Introduction
Earth-Sun Relationship
The Sun & its Energy
Earth & its Atmosphere
Solar Radiation in the Atmosphere
Atmospheric Environmental Concerns
Ozone Depletion
Global Climate Change
Regional Concerns
Exercises
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The Greenhouse Effect & Global Climate Change

Solar Radiation Absorption, Balance, and the Natural Greenhouse Effect

The atmosphere plays a role similar to that of a greenhouse. When solar radiation falls on the atmosphere, part of it is transmitted and part of it is immediately reflected back into space. As the transmitted radiation travels toward the Earth's surface, different regions of the spectrum are absorbed by the molecules of the atmosphere. Energy that is not reflected or absorbed by the atmosphere falls on the surface of the Earth. In turn, the energy that falls on the surface of the Earth is either reflected back into the atmosphere or absorbed by the surface of the Earth. For every 100 units of solar radiation falling on the Earth and its atmosphere, 25 units are reflected by the atmosphere, and 25 units are absorbed by the atmosphere. The remaining 50 units fall on the surface of the Earth. Of these 50 units, 5 units are reflected by the surface of the Earth, and 45 units are absorbed. So altogether, approximately 30% of the incident energy is reflected by the atmosphere and the surface of the earth. This portion is known as the "albedo." Thus, the atmosphere absorbs approximately 25% of the radiation, and the earth's surface absorbs 45%.

Some of the energy absorbed by the Earth is used to evaporate water, driving the water cycle. The Earth then re-radiates some of the remaining energy. This re-radiated energy contains more infrared than the original incoming radiation. So now the total amount of radiation present in the atmosphere, on the whole, contains longer in wavelength ranges than the original solar radiation that came to the Earth. Certain gases in the atmosphere (known as greenhouse gases) do not allow this longer wavelength radiation to pass through as easily as the shorter-wavelengths that entered. These gases absorb, retain, and re-radiate the infrared, keeping a "warm blanket" around the Earth that prevents sudden cooling and heating effects each time the face of the Earth rotates away from or towards the sun. The two components of the atmosphere primarily responsible for the natural greenhouse effect are H2O and CO2. This moderating effect - one that is a net result of visible radiation being transformed increasingly into infrared and shorter infrared to longer is called the "natural greenhouse effect."


Figure 9 : Solar energy balance.
[© Steve Schneider (1989)
"The Greenhouse Effect: Science and Policy." Science 243: 771-81.]

Most light energy that reaches the Earth (approximately 90%) is absorbed by the surface of the earth and other objects on it. Energy is then converted to long-wave radiation and re-emitted, as described in the example of the greenhouse above.


Figure 10: Energy balance of solar radiation.
(from Climate Change Impacts on the United States, 2000.
Reprinted with the permission of Cambridge University Press.)

 

The phenomenon we described above is known as the natural greenhouse effect, and is responsible for keeping the temperature of the Earth a full 33° C warmer than it would be otherwise. It creates a climate in which humans and other life forms can live under relatively hospitable conditions. However, human activities are causing a rapid increase in the concentrations of greenhouse gases, and we are now facing an "enhanced" greenhouse effect. The result of the enhanced greenhouse effect is an increase in the global average surface temperature of the Earth -- and possible changes in climate on a global scale.

Figure 11: Scheme of greenhouse effect. (From John R. Herman and Richard A. Goldberg:
Sun, Weather, and Climate. Washington D.C.: NASA, 1978.)

 

Global Climate Change

We noted earlier that even slight increases in the concentrations of greenhouse gases in the atmosphere result in more heat being trapped. In this section, we will summarize the scientific evidence that the increase in concentration of greenhouse gases impacts global climate conditions. We will also discuss the human activities that are causing the increase, and the policies that have been put in place to slow or reduce the effects of global climate change.

Recall from the previous section that there is a natural greenhouse effect that is necessary to maintain temperatures warm enough to sustain current ecosystems. This "temperature bath" occurs due to the absorption of short-wave (visible) solar radiation by surfaces on the Earth, and the subsequent transformation of that radiation into longer-wave infrared. Infrared is then absorbed and "trapped" by greenhouse gases, causing the troposphere to maintain a significantly warmer temperature than it would without this effect. Natural sources of greenhouse gases are part of a balanced chemical cycle that has been relatively steady during the time of human evolution to the present form.

As current ecosystems evolved over long time periods to acclimate to the environmental temperature, a permanent increase of even 1° on the average can be very disruptive, especially when this change occurs too quickly for the system to co-evolve.

Geologic evidence has been used to understand the correlation between the amount of CO2 in the atmosphere and global temperature and climate. For example, it is believed that when life began around 4 billion years ago, the sun was about 30% fainter than it is today. However, much higher levels of CO2 in the air (about 1000 times what they are today) made for enough warmth on the surface of the earth so at least some regions were above the freezing point of water, and began to provide conditions necessary for life to emerge. Analysis of CO2 in the frozen layers of ice in Antarctica provides evidence that over the past 160,000 years, climatic change and levels of carbon dioxide are closely related.

