2.6.1 Weighing up the evidence: the full cast of suspects

Figure 36 (again adapted from the TAR) takes your thoughts on Question 11 on a stage. It gives estimates of the cumulative effect since pre-industrial times of the various climate change agents, with the contributions expressed in terms of radiative forcing. Note that the figure also includes yet another device for communicating the IPCC's confidence in a particular finding – an indication of the ‘level of scientific understanding’ that accompanies each estimate. This reflects the autho
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2.5.2 Environmental indicators

The notion of a link between climatic conditions and the behaviour of plants and animals (e.g. the growth of trees or coral) and the composition of natural communities or ecosystems (the type of vegetation in a given area, say) is fundamental to the use of proxy data to reconstruct past climates. Some examples of biological responses to recent climate change were included in Box 9. Here we should be wary of jumping to conclusions. Such changes involve complex living systems that can respond i
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2.2.1 Long-term rhythms in the climate

The instrumental record referred to above is based on direct temperature measurements (using thermometers), and extends back only 150 years or so. Temperatures further back in time are reconstructed from a variety of proxy data. These include historical documents, together with natural archives of climate-sensitive phenomena, such as the growth or retreat of glaciers, tree rings, corals, sediments and ice cores (see Author(s): The Open University

2.2 Records of the Earth's temperature

To put the temperature records reported by the IPCC in context, we start with a longer-term geological perspective on the Earth's GMST.


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2.1 Preamble

Here are some quotes from the ‘Summary for Policymakers’ (SPM) included in the report from the scientific working group in the IPCC TAR (IPCC, 2001a):

  • The Earth's climate system has demonstrably changed on both global and regional scales since the pre-industrial era, with some of these changes attributable to human activities.

  • Globally, it is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instru
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1.7 Summary

  1. Figure 12 summarises the ways in which the Earth's surface and atmosphere gain and lose energy. The main points are as follows:

     

    • A proportion (the planetary albedo) of the incoming shortwave radiation from the Sun is reflected (or scattered) directly back to space, mainly by clouds and the Earth's surface (especially snow and ice cover), but also by aerosols (e.g. dust, salt particles, etc.). Most of the re
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1.6 The human impact on the atmosphere: the coming of the industrial age

There is no doubt that CO2 is accumulating in the atmosphere. The record from Mauna Loa charts a continuing rise in CO2 concentration since measurements began in 1958, when the level was 315 ppm; the value had reached about 370 ppm by the end of the 20th century, and hit more than 378 ppm in 2004. Important as changes in atmospheric CO2 undoubtedly are (see below), we need to be aware that this is not the whole story of human-induced greenhouse forcing. In par
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1.5 ‘Radiative forcing’ as an agent of climate change

Since its first major report in 1990, the IPCC has used the concept of ‘radiative forcing’ as a simple measure of the importance of a potential climate change mechanism. The basic idea is straightforward. Any factor that disturbs the radiation balance at the top of the atmosphere has the potential to ‘force’ the global climate to change: it will either warm up or cool down until a balance is restored. The perturbation to the energy balance of the whole Earth-atmosphere system i
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1.3.4 The role of convection in the atmosphere

We come now to our final refinement to the simple picture in Figure 7. Recall that the troposphere is heated from below, with temperature then falling with increasing altitude. This situation sets the scene for the onset of convection – the bulk flow or circulation of a fluid driven by differences in temperature. Convection in the atmosphere plays a vital role in two further mechanisms – quite apart from the emission of longwave radiation – whereby energy is transferred from the
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1.3.3 The role of clouds

We have already identified one role that clouds play in the Earth's climate: they are highly reflective (Section 1.2.1). At any given time, about half of our planet is covered by clouds; the sunlight they reflect back to space accounts for about 55% of the total planetary albedo. However, clouds also absorb and re-emit outgoing longwave radiatio
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1.3.2 The fate of incoming solar radiation

Look back at Figure 7. In this schematic representation, what is the fate of incoming solar radiation?

Answer

It is either reflected back to space (31 units) or absorbed by the su
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1.3 Energy flows within the Earth-atmosphere system

Before we focus on the enhanced greenhouse effect, we need to refine the schematic representation in Figure 7 and draw in some of the other processes that influence the Earth's temperature – not only at the surface, but also at different levels within the atmosphere.


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1.2.2 Bringing in the atmosphere: the natural greenhouse effect

As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at the Earth's surface.

(Tyndall, 1862, quoted in Weart, 2004)

Thus, writing in 1862, John Tyndall (Figure 6) described the key to our modern understanding of why the Earth's surface is so much warmer than t
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1.2.1 Heating and cooling the Earth: the overall radiation balance

The Sun emits electromagnetic radiation with a range of wavelengths, but its peak emission is in the visible band – the sunlight that allows us to see. The wavelength of radiation has important climatic implications, as we shall see shortly. For now, we are mainly interested in the overall rate at which energy in the form of solar radiation reaches the Earth.

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1.1 Introduction

At the beginning of the 21st century, terms such as the ‘greenhouse effect’, ‘greenhouse gases’ and ‘greenhouse warming’ are printed or spoken thousands of times a week in the context of climate change caused by human activities. This section is designed to consolidate your understanding of the basic science behind these terms, and then to review what is known about the human impact on the composition of the atmosphere since the dawn of the industrial age, commonly put (in this co
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Learning outcomes

Having studied this unit you should:

  • understand the physical basis of the natural greenhouse effect, including the meaning of the term radiative forcing;

  • know something of the way various human activities are increasing emmissions of the natural greenhouse gases, and are also contributing to sulphate aerosols in the troposphere;

  • be aware of the difficulties involved in the detection of any unusual global warming ‘signal’ above the ‘background
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Introduction

This unit explores the topic of climate change and global warming. We will begin by exploring how the Earth’s global mean surface temperature is determined through a global “balancing act” of the rate of energy that comes from the Sun and the rate at which the planet returns that energy into space. We will also discuss the natural greenhouse effect, and how this contributes to a balanced global climate. We will then go on to consider the human impact on the atmosphere, including the imp
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Acknowledgements

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

The content acknowledged below is Proprietary and used under licence (not subject to Creative Commons licence). See Terms and Conditions.

Text

Box 1: ‘O sweet spontaneous’. Copyright 1923. Trustees for the E E Cummings Trust.

Box 5: Maugh, T H (2008) ‘The MIT Meteorologist’s theory
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References

Capra, F. (1996) The Web of Life. Used by permission of Doubleday, a division of Random House, Inc., and, in the UK, reprinted by permission of HarperCollins Publishers Ltd.
Capra, F. (2002) The Hidden Connections: Integrating the Biological, Cognitive, and Social Dimension of Life into a Science of Sustainability. Used by permission of Doubleday, a division of Random House, Inc., and, in the UK, reprin
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2.3 Citizens in conversation with nature and experts

Before leaving office in 2008, Sir David King (the ex-Chief Scientific Advisor to the UK Government) introduced an ethical code for scientists. This drew particularly on his experience in working across the scientific–political divide on issues of climate change. The code comprises three attributes of scientific endeavour: rigour, representation and responsibility (Author(s): The Open University