GLOBAL WARMING

THE COMPLETE BRIEFING

JOHN HOUGHTON

Co-Chairman of the Scientific Assessment Working Group of the Intergovernmental Panel on Climate Change

CAMBRIDGE UNIVERSITY PRESS REPRINTED 1999

PART II

Chapter 4: Climates of the Past (continued)

The past million years

Deep cores have been drilled out of the ice at several locations in both Greenland and Antarctica. The longest and most recent core reached a depth of over 2.5 km. The ice at the bottom fell as snow over 200,000 years ago.
Small bubbles of air are trapped within the ice. Analysis of the composition of that air shows what was present in the atmosphere for the time at which the ice was formed – gases such as carbon dioxide or methane.
A temperature record for the polar regions can therefore be constructed from analyses of the ice cores.
Reconstruction from a Vostok core shows that the last major ice age began about 120,000 years ago and ended about 20,000 years ago. It also demonstrates the close connections which exist between temperature and carbon dioxide and methane concentrations.
To go further back, over the past million years, the composition of ocean sediments can be investigated to yield information. From oxygen isotope and other data we can deduce that the sea level at the last glacial maximum, 20,000 years ago, was about 120m lower than today.
Over the last million years six or seven major ice ages can be identified with warmer periods in between, the period between these major ice ages being approximately 100,000 years. Other cycles are also evident in the record.
So far as is known the output of the sun has not changed to any significant extent over the last million years or so. But because of variations in the Earth’s orbit, the distribution of solar radiation has varied in a more or less regular way during the last millennium.
Three regular variations occur in the orbit of the Earth around the sun. Although nearly circular, the Earth’s orbit is actually an ellipse. The eccentricity of the ellipse varies with a period of about 100,000 years; that is the slowest of the three variations.
The Earth also spins on its own axis, the axis of spin being tilted with respect to the axis of the Earth’s orbit, the angle of tilt varying between 21.6° and 24.5° (currently it is 23.5°) with a period of about 41,000 years.
The third variation is of the time of the year when the earth is closest to the sun with a period of about 23,000 years.
Although the total quantity of solar radiation reaching the Earth varies very little, the distribution of that radiation with latitude and season over the Earth’s surface changes considerably, being especially large in the polar regions.
In 1867 James Croll pointed out that the major ice ages of the past might be linked with these regular variations in the seasonal distribution of radiation reaching the earth.
Milankovitch showed, from inspection by eye, a significant connection between the variations of polar summer sunshine and global ice volume. Careful study of the correlation of the two curves provides support for the Milankovitch theory.
More careful study of the relationship between the ice ages and the Earth’s orbital variations shows that the size of the climate changes is larger than might be expected from forcing by the radiation changes alone. Positive feedback processes have to be introduced to explain the climate variations.
As we shall see in later chapters, climates of the past cannot be modelled successfully without taking into account the influence on climate of variation in carbon dioxide concentration.

How stable has past climate been?

The growth and recession of the large polar ice-sheets between the ice ages and the intervening warmer interglacial periods have taken on average many thousands of years.
Over the last 8,000 years or so, which can be investigated more thoroughly than earlier epochs, the climate has shown some slow changes but no dramatic fluctuations. There is substantial evidence that the last 8,000 years have been unusually stable.
In 1993, Professor Dansgaard found evidence that rapid variations of the Arctic temperature frequently occurred during the glacial period up to 100,000 years ago.
These variations have been up to five or six degrees Celsius and have sometimes occurred over periods of less than a hundred years.
For 6,000 years before the start of the Younger Dryas event the earth had been warming up after the end of the last ice age. But then the climate swung back again into much colder conditions similar to those at the end of the last ice age.
At the end of the event, 10,700 years ago, the warming in the Arctic of about 7°C occurred over only about 50 years and was associated with decreased storminess and an increase of precipitation of about 50%.
Two main possible reasons have been suggested: major breakups of ice-sheets occurred releasing massive numbers of icebergs into the north Atlantic; and that the ocean circulation in the north Atlantic region has been strongly affected by injections of fresh water from the melting ice.
At present the ocean circulation here is strongly influenced by cold salty water sinking to deep ocean levels because its saltiness makes it dense; this sinking process is part of the ‘conveyor belt’ which is the major feature of the circulation of deep ocean water around the world.
Large quantities of fresh water from the melting of ice would make the water less salty, preventing it from sinking and thereby altering the whole Atlantic circulation.
This link between the melting of ice and the ocean circulation is a key feature of the explanation put forward by Professor Wallace Broecker for the Younger Dryas event.
As the great ice-sheet over North America began to melt at the end of the last ice age, the melt water at first drained through the Mississippi into the Gulf of Mexico.
Eventually, however, the retreat of the ice opened up a channel for the water in the region of the St Lawrence river. This influx of fresh water into the north Atlantic reduced its saltiness, thus, Broecker postulates, cutting off the formation of deep water and that part of the ocean ‘conveyor belt’.
Warm water was therefore prevented from flowing northward, resulting in a reversal to much colder conditions. The suggestion is also that a reversal of this process with the starting up of the Atlantic ‘conveyor belt’ could lead to a sudden onset of warmer conditions.
Our perspective regarding the possibilities of future climate change needs to take into account the rapid climate changes which have occurred in the past.
I move on in the next chapter to describe how, through computer models of the climate, predictions can be made about what climate change can be expected in the future.

