Impact of Greenhouse Gases

By volume (rounded), the gas concentration of dry air includes 78% nitrogen (N₂) and 21% oxygen (O₂) with the remaining made up of gases including argon (1%), water vapor (0-1%), carbon dioxide (.04%), and other trace gases. Nitrogen and oxygen gases are transparent to both incoming solar radiation and outgoing longwave radiation from the earth's surface. Therefore, these gases do not play a role in establishing atmospheric temperature. Atmospheric temperature is influenced by gases that absorb outgoing radiation and these gases are called greenhouse gases. Although greenhouse gas concentrations appear to be small (less than one percent), their effect is certainly not.

Figure 2.1 (IPCC, 2007) shows the role that greenhouse gases play in the atmosphere. Solar radiation is primarily shortwave radiation which is transparent to greenhouse gases. Incoming solar radiation passes through these gases as if they were not present so the concentration of greenhouse gases does not directly influence incoming sunlight. The sunlight is absorbed by the Earth and atmosphere. Heat from the surface radiates up into the atmosphere in the form of infrared energy (longwave radiation). Greenhouse gases do absorb longwave radiation so the concentration of these gases is very important in determining how much energy the atmosphere absorbs. Increasing greenhouse gases causes an increase in atmospheric temperature. The greenhouse effect from natural greenhouse gas concentrations prior to the Industrial Revolution has kept the Earth's surface about 33 ^oC warmer than with an atmosphere with no greenhouse gases. For the detailed physics of the greenhouse effect please see Arthur Smith's Proof of the Atmospheric Greenhouse Effect (.PDF).

Figure 2.1: Role of greenhouse gases in the atmosphere

The most abundant greenhouse gases in the atmosphere in order of concentration are:

1. water vapor (H₂O)
2. carbon dioxide (CO₂)
3. methane (CH₄)
4. nitrous oxide (N₂O)
5. ozone (O₃)
6. CFCs

Not all greenhouse gases are equally effective at absorbing energy. When one ranks these gases by how efficient they are at absorbing longwave radiation the order becomes: (IPCC, 2007)

1. water vapor: 36–70%
2. carbon dioxide: 9-26%
3. methane: 4–9%
4. ozone: 3–7%

Because clouds are also very efficient absorbers of longwave radiation, the relative abundance of and type of clouds influences the absorption range of water vapor and subsequently carbon dioxide. Because of the impact of clouds, there is a large range of absorption values in these two gases. Water vapor is naturally cycled into and out of the atmosphere on a relatively short time cycle so it is not considered to be a major component of long-term climate change. Therefore, the concentrations of CO₂ and CH₄ are the main drivers of greenhouse gas induced climate change.

An excellent illustrated tutorial about greenhouse gases can be found at Windows to the Universe: The Greenhouse Effect & Greenhouse Gases by University Corporation for Atmospheric Research - UCAR.

Are Greenhouse Gases Increasing?

The answer depends on the time frame being considered. Figure 2.2 (Ruddiman, 2001) shows that CO₂ concentrations have cycled between high (300 ppm) and low (190 ppm) values with a period of approximately 100,000 years and that the climate has closely followed this cycle - high levels of CO₂ occur in warmer periods and low levels of CO₂ occur in cooler periods.

Figure 2.2: CO₂ concentration of the atmosphere vs. time

However, Figures 2.3, 2.4, and 2.5 (Ruddiman, 2001), Figure 2.6 (IPCC, 2007) and Figure 2.7 (CO2 Now, 2009) reveal that concentrations of CO₂ and CH₄ have risen dramatically in the past century with values that are far greater than the normal range. For hundreds of thousands of years, CO₂ values have ranged between 190 ppm and 300 ppm while CH₄ values have ranged between 350 ppb and 750 ppb. Research from Tripati, Roberts, and Eagle (2009) shows that the CO₂ levels today have not appeared in the climate record for the past 15 million years! Present day values of both of these greenhouse gases have far exceeded these ranges and have done so at an extraordinarily fast rate.

Figure 2.3: Short-term CO₂ and CH₄ concentrations of the atmosphere

Figure 2.4: Present day CO₂ concentration far exceeds natural values

Figure 2.5: Present day CH₄ concentration far exceeds natural values

Figure 2.6: Concentrations of greenhouse gases from year 0 to 2005

Figure 2.7: Current level of CO₂ measured directly from the Earth's atmosphere at the Mauna Loa Observatory in Hawaii

What is so different about the present day that could cause these unprecedented increases in greenhouse gases at unprecedented rates that was not present for hundreds of thousands of years? The answer is humans (anthropogenic forcing as it is known in the scientific literature.) In the past 250 years, 1,200 billion tons of CO₂ have been released to the atmosphere. Most of this is from fossil fuel emissions and half of these emissions has occurred only since the mid-1970s. (Romm, 2007) This sudden increase in the past few decades is the reason climate change has accelerated since the 1970s. To put the CO₂ increases in proper perspective, view Figure 2.8 below from the UK Met Office which shows CO₂ concentrations over the past 400,000 years vs. the projected concentration by the year 2100.

