Other Climate Forcing Agents

Aerosols. Small liquid or solid particles suspended in the atmosphere—aerosols—can be composed of many different chemicals, come from many different sources (including both natural sources and human activities), and have a wide range of effects. Fossil fuel burning, industrial activities, land use change, and other human activities have generally increased the number of aerosol particles in the atmosphere, especially over and downwind of industrialized counties. The net climate forcing associated with aerosols is estimated to be -1.2 W/m2 (Forster et al., 2007; see also Murphy et al., 2009), which offsets roughly one-third of the total positive forcing associated with human emissions of GHGs (see Figure 6.4). However, the forcing associated with aerosols is more uncertain than the forcing associated with GHGs, in part because the global distribution and composition of aerosols are not very well known and in part because of the diversity and complexity of aerosol radiative effects.

Two separate types of effects contribute to the net cooling associated with aerosols: (1) a "direct effect," which occurs because most aerosols scatter a portion of the incoming sunlight that strikes them back to space, and (2) "indirect effects," which arise because aerosols play an important role in the formation and properties of cloud droplets, and on average the increasing number of aerosols have caused clouds to reflect more sunlight back to space. Certain kinds of aerosols, including dust particles

5 Exhaust from jet aircraft also adds water vapor to the stratosphere, which can both directly contribute to the greenhouse effect and also form linear contrails, which tend to warm the Earth slightly. While contrails were once thought to potentially contribute a significant climate forcing, more recent estimates—including some based on measurements taken during the days following the September 11 attacks, when air travel over North America was sharply curtailed—show that aircraft exhaust has only a small effect on climate forcing, although contrails do appear to have a discernible effect on regional day-night temperature differences (Travis et al., 2002).

and black carbon (soot), absorb both incoming solar energy and the outgoing infrared energy emitted by the Earth. These aerosols tend to warm the atmosphere, offsetting some (but not all) of the cooling associated with the direct and indirect effects. Black carbon particles that settle on snow and ice surfaces can also accelerate melting; however, this positive forcing is typically included in estimates of the forcing associated with land use change, which is discussed below.

It is worth noting the sources of a few key types of aerosols to illustrate their diversity: Dust and some organic aerosols arise from natural processes, but some human activities such as land use change also lead to changes in the abundance of these species. Black carbon particles are produced from the burning of both fossil fuels and vegetation. Sulfate (SO4) aerosols—which are a major contributor to the aerosol direct and indirect effects—have three notable sources: fossil fuel burning, marine phytoplank-ton, and volcanoes. The composition and size of each of these aerosol species affect how they absorb or scatter radiation, how much water vapor they absorb, how effectively they act to form cloud droplets, and how long they reside in the atmosphere— although in general most aerosols only remain in the atmosphere for a few weeks on average.

In addition to their role in global climate forcing, aerosols also have a number of other important environmental effects. The same industrial emissions that give rise to SO4 aerosols also contribute to acid rain, which has a major detrimental effect on certain ecosystems. One of the major objectives—and successes—of the 1990 Clean Air Act (P.L. 101-549) was to reduce the amount of sulfur emissions in the United States. Similar laws in Europe have also been successful in reducing SO4 aerosol concentrations (Saltman et al., 2005). The relationship between aerosols and cloud formation also means that changes in aerosols play an important role in modulating precipitation processes (see Chapters 8 and 15). Also, many aerosols are associated with negative impacts on public health, as discussed in further detail in Chapter 11.

Finally, aerosol emissions represent an important dilemma facing policy makers trying to limit the magnitude of future climate change: If aerosol emissions are reduced for health reasons, or as a result of actions taken to reduce GHG emissions, the net negative climate forcing associated with aerosols would decline much more rapidly than the positive forcing associated with GHGs due to the much shorter atmospheric lifetime of aerosols, and this could potentially lead to a rapid acceleration of global warming (see, e.g., Arneth et al., 2009). Understanding the many and diverse effects of aerosols is also important for helping policy makers evaluate proposals to artificially increase the amount of aerosols in the stratosphere in an attempt to offset global warming (see Chapter 15).

Changes in land cover and land use. Human modifications of the land surface can have a strong local or even regional effect on climate. One notable example is the "urban heat island" effect on temperatures, described below and in Chapter 12. Globally, land cover and land use changes are important sources of several GHGs, such as the release of CO2 from deforestation or CH4 from rice paddies. Land use and land cover change can also yield a global climate forcing by altering the reflectivity of the Earth's surface—for example, by replacing forests (which absorb most incident sunlight) with cropland (which is generally somewhat more reflective). Satellite measurements provide an excellent record of how changes in land cover have influenced surface reflectivity over the last few decades, although in some cases there is uncertainty as to whether observed changes are directly human-induced, part of a feedback process, or attributable to natural changes. To estimate global patterns of land use change for the last several hundred years, scientists use historical and paleoecological records combined with land use models that can simulate changes in vegetation over time in response to both climatic and nonclimatic effects.