Chapter 5: Modelling the Climate

Chapter 2 looked at the greenhouse effect in terms of a simple radiation balance. That gave an estimate of the rise in the average temperature at the surface of the Earth as greenhouse gases increase. But any change in climate will not be distributed uniformly everywhere; the climate system is much more complicated than that. More detail in climate change prediction requires very much more elaborate calculations using computers. The problem is so vast that the fastest and largest computers available are needed. But before computers can be set to work on the calculation, a model of the climate must be set up for them to use. A model of the weather as used for weather forecasting will be used to explain what is meant by a numerical model on a computer, followed by a description of the increase in elaboration required to include all parts of the climate system in the model.

Chapter 6: Climate Change under Business-as-usual

The last chapter showed that the most effective tool we possess for the prediction of future climate change due to human activities is the climate model. This chapter will describe the predictions of models for likely climate change next century. It will also consider other factors which might lead to climate change and assess their importance relative to the effect of greenhouse gases.

To summarize this chapter:

The increase in greenhouse gases is by far the largest of the factors which can lead to climate change during the next century.
The likely climate change for a business-as-usual scenario of greenhouse gas emissions has been described in terms of global average temperature and in terms of regional change of temperature and precipitation and the occurrence of extremes.
The rate of change is likely to be larger than the Earth has seen at any time during the past 10,000 years.
The changes which are likely to have the greatest impact will be changes in the frequencies, intensities and locations of climate extremes, especially droughts and floods.
Sufficient fossil fuel reserves are available to provide for continuing growth in fossil fuel emissions of carbon dioxide well into the twenty-second century. If this occurred the climate change could be very large indeed and have unpredictable features or ‘surprises’.

The next chapter will look at the impact of such changes on sea level, on water, on food supplies and on human health. Later chapters of the book will then suggest what action might be taken to slow down and eventually to terminate the rate of change.

Chapter 7: The Impacts of Climate Change

The last two chapters have detailed the climate change which we can expect next century because of human activities in terms of temperature and rainfall. To be useful to human communities, these details need to be turned into descriptions of the impact of climate change on human resources and activities. The questions to which we want answers are: how much sea level rise and what effect will that have?; how much will water resources be affected?; what will be the impact on agriculture and food supply?; will natural ecosystems suffer damage and how will human health be affected? This chapter considers these questions.

How much will sea level rise?

There is plenty of evidence for large changes in sea level during the Earth’s past history.
Various contributions to the likely sea-level rise next century are shown in fig. 7.1. The largest comes from thermal expansion of water in the oceans. The other main contribution comes from the melting of glaciers.
Estimates for the average sea-level rise under the business-as-usual scenario is 12 cm by 2030 and 50 cm by 2100. Sea-level rise will not be uniform over the globe.

The impacts of sea-level rise

Half of humanity inhabits the coastal zones around the world. Within these, the lowest lying are some of the most fertile and densely populated. To people living in these areas, even a fraction of a metre increase in sea level can add enormously to their problems.
Some of the areas which are especially vulnerable are Bangladesh and similar delta areas, the Netherlands and the small low-lying islands in the Pacific and other oceans.
It is quite impractical to consider full protection of the long and complicated coastline of Bangladesh from sea-level rise. Substantial amounts of good agricultural land will be lost.
Half the country’s economy comes from agriculture. 85% of the nation’s population depends on agriculture for its livelihood. Many of these people are at the very edge of subsistence.
Bangladesh is extremely prone to storm surges. Every year, on average, at least one major cyclone attacks Bangladesh. The storm surge in 1970 claimed the lives of over a quarter of a million people.
At the present time, it is estimated that saltwater intrusion extends seasonally inland over 150 km. With a one metre rise in sea level, the area affected could double, with a large effect on fresh water.
A similar situation exists in the Nile delta of Egypt. Many other examples of vulnerable delta regions can be given.

The impact on fresh water resources

The global water cycle is a fundamental component of the climate system. Water is also a key substance for humankind; to drink, for the production of food, for health and hygiene, for industry and transport.
Water use averaged per capita varies from 1,000 cubic metres (220,000 imperial gallons) per year to over 50,000 cubic metres (11 million imperial gallons).
Those in very poor areas may walk many hours each day to fetch a few gallons.
The demands of increased populations and the desire for higher standards of living have brought with them much greater requirements for fresh water.
Two-thirds of human water use is for agriculture, much of it for irrigation; about a quarter is used by industry and 10% is used domestically. Non-replaceable ancient water is being tapped for current use.
With this rapid growth of demand comes greatly increased vulnerability regarding water supplies.
Many of the world’s major sources of water are shared. The Danube passes through 12 countries, the Nile through 9, the Ganges-Bramaputra through 5.
The availability of fresh water will be substantially changed in a world affected by global warming. The increase in temperature means higher evaporation, when some parts of the world will have less rainfall, especially in summer, resulting in less run-off.
Other parts of the world will have increased precipitation. During the period 1980-85, more than 160 major floods were recorded.
The effects of droughts tend to be felt over a long period of time. Droughts and floods are likely to occur in locations where, at present, such disasters are rare. Extensive deforestation can lead to large changes in rainfall.
Irrigation is applied to one-sixth of the world’s farmland which produces about one-third of the world’s crops. Microirrigation techniques provide large opportunities for water conservation. Management of existing infrastructure can be improved.