Figure 2.8: Carbon dioxide concentrations vs. time

The influence of a factor that can cause climate change, such as a greenhouse gas, is often evaluated in terms of its radiative forcing. Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered. Figure 2.9 (IPCC, 2007) shows the radiative forcing of climate between 1750 and 2005 with the net forcing due to human activities being positive (global warming). The black error bars reveal the range of uncertainty in the various values. One can see that radiative forcing due to clouds is the most uncertain. However, even if clouds are assumed to have the maximum cooling value, there would still be a net warming of the climate due to human activities.

Figure 2.9: Radiative forcing of climate between 1750 and 2005

There is a common misconception that the concentration levels of carbon dioxide are so small that they could not possibly be causing global warming. It is important to note that non-greenhouse gases that make up more than 99% of the Earth's atmosphere do not absorb outgoing longwave radiation so these gases cannot keep the atmosphere warm. As mentioned previously, the natural greenhouse effect (from gas concentrations before the Industrial Revolution) has kept the Earth's surface about 33 ^oC warmer than with an atmosphere with no greenhouse gases. Pre-Industrial Revolution CO₂ levels ranged between 190 ppm and 300 ppm. Today they are rapidly approaching 400 ppm. Because levels of carbon dioxide are well above natural levels, it should not be hard to see how these increases could cause temperatures to rise at least a few ^oC.

Measuring the Greenhouse Effect:

In their paper Spectral signatures of climate change in the Earth’s infrared spectrum between 1970 and 2006, Chen et al. (2007) examined data from the following sources: 2006 from the Tropospheric Emission Spectrometer (TES) on the AURA satellite, 2003 from the Atmospheric Infrared Sounder (AIRS), 1997 from the Interferometric Monitor of Greenhouse gases (IMG), and 1970 from the Infrared Interferometer Spectrometer (IRIS). All four of these satellite instruments measure the outgoing longwave (LW) radiation from the top of the atmosphere. Comparison of observed outgoing LW to modelled outgoing LW was used to test the ability to model processes in the atmosphere that affect outgoing longwave radiation.

Figure 2.10: Observed difference spectrum (black line) between 2006 and 1970 (TES – IRIS) and the simulated difference spectrum (red line) for the same time interval. (ibid)

Chen et al. showed that increased CO₂ is preventing LW radiation from escaping the atmosphere and this decreasing LW radiation is accurately being predicted by models.

Wang & Liang (2009) estimated downwelling LW radiation using two widely accepted methods under both clear and cloudy conditions, using meteorological observations from 1996 to 2007 at 36 globally distributed sites, operated by the Surface Radiation Budget Network (SURFRAD), AmeriFlux, and AsiaFlux Projects. These locations included North America (20 sites), Asia (12 sites), Australia (2 sites), Africa (1 site), and Europe (1 site). Latitudes for these sites range from 0^o to 50^o N/S; elevation ranges from 98 to 4700 m, and six different land cover types were represented (deserts, semideserts, croplands, grasslands, forests, and wetlands).

The decadal variations in global downwelling LW under both clear and cloudy conditions at about 3200 stations from 1973 to 2008 were presented. Wang & Liang found that daily downwelling LW increased at an average rate of 2.2 W/m² per decade from 1973 to 2008. The rising trend results from increases in air temperature, atmospheric water vapor, and CO₂ concentration.

Evans & Puckrin (2006) measured the downward radiative flux for several important greenhouse gases. Their measurements showed that the greenhouse effect from trace gases in the atmosphere is real and adds significantly to global warming. Their data indicates that an energy flux imbalance of 3.5 W/m² has been created by anthropogenic emissions of greenhouse gases since 1850. This compares favorably with a modeled prediction of 2.55 W/m².

They concluded: "This experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming."

More about these studies can be found at Skepticalscience.com: How do we know CO2 is causing warming?

