The Climate System

1.1 Introduction

We define climate as the mean state of the atmosphere, averaged over several years and all seasons, with particular emphasis on those factors that affect the temperature of the Earth's surface, in both a local and a global mean sense. The climate system includes the Sun, the Earth's surface and some aspects of its interior, for example that which gives rise to outgassing and volcanism, as well as the atmosphere, hydrosphere, biosphere and cryosphere. In this chapter, we provide an overview of the climate system and the main ways in which it is observed and predicted to behave. The overall goal of climate research, so far as the physical sciences are concerned, is to refine observational techniques and the theoretical underpinning of climate models to the point where we fully understand the present climate and are able to make meaningful predictions of its future evolution.

Much of the current level of interest in the Earth's climate has to do with the fact that it is perceptibly changing at present, and that the accumulated change over the next century is thought likely to have serious consequences for the habitability of the planet as a whole. In considering this scenario, it is usual to distinguish between natural and anthropogenic effects on the climate. In this book, where we are mainly concerned with the physics of the problem and in particular the role of radiation in the climate system, the distinction is more one of degree than of kind. Popular concern about 'global warming', for instance, is based mainly on relatively small (although still serious in social terms) changes in the energy balance at the surface due to atmospheric compositional changes. This anthropogenic contribution to the 'greenhouse effect' is smaller than the natural 'greenhouse', which renders the Earth habitable in the first place, and largely indistinguishable in terms of the physical processes at work.

In this chapter, we give an overview of these processes, and the principal engines of change, in order that the theory of atmospheric radiation set out in the following pages may be set in the context of current interest in, and concern about, the climate on the Earth. Figure 1.1 presents a well-known diagram from the Intergovernmental Panel on Climate Change, or IPCC, an authoritative, global body charged with obtaining a balanced consensus on the problem of global change for use ultimately by politicians and other policymakers seeking to define and tackle the problem. It summarizes the main factors involved, and estimates the

Halocarbons

Aerosols

Tropospheric ozone

Stratospheric ozone

Black carbon from fossil fuel burning

Mineral dust

Aviation-induced

Aviation-induced

Organic carbon Biomass Sulphate from burning fossil fuel burning

Contrails cirrus

Land-

Aer

osol

use

ind

rect

(albedo)

eff

ect

Organic carbon Biomass Sulphate from burning fossil fuel burning

High Medium Medium Low Very Very Very Very Very Very Very Very Low Low Low Low Low Low Low Low

Level of Scientific Understanding

FlG. 1.1. The global mean radiative forcing of the climate system for the year 2000, relative to 1750. (Source: IPCC 2001)

magnitude (and direction) of their contribution to climate change in terms of a factor called radiative forcing (see §1.5). In the following sections we discuss each of these in enough detail to provide the necessary background to the later chapters in which the relevant theory and experimental application of atmospheric radiation are developed.

1.2 Solar radiation

The Earth's climate is primarily determined by the amount of solar radiation flux (W m~2) incident at the top of the atmosphere (TOA) and its spectral distribution, which for the present Sun is about 5% in the ultraviolet, 55% in the visible, and 40% in the infra-red part of the electromagnetic spectrum. The amount of energy from the Sun ultimately determines the surface temperature of the Earth, while the solar spectral flux, especially the ultraviolet flux, affects the atmospheric molecular composition.

Over a span of billions of years, the Earth's climate is affected by the evolution of the Sun, which like other stars changes its luminosity and the spectral distribution of its radiation output with age. The solar energy that warms the Earth has increased by about 30% from the time the Sun became a main-sequence star some 4.6 billion years ago, to a phase where the hydrogen is converted to helium in the stellar core resulting in long-term stable solar radiative emission. However, small changes associated with the solar cycle and the number of sunspots are seen to occur, and a change of just 1% in the solar luminosity can alter the Earth's mean surface temperature by 1 degree, a significant amount in human terms.

In addition to any changes in the output from the Sun itself, changes in the solar energy that is absorbed by the Earth's atmosphere-surface system can arise from many factors related to the reflectivity (albedo) of the planet, and from slow variations in the Earth's orbit around the Sun. Croll (1867a,b) and Milankovitch (1941), proposed theories linking the Earth's climate record, particularly the ice ages, with periodic changes in its elliptical orbit around the Sun. Milankovitch proposed that variations related to the obliquity of the Earth's axis of rotation and the eccentricity of its orbit could change the climate on timescales with periods ranging from 41000 years for obliquity variations, as high as 413000 years for eccentricity variations, and about 21000 years for precession of the equinoxes or variations in the Earth's longitude of perihelion (§5.6). The short-term variations affect the distribution of solar energy over the globe, while the longer eccentricity variations affect the amount of solar energy reaching the Earth. Milankovitch considered the combination of variations in the amount and distribution patterns of incoming radiation to be critical factors for the formation of continental ice sheets that could trigger ice ages.