Most recent published estimates of the global climate forcing associated with land use and land cover change are in the range of -0.1 to -0.3 W/m2, although some estimates are as large as -0.5 W/m2, while others indicate a small positive net forcing (Forster et al., 2007). As noted above, an additional land-surface effect is the deposition of black carbon aerosols (soot) on white snow and ice surfaces, which leads to melting and has been estimated to yield a positive forcing of up to 0.2 W/m2, although more recent estimates have suggested a somewhat smaller warming effect (Hansen et al., 2005). Thus, the total climate forcing associated with modifications to the land surface due to human activities since 1750 could potentially be positive or negative, but the balance of evidence seems to suggest a slight cooling effect.

Changes in solar radiation. As discussed in the next section, even small variations in the amount or distribution of energy received from the sun can have a major influence on Earth's climate when they persist for many thousands of years. However, satellite measurements of solar output show no net increase in solar forcing over the last 30 years, only small periodic variations associated with the 11 -year solar cycle (Figure 6.9). Changes in solar activity prior to the satellite era are estimated based on a variety of techniques including observations of sunspot numbers, which correspond roughly with solar output (Figure 6.10). The available evidence suggest that solar activity has been roughly constant (aside from the 11 -year solar cycle) since the mid-20th century but that it increased slightly during the late 19th and early 20th centuries. The total solar forcing since 1750 is estimated to be less than 0.3 W/m2 (Forster et al., 2007).

Cosmic rays. Finally, it has been proposed that cosmic rays might influence Earth's cli-

FIGURE 6.9 Solar irradiance observed at the top of the Earth's atmosphere by satellites. There is no overall trend in irradiance since 1979, but the ~11-year solar cycle produces small variations in irradiance of roughly 1.5 W/m2. Due to the geometry of the Earth and the reflection of some of the incoming sunlight back to space, this 1.5 W/m2 variation in irradiance corresponds to a periodic oscillation in climate "forcing" of around 0.3 W/m2 (although climate forcing is usually defined as the overall change in forcing since 1750). SOURCE: Lean and Woods (in press).

FIGURE 6.9 Solar irradiance observed at the top of the Earth's atmosphere by satellites. There is no overall trend in irradiance since 1979, but the ~11-year solar cycle produces small variations in irradiance of roughly 1.5 W/m2. Due to the geometry of the Earth and the reflection of some of the incoming sunlight back to space, this 1.5 W/m2 variation in irradiance corresponds to a periodic oscillation in climate "forcing" of around 0.3 W/m2 (although climate forcing is usually defined as the overall change in forcing since 1750). SOURCE: Lean and Woods (in press).

FIGURE 6.10 Estimated variations in solar irradiance at the top of the atmosphere by three different research teams during (top) the last 400 years based on (bottom) observations of sunspot numbers. All three irradiance reconstructions indicate drops in solar output during extended periods with low sunspot numbers, especially the Maunder and Dalton minimums (which are indicated in the bottom panel), and an increase in solar irradiance during the first several decades of the 20th century. The estimated total climate forcing associated with changes in solar irradiance since 1750 is 0.3 W/m2. (As noted in the caption for Figure 6.9, the climate forcing associated with solar irradiance changes must be scaled to account for Earth's geometry and the reflection of some of the incident solar energy back to space.) SOURCE: Lean and Woods (in press).

FIGURE 6.10 Estimated variations in solar irradiance at the top of the atmosphere by three different research teams during (top) the last 400 years based on (bottom) observations of sunspot numbers. All three irradiance reconstructions indicate drops in solar output during extended periods with low sunspot numbers, especially the Maunder and Dalton minimums (which are indicated in the bottom panel), and an increase in solar irradiance during the first several decades of the 20th century. The estimated total climate forcing associated with changes in solar irradiance since 1750 is 0.3 W/m2. (As noted in the caption for Figure 6.9, the climate forcing associated with solar irradiance changes must be scaled to account for Earth's geometry and the reflection of some of the incident solar energy back to space.) SOURCE: Lean and Woods (in press).

mate by modifying cloud properties (Shaviv, 2002; Svensmark, 1998, 2006) or through a variety of other mechanisms (Gray et al., 2005). Cosmic rays are influenced by solar activity, so it is difficult to study the effect of cosmic rays in isolation. However, direct observations of cosmic ray fluxes do not show any net change over the last several decades (Benestad, 2005), and a plausible physical mechanism linking changes in cosmic rays to changes in climate has not been demonstrated. Hence, cosmic rays are not regarded as an important climate forcing (Forster et al., 2007).

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