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 I

Introduction to the First Edition

• It is only during the last thirty years or so that human activities have been of such a kind or on a sufficiently large scale that their effects can be significant globally. Because the problems are global, all nations have to be involved in their solution.
• The intergovernmental body which has been set up to assess the problem of global warming is the Intergovernmental Panel on Climate Change (IPCC), formed in 1988.
• Their first scientific report was published at the end of May 1990. Much of the continuing assessment of climate change has been focused on the IPCC and its three working groups dealing respectively with science, impacts and response strategies.

Introduction to the Second Edition by John Houghton, 1997

• Since the publication of the first edition nearly three years ago, interest in the issue of Global Warming and concern about it has continued to grow. This revised edition takes into account the further information from the 1995 IPCC reports.
• Some of my colleagues sometimes comment on how formidable is the task of stewardship of the Earth feeling that it is perhaps beyond the capability of the human race to tackle it adequately.
• I feel optimistic for three main reasons: I have seen how the world’s scientists have worked closely and responsibly to provide a consensus; the technologies required to provide greater efficiency in the use of fossil fuels and for their replacement with renewable sources of energy are available and affordable; my belief in God’s commitment to the material world coupled with his offer of partnership in caring for it, makes stewardship of the Earth an especially exciting and challenging activity.

Chapter 1: Global Warming and Climate Change

• Many new things have happened in the last seventy years that could not have been predicted in the 1920s. The pace of change is such that even more novelty can be expected in the next seventy.
• The last million years have seen a succession of major ice ages interspersed with warmer periods. The last of these ice ages began to come to an end about 20,000 years ago and we are now in what is called an interglacial period.
• The climate of a region is its average weather over a period of a few months, a season or a few years. Most of the worst disasters in the world are weather- or climate-related.

The 1980s: a remarkable decade

• The 1980s were unusually warm. Globally speaking, the decade has been the warmest since accurate records began over one hundred years ago and unusually warm years have continued into the 1990s.
• 1995 was the warmest on record and eight of the nine warmest years in the record occurred in the 1980s and early 1990s.
• The period has been remarkable for the frequency and intensity of extremes of weather and climate. About eighty hurricanes and typhoons occur around the tropical oceans each year.
• Low-lying areas such as Bangladesh are particularly vulnerable to the storm surges associated with tropical cyclones.
• Because of the likely locations of floods and droughts, they often bear most heavily on the most vulnerable in the world, who can have little resilience to major disasters.

The El Niňo event

• Rainfall patterns which lead to floods and droughts in tropical and semi-tropical areas are strongly influenced by the surface temperature of the oceans. About every three to five years a large area of warmer water appears and persists for a year or more, known as the El Niňo.
• A particularly intense El Niňo occurred in 1982-83 when surface temperatures were 7 °C above the average causing droughts and floods in almost all the continents.

The effect of volcanic eruptions on temperature extremes

• Volcanoes inject enormous quantities of dust and gases into the upper atmosphere.
• One of the largest volcanic eruptions this century was that from Mount Pinatubo in the Philippines on 12 June 1991 which injected about 20 million tonnes of sulphur dioxide into the stratosphere together with enormous amounts of dust.
• The amount of radiation from the sun reaching the lower atmosphere fell by about 2% and global average temperatures fell by about a quarter of a degree Celcius for two years.

Vulnerable to change

• Any large change to the average climate tends to bring stress to human communities. Changes in climate which indicate a genuine long-term trend can only be identified after many years.
• Because of human activities especially the burning of fossil fuels, carbon dioxide in the atmosphere has been increasing over the past two hundred years and more substantially over the past fifty years.
• The generally cold period worldwide during the 1960s and early 1970s caused speculation that the world was heading for an ice age, but the cold trend soon came to an end.
• Later chapters will look in detail at the science of global warming and at the climate changes that we can expect, as well as investigating how these changes fit in with the recent climate record. Here, however, is a brief outline of current scientific thinking on the problem.

The problem of global warming

• Human industry and other activities such as deforestation are emitting increasing quantities of gases, in particular the gas carbon dioxide, into the atmosphere.
• Every year these emissions add a further 7,000 million tonnes, much of which is likely to remain there for a period of a hundred years or more.
• Increased carbon dioxide acts like a blanket over the surface, keeping it warmer than it would be otherwise. With the increased temperature the amount of water vapour in the atmosphere also increases, providing more of a blanket effect and causing it to be even warmer.
• An increase in global temperature will lead to global climate change. With the rapid change taking place in the world’s industry the change is unlikely to be either small or slow.
• In the absence of efforts to curb the rise in the emissions of carbon dioxide, I estimate that the global average temperature will rise by a quarter of a degree Celcius every ten years – faster than the global average temperature has changed at any time over the past ten thousand years.
• As there is a difference in global average temperature of only about five or six degrees between the coldest part of an ice age and the warm period in between ice ages, we can see that a few degrees in this global average can represent a big change in climate.
• Substantial uncertainty remains about just how large the warming will be and which regions will be most affected and in what way.

