11 Radiative Forcing

Pallath Pradeep Kumar

1. Learning outcomes
2. Introduction
2.1 . Radiative forcing
2.2 . Positive and negative radiative forcing
3. Radiative forcing due to greenhouse gases (GHGs)
3.1 . Carbon-di-oxide
3.2 . Nitrogen species
3.3 . Methane
3.4 . Halocarbons
3.5 . Ozone
3.6 . Water Vapor
4. Radiative forcing due to aerosols
4.1 . Direct and Indirect radiative forcing
4.2 . Natural aerosols
4.3 . Anthropogenic aerosols
5. Albedo Effects
5.1 . Land surface
5.2 . Ocean
5.3 . Cryosphere
6. Feedback effect from clouds
6.1 . Low and middle clouds
6.2 . High clouds and contrails
6.3 . Vertically developing clouds
7. Solar effects
7.1 . Sunspot cycle
7.2 . Orbital variations
7.3 . Cosmic rays
8. Global Warming Potential
9. Summary

1. Learning outcomes

• After studying this module, you shall be able to:
• Know what Radiative Forcing is
• Learn about the positive and negative radiative forcing
• Understand the radiative forcing due to greenhouse gases, aerosols, clouds
• Know the contribution of Land, Ocean and Cryosphere to the radiative forcing. Know about the solar effects on the radiative forcing

2. Introduction

2.1. Radiative forcing

The earth’s weather systems are basically driven by the energy it receives from the sun. The climate of a place is the long time average of the components that define the weather like temperature, precipitation, winds, sea surface temperature etc. The earth receives energy from the sun mostly in the ultraviolet and visible region of the electromagnetic spectrum and this energy which the earth intercepts is only a fraction of the energy emitted by the sun. The radiant flux emitted by the sun is of the order of 1026 watts and that intercepted by the earth is only of the order of 1017 watts. The earth radiates like a black body and emits energy in the infrared part of the electromagnetic spectrum back to outer space (Fig. 1). The difference in the net incoming solar radiation and the outgoing terrestrial radiation is the radiative forcing. Radiative forcing from an increase of solar intensity since 1750 is about +0.12 watts m-². The term “Radiative forcing” can also be defined as a direct measure of the instantaneous globally averaged heating rate due to a given quantity of a greenhouse gas. Over a long period of time the incoming solar radiation and the outgoing infrared radiation from the Earth has to balance each other. This is called as the energy balance (can write radiation balance instead). When this is disturbed the overall effect maybe warming of the earth or cooling of the earth. That is what the climate scientists term as climate change.

The distribution of incoming solar radiation is shown in Figure 2a. A fraction of the incoming solar radiation is reflected by the air molecules, aerosols, clouds, land, ocean and cryosphere (frozen water on the earth) which contributes to the albedo (the radiation reflected back without absorption) of the planet. The remaining fraction gets absorbed by the clouds, aerosols, atmospheric gases, land and ocean. Fig. 2b shows the emission from the earth’s surface, clouds and atmospheric gases.

2.2.  Positive and negative radiative forcing

The contribution to the radiative forcing can come from a variety of agents like Green House Gases, Aerosols, Land surface, Ocean, Cryosphere, Clouds and even variations from the Sun. The most common variable that is used to measure the effect of radiative forcing is the surface temperature. If the surface temperature is warming then we can call it as the positive radiative forcing and if it is cooling then we can term it as negative radiative forcing. Sometimes the radiative forcing is also determined by measuring the difference in flux at the Top of the Atmosphere (TOA). In view of large changes noticed in the tropical TOA radiation by the Earth Radiation Budget Satellite (ERBS), the TOA radiation budgets are also assessed now (Hartmann et al., 2013). It is also possible to determine the contribution towards radiative forcing coming from individual agents, like different types of green house gases, different types of aerosols, different types of clouds and albedo changes (Myhre et al., 2013).