Impact on agriculture and food supply

Every farmer understands the need to grow crops or rear animals suited to the local climate.
Between the mid-1960s and the mid 1980s global food production rose by an average annual rate of 2.4% – faster than global population. Grain production grew at an annual rate of 2.9%.
Two of the primary crops grown in Peru are very sensitive to the amount and timing of rainfall and are strongly affected by El Niňo events. Improved weather forecasting and planning by farmers has resulted in improved food production following El Niňo events.
Production in developed countries with relatively stable populations may well increase, whereas that in many developing countries (where large population increases are occurring) is likely to decline as a result of climate change.
The disparity between nations will tend to become larger, as will the number of those at risk from hunger.
Firstly, there is large need for technical advances in agriculture in developing countries requiring investment and widespread local training. Secondly, improvements need to be made in the availability and management of water for irrigation, especially in arid or semi-arid areas of the world.
There is yet no strong evidence that the effect of climate change on global food supply is likely to be large. What needs urgent research is how well world agriculture will respond to extremes, such as prolonged droughts.
The surplus of food in developed countries is likely to increase, while developing countries will face large population increases coupled with a likely relative decrease in food production. Such a situation will raise enormous problems and serious deprivation especially in the developing world.
Agriculture is the main source of employment in developing countries. People will tend to migrate to places where they might be employed in agriculture. With pressures of rising populations such movement is likely to be increasingly difficult and we can expect large numbers of environmental refugees.

The impact on natural ecosystems

A little over 10% of the world’s land area is under cultivation. The rest is unmanaged by humans. Of this about 30% is natural forest.
Changes in climate alter the suitability of a region for different species. In the past these changes took place over thousands of years. With global warming similar changes occur over a few decades. Most ecosystems cannot respond or migrate that fast.

The impact on human health

Many of the factors that lead to a deteriorated environment also lead to poor health. Pollution of the atmosphere, polluted or inadequate water supplies, and poor soil (leading to poor crops and inadequate nutrition) all present dangers to human health and wellbeing and assist the spread of disease.
The main direct effect of climate change on humans will be that of heat stress in the extreme high temperatures that will become more frequent and more widespread.
A further likely impact of climate change on health is the increased spread of diseases in a warmer world. The potential impact of climate change on human health could be large.

Chapter 8: Why should we be concerned?

I have been describing the likely changes in climate which may occur as a result of human activities, and the impact these may have in different parts of the world. But large and potentially devastating changes are likely to be a generation or more away. So why should we be concerned? What responsibility, if any, do we have for the planet as a whole and the great variety of other forms of life which inhabit it? And does our scientific knowledge in any way match up with other insights, for instance ethical and religious ones, regarding our relationship with our environment? In this chapter I want to digress from the detailed consideration of global warming (to which I shall return) in order to briefly explore these fundamental questions and to present something of my personal viewpoint on them.

Chapter 9: Weighing the Uncertainty

This book is intended to present clearly the current scientific position on global warming. A key part of this presentation must concern the uncertainty associated with all parts of the scientific description, especially with the prediction of future climate change, which forms an essential consideration when decisions regarding action are being undertaken. However, uncertainty is a relative term; utter certainty is not often demanded on everyday matters as a prerequisite for action. Here the issues are complex; we need to consider how uncertainty is weighed against the cost of possible action.

Chapter 10: A Strategy for Action to Slow and Stabilize Climate Change

Following the awareness of the problems of climate change aroused by the IPCC scientific assessments, the necessity of international action has been recognized. In this chapter I address the forms that action could take.

Chapter 11: Energy and Transport for the Future

We flick a switch and energy flows. Energy is provided so easily for the developed world that thought is rarely given to where it comes from, whether it will ever run out or whether it is harming the environment. Energy is also cheap enough that little serious attention is given to conserving it. However, most of the world’s energy comes from the burning of fossil fuels which generates a large proportion of the greenhouse gas emissions into the atmosphere. If these emissions are to be reduced, a large proportion of the reduction will have to occur in the energy sector. There is need, therefore, to concentrate the minds of policymakers and indeed of everyone on our energy requirements and usage. This chapter looks at how future energy might be provided in a sustainable manner.

Chapter 12: The Global Village

The preceding chapters have considered the various strands of the global warming story and the action that should be taken. In this last chapter I want first to present some of the challenges of global warming, especially those which arise because of its global nature. I then want to put global warming in the context of other major global problems faced by humankind.

Global warming, climate change, weather extremes