1.3 The atmosphere

The pressure and composition of the present-day atmosphere were determined by outgassing from the Earth's interior, primarily through volcanic activity, with an unknown admixture of mainly volatile material due to cometary impacts. The result, the atmosphere we see today, consists almost entirely of three major constituents, nitrogen (78.1%), oxygen (20.9%), and argon (0.93%), which have not varied significantly in abundance over the last several millennia at least, and are not expected to vary much in the foreseeable future. Furthermore, these three gases each contain atoms of a single element and so possess molecular symmetries that prevent them from absorbing or emitting infra-red radiation under most conditions of temperature and pressure found in the Earth's atmosphere. These gases do affect the propagation of solar radiation through the process of Rayleigh scattering, and hence contribute to global cooling of the planet despite the fact that they do not contribute directly to the greenhouse effect. They do moderate the greenhouse effect indirectly, as the atmospheric pressure they produce affects the efficiency by which the infra-red-active molecules absorb the Earth's thermal radiation emitted to space. For instance, although Mars has more of the greenhouse gas CO2 in its atmosphere, its greenhouse effect is weaker than on the Earth, due to Mars' very low atmospheric pressure of only about 7 mbar, less than 1% of that on the Earth.

In the present terrestrial atmosphere, the trace gases, water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3), are

Table 1.1 Pre-industrial (1750) and present (1998) abundances of well-mixed greenhouse gases, their annual increase rate and atmospheric lifetime. (After IPCC 2001)

co2

ch4

n2o

CFC-11

Pre-industrial concentration

« 280 ppm

« 700 ppb

« 270 ppb

zero

Concentration 1998

365 ppm

1745 ppb

314 ppb

268 ppt

Concentration change per year6

1.5 ppm a

7.G ppb a

G.8 ppb

-1.4 ppt

Atmospheric lifetime (year)

5 to 200 c

12

114

45

a Rate has fluctuated between 0.9 ppm/year and 2.8 ppm/year for CO2 and between 0 and 13 ppb/year for CH4 over the period 1990 to 1999. b Rate is calculated over the period 1990 to 1999.

c No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes.

a Rate has fluctuated between 0.9 ppm/year and 2.8 ppm/year for CO2 and between 0 and 13 ppb/year for CH4 over the period 1990 to 1999. b Rate is calculated over the period 1990 to 1999.

c No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes.

the main greenhouse gases. The man-made chloroflurocarbons (CFCs), associated with cosmetic sprays and refrigeration, also contribute to the present greenhouse effect. These are minor constituents of the atmosphere in terms of abundance (e.g. in units ppm, i.e. parts per million by volume), and, unlike the major constituents, these abundances are prone to change. Carbon dioxide, methane, nitrous oxide and CFCs have increased systematically within the past century as shown in Table 1.1. Water-vapour abundance is largely controlled by the ocean-surface temperature and transport processes, and the global distribution of its atmospheric concentration is highly variable.

The presence of liquid water on the Earth is a primary determinant of the establishment of life. Life itself has changed the climate, for instance, by modifying the Earth's albedo by covering the land surface with plants that have a variety of reflecting properties. Biological decay has changed the absorption of ultraviolet radiation within the oceans, pure water being mainly a scatterer of solar radiation. Liquid oceans are weak reflectors of solar radiation whereas the presence of the condensed phase in the form of snow and ice rapidly changes the surface to an efficient solar reflector, with dramatic consequences for the climate.

The oceans are the main source of atmospheric water vapour, through evaporation, which depends on the net heat (both solar and terrestrial) absorbed at the surface and stored at all depths. The relatively high heat-storage capacity of the oceans buffers the Earth against rapid climatic changes, while the atmospheric water vapour they produce plays a crucial role in the control of climatic variability and change. It absorbs significant amounts of solar ultraviolet and near-infra-red radiation, and of the infra-red radiation emitted by the Earth towards space. As the atmospheric temperature falls with increased height in the troposphere, water vapour condenses to form a variety of cloud types that increase the Earth's albedo through multiple scattering of photons. Clouds also act to warm the Earth by preventing direct surface and lower-atmospheric thermal emission to space. Overall, the role of clouds is complex and the radiative role of clouds is one of the most difficult processes to simulate in climate models.