Climate Change Feedbacks and Climate Sensitivity:

A climate forcing mechanism such as CO₂ is one that will cause a change in climate. A feedback mechanism is one in which the forced change is either amplified (positive feedback) or dampened (negative feedback). A review of the literature by Bony et al. (2006) shows that there are four major climate change feedbacks. These are listed below along with the estimates of their radiative feedback in parentheses:

• Water Vapor (1.80 ± 0.18 W/m²/K): Water vapor is a very important positive feedback mechanism. When the air gets warmer, the saturation vapor pressure of water increases. That means that more water vapor can be present in warmer air. Because the average relative humidity of the climate is conserved, a warmer climate means that there will be more water vapor in the air. In turn, this causes a greater greenhouse effect which amplifies the initial warming caused by increasing industrial greenhouse gases. This water vapor feedback essentially doubles the warming caused by greenhouse gas forcing. (Note: Water vapor molecules typically spend about 10 days in the atmosphere {while elevated CO₂ concentrations can remain for hundreds to thousands of years} so water vapor cannot be a climate change forcing mechanism like CO₂.) See: A Matter of Humidity by Dessler and Sherwood (2009) for more information.

• Lapse Rate (-0.84 ± 0.26 W/m²/K): The tropospheric lapse rate (rate of change of temperature with height) affects the emission of LW radiation to space. If the troposphere warms uniformly, there is no radiative feedback whereas if there is a larger decrease in temperature with height there will be a greater greenhouse effect. An atmosphere that warms more in the lower troposphere will produce a larger positive feedback whereas an atmosphere that warms faster at higher altitudes will produce a negative feedback.

• Clouds (0.69 ± 0.38 W/m²/K): Cloud feedbacks are the most uncertain but progress has been made in recent years to understand the magnitude of the cloud feedback. Clouds are effective at absorbing and emitting LW radiation and are also affective at reflecting SW radiation. The feedback from clouds is influenced by cloud amount, cloud height and vertical profile, optical depth, liquid and ice water contents, and particle sizes. (Stephens, 2005) For some climate models, cloud feedback is positive and comparable in strength to the combined “water vapor plus lapse rate” feedback while for other models, cloud feedback is close to neutral. (Soden and Held, 2006)

• Surface Albedo (0.26 ± 0.08 W/m²/K): Albedo is defined as the percentage of incoming SW radiation from the sun that is reflected. In a warmer climate, highly reflective snow and ice melt away and leave less reflective surfaces such as water and land exposed below. These lower albedo surfaces will absorb more incoming radiation than the snow and ice that were above resulting in a positive feedback.

Climate sensitivity is the term used to describe the equilibrium global surface air temperature change due to a doubling of CO₂ from 280 ppm (pre-Industrial Revolution) to 560 ppm. It is usually given as a ^oC change per W/m² forcing. According to the IPCC (2007): "climate sensitivity is likely to be in the range 2 to 4.5^oC with a best estimate of about 3^oC, and is very unlikely to be less than 1.5^°C. Values substantially higher than 4.5^oC cannot be excluded, but agreement of models with observations is not as good for those values." The forcing from doubled CO₂ is around 4 W/m² and so a sensitivity of 3^oC for a doubling is equivalent to a sensitivity of 0.75 ^oC/W/m². (Schmidt, 2004) "Climate sensitivity is the largest source of uncertainty in projections of climate change beyond a few decades and is therefore an important diagnostic in climate modelling." (Knutti & Hegerl, 2008)

Climate sensitivity cannot be measured directly but estimates can be made by observing climate change in the past or from short-term changes caused by volcanic eruptions. Beginning in the 1960s early GCMs showed a sensitivity in the range of 1.5 - 4.5^oC. Current models show a range of 2.1 - 4.4^oC. This confirms that model simulations of climate feedbacks are quite robust. (Ibid)

Figure 2.11 (Ibid) shows the distribution and ranges of climate sensitivity from different lines of evidence. Most likely values are denoted by circles, likely range values (more than 66% probability) by bars, and very likely ranges (more than 90% probability) by lines. The IPCC (2007) most likely range and most likely value are denoted by the vertical gray bar and black line, respectively.

Figure 2.11: Climate sensitivity values from various lines of evidence (Click for larger image)

Knutti and Hegerl (2008) and IPCC (2007) conclude that various observations show a climate sensitivity value of about 3^oC, with a likely range of about 2 – 4.5^oC. Furthermore, the lower value of climate sensitivity of 2^oC is fairly well constrained which means that if emissions are not stabilized very soon, significant global warming is inevitable. According to Synthesis Report from the Climate Change Congress - University of Copenhagen (Richardson et al., 2009):

"Recent observations show that societies and ecosystems are highly vulnerable to even modest levels of climate change, with poor nations and communities, ecosystem services and biodiversity particularly at risk. Temperature rises above 2^oC will be difficult for contemporary societies to cope with, and are likely to cause major societal and environmental disruptions through the rest of the century and beyond."

