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 Raymond T. Pierrehumbert's Infrared radiation and planetary temperature and Arthur Smith's Proof of the Atmospheric Greenhouse Effect (both are .PDF).
Figure 2.1: Role of greenhouse gases in the atmosphere
The most abundant greenhouse gases in the atmosphere in order of concentration are:
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)
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. Therefore, there is a large range of absorption values in these two gases.
According to Schmidt, et al. (2010), and summarized by Chris Colose, the total greenhouse effect between various radiatively active substances in the atmosphere is:
Under clear sky conditions:
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.
Just how do greenhouse gases change the climate? Use the Java applet from University of Colorado PhET Interactive Simulations and select the level of atmospheric greenhouse gases during an ice age, in the year 1750, today, or some time in the future and see how the Earth's temperature changes. Add clouds or panes of glass.
Greenhouse effect Java simulator from PhET
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), Figure 2.6a Trenberth, 2009) 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
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)
According to Hofmann, Butler, and Tans (2009) the growth rate of CO2 concentrations has been doubling about every 30 years since 1800. At this rate, CO2 concentration will reach 560 ppm (doubling pre-Industral Revolution values) by the year 2050. 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 (USGRP, 2010) shows CO2 concentrations over the past 800,000 years vs. the projected concentration by the year 2100. After major deglaciation, it takes about 4,000 years for CO2 to increase 100 ppm. At 2 ppm rise per year, humans are increasing CO2 at a rate that is about 80 times that of the fastest natural rate and almost 2000 times the average rate over the past hundreds of thousands of years!
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 changes in aerosols, especially their impact on clouds, is the most uncertain. However, even if the cloud albedo effect is 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 0o to 50o 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:
For an excellent primer on this topic please see: Introduction to feedbacks by Chris Colose.
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:
Despite the large uncertainty in the magnitude of cloud feedbacks, the overall picture of feedbacks in a warmer world is one that is positive - meaning that greenhouse gas warming will be enhanced by these mechanisms. A superb tutorial on forcing and feedbacks can be read at Chris Colose's: Re-visiting climate forcing/feedback concepts
|Peter Sinclair's Climate Crock of the Week: The Big Mist
Watch this video to learn about the role of water vapor in the atmosphere.
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 4.5oC with a best estimate of about 3oC, and is very unlikely to be less than 1.5oC. Values substantially higher than 4.5oC 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 3oC 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 - 4.5oC. Current models show a range of 2.1 - 4.4oC. 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 3oC, with a likely range of about 2 � 4.5oC. Furthermore, the lower value of climate sensitivity of 2oC 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 2oC 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."
The climate sensitivities discussed above are considered fast-feedbacks. Slower feedbacks such as the albedo feedback due to the melting of large ice sheets in Greenland and Antarctica were not considered by the IPCC. (Hansen, et al., 2008) When these feedbacks are included, the sensitivity value increases to approximately 6 oC. Hanson et al. explain:Paleoclimate data show that climate sensitivity is ~3oC for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6 oC for doubled CO2 for the range of climate states between glacial conditions and icefree Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 425±75 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.
Humanity today, collectively, must face the uncomfortable fact that industrial civilization itself has become the principal driver of global climate. If we stay our present course, using fossil fuels to feed a growing appetite for energy-intensive life styles, we will soon leave the climate of the Holocene, the world of prior human history. The eventual response to doubling preindustrial atmospheric CO2 likely would be a nearly ice-free planet.
Humanity's task of moderating human-caused global climate change is urgent. Ocean and ice sheet inertias provide a buffer delaying full response by centuries, but there is a danger that human-made forcings could drive the climate system beyond tipping points such that change proceeds out of our control. The time available to reduce the human-made forcing is uncertain, because models of the global system and critical components such as ice sheets are inadequate. However, climate response time is surely less than the atmospheric lifetime of the human-caused perturbation of CO2. Thus remaining fossil fuel reserves should not be exploited without a plan for retrieval and disposal of resulting atmospheric CO2.
Paleoclimate evidence and ongoing global changes imply that today's CO2, about 385 ppm, is already too high to maintain the climate to which humanity, wildlife, and the rest of the biosphere are adapted. Realization that we must reduce the current CO2 amount has a bright side: effects that had begun to seem inevitable, including impacts of ocean acidification, loss of fresh water supplies, and shifting of climatic zones, may be averted by the necessity of finding an energy course beyond fossil fuels sooner than would otherwise have occurred.
To summarize, Hanson et al. believe that it is quite possible Earth could end up ice free with CO2 levels of 350 ppm which is well below where we currently are. Because the melting of Antarctic ice takes centuries there is time to lower the "tipping point" level of CO2 before it is too late. When Antarctica was last ice-free, sea levels were 70m (~230 feet) higher than today.
So where are we likely headed under current emission proposals? According to The Climate Scoreboard it is very unsettling. Assuming countries actually enforce their proposed emission targets (which is unlikely), the climate is likely to be about 4 oC warmer by the year 2100 (Fig. 2.12).
