Changes in Atmospheric Composition
In addition to the observed phenomena described above, scientists have accumulated other relevant environmental recordings. For example, levels of several trace gases in our atmosphere have been rising and continue to rise. This change in atmospheric gas composition may be the most serious threat to the global well-being of Earth's environment. Information about this has been nicely summarized in an October, 1997 pamphlet entitled Climate Change: State of Knowledge, prepared and distributed by the President's Office of Science and Technology (OST), from which information and selected illustrations on this page have been extracted.
One of these gases, carbon dioxide (CO2) has been increasing at an accelerating rate since the middle of the last century, after many, many years of essentially stable levels. Why is this? A simple explanation is that the Industrial Revolution began at about the time these increases started. With that social upheaval came the use of biomass and coal for fuel to support these new industries. Burning such material generates CO2.
Other trace gases have been rising, as well. Carbon Monoxide (CO) is particularly deletorious. Methane (CH4), from rice paddy production and enteric fermentation, is increasing, as are chlorofluorocarbons (CFCs) that have been used for many years as a refrigerant and to produce foam. Methane is a much more potent greenhouse gas that CO2.
This pie chart shows the relative contributions of the different gases involved in atmospheric contamination:
For those unfamiliar with carbon dioxide and its properties, consult this Wikipedia website
As you will learn shortly, CO2 is generally considered to be the prime culprit in any supposed global warming. For the next few paragraphs, we will divert from this theme to review some basic facts that provide a background for discussing CO2's role in moderating the atmosphere:
As we saw in the Section 14 subsection that presented a "short course" in meteorology, the Earth's atmosphere obtains almost all its thermal energy from the Sun. This diagram is based on the general blackbody radiation curve (see Section 9). It shows the peak wavelength for radiation input from the Sun and the peak wavelength of the Earth itself as a thermal body of average temperature of 288 degree Kelvin. (The two curves have the same height in the plots; in fact the height for Earth is greatly subdued, indicating its energy output is much lower.)
The next two illustrations are plots of the so-called radiation budget that balances incoming with outgoing radiation, first in percents and then in power terms.
The reason for showing the above two illustrations is just to call attention to the complexities involved in the physics of the atmosphere. Various processes and interactions are involved in heating up the atmosphere. Changes in both short-term and long-term temperatures depend not only on adding a single heat-absorbing gas but on other factors that either exacerbate or mitigate the thermal state of the Earth's air.
This next diagram shows the absorbing bands associated with water and carbon dioxide, the two princpal "contaminants" in the atmosphere besides the primary constituents nitrogen and oxygen. The total amount of water vapor in the atmosphere remains essentially constant although the distribution (locations) at any one time do vary. The amount of carbon dioxide added or subtracted through natural processes (controlled mainly by vegetation) is seasonal but the overall level as annualized is fairly constant.
In almost all discussions of global warming caused by CO2, the plots used to demonstrate this effect show temperature versus time (years). This results in two kinds of plots: 1) one that shows variations (ups and downs) over long periods; and 2) one that shows a steady increase in recent years. This is pertinent information but a third plot would be equally informative: one that simply shows how temperature increases with concentration (the amount of CO2 in a fixed volume). This is a straightforward type of measurement in Physics. Yet the writer had great difficulty in finding such a plot after searching through hundreds of images on Google. The one shown below consists of measurements made on a mixture of Nitrogen and Oxygen (in atmospheric proportions) containing also variable amounts of CO2-gas in a chamber that was subjected to thermal radiation; the three plots refer to different rates of allowed temperature increases:
The irrefutable message from this plot: experimentally, it is clearly demonstrated that the addition of increasing amounts of CO2 to an otherwise constant atmosphere causes a systematic rise in gas temperature. The ranges shown in the above plot are pertinent to levels that might be attained in the future if carbon dioxide emissions are not curbed.
This is a good moment in our discussion to place the CO2 variations into the context of their distribution in the atmosphere, land vegetation, and the oceans in terms of the total amounts of carbon itself (but excluding the carbon found in limestone and dolomite rocks on the continents); a variant of this is shown on page 16-4. Note that the bulk of the carbon is found in the oceans, mainly in the form of carbonate-based shells of plants and animals, but also as dissolved carbonate and bicarbonate ions (see below):
Now, back to other aspects of information relevant to global warming. Later in this Section we will familiarize you with dedicated satellites that gather data on atmospheric gases and other contributors to global warming. This bears on a main thesis in this Tutorial, that satellites are proving to be a powerful, almost unique, tool for monitoring the whole Earth at the global scale. For now, we will mention two such observers. The first satellite is Aqua, discussed on page 16-11. Here are the results obtained by the AIRS sensor on the Aqua satellite which led to a global map of carbon dioxide at a height of 8 kilometers (5 miles) above the Earth's surface; the upper image applies to July of 2003 whereas the lower is five years later, showing the spread of CO2 in the mid-troposphere:
NASA built a sophisticated satellite, the Orbiting Carbon Observer, which would have provided valuable data on CO2 distribution in the lower atmosphere. Unhappily, the launch on February 24, 2009 failed when during the ascent the fairing that covered the satellite did not jettison properly. But by good fortune the Japanese space program JAXA had just shortly before successfully launched GOSAT (also called Ibuki) which is designed to make similar CO2 measurements. Here is a drawing of the satellite and below it the first data set released (the satellite is still undergoing adjustments):
In June 2009, the first maps of CO2 and CH4 distributions worldwide were released, as shown here (more comprehensive plots will follow):
These new data sets will be invaluable. Scientists need to establish a long-term data base for global warming, which includes temperatures from previous centuries. One way to estimate past temperatures is from CO2 measurements of air trapped in glacial ice that has been accumulating for thousands of years. The plot below shows the general trend in rising CO2 during the last 160,000 years (includes recent ice ages), with estimates of temperatures (derived from a CO2 model) shown for comparison.
This may be a very important graph bearing on the arguments for and against global warming. The graph shows a temperature maximum about 130,000 years ago, with a steady decline thereafter until a rapid rise beginning about 15,000 years ago. This rise resembles the same increase leading to the 130,000 peak. The question that must be asked: Is this just a repeat in the normal cycle associated with Pleistocene glaciation? The follow-up question: Is there any strong evidence that the more rapid temperature rise of the last 50 years (or, going back to the Industrial Revolution) is abnormal? The closest to the truth may be that there is now a temperature spike added to a trend that has natural causes related to glacial processes.
Even more impressive is the correlation between CO2 content in cored ice and derived temperatures during the last 400,000 years, as shown in these plots, both using data from the Vostok ice core (Antarctic):
Note: These high and low range values of temperature correlate well with the periods of continental glaciation in the northern hemisphere and the warm interglacial intervals over this extended time period.
The next graphs provide more details on the increasing levels of CO2 during the last 140 years. The top shows carbon dioxide concentration increases, based on ice core measurement until 1960 and Mauna Loa Observatory measurements thereafter. Below it is the measured temperature changes averaged for the entire world; the trend upwards, amounting to about 1.5° F, shows some irregularities (not smoothly cyclic) which result from other climatic factors. The Mauna Loa level in 2006 was 384 ppm.