3. Radiative forcing due to greenhouse gases (GHGs)

Radiative forcing effect of green house gases (GHGs) mainly happens through the process of trapping the outgoing terrestrial radiation. Those gases which are weak absorbers in the visible region but have strong absorption in the infrared region are termed as the greenhouse gases. A gas which is a strong absorber of infrared radiation will also be a strong emitter of radiation in the infrared. GHG absorbs the infrared radiation emitted by the earth and reemits again in the infrared. The blanket of infrared gases enveloping the earth thus traps the infrared radiation. Trapping of the terrestrial radiation by the GHG has mostly a warming effect on the earth. In the absence of GHG the infrared radiation emitted by the earth would have completely escaped to space. Roles of some of the important GHGs in radiative forcing is discussed here.

3.1. Carbon-di-oxide

CO2 occurs naturally in the atmosphere through biogenic emissions and through human respiratory processes. However, from the time of industrialization and the use of fossil fuels like coal and petroleum products the CO2 level in the atmosphere has been rising very rapidly. From a pre-industrial level of 280 ppm the present day ambient atmospheric CO2 stands at 400 ppm (Dalling et al., 2016). The main contribution for this increase has come from the burning of fossil fuels. Increase in CO2 contributes to positive radiative forcing.

3.2.Nitrogen species

Oxides of Nitrogen like NO, N2O, NO2 are found in the atmosphere. The residence time of each of them in the atmosphere varies. NO2 occurs naturally from soils, ocean and also from anthropogenic activities like use of fertilizers, biomass burning etc. This constituent GHG has also been steadily increasing in the atmosphere and has a positive radiative forcing. Among many sources of NO and NO2 (NOx) in the atmosphere one of the source is the lightning activity. NOx modulates the ozone levels in the atmosphere. As the residence time of NOx in the atmosphere is very short, their contribution to the radiative forcing cannot be quantified with certainty as to whether they contribute more to the positive radiative forcing or towards the negative radiative forcing.

3.3.Methane

Methane emissions can come from biogenic process, geological process or due to incomplete combustion process of biomass or biofuels (Cias et al., 2013). The biogenic sources can be from wetlands, rice fields, landfills, waste etc. The surface methane concentration has increased by 150% since the pre-industrial period (Myhre et. al., 2013). Methane contributes to the positive radiative forcing.

3.4. Halocarbons

Halocarbons are compounds containing Chlorine, Fluorine and Carbon. The Chlorofluorocarbon or CFC as they are commonly referred, were used widely in the refrigeration industry and propellant for aerosol spray cans. These compounds were ending up in the stratosphere and because of their long lifetime they were depleting the stratospheric ozone. In the 1980’s the Ozone hole was discovered over Antarctica where the ozone levels went very low during the Southern hemisphere spring. Because of the Montreal protocol the emissions have sharply decreased over the last two decades. The direct radiative forcing by halocarbons is positive and indirectly it destroys the ozone in the stratosphere.

3.5.Ozone

The radiative forcing due to Ozone has to be understood in terms of tropospheric ozone and stratospheric ozone. The ozone in the troposphere accounts for only about 10% of the total columnar ozone. Ozone in the atmosphere is a result of photochemical reactions. The radiative forcing of tropospheric ozone is due to Methane, oxides of Nitrogen and Volatile Organic Compounds. While in the Stratosphere it is due to depletion of ozone by halocarbons (Myhre et al., 2013). If ozone increases in the stratosphere it will contribute to a warming effect and in the troposphere its behavior is opposite.

3.6.Water vapor

Although it is a very important Green House Gas the Radiative Forcing due to it is never computed as it is highly variable in space and time. The radiative forces of other gases can also control the amount of water vapor in the atmosphere. Another main contribution from water vapor is through cloud formation process and clouds participate in the feedback processes. Water vapor is present in small quantity in the stratosphere where it forms mainly from the oxidation of methane. The radiative forcing due to water vapor in the stratosphere is positive but very small.

