Cotton W.R., Pielke R.A. Human Impacts on Weather and Climate

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Concluding remarks 149

For some incomprehensible reason, funding of inadvertent weather modification

has apparently been tied to funding of advertent weather modification. To some

extent, funding for inadvertent weather modification fell through the cracks of the

funding agencies. It does not fit into the mission-oriented agencies that have sup-

ported planned weather modification, so little support came from those agencies.

It does fit within the mission of the Environmental Protection Agency (EPA), but

the research program there, with respect to inadvertent local and regional weather

and climate changes, was seriously weakened under the Reagan administration.

Because inadvertent modification and planned weather modification were placed

within the same program in the National Science Foundation, cuts in the planned

weather modification program also cut the already meager program in inadvertent

weather modification. Again, the lack of a lead agency or a funded, coordinated

program in weather modification appears to be limiting progress in furthering

our quantitative understanding of deliberate and inadvertent human impacts on

weather and climate on the local and regional scale.

Part III

Human impacts on global climate

Overview of global climate forcings and feedbacks

8.1 Overview

The Earth’s global climate system consists of the atmosphere, oceans, land, and

continental glaciers as illustrated in Fig. 8.1. In this framework, variables such as

salinity, soil moisture, and flora are integral to the functioning of this dynamic

system. All of these variables are climate variables. Several of these climate

forcings were discussed in a regional context in Chapters 5 through 7. Here

we present a global perspective. This definition of climate is broader than the

definition of climate as long-term weather statistics (Pielke, 1998).

A climate forcing is defined as “an energy imbalance imposed on the climate

system either externally or by human activities” (National Research Council,

2005). Climate forcing can be separated into radiative and non-radiative forc-

ing following the definitions provided in National Research Council (2005). A

radiative forcing is reported in the climate change scientific literature as a change

in energy flux at the tropopause, calculated in units of watts per square meter.

A non-radiative forcing is a climate forcing that creates an energy imbalance that

does not immediately involve radiation. An example is the increasing latent heat

flux resulting from agricultural irrigation. A direct forcing is a climate forcing that

directly affects the radiative budget of the Earth’s climate system. For example,

this perturbation may be due to a change in concentration of the radiatively active

gases, a change in solar radiation reaching the Earth, or changes in surface albedo.

A climate feedback is an amplification or dampening of the climate response to

a specific forcing due to changes in the atmosphere, oceans, land, or continental

glaciers. These definitions are summarized in Appendix C as given in National

Research Council (2005).

Figure 8.2 schematically illustrates the relation of climate forcings to the climate

response. Also included in this figure are identified forcings, which are discussed

in this chapter.

153

154 Overview of global climate forcings and feedbacks

The climate system

Atmosphere

Cryosphere

Land

Sun

Oceans

– Temperature

– Currents

– Salinity

– Marine

biota

– Temperature

– Humidity, clouds, and winds

– Precipitation

– Atmospheric trace gas and

aerosol distribution

– Snow cover

– Ice cover

– Temperature

– Soil moisture

– Fauna and

flora

Volcanoes

Figure 8.1 The climate system, consisting of the atmosphere, oceans, and land.

Critical state variables for each sphere of the climate system are listed in the

boxes. For the purposes of this report, the Sun, volcanic emissions, and human-

caused emissions of greenhouse gases or changes to the land surface are consid-

ered external to the climate system. From National Research Council (2005).

NATURAL

PROCESSES

Sun, orbit, volcanoes

HUMAN

ACTIVITIES

FORCING AGENTS

CHANGE IN CLIMATE

SYSTEM COMPONENTS

CLIMATE RESPONSE

Temperature, precipitation, vegetation, etc.

• Emissions of greenhouse gases and precursors, aerosols and

precursors, and biogeochemically active gases

• Solar irradiance and insolation changes

• Land-cover changes

• Fuel usage

• Industrial practices

• Agricultural

practices

• Atmospheric lapse rate

• Atmospheric composition

• Evapotranspiration flux

Societal

impacts

Direct

radiative

forcings

Indirect

radiative

forcings

Feedback

Non-radiative

forcings

Figure 8.2 Conceptual framework for the climate forcings that impact the

physical components of the climate system under present-day conditions. From

National Research Council (2005).

