Category Acid Deposition and Energy Use

Changes in climate have occurred in the distant past as the distribution of continents and their landscapes have changed, as the so-called Milanko – vitch changes in the orbit of the earth and the earth’s tilt relative to the ecliptic plane have varied the insolation received on earth, and as the composition of the atmosphere has changed, all through natural processes. Recent evidence obtained from ice cores drilled through the Greenland ice sheet indicates that changes in climate may often have been quite rapid and major and not associated with any known external forces.

Observations of surface temperature show a global mean warming of approximately 0.7°C during the past 100 years...

Aerosols occur in the atmosphere from natural causes; for instance, they are blown off the surface of deserts or dry regions. As a result of the eruption of Mt. Pinatubo in the Philippines in June 1991, considerable amounts of aerosol were added to the stratosphere that, for approximately 2 years, scat­tered solar radiation leading to a loss of radiation and a cooling at the surface.

As noted previously, human activities also affect the amount of aerosol in the atmosphere. The main direct effect of aerosols, such as sulfate particles, is the scattering of some solar radiation back to space, which tends to cool the earth’s surface...

The amount of carbon dioxide in the atmosphere has increased by more than 30% in the past two centuries since the beginning of the industrial revolution, an increase that is known to be in part due to combustion of fossil fuels and the removal of forests. Most of this increase has occurred since World War II. Because carbon dioxide is recycled through the atmosphere many times before it is finally removed, it has a long lifetime exceeding 100 years. Thus, emissions lead to a buildup in concen­trations in the atmosphere. In the absence of controls, it is projected that the rate of increase in carbon dioxide may accelerate and concentrations could double from pre-industrial values within approximately the next 60 years.

If the amount of carbon dioxide in the atmosphere were suddenly doubled...

1.1 Human Influences

Climate can vary for many reasons, and in particular, human activities can lead to changes in several ways. However, to place human influences in perspective, it is worthwhile to compare the output from a power plant with that from the sun. The largest power plants that exist are on the order of 1000 MW, and these service human needs for electricity in appli­ances and heating that use power in units of kilowatts. Figure 2 shows that the energy received at the top of the atmosphere is 342 W m~2 on an annual mean basis averaged over the globe. This is equivalent to 175 PW, although only approximately 120 PW is absorbed. One petawatt is 1 billion MW or 1 million huge power plants...

The climate system becomes more involved as the components interact. A striking example is a phenomenon that would not occur without interac­tions between the atmosphere and ocean, El Nino, which consists of a warming of the surface waters of the tropical Pacific Ocean. It takes place from the International Dateline to the west coast of South America and results in changes in the local and regional ecology. Historically, El Ninos have oc­curred approximately every 3-7 years and alternated with the opposite phases of below average tempera­tures in the tropical Pacific, called La Nina. In the atmosphere, a pattern of change called the Southern

Oscillation is closely linked with these ocean changes so that scientists refer to the total phenomenon as ENSO...

The different components of the climate system contribute on different timescales to climate varia­tions and change. The atmosphere and oceans are fluid systems and can move heat through convection and advection in which the heat is carried by the currents, whether small-scale short-lived eddies or large-scale atmospheric jet streams or ocean currents. Changes in phase of water, from ice to liquid to water vapor, affect the storage of heat. However, even ignoring these complexities, many facets of the climate are determined simply by the heat capacity of the different components of the climate system. The total heat capacity considers the mass involved as well as its capacity for holding heat, as measured by the specific heat of each substance.

The atmosphere does not have much capability...

Major ice sheets, such as those over Antarctica and Greenland, have a large heat capacity but, like land, the penetration of heat occurs primarily through conduction so that the mass involved in changes from year to year is small. Temperature profiles can be taken directly from bore holes into ice and it is estimated that terrestrial heat flow is 51 mW/m2. On century timescales, however, the ice sheet heat capacity becomes important. Unlike land, the ice can melt, which has major consequences through changes in sea level on longer timescales.

Sea ice is an active component of the climate system that is important because it has a high albedo. A warming that reduces sea ice, reduces the albedo and hence enhances the absorption of solar radiation, amplifying the original warming...

The heat penetration into land is limited and slow, as it occurs mainly through conduction, except where water plays a role. Temperature profiles taken from bore holes into land or ice caps provide a coarse estimate of temperatures in years long past. Conse­quently, surface air temperature changes over land occur much faster and are much larger than those over the oceans for the same heating, and because we live on land, this directly affects human activities. The land surface encompasses an enormous variety of topographical features and soils, differing slopes (which influence runoff and radiation received), and water capacity. The highly heterogeneous vegetative cover is a mixture of natural and managed ecosys­tems that vary on very small spatial scales...