4. Radiative forcing due to aerosols

Aerosols are micron or sub-micron sized solid or liquid particles in the atmosphere. Their role in modulating the earth’s climate through radiative forcing has been recognized for a long time.

4.1.Direct and indirect radiative forcing

The direct radiative forcing effect of Aerosol is through scattering or absorption of solar and terrestrial radiation. Aerosols contribute to both positive and negative radiative forcing and the contribution depends on the physical and chemical properties of the aerosols. As aerosols are highly variable in space and time and constantly changing its properties and size it has a very large uncertainty in terms of radiative forcing.

The indirect effect of aerosols comes through aerosol-cloud interaction. The indirect radiative effect is through modifying the microphysical and radiative properties of clouds (Ramanathan et al., 2001). A subsection of aerosol participates in the cloud nucleation process and they are called as Cloud Condensation Nuclei (CCN). The CCN controls the cloud drop size and the droplet number concentration which in turn affects the cloud reflectivity and cloud optical depth thus leading to changes in cloud albedo. This indirect effect is also known as the Twomey effect (Twomey, 1974). A small section of aerosols also act as centers for Ice Nucleation in the clouds. The amount of ice in the cloud also affects the cloud reflectivity and thus contributes to the radiative forcing.

4.2. Natural aerosols

Aerosols are produced naturally through a number of mechanism like gas to particle conversion, windblown dust, sea salt spray. The windblown dust from the Arabian Desert is transported long distances. There are other natural sources that generate aerosols, like plants, pollens, fungi etc. From the ocean source other than sea salt is the dimethylsulphide (DMS) which are emitted from the oceanic biogenic sources.

Aerosols which result from volcanic ash end up in the stratosphere and have a long residence time. They block the solar radiation and have a negative radiative forcing. The most important volcanic eruptions in the recent times were that from El Chichon, Mexico in 1982 and that from Mt. Pinatubo in the Philippines in 1991. The aerosols from this eruption were present in the stratosphere for several months having a profound effect on the radiative balance.

4.3.Anthropogenic aerosols

There are number of ways the anthropogenic aerosols are produced starting from the transport sector which also includes fossil fuel burning, to biomass burning. The main compounds that come from the anthropogenic sources are carbon and sulfate aerosols. In the carbonaceous aerosols we have black carbon and organic carbon which can form due to incomplete combustion. The black carbon can absorb the solar radiation and thus can warm the atmosphere and contribute to positive radiative forcing. The organic carbon is called as the brown carbon and its radiative forcing at the top of the atmosphere (TOA) is positive while at the surface it is negative (Feng et al., 2013).

5. Albedo effects

Albedo is basically the amount of radiation that is reflected back without absorption. The solar radiation reaching the earth’s surface can be reflected back from the land, sea and the frozen water on the earth.

5.1. Land surface

There is a large variation on the type of coverage the earth’s surface has, varying from desert sand to forests to green lands. The solar radiation which is reflected back varies considerably depending upon whether it is from desert sand, tree canopy or green grass. It also depends upon the state of the vegetation.

5.2.Ocean

The reflection of solar radiation from sea surface and ocean depends on the angle of the sun. The sea ice which forms in the Polar Regions has a very high albedo and thus contributes to a negative radiative forcing.

5.3. Cryosphere

It comprises of frozen water on the earth at the Polar Regions, sea ice, and frozen rivers, glaciers on the mountains, ice sheets and permafrost. The reflection of solar radiation depends on the condition of the snow or ice surface. Freshly formed snow cover has a very high reflectivity reaching more than 90%, so their radiative forcing factor is very high.

6. Feedback effect from clouds

Clouds have two type of effects on the radiative balance. The first is the albedo effect where the incoming solar radiation is reflected back and thus will contribute to a cooling effect. The second one is the Greenhouse effect where the terrestrial radiation is absorbed and re-emitted back to the earth contributing to the warming effect.