Atmospheric radiation 155

8.2 Atmospheric radiation

The principal source of energy driving the atmospheric/ocean system comes from

the Sun. In order to understand the potential for human impact on global climate,

we must have at least a general understanding of how the Sun’s energy is dis-

tributed through the atmosphere–Earth–ocean system. The Sun emits most of its

radiant energy over wavelengths ranging from 0.2 to 18 m, with a peak in inten-

sity at 0 470 m. As shown in Fig. 8.3, the spectrum of energy emitted by the Sun

closely approximates the energy spectrum emitted by a perfect absorber–emitter

substance (or what we call a blackbody) having a temperature of 5900 K. On

the other hand the spectrum of energy emitted by the Earth emits radiant energy

with a peak in spectral density at wavelengths between 10 and 15 m. Thus the

combination of radiation energy emitted by the Sun as received by the Earth at

its orbital distance and emitted by the Earth span from less than 02 m to greater

than 50 m. The spectrum of energy emitted by the Sun and the Earth, however,

exhibit very little overlap. We therefore refer to the energy emitted by the Sun as

shortwave radiation and the energy emitted by the Earth and its atmosphere as

longwave radiation.

Solar irradiation curve outside atmosphere

Solar irradiation curve at sea level

Curve for blackbody at 5900 K

, H

O, CO

0.25

0.20

0.15

0.10

0.05

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2

WAVELENGTH (μm)

(W m

–2

–1

)

Figure 8.3 Spectral distribution curves related to the Sun; shaded areas indicate

absorption at sea level, due to the atmospheric constituents shown. From Gast

et al. (1965). © McGraw-Hill; Reprinted by permission.

156 Overview of global climate forcings and feedbacks

As atmospheric radiation passes through the atmosphere it interacts with the

gases and particulates. Some of the radiant energy is absorbed, some scattered,

and some unaffected. The amount of energy absorbed and scattered varies with

the wavelength of the radiant energy, the type of gases, and the size and chemical

composition of the particles. The absorbed energy is converted to heat in the

atmosphere and that energy is reradiated at a wavelength and intensity that corre-

sponds to the temperature and composition of the absorbing gases or particulates.

The amount of energy that is scattered, however, is simply redirected in space (or

reflected) with no change in intensity. The amount of energy scattered and the

directions in which it is scattered depends on the wavelength of the radiation as

well as the types of gases and size and composition of particles.

8.2.1 Absorption and scattering by gases

As shown in Fig. 8.3, the energy spectrum incident at the top of the atmosphere is

significantly attenuated by the time it reaches the Earth’s surface at sea level. It is

not attenuated uniformly across the spectrum, however, but instead major energy

losses occur over rather narrow bands, called absorption bands. In a cloud-free

clean atmosphere the primary absorbers of shortwave radiation are ozone and

water vapor. Airborne particles, or aerosols, in typical non-polluted air contribute

less to absorption. At wavelengths less than 03 m, oxygen and nitrogen absorb

nearly all the incoming solar radiation in the upper atmosphere. Between 0.3 and

08 m, however, little gaseous absorption occurs. Only weak absorption by ozone

takes place over this spectral range. At wavelengths less than 08 m Rayleigh

scattering by oxygen and nitrogen molecules also depletes the radiant energy that

reaches the surface. At longer wavelengths absorption in various water vapor

bands is quite pronounced while weak absorption by carbon dioxide and ozone

also occurs.

Absorption of longwave radiation occurs in a series of bands. The principal nat-

ural absorbers of longwave radiation are water vapor, carbon dioxide, and ozone.

Figure 8.4 illustrates the infrared spectrum obtained by a scanning interferometer

looking downward from a satellite over a desert. Strong absorption by carbon

dioxide at a wavelength band centered at 147 m is shown by the emission of

radiance at a temperature of 220 K which is representative of stratospheric tem-

peratures. The stratosphere contributes mainly to the peak of this band. Water

vapor absorption bands at 1.4, 1.9, 2.7, and 63 m, and greater than 20 m,

cause emissions corresponding to midtropospheric temperatures. Little absorption

is evident in the region called the atmospheric window between 8 and 14 m.

