Climate Change, Radiative Forcing, Aerosols, and Global Warming Potential of Gases in Our Atmosphere

Natural Climate Change Radiative Forcing Aerosols Global Warming Potential Summary

Driving Questions:

  • What has controlled past changes in Earth's temperature?
  • What processes lead to temperature change?
  • What gases and aerosols comprise the atmosphere and how do they influence the temperature?
  • What are the global warming potentials for greenhouse gases?

1. Natural Climate Change

We believe that the temperature of the earth has varied wildly over the evolution of the earth. Figure 1 shows an estimate of temperature changes as complied by Scotese. So how can it be that the climate has changed so over the ages and what processes could lead to these changes?

Figure 1. Estimated changes in global temperature.

The processes for changing climate naturally over long timescales include:

  1. Plate tectonics
  2. Long-term carbon cycle
  3. Solar Variations (sun's luminosity), and
  4. Orbital Variations (which will be discussed in a later lecture)

2. Radiative Forcing

The temperature of the Earth's surface and atmosphere are dictated by a balance between incoming energy and outgoing energy. When more energy is received than lost, temperatures rise. The Earth's surface, for example, absorbs radiation from the Sun. This energy is then redistributed by the atmospheric and oceanic circulations and radiated back to space at longer (infrared) wavelengths. For the annual mean and for the Earth as a whole, the incoming solar radiation energy is balanced approximately by the outgoing terrestrial radiation. Any factor that alters the radiation received from the Sun or lost to space, or that alters the redistribution of energy within the atmosphere and between the atmosphere, land, and ocean, can affect climate. A change in the net radiative energy available to the global Earth-atmosphere system is termed a radiative forcing. Positive radiative forcings tend to warm the Earth’s surface and lower atmosphere. Negative radiative forcings tend to cool them.

The Greenhouse Effect

Increases in the concentrations of greenhouse gases will reduce the efficiency with which the Earth’s surface radiates to space. More of the outgoing terrestrial radiation from the surface is absorbed by the atmosphere and re-emitted at higher altitudes and lower temperatures. This results in a positive radiative forcing that tends to warm the lower atmosphere and surface. Because less heat escapes to space, this is the enhanced greenhouse effect – an enhancement of an effect that has operated in the Earth’s atmosphere for billions of years due to the presence of naturally occurring greenhouse gases: water vapor, carbon dioxide, ozone, methane and nitrous oxide. The amount of radiative forcing depends on the size of the increase in concentration of each greenhouse gas, the radiative properties of the gases involved (indicated by their global warming potential), and the concentrations of other greenhouse gases already present in the atmosphere. Further, many greenhouse gases reside in the atmosphere for centuries after being emitted, thereby introducing a long-term commitment to positive radiative forcing.

3. Aerosols

Anthropogenic aerosols (microscopic airborne particles or droplets) in the troposphere, such as those derived from fossil fuel and biomass burning, can reflect solar radiation, which leads to a cooling tendency in the climate system. Because it can absorb solar radiation, black carbon (soot) aerosol tends to warm the climate system.

Sources of aerosols include: Biomass combustion (fires); Wind-blown dust; Salt from sea spray; Volcanoes; and Industrial pollution. Aerosols have both a direct and an indirect effect on albedo in the atmosphere or on the ground surface.

1. Direct effect: aerosols reflect and scatter incoming solar radiation, causing an increase in albedo (leads to negative radiative forcing = net cooling).

2. Indirect effect: aerosols influence cloud properties (albedo, height, droplet size, causing net cooling). A particularly important indirect effect is that sulfate aerosols derived from sulfur gases emitted from volcanoes and in pollution, act as cloud condensation nuclei (provide a tiny nucleus for water vapor to condense around and form a water droplet) and in doing so they increase the thickness or optical depth of clouds, increasing their albedo. A second important indirect effect is that the black carbon in soot that comes from pollution or burning, will decrease the albedo of surfaces and especially of ice or snow (leads to a net warming).

