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Global Warming and Radiative Forcing

February 14th, 2008 by atripp3 · 5 Comments

As I’m sure many of you are aware, the issue of global warming has been deeply embedded within the concerns of nations across the world. With an issue that is so widespread, you would think that the causes would be common knowledge to everyone. However, from my experience over the past few years, I have found that most people are rather confused about the warming of the climate and whether it is a problem or not. Typically, some are convinced that climate warming is a natural process, and the “global warming” issue doesn’t really exist. Others seem to think that it is an issue and humans are accelerating it by the burning of fossil fuels. Because of the confusion, I have decided to explore the issue further to get an idea whether climate warming is due more to nature or humans.

The following is a brief overview of the section entitled “Concept of Radiative Forcing” that appears in the report “Climate Change 2007: The Physical Science Basis” written by Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). It discusses what is referred to as Radiative Forcing and how it is used to determine whether or not a specific agent, such as Carbon Dioxide, affects the global climate.

Radiative Forcing, or RF, is specifically defined as the “change in net irradiance at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values.’” In layman’s terms, RF refers to an imbalance between incoming solar radiation and outgoing infrared radiation that causes the Earth’s radiative balance to stray away from its normal state. This straying causes changes in global temperatures. The concept of Radiative Forcing is useful because a linear relationship has been determined between the global mean equilibrium surface temperature changes and the amount of RF. Furthermore, the units of Radiative Forcing are watts per square meter, and, if the value is positive (negative), it has a warming (cooling) effect on the climate.

As stated above, RF is an important tool used to determine the effects that greenhouse gases, aerosols, and clouds have on climate change. It is necessary, however, to state that RF does not depict the climate response in its entirety. There are several parameters related to climate change that exist, but they are greatly variable and complex. Since RF is easy to calculate, it provides a general, yet respectable, estimate of how the climate will respond to changes in greenhouse gas concentrations and various other agents.

Another important aspect to touch on is the effects of “boundary conditions,” such as variations in orbital parameters, ice sheets, and continents, on RF calculations. An example would be the effects of the Milankovitch Cycles on the energy budget of the Earth. Surely, the variations in the Earth’s orbital parameters will cause a change in the amount of forcing. However, because this change occurs over such a long period of time, it is assumed to be constant. That being said, RF calculations are based on shorter-term changes in forcing due to natural and human-caused events.

So how is Radiative Forcing calculated? For the most part, it is estimated using data from what is referred to as General Circulation Models (GCM’s). These models use numerous methodologies which include instantaneous RF (temperatures throughout the atmosphere are fixed), stratospheric-adjusted RF (allows for the adjustment of stratospheric temperatures), zero-surface-temperature-change RF (allows for the adjustment of atmospheric temperatures, but fixes the surface temperature), and equilibrium climate response (allows both the atmosphere and surface temperatures to adjust to equilibrium causing a surface temperature change). (For an illustrated representation of the methodologies, see Figure 2.2 in the following link: http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf).

Now that the definitions and formalities have been covered, it is time for the application. In other words, I would like to briefly discuss whether global warming is a result of human activities or natural influences by employing RF values. It is common knowledge that human activities directly affect the amounts of greenhouse gases and aerosols in the atmosphere. But to what extent do each of the greenhouse gases and aerosols cause radiative forcing? Is this greater than the forcing caused by natural processes such as volcanic eruptions and solar changes?

Between the start of the Industrial Era and roughly present time (~1750 – 2005) the values of Radiative Forcing for various atmospheric agents are as follows:

Human-induced agents:

  • Carbon Dioxide has an average RF value of 1.66 W/m² with a range between 1.49 and 1.83 W/m².
  • Methane has an average RF value of 0.48 W/m² with a range of 0.43 to 0.53 W/m².
  • Nitrous Oxide has a value of 0.16 W/m² with a range of 0.14 to 0.18 W/m².
  • Halocarbon gases have an average value of 0.34 W/m² with a range of 0.31 to 0.37 W/m².
  • Tropospheric Ozone has an average of 0.35 W/m², ranging from 0.25 to 0.65 W/m².
    • Although not a result of human activity, Stratospheric Ozone averages -0.05 W/m² [-0.15 to 0.05 W/m²].
  • Stratospheric Water Vapor from the breakdown of Methane averages 0.07 W/m² [0.02 to 0.12 W/m²].
  • Aerosols have two categories (overall possessing a negative RF value):
    • Direct Aerosols (due to reflection/absorption of solar and infrared radiation) have an average RF of -0.5 W/m² [-0.9 to -0.1 W/m²].
    • Aerosols causing Cloud Albedo Effects average -0.7 W/m² [-1.8 to -0.3 W/m²].
  • Linear Contrails (Yes, even these have a minute effect.) have a value of 0.01 W/m² [0.003 to 0.03 W/m²].

Overall, the total net anthropogenic Radiative Forcing is equal to an average value of 1.6 W/m² [0.6 to 2.4 W/m²]. This means a warming of the climate.