Here the satellite observed radiance corresponds to the surface temperature of

the desert except for a slight depression in magnitude due to departures of the

Atmospheric radiation 157

Wavelength (μm)

15 10 8 7

0.010

0.0050

0.015

400 600 1000 1200 1400 1600800

band

Wavenumber (cm

–1

)

O Rotation band Atmospheric window H

O 6.3 μm band

320 K

280 K

240 K

200 K

Radiance (mW cm

–2

–1

)

Figure 8.4 Atmospheric spectrum obtained with a scanning interferometer on

board the Nimbus 4 satellite. The interferometer viewed the Earth vertically as

the satellite was passing over the North African desert. After Hänel et al. (1972).

American Geophysical Union.

emittance of sand from its block body value of unity. A distinct ozone absorption

band is evident at about 96 m in the middle of the window. Although not very

evident in the figure, weak continuous absorption also occurs across the window

believed to be due to the presence of clusters or dimers of water vapor molecules.

In summary, the principal natural gaseous absorbers of terrestrial radiation are

carbon dioxide and water vapor. These are the so-called principal “greenhouse”

gases. The basic greenhouse concept was probably first proposed by Fourier

(1827). Later, Arrhenius (1896) calculated the effects of varying carbon dioxide

concentrations on surface temperatures and even included the impact of water

vapor absorption in his calculations. He estimated that a 2.5- to 3-fold increase in

carbon dioxide would increase temperatures in Arctic regions by 8–9



C. His intent

was to examine the impact of greenhouse gases on ice ages and glaciation and

was not particularly concerned about human sources. As early as 1938, Callendar

(1938) became concerned about the impacts of anthropogenic emissions of carbon

dioxide on global temperatures. Therefore, the basic concept that anthropogenic

sources of carbon dioxide and other greenhouse gases can warm the atmosphere

is not new, but reports suggesting that global average surface temperatures are

158 Overview of global climate forcings and feedbacks

rising has caused a great deal of concern about potential human impact on climate,

and resulted in major research activities in global climate change.

In the absence of so-called greenhouse gases, the average surface temperature

of the Earth would be over 30



C cooler than it is today. This is because the

greenhouse gases absorb upwelling terrestrial radiation and radiate the absorbed

gases, both upward and downward, causing a net gain in energy (reduced loss)

at the Earth’s surface. Without those greenhouse gases, a larger fraction of the

upwelling terrestrial radiation would escape through the atmospheric window to

space. The major greenhouse gas is water vapor which varies naturally in space

and time due the Earth’s hydrological cycle (e.g., see Randall and Tjemkes, 1991).

The second most important greenhouse gas is carbon dioxide. In contrast to water

vapor, carbon dioxide is rather uniformly distributed throughout the troposphere,

although the radiative forcing associated with it is more heterogeneous as a result

of spatial (e.g., latitudinal) and temporal variations in tropospheric temperature

and water vapor concentrations, and in surface emissions and absorption.

We discuss the greenhouse effect further in Subsection 8.2.4. Other strong

absorbers of terrestrial radiation are methane, nitrous oxide, and chlorofluoro-

carbons (CFCs). Currently the concentrations of these gases are so small that

their contributions to infrared absorption cannot be easily detected by satellite.

The concentrations of CFCs, for example, are six orders of magnitude (a million

times) smaller than carbon dioxide. The concern about CFCs is that per molecule

they are 20 000 times more effective at absorbing infrared radiation than carbon

dioxide.

8.2.2 Absorption and scattering by aerosols

The radiative properties of aerosol particles is a complicated function of their

chemistry, shape, and size spectra. Moreover, if the aerosol particles are hygro-

scopic, their radiative properties change with the relative humidity of the air.

At relative humidities greater than 70%, the hygroscopic particles (called haze)

take on water vapor molecules and swell in size, thus changing their radiative

properties not only because of size effects but also because of changes in their

complex indices of refraction as the water-solution–particle mixture changes in

relative amounts.

Dry aerosol particles in high concentrations such as over major polluted urban

areas and over deserts can cause substantial absorption and scattering of solar

radiation. In some polluted boundary layers, aerosol absorption has been estimated

to result in heating rates on the order of a few tenths to several degrees per

hour (Braslau and Dave, 1975; Welch and Zdunkowski, 1976). In the Saharan

dust layer, aerosol absorption has been calculated to produce a heating of 1–2