Changes in aerosol concentrations can alter cloud amount and cloud reflectivity through their effect on cloud properties and lifetimes. In most cases, tropospheric aerosols tend to produce a negative radiative forcing and a cooler climate. They have a much shorter lifetime (days to weeks) than most greenhouse gases (decades to centuries), and, as a result, their concentrations respond much more quickly to changes in emissions. Volcanic activity can inject large amounts of sulphur-containing gases (primarily sulfur dioxide) into the stratosphere, which are transformed into sulfate aerosols. Individual eruptions can produce a large, but transitory, negative radiative forcing, tending to cool the Earth’s surface and lower atmosphere over periods of a few years. For example, the large volcanic eruption of Mt. Pinatubo in the Phillipines in 1991 caused the release of sulfur dioxide that produced sulfate aerosols, which stayed in Earth's atmosphere for over 2 years and caused a short-term cooling of 0.5 degC in Earth's surface temperature.

Changes in Radiative Forcing

When radiative forcing changes, the climate system responds on various time-scales. The longest of these are due to the large heat capacity of the deep ocean and dynamic adjustment of the ice sheets. This means that the transient response to a change (either positive or negative) may last for thousands of years. Any changes in the radiative balance of the Earth, including those due to an increase in greenhouse gases or in aerosols, will alter the global hydrological cycle and atmospheric and oceanic circulation, thereby affecting weather patterns and regional temperatures and precipitation.

   

4. Global Warming Potential

The Global Warming Potential (GWP) of a greenhouse gas is the ratio of global warming, or radiative forcing – both direct and indirect – from one unit mass of a greenhouse gas to that of one unit mass of carbon dioxide over a period of time. Hence this is a measure of the potential for global warming per unit mass relative to carbon dioxide. This allows for different gases to be compared on the same, level playing field.

The Global Warming Potential of any gas is determined by three main factors:

1. The amount of absorption of infrared radiation by a gas

2. The spectral location of its absorbing wavelengths (if it absorbs in a range of high longwave emission from Earth, its GWP is higher)

3. The atmospheric lifetime of the gas (the longer it stays in the atmosphere, the greater the GWP)

Atmospheric Lifetime

The residence time of a gas in the atmosphere (the residence time, Rt, is the average time that any material spends in a pool or location before leaving) is called its Atmospheric Lifetime. These lifetimes range from months to centuries, and are important in determining the overall GWP of a gas. But they also provide information on the behavior of a system, and the residence time of a gas has implications for its overall effect. For example, we see that CO2 has a lifetime of 50-200 years in the atmosphere. The implication of that long residence time is that even if we were stop human emissions of CO2 into the atmosphere right now, our future climate would still be affected by the amount of CO2 we have already put into the atmosphere. It is like raking leaves into a big pile - once you stop raking the pile doesn't get any bigger, but, it also doesn't go away immediately. The same thing occurs with CO2 in the atmosphere -- because it has a long residence time, it will persist and continue to warm our planet, creating a "legacy effect" of past CO2 emissions that future generations will bear.

GWP Comparisons

Global Warming Potentials are presented in Table 1 for an expanded set of gases compared to what was shown in lecture. GWPs are a measure of the relative radiative effect of a given substance compared to CO2, integrated over a chosen time horizon. New categories of gases in Table 1 include fluorinated organic molecules, many of which are ethers that are proposed as halocarbon substitutes. Some of the GWPs have larger uncertainties than that of others, particularly for those gases where detailed laboratory data on lifetimes are not yet available. The direct GWPs have been calculated relative to CO2 using an improved calculation of the CO2 radiative forcing, the SAR response function for a CO2 pulse, and new values for the radiative forcing and lifetimes for a number of halocarbons. Indirect GWPs, resulting from indirect radiative forcing effects, are also estimated for some new gases, including carbon monoxide. The direct GWPs for those species whose lifetimes are well characterized are estimated to be accurate within ±35%, but the indirect GWPs are less certain.

