankovich. The cycles are known as Milankovich cycles.

The wobbles in the motion of Earth slightly alter the amount of heat arriving from the Sun, which in turn can kick-start or stop a period of glaciation (icing up). However, this isn't the whole story. The situation is more complex and also involves the amount of carbon dioxide and methane (the greenhouse gases) contained in the atmosphere.

From analysing the gas bubbles trapped in Greenland ice, it's apparent that the level of greenhouse gases in the atmosphere has been intimately linked to climate change for at least 500,000 years. Changing levels of carbon dioxide and methane and changing temperatures follow the same patterns of oscillation or swing. Higher levels of greenhouse gases are present at higher temperatures. Why this pattern?

It appears the slight temperature change caused by the Milankovich cycles may effect how carbon is stored on Earth. When carbon is released from its natural stores (oceans, coal, oil, gas and plants) into the atmosphere it quickly combines with oxygen to make carbon dioxide gas. But when carbon is stored it's not free to make carbon dioxide gas. Change Earth's temperature slightly and carbon can move more rapidly from store to atmosphere or the other way around, depending on the conditions.

For example it can go from a large biomass, let's say a forest, into the atmosphere, or from the atmosphere into the ocean. The amount of methane and carbon dioxide in the atmosphere affect global temperatures because both are potent greenhouse gases, that is, they act like insulation. So while the Milankovich cycles may initiate a period of cooling or warming, it's really the distribution of carbon that controls climatic conditions. This conclusion is very significant when considering how we may disrupt climate cycles.

Out of the frying pan ...

Normally the carbon in coal, gas and oil remains locked in rocks for many millions of years. But that's all changed in the last few hundred years. Each year we pour around six billion tonnes of carbon dioxide into the atmosphere through burning fossil fuels (coal, gas and oil). Carbon that was safely locked away is being dumped into the atmosphere at a phenomenal speed. The Greenland gas bubbles show the level of carbon dioxide in the atmosphere has swung between 170 and 280 parts per million during the past ice age, as carbon moved in and out of natural stores. Today, levels stand at 370 parts per million.

Should we worry about this fact? After all, it's clear that the climate can swing rapidly from hot to cold without our intervention. Perhaps dumping carbon dioxide into the atmosphere may actually be beneficial by preventing the next glacial period of the ice age?

The answer is we should indeed be concerned. Recent studies indicate that human activities are upsetting the natural carbon cycle. This meddling is likely to cause considerable climatic chaos in the future and could disrupt natural climate cycles for thousands of years.

While we pump six billion tonnes of carbon dioxide into the atmosphere yearly, the measured atmospheric increase is only three billion tonnes - half our emissions are going somewhere else. That somewhere else has recently been discovered - the rainforests. To the astonishment of scientists, within the last few decades the Amazon rainforest has started growing rapidly, feeding and locking carbon dioxide from the atmosphere into its leaves at a much faster rate than is released by natural decay. It appears we are actually fertilising the trees with our fossil fuel carbon emissions.

The effect of this forced fertilisation of the rainforest is to slow global warming, protecting us from the full effects of our own emissions. Such rapid growth of the rainforests is curbing the rise of carbon dioxide in the atmosphere and although it sounds like a perfect situation, it's very much a false economy. Global temperatures are still predicted to rise and areas that are now covered in rainforest may soon change, with potentially catastrophic consequences.

... into the fire

Surrounding the Amazon rainforest is the grass and scrub savannah. Savannah differs from the rainforest in that it gets less rainfall, and the vegetation there dries out during the summer. This drying greatly increases the risk of fire and savannah naturally burns about twice a decade. Rising temperatures in the near future are likely to dry the wetter rainforests and this, of course, will increase the threat of major fires there.

The rainforests are thought to contain five to ten times more carbon than the less vegetated savannah. Should the rainforests succumb to major fires the carbon they have been soaking up for many years would be quickly released into the atmosphere. Furthermore, the carbon store that was protecting us from the full effect of our carbon emissions has now gone up in smoke! The effect? Another rapid rise in global temperature.

It's possible the chain of events may not stop there. The rapid increase in global warming owing to the burning of the rainforests may well interfere with another store of carbon - methane hydrate.

Methane hydrate is a solid substance formed from rotting organic matter and is typically found trapped within shallow ocean sediments. It's estimated that there is as much methane hydrate trapped in ocean sediments as there is oil, coal and gas put together. The problem is that methane hydrate only remains solid under a very narrow range of pressures and temperatures.

Global warming will heat the oceans and could render the hydrates unstable, releasing vast quantities of methane into the atmosphere. Since methane is about 60 times more effective as a greenhouse gas than carbon dioxide the threat would be colossal. It could tip the climatic scales in such a way as to suppress any further period of global cooling for tens or hundreds of thousands of years.

These human-induced climate scenarios are based on the latest and most sophisticated computer models. What's more, there is strong evidence from the geological record that a catastrophic carbon release (perhaps from methane hydrates) occurred around 55 million years ago. Then, global temperatures rocketed by 15°C, and took some 60,000 years to return to their normal levels.

Could the human race survive a rapid global temperature increase of 15°C? And do we want to find out?

Find out more

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Websites

A Brief History of Environmentalism
A whistlestop tour of environmentalism. From the lone pioneers camped out in the American wildernesses to 65,000 people from all over the world gathering at the Johannesburg Earth Summit.

Green Nature
http://greennature.com/article821.html
Article on climate modelling, the African Rainforest and Africa Savanna.

Milankovich Cycles in Paleoclimate
http://deschutes.gso.uri.edu/
~rutherfo/milankovitch.html
Milankovich cycles are cycles in the Earth's orbit that influence the amount of solar radiation striking different parts of the Earth at different times of year. They are named after a Serbian mathematician, Milutin Milankovitch, who explained how these orbital cycles cause the advance and retreat of the polar ice caps.

Nuclear Power – Cure For Global Warming?
Greenhouse gases are on the rise. So are global temperatures and sea levels. Our use of fossil fuels is to blame. Could it be time to say 'Nuclear Power? Yes please.'

The Politics of Carbon
Scientists calculate that the only way to prevent devastating climate change is to reduce global carbon emissions by about 60% from today's levels by 2050. How will the international community tackle this? Can the politicians pull it off?

UK Weather 2080
Global warming is changing the climate. What will become of the Great British weather? Powerful computers have been attempting to predict this. You can find out here what's in store for us.

Books

	Science at the Edge: Global warming by Sally Morgan (Heinemann, 2002) This series investigates scientific developments and theories that give rise to popular and widespread interest, concern and controversy. Buy from Amazon
	Global Warming by Chris Oxlade (Franklyn Watts, 2002) Basic guide to what causes this potentially catastrophic climate change. Buy from Amazon
	The No Nonsense Guide to Climate Change by Dinyar Godrej (Verso Books, 2001) Explores the history of climate change and the symptoms of today's global warming. Investigates the contribution of humans to the temperature change and asks what we can do about it. Buy from Amazon

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