Carbon dioxide is an important atmospheric minor constituent because its strong infra-red absorption bands make it a major contributor to the energy budget of the atmosphere and surface of the Earth. Because it has a long lifetime, the abundance of CO2 tends to be nearly constant to a considerable height, although a small discontinuity in the mixing ratio has been measured at the tropopause, and its vertical distribution remains rather poorly known in the mesosphere and above where dissociation takes place. Measurements have been made in the mesosphere and lower thermosphere using rocket-borne mass spectrometers that show a sharp decline in CO2 abundance between about 90 and 110 km, from a value not greatly different from that at the tropopause to more than an order of magnitude less. This high-altitude decline in CO2 is expected on theoretical grounds due to photolysis by solar ultraviolet energy of wavelength 230 nm and shorter, leading to the production of carbon monoxide and atomic oxygen. Carbon monoxide, CO, is also produced in the stratosphere, along with water vapour, by the photochemical oxidation of methane. The role of CO in the heating and cooling of the atmosphere is much less than for CO2, but its abundance is obviously of interest because of its relevance to an understanding of the CO2 budget, and as a tracer of the atmospheric circulation at stratospheric and mesospheric levels, for which little data exists.

Changes in the mean annual concentration of CO2 with time are one of the most cited causes of recent climate change. Measurements at Mauna Loa in Hawaii, made far from the main anthropogenic sources and therefore considered globally representative, show an increase from 315.98 parts per million by volume (ppmv) of dry air in 1959 to 377.38 ppmv in 2004, an average annual increase of 1.4 ppmv per year. Over the whole of the industrial era, i.e. from about 1750 to the present day, the atmospheric concentration of carbon dioxide has increased by 31%; the rate of increase is accelerating and has been about 0.4% per year over the past two decades. The rise is largely attributed to anthropogenic emissions due to fossil-fuel burning and deforestation. About half of the anthropogenic CO2 emissions are sequestered by processes in the oceans and the land surface, which reduces or at least delays the potential greenhouse effect of these emissions. The net result is that the current concentration of atmospheric CO2 has not been exceeded during the past 420000 years, while the present rate of increase is unprecedented during at least the past 20000 years.

Ozone is formed in the atmosphere by photochemical processes involving a number of natural and athropogenic species. Its residence time in the atmosphere is relatively short, varying from weeks to months, and it exhibits significant spatial variability. In the ultraviolet spectral region, where it has strong absorption, there are no other molecules with significant atmospheric concentration to control the ultraviolet radiation that reaches the Earth's surface. Since it is formed from molecular and atomic oxygen, ozone was most likely not present in significant levels in the anoxic early terrestrial atmosphere (some 4 billions years ago) to provide a shield for early biological evolution.

The total amount of O3 in the lower atmosphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic gas emissions that result in the production of ozone. The atmospheric concentrations of many of the halocar-bon gases that are both ozone-depleting and greenhouse gases (e.g. CFCl3 and CF2Cl2), are either increasing more slowly or decreasing since 1995, in response to reduced emissions under the regulations of the Montreal Protocol. Their replacements (e.g. CHF2Cl and CF3CH2F) and some other synthetic compounds (e.g. perfluorocarbons, PFCs, and sulphur hexafluoride, SF6) are also greenhouse gases, and their concentrations are currently increasing.

Methane has increased by 151% since 1750 and its abundance continues to grow; the present CH4 concentration has not been exceeded during the past 420000 years. Methane is an important greenhouse gas, which may have had a much higher abundance in the atmosphere of the early Earth. Its role is made complicated by photochemical processes, as can be appreciated from its present role in the atmosphere of Titan, a moon of Saturn that has a thick atmosphere containing a mixture of aerosols or haze. The haze is generated by the photodissociation of methane that results in complex hydrocarbons that eventually polymerize to form large aerosol particles. These particles have the ability to absorb and scatter solar radiation from the ultraviolet all the way to the infra-red. On the Earth, methane also controls the amount of water vapour in the upper atmosphere and so plays an important indirect role in the temperature of the upper atmosphere.

Globally representative measurements of the concentration of CH4 in the atmosphere have been made since 1983, and the record of atmospheric concentrations has been extended to earlier times from air extracted from ice cores and fern layers. About half of the current CH4 emissions are anthropogenic, from fossil fuels, cattle, rice agriculture, and landfills. Carbon monoxide (CO) emissions can result in an increase in methane, while increases in NO and NO2, together referred to as NOK, result in a decrease in methane, and in an increase in tro-pospheric ozone. Emission rates from biospheric sources are inherently highly variable. Methane is removed from the atmosphere by photochemical reactions.

Nitrous oxide has increased by 17% since 1750 and continues to increase; the present N2 O concentration has not been exceeded during at least the past thousand years. About a third of current N2O emissions are anthropogenic (agriculture, biomass burning, and industrial activities). Natural sources include soils (about 65%) and the oceans (about 30%).

1.4 Clouds and aerosols

Clouds cover on average about 60% of the globe (Fig. 1.2) and play a crucial role in determining the climate by acting both as reflectors to solar radiation and absorbers of terrestrial radiation. They constitute the biggest uncertainty in climate-change predictions because of difficulties in simulating changes in the sP

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