Table 1. Direct Global Warming Potentials (GWPs) relative to carbon dioxide (for gases for which the lifetimes have been adequately characterized). GWPs are an index for estimating relative global warming contribution due to atmospheric emission of a kg of a particular greenhouse gas compared to emission of a kg of carbon dioxide. GWPs calculated for different time horizons show the effects of atmospheric lifetimes of the different gases.
    Lifetime Global Warming Potential
    (years) (Time Horizon in Years)
 GAS     20 yrs 100 yrs 500 yrs
Carbon Dioxide CO2
50-200 
1
1
1
Methane CH4
12.0
62
23
7
Nitrous Oxide N2O
114
275
296
156
Chlorofluorocarbons
 
 
 
 
CFC-11  
55
4500
3400
1400
CFC-12  
116
7100
7100
4100
CFC-115  
550
5500
7000
8500
Hydrofluorocarbons
 
 
 
 
HFC-23 CHF3
260
9400
12000
10000
HFC-32 CH2F2
5
1800
550
170
HFC-41 CH3F
2.6
330
97
30
HFC-125 CHF2CF3
29
5900
3400
1100
HFC-134 CHF2CHF2
9.6
3200
1100
330
HFC-134a CH2FCF3
13.8
3300
1300
400
HFC-143 CHF2CH2F
3.4
1100
330
100
HFC-143a CF3CH3
52
5500
4300
1600
HFC-152 CH2FCH2F
0.5
140
43
13
HFC-152a CH3CHF2
1.4
410
120
37
HFC-161 CH3CH2F
0.3
40
12
4
HFC-227ea CF3CHFCF3
33
5600
3500
1100
HFC-236cb CH2FCF2CF3
13.2
3300
1300
390
HFC-236ea CHF2CHFCF3
10
3600
1200
390
HFC-236fa CF3CH2CF3
220
7500
9400
7100
HFC-245ca CH2FCF2CHF2
5.9
2100
640
200
HFC-245fa CHF2CH2CF3
7.2
3000
950
300
HFC-365mfc CF3CH2CF2CH3
9.9
2600
890
280
HFC-43-10mee CF3CHFCHFCF2CF3
15
3700
1500
470
Fully fluorinated species
 
 
 
 
SF6  
3200
15100
22200
32400
CF4  
50000
3900
5700
8900
C2F6  
10000
8000
11900
18000
C3F8  
2600
5900
8600
12400
C4F10  
2600
5900
8600
12400
c-C4F8  
3200
6800
10000
14500
C5F12  
4100
6000
8900
13200
C6F14  
3200
6100
9000
13200
Ethers and Halogenated Ethers
 
 
 
 
CH3OCH3  
0.015
1
1
<<1
HFE-125 CF3OCHF2
150
12900
14900
9200
HFE-134 CHF2OCHF2
26.2
10500
6100
2000
HFE-143a CH3OCF3
4.4
2500
750
230
HCFE-235da2 CF3CHClOCHF2
2.6
1100
340
110
HFE-245fa2 CF3CH2OCHF2
4.4
1900
570
180
HFE-254cb2 CHF2CF2OCH3
0.22
99
30
9
HFE-7100 C4F9OCH3
5
1300
390
120
HFE-7200 C4F9OC2H5
0.77
190
55
17
H-Galden 1040x CHF2OCF2OC2F4OCHF2
6.3
5900
1800
560
HG-10 CHF2OCF2OCHF2
12.1
7500
2700
850
HG-01 CHF2OCF2CF2OCHF2
6.2
4700
1500
450

Summary

  • Greenhouse gases selective absorb infrared radiation, thus trapping energy in the atmosphere.
  • Aerosols (except soot or black carbon) have a net cooling effect on the atmosphere because they increase Earth's albedo, reflecting incoming radiation back out to space. Soot (black carbon) can decrease surface albedo, especially for snow or ice, and thus slightly warm the Earth.
  • The atmosphere radiates energy to the surface at an average rate greater than the rate of incoming solar radiation.
  • Each greenhouse gas is characterized by its atmospheric lifetime and global warming potential.

Self Test

All materials © the Regents of the University of Michigan unless noted otherwise.