Stratospheric Cooling

Figure 2.12 (Atkins, 2008) shows the vertical temperature profile of the atmosphere. Increasing greenhouse gases should result in a warmer troposphere and a cooler stratosphere.

Figure 2.12: Vertical temperature profile of the atmosphere

As discussed above, greenhouse gas emissions are very effective at trapping the outgoing IR radiation. As these greenhouse gases increase, more heat will be trapped in the troposphere which means there will be less incoming heat into the stratosphere above. Furthermore, the greenhouse gases in the stratosphere will still be very effective at emitting their heat into the regions above. The net effect is that the stratosphere will be emitting more heat upward than it receives from below resulting in a cooler stratosphere.

Ozone, which is concentrated in the lower stratosphere (Figure 2.13), absorbs incoming UV radiation from the sun which results in a warming of the stratosphere. Ozone also acts as a greenhouse gas. One complication in identifying the role of increased greenhouse gases and the subsequent cooling of the stratosphere is that loss of ozone due to ozone depleting chemicals from industry (ozone hole) will cause a cooling trend also. Figures 2.14 (Uherek, 2006) and 2.15 (Ajavon et al., 2007) show how carbon dioxide is cooling the stratosphere.

Figure 2.13: Ozone Concentration vs. Height (Ajavon, et al., 2007)

Figure 2.14: Stratospheric cooling rates: The picture shows how water, cabon dioxide and ozone contribute to longwave cooling in the stratosphere. Colours from blue through red, yellow and to green show increasing cooling, grey areas show warming of the stratosphere. The tropopause is shown as dotted line (the troposphere below and the stratosphere above). For CO2 it is obvious that there is no cooling in the troposphere, but a strong cooling effect in the stratosphere. Ozone, on the other hand, cools the upper stratosphere but warms the lower stratosphere. (ibid)

Figure 2.15: Stratospheric Temperature Anomolies vs. Height

Figure 2.16 (Ajavon et al., 2007) shows that global ozone concentrations have leveled off after years of decline and are on the rise since 1993. This suggests that the portion of stratospheric cooling caused by ozone depletion should have ended and warming would result in the absence of other forcing mechanisms.

Figure 2.16: Global Total Ozone Change

Randel et al. (2009) updated the analysis of observed stratospheric temperature variability and trends on the basis of satellite, radiosonde, and lidar observations. Their research reveals that temperature changes in the lower stratosphere show cooling of ~0.5 K/decade over much of the globe for 1979–2007. This cooling of the lower stratosphere did not occur in a straight line but as two downward steps in temperature that are coincident with the end of the warming associated with the El Chichon and Pinatubo volcanic eruptions. Significant warming events occurred in the stratosphere following the volcanic eruptions of Agung (March 1963), El Chichon (April 1982) and Mt. Pinatubo (June 1991). To avoid a significant influence on trend results, data were omitted for two years following each eruption in the analysis. In the global mean, the lower stratosphere has not noticeably cooled since 1995. This is no surprise because ozone levels are increasing which should be causing a warming trend.

In the middle and upper stratosphere there was mean cooling of 0.5–1.5 K/decade during 1979–2005, with the greatest cooling in the upper stratosphere near 40–50 km. Recalling Figure 2.13 above, ozone concentration above 35 km is minimal so ozone depletion is much less a factor at these levels than cooling due to CO₂.

The 11 year solar (sunspot) cycle also influences stratospheric temperatures. Randel et al. also observed statistically significant positive values near ~0.5 K in the lower stratosphere for both sets of satellite data and for the different radiosonde data sets with a maximum value of ~1 K in the upper stratosphere.

Model calculations suggest that the upper stratosphere trends are due, about equally, to decreases in ozone and increases in well-mixed greenhouse gases. (Ajavon et al., 2007) As these greenhouse gases increase, more heat will be trapped in the troposphere which means there will be less incoming heat into the stratosphere above. All climate models predict further cooling of the stratosphere as the year 2100 approaches even though full ozone recovery is predicted to occur around 2060. (Ibid)

Next: The Smoking Gun for Humans

• Introduction
• The Scientific Consensus
• Determining the Climate Record
• Climate Change: The Smoking Gun for Humans
• Natural Causes of Climate Change
• Global Cooling?
• Climate Models & Accuracy
• Modern Day Climate Change
• The Global Warming Denial Machine
• Summary of Key Points
• Suggested Reading
• Works Cited

Last updated: 11/01/09