Uncertainty and response

• Until predictions improve to the point where they can be used as a clear guide to action, politicians and others making decisions are faced with the need to weigh scientific uncertainty against the cost of the various actions which could be taken in response to the threat of climate change.
• Some actions can be taken at little cost (or even at a net saving) such as programmes to conserve and save energy, reducing deforestation and planting trees. A large shift to renewable energy sources will take some time, but a start can be made.
• In the following chapters I shall first explain the science of global warming, the evidence for it and the current state of the art regarding climate prediction. I shall then go on to say what is known about the likely impacts of climate change on human life – on water and food supplies for instance.
• The questions of why we should be concerned for the environment and what action should be taken in the face of scientific uncertainty is followed by consideration of the technical possibilities for the large reductions in the emissions of carbon dioxide and how these might affect our energy sources and usage, including means of transport. Finally I address the issue of the ‘global village’
• It is clear from our current scientific understanding that global warming poses a global challenge, which must be met by global solutions.

Chapter 2: The Greenhouse Effect

The basic principle of global warming can be understood by considering the radiation energy from the sun which warms the earth’s surface and the thermal radiation from the Earth and the atmosphere which is radiated out to space. On balance these two radiation streams must balance. If the balance is disturbed (for instance by an increase in atmospheric carbon dioxide) it can be restored by an increase in the Earth’s surface temperature.

Chapter 3: The Greenhouse Gases

The greenhouse gases are those gases in the atmosphere which, by absorbing thermal radiation emitted by the Earth’s surface, have a blanketing effect upon it. The most important of the greenhouse gases is water vapour, but its amount in the atmosphere is not changing directly because of human activities. The important greenhouse gases which are directly influenced by human activities are carbon dioxide, methane, nitrous oxide, the chlorofluorocarbons (CFCs) and ozone. This chapter will describe what is known about the origin of these gases, how their concentration in the atmosphere is changing and how it is controlled. Also considered will be particles in the atmosphere of anthropogenic origin which can cool the surface.

Chapter 4: Climates of the Past

To obtain some perspective against which to view future climate change, it is helpful to look at some of the climate changes which have occurred in the past. This chapter will briefly consider climatic records and climate changes in three periods: the last hundred years, then the last thousand years and finally the last million years. At the end of the chapter some interesting recent evidence for the existence of relatively rapid climate change at various times during the past one or two hundred thousand years will be presented.

The last hundred years

• The 1980s and early 1990s have brought some unusually warm years for the globe as a whole (see Chapter 1); eight of the nine warmest years during the past century have occurred during this period (up to and including 1995).
• ‘Warmest’ means only a few tenths of a degree Celcius at most, but in terms of global averages such differences are quite significant. An increase of between 0.3 °C and 0.6 °C has taken place since 1860. Some periods of cooling as well as warming have taken place.
• How can a global average changing by a few tenths of a degree have any meaning?
• To estimate the changes over land, weather stations are chosen where consistent observations have been taken from the same location over a substantial proportion of the whole 130 year period.
• Changes in sea surface temperature have been estimated by processing over sixty million observations from ships over the same period.
• All the observations are then located within a grid of squares covering the Earth’s surface. Observations within each square are averaged; the global average is obtained by averaging (after weighting them by area) the averages for each of the squares.
• Good agreement has been achieved between analyses carried out at different centers.
• During the past twenty years or so observations have been made available from satellites orbiting around the earth, a comparatively short period in climate terms.
• A careful analysis confirms that there is satisfactory agreement between the satellite and surface measurements.
• Some variability arises through causes external to the atmosphere and the oceans. The low global average in 1992, compared with 1990, 1991, 1994 and 1995 is almost certainly due to the Pinatubo volcanic eruption.
• The warming during the 20^th century has not been uniform over the globe and there have been some areas of cooling. There is evidence that precipitation has increased over land in high latitudes of the northern hemisphere especially in winter.
• The broad features of these changes in temperature and precipitation are consistent with what is expected because of the influence of increased greenhouse gases (see Chapter 6).
• Minimum temperatures over land have increased about twice as much as maximum temperatures. An increase in cloud cover tends to obstruct daytime sunshine and to reduce the escape of terrestrial radiation at night.
• The increase from 1910 to 1940 is too rapid to have been due to the rather small increase in greenhouse gases during that period.
• From ice core data the 20^th century temperatures are at least as warm as any century since 1400, and at some sites the 20^th century appears to have been warmer than any century for some thousands of years.
• The balance of evidence suggests that there is discernible human influence on global climate.
• As greenhouse gases increase further, the amount of warming is expected to swamp the natural variation in climate.
• Over the last hundred years sea level has risen by between 10 and 25 cm, due to thermal expansion of ocean water and from glaciers retreating.

The last thousand years

• Professor Hubert Lamb of the Climatic Research Unit of the University of East Anglia has derived a record of the average temperatures for central England for the past thousand years.
• Its main features are of a medieval warm period between AD1100 and 1300 when vines were grown as far north as Yorkshire, and the ‘Little Ice Age’ between AD1400 and 1850, during which freezing of the Thames in winter was not uncommon. Evidence suggests that the ‘Little Ice Age’ could have been experienced worldwide.
• One of the largest eruptions during the period was that of Tambora in Indonesia in April 1815, which was followed in many places by two exceptionally cold years, described as the ‘year without a summer’. But the effect on the climate even of an eruption of the magnitude of Tambora only lasts a few years at most.
• Although accurate direct measurements of solar output are not available apart from those made during the last two decades from satellite instruments, other evidence suggests that the solar output compared with its value today could have been significantly lower during the Maunder Minimum in the 17^th century – a period when almost no sunspots were recorded.