Now for the naturally-induced RF agents:

  • Solar Irradiance has a RF value of 0.12 W/m² that ranges from 0.06 to 0.30 W/m².
  • Volcanic Aerosols in the atmosphere are currently negligible due the fact that the last major eruption was Mt. Pinatubo in 1991.

This makes the total net natural RF equal to the Solar Irradiance RF values.

(For an illustration, please see Figure SPM.2 in the following link: http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf).

As you can see, in today’s atmosphere, the Radiative Forcing caused by human activities are much more influential on climate change than the RF caused by natural means. Thus, it appears as though humans may want to reconsider the amount of greenhouse gases that they pump into the Earth’s atmosphere because it obviously has a measurable effect on climate change.

Personally, I found this article to be helpful in furthering my understanding of our warming climate. I have always been curious as to the exact quantitative effects of burning fossil fuels. Now that I know them, I am in complete agreement with the conclusions that the IPCC has published. Moreover, this report is quite detailed, and I am sure if I had the time to thoroughly examine its 800 or more pages, I would be fully aware of the global warming issue. That being said, I must add that I found parts of the report quite hard to read because it constantly referenced different sections of different chapters. It is almost as if you need to read the entire report to fully understand the individual sections.

I did have questions regarding the presence of water vapor in the atmosphere. If the atmosphere warms, and thus, is able to “hold,” if you will, more water vapor, wouldn’t that lead to a small negative feedback in the warming trend? In other words, wouldn’t more water vapor in the atmosphere lead to more clouds, and, thus, a higher albedo?

Thanks for tuning in,

Adam “CSP” Tripp

Tags: global warming · MTR3440

5 responses so far ↓

  • 1 Sean Davis // Feb 14, 2008 at 9:20 am


    On your question regarding water vapor, there are two related issues to consider.

    1) WV Feedback — As you correctly point out, as air warms it can “hold” more water, but that doesn’t necessarily say anything about whether or not more clouds will form. In other words, yes — it can hold more water, but IF the absolute humidity remains constant as you increase the temperature, then the relative humidity drops, which would lead to less cloud formation. However, climate models — from the most basic to the most complex — tell us that the relative humidity stays almost constant as you crank up the temperature. In doing this, the absolute humidity goes up. As WV is the single most potent greenhouse gas in the atmosphere, this forms a positive feedback whereby increases in CO2 increase WV and add to the RF that you would otherwise have from CO2. This “extra” forcing amounts to about 50% of the CO2 only value. I’m not sure if the IPCC numbers include the WV-component that goes on top of the CO2-only forcing — I think they don’t, given the definition of RF you give above.

    2) Cloud Feedbacks — This is the million dollar question in climate science, as far as I’m concerned. Models vary even among the sign of the WV feedback, although as I recall most have a positive cloud feedback. See the post here for more information. I’m not sure how reliable GCM cloud feedbacks are, as clouds are not resolved explicitly in the models. One thing’s for sure.. this is an issue that will be around for some time to come …

  • 2 OVER UNITY // Feb 15, 2008 at 4:48 am

    I think that what we need to be more aware of is that we are not dealing with an inrease in ethalpy as the misnomre ‘global warming’ suggests, but rather an increase in entropy – leading to greater and faster fluctuation and chaos in the the wheather system..

  • 3 podo // Mar 20, 2009 at 4:58 pm

    Here’s something I don’t understand: The average emission from the earth’s surface that escapes to space is about 250 W/m2, while the blackbody radiation expected for a surface at 15C is about 390 W/m2. Thus, greenhouse gases prevent the escape of about 140 W/m2, producing a temperature rise of 30C. Changes in CO2 to date account for only about 1% of 140, so we might expect the delta-CO2 to have produced a 0.3 degree change in average climate. This is simplistic, but is it wrong???

  • 4 A Simple Student // Jun 17, 2009 at 10:17 pm

    I have a few fairly simple questions:

    1. What initiated the current warming trend that started around 1600-1700s (little ice age) which is before the 1750′s industrial revolution?

    2. Once the temperature started to rise during the little ice age, how did that affect the atmospheric H20 in turn, now did/would that affect global temperature?

    3. How come the effects of the 1% of atmospheric water vapor is omitted in most reports? Since it is the most abundant green house gas at 10,000 ppm (I understand it is a variable gas but this does not mean negligible) compared to CO2 at 385 ppm and CH4′s 1.7ppm wouldn’t it be an important part of the global warming papers?

    I look forward to your answers.

  • 5 tanya // Jun 22, 2009 at 11:35 am

    To A Simple Student,

    I can answer #3 right now, the other two I’d have to think about for a second.
    Water Vapor isn’t mentioned much because humans are not directly changing the about H2O in the atmosphere. Yes it is variable and yes it is not negligible, but any changes that occur are due to natural forcing not human forcings. So the total amount of water is constant, now how much is in the atmosphere can change with a change in temperature, along with how much fresh water is available in certain areas. Thus it is indirectly affected by an increase in CO2, but not directly. This is why it isn’t discussed that much. CO2 is the source, the reason why many other things in on the plant, like H2O, may and/or are changing.
    Hope this helps. I’ll try to get to the other two questions later.


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