6.1. Low and middle clouds

The low and middle level clouds reflect the solar radiation due to high water content in them. So their albedo effect is very high and since their height is low their greenhouse effect is low. Thus they contribute more to the Cooling effect.

6.2. High clouds and contrails

High clouds which are of Cirrus type are very thin, have ice crystals but practically no liquid water in them. These clouds allow the short wave radiation from the sun to pass through them and contribute very little to the albedo effect. They trap the outgoing longwave radiation and thus contribute more to the greenhouse effect causing warming of the earth. Contrails are condensation trails left by aircraft and they have ice content which are basically formed on the particles from the aircraft exhaust. They can merge with the cirrus clouds and have a very long life time. They contribute to the warming effect. Figure 4 shows the Cirrus Clouds and also the condensation trail left by an aircraft.

These clouds are mostly cumulus and cumulonimbus types. As clouds contribute to both albedo effect and greenhouse effect, the effect from tall clouds depends on their vertical structure. In general, as the radiative forcing from clouds depends upon the cloud type it is difficult to quantify whether the overall effect from these clouds will be positive or negative. Figure-5 shows a typical vertical developing cumulonimbus cloud.

7.1. Sunspot cycle

Sunspots are dark spots on the surface of the sun and their number varies from maximum to minimum approximately in an 11 year cycle. The solar emission although not very significant can change during this cycle. Their effect on radiative forcing is not very clear.

7.2. Orbital variations

The radiation reaching the earth can change due to orbital variations of the earth more commonly known as the Milankovich variations. They are due to the changes in eccentricity, changes in obliquity (tilt of earth’s axis) and precision of the earth’s orbit around the sun (Henderson-Sellers and McGuffie, 1987). Their effect on the earth’s climate is in thousands of years.

7.3.Cosmic rays

Cosmic Rays are very high energetic particles that are continuously bombarding the earth. As their energies are very high they generate ions in the atmosphere. These ions have the ability to attract the aerosols in the atmosphere and thus form CCN particles. The CCN can modulate the clouds which can influence the radiative forcing.. Radiative forcing ranges of several GHG and other factors are presented in Fig.6.

8. Global Warming Potential

The Global Warming Potential or GWP is a term which we often encounter in literatures related to global warming and climate change. GWP is basically an index which describes the radiative characteristics of well mixed greenhouse gases (the timescale of removal of a well mixed gas is much longer than its mixing time in the atmosphere; examples include carbon dioxide, methane, nitrous oxide chlorocarbons). The term GWP is closely associated with radiative forcing in the sense, the former basically predicts radiative forcing of a gas on a given time horizon – say, 20, 100 or 500 years from now (table 1).

Global warming potential accounts for the following:

• The radiative forcing for a known amount of a gas (W m-2) The emission rate of the gas
• The lifetime of the gas in the atmosphere
• The indirect effects of the gas on radiative forcing

Similar to GWP is Global Temperature change Potential which is another way to quantify the ratio change from a substance relative to that of CO2, in global mean surface temperature, used for a specific time span.

Table 1: Life time, present atmospheric concentration and global warming potentials of some important greenhouse gases at different time horizons

9. Summary

• Radiative forcing is the difference between the net incoming solar radiation and the outgoing terrestrial radiation from the Earth.
• Radiative forcing contributions can come from a variety of agents like Green House 
• Gases, Aerosols, Land surface, Ocean, cryosphere, clouds and even variations from sun.
• Radiative forcing effect of Greenhouse gases (GHG) mainly happens through the process of trapping the outgoing terrestrial radiation.
• Aerosols are micron or sub-micron sized solid or liquid particles in the atmosphere and modulates the Earth’s climate through radiative forcing.
• Albedo is the amount of radiation that is reflected back without absorption.
• Clouds can contribute through either positive or negative feedback to the radiative balance. Solar effects can come through the eleven year sun spot cycle, the orbital variations of the earth or the cosmic rays.