The yellow "business-as-usual" line represents the estimated global temperature increase in 2100 if greenhouse gas emissions are not reduced. The blue "proposals" line represents the estimated global temperature increase in 2100 if the current proposals were enacted. The shaded blue curve shows the uncertainty in the climate system's response to emissions. The green "goals" line represents the goal of limiting the temperature increase to 1.5o-2.0oC.
According to Dr. Joseph Romm (2010) at the award-winning Climate Progress blog:
"Given that the anti-science, pro-pollution forces seem to be succeeding in their fight to keep us on our current emissions path, it’s no surprise that multiple recent analyses conclude that we face a temperature rise that is far, far beyond dangerous:"
Figure 2.13 (Atkins, 2008) shows the vertical temperature profile of the atmosphere. Increasing greenhouse gases should result in a warmer troposphere and a cooler stratosphere.
Figure 2.13: Vertical temperature profile of the atmosphere
The stratosphere is found between the tropopause to about 51 km or so. The tropopause heights varies between 8 km and 18 km due to latitude and season. Cooler latitudes and seasons result in a lower tropopause height.
The greatest ozone density is around 20-25 km because the ozone destruction processes are slower there. Maximum incoming solar heating occurs there but the greatest temperatures are near 50 km where the low density air requires very little energy to raise its temperature (think KE=½ mv2 and fewer collisions with fewer air molecules yield greater v). Therefore, there is a temperature inversion (warming with height) throughout the stratosphere above the ozone maximum. This inversion severely inhibits convection (mixing) so radiative and conductive processes dominate. The inversion also means that air from the troposphere has difficulty entering the stratosphere so heat is not easily transported between the two by convection or conduction. (Note: Strong thunderstorms can transport gases such as water vapor up across the tropopause and there are breaks in the jet stream westerlies allowing interchange of stratospheric and tropospheric air. Also, gravity waves can help mix the air in the stratosphere.) One can assume that radiative transfer must dominate between these two spheres.
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 2.14), absorbs incoming UV radiation from the sun which results in a warming of the stratosphere.
Figure 2.14: Ozone Concentration vs. Height (Ajavon, et al., 2007)
Ozone also acts as a greenhouse gas. However, ozone's ability to absorb incoming solar radiation (ultra-violet) is the dominant radiative forcing mechanism. 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 because less incoming solar radation will be absorbed. Figures 2.15 (Uherek, 2006) and 2.16 (Ajavon et al., 2007) show how carbon dioxide is cooling the stratosphere.
|Figure 2.15: Stratospheric cooling rates: The picture shows how water, carbon dioxide and ozone contribute to longwave cooling in the stratosphere. Colors 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 2.16: Stratospheric Temperature Anomalies vs. Height
Figure 2.17 (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 because of the increased absorption of incoming SW radiation from the sun.
Figure 2.17: 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 increases incoming solar radiation absorption.
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 2.14 and 2.15 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) All climate models predict further cooling of the stratosphere as the year 2100 approaches due to increased GHGs even though full ozone recovery is predicted to occur around 2060. (Ibid)
Schwarzkopf & Ramaswamy (2008) used an atmosphere-ocean climate model to investigate the evolution of stratospheric temperatures over the twentieth century. They modeled known anthropogenic and natural forcing agents. In the global lower-to-middle stratosphere (20-30 km) their simulations produce a sustained, significant cooling by 1920, earlier than in any lower atmospheric region, largely resulting from carbon dioxide increases. After 1979, stratospheric ozone decreases reinforced the cooling. The result of various forcing mechanisms appears below in Figure 2.18:
Figure 2.18: Evolution of the global, annual-mean temperature change relative to 1861 due to changes in radiative forcing agents at (a) 10 mb; (b) 30 mb; and (c) 50 mb. The forcing agents are "All Forces (AllForc)"; anthropogenic only ("Anth"); natural only - volcanoes and solar ("Nat"); GHGs and Ozone ("WmGhgO3"); carbon dioxide only ("CO2").
The results indicate that natural forcing mechanisms cannot cause stratospheric cooling while increased CO2 would be responsible for most of the cooling in the upper stratosphere and a significant portion of the lower region of the statosphere. Fig. 2.19 (Ibid) shows the evolution of the global stratospheric temperature change relative to 1861 due to changes in all known human plus natural forcing agents. The hatched area denotes the time-height region in which the temperature change from 1861 exceeds twice the standard deviation of annually averaged temperatures for years 1001�2000 of the unforced model simulation.
Figure 2.19: Evolution of the global, annual-mean temperature change relative to 1861 due to all known forcing agents
Increased GHGs, particularly CO2, will cause substantial warming in the lowest portion of the atmosphere (troposphere) while causing cooling in the higher portion of the atmosphere (stratosphere). Natural forcing mechanisms cannot do so. Model projections appear to closely follow actual observations as described by Randel et al. (2009). This coupling of a warmer lower atmosphere and a cooling upper atmosphere is a strong signature for anthropogenic global warming (AGW).
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Scott A. Mandia
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