The greenhouse effect can become a "problem" when the amount of heat-absorbing gases in the atmosphere rapidly rises far above the levels at which they have been historically present. Since the Industrial Revolution, there has been a high rate of increase in the concentration of greenhouse gases, due in large part to the combustion of fossil fuels and the destruction of large plant systems such as tropical forests. Carbon dioxide concentrations, for example, have risen by 30% since the late 1800's. Furthermore, scientists predict that CO2 concentration will continue to rise, likely reaching 2 to 3 times its pre-industrial level by 2100.

The hypothesis that the known increase in greenhouse gas concentrations has led and will continue to lead to changes in the Earth's climate has been hotly debated in the past decade. However, a vast majority of scientists are now in agreement that evidence is sufficiently strong to prove the relationship. They are now mostly concerned with how to predict the impacts and scale of climate change, and how society can adapt to and minimize the harmful effects of these changes.

Reliable temperature records only exist of the last century or so, and scientists use paleohydrologic studies to extract longer-term records. These data show that global average surface temperature can vary greatly over short periods of time. For example, there was an apparent temporary cooling during the 1940's, 50's, and 60's. However, the past century has seen an overall increase in temperature by 1° F (or 0.6° C), with about half of that increase occurring since the late 1970's. Seventeen of the eighteen warmest years of the 20th century occurred between 1980 and 2000.

These studies have also shown a positive correlation between greenhouse gas concentration and temperature, as shown by Figures 12.1 and 12.2. Notice on each graph that the blue line indicates readings taken from historical records, tree rings, corals, and air trapped in Antarctic ice, while the shift to a purple line indicates temperatures and CO2 concentrations directly measured and recorded.

Figure 12.1: Graph of temperature change over past 1000 years.
(from Climate Change Impacts on the United States, 2000.
Reprinted with the permission of Cambridge University Press.)

Figure 12.2: Atmospheric CO2 concentration over past 1000 years.
(from Climate Change Impacts on the United States, 2000.
Reprinted with the permission of Cambridge University Press.)

Figure 12.3: Carbon emissions over past 1000 years.
(from Climate Change Impacts on the United States, 2000.
Reprinted with the permission of Cambridge University Press.)

 

Policy

Since fossil fuel combustion is one of the primary sources of greenhouse gases, fuel use is indicative of those countries that have contributed most to global climate change. The US leads the world in the gross amount of carbon emissions from fossil fuels, followed by China. Overall, developing countries contribute a very small percentage as compared to industrialized nations. In speaking of CO2 emissions, we normally speak in terms of carbon emitted, rather than CO2 emitted. The US is also the leader in terms of the amount of carbon dioxide emitted per person, while China is the leader in terms of carbon emitted per dollar GNP.

While developed countries produce the vast majority of the carbon emissions from fossil fuel use, they often use more environmentally efficient technology. As highly-populated developing countries are becoming more industrialized, we risk a further leap in greenhouse gas concentrations due to the use of outdated (cheaper) technology.

There have been many international meetings regarding climate change since 1979 and they continue today. The most recent meeting that resulted in international agreement was the Kyoto conference in 1997. The debate there was between industrialized and the developing nations. The points made by each are as follows:

  • Industrialized countries claim that most of the population growth is occurring in the developing countries, and most of the negative effects of climate change will affect those countries. Therefore, the developing world should be active participants in curtails on greenhouse gas emissions.

  • Developing countries claim that industrialized countries caused these problems as they achieved their socioeconomic status. As such, the industrialized countries should be solely responsible for minimizing emissions etc., since emission limits would slow development in the developing countries.

The following preliminary agreement was reached as part of the Kyoto Treaty:

  • 38 industrialized countries agreed to lower their greenhouse gas emissions by a combined 5.2% of 1990 levels by 2008. Of this, the US agreed to 7%, Japan to 7%, and Europe to 8%.
  • There were no emissions reductions for the developing countries.
  • An analysis of these agreed reductions would mean that due to population growth, the USA would have to reduce its carbon emissions by 33% of what it would have been in 2008. However, because the US lobbied for the inclusion of the effects of carbon sinks (or the "recapturing" of carbon by new forest growth, etc.) the reduction of actual emissions of greenhouse gases by 2008 only amounts to 2-3% less what the emissions were in 1990.
  • In spite of the efforts of industrialized nations to reduce greenhouse gas emissions, it is understood that the overall atmospheric concentration of greenhouse gases would still increase due to the impacts of the developing world.

In order for this initial agreement to be considered binding, at least 55 countries must have the Treaty officially ratified and put in place by their individual governments. Furthermore, the 55 countries that ratify the Treaty must produce at least 55% of the world's greenhouse gas emissions. Otherwise, the Kyoto Treaty is not binding to the countries that signed it, and results will not be achieved.

Although the US signed the Kyoto Treaty within a year of its proposal, it has yet to be put before the Senate, where it would need a 2/3 vote to ratified. Shortly after taking office in 2001, President Bush suggested that the US may be withdrawing from the Kyoto treaty. His administration points to the "energy crisis" in California, possible threats to the economy, and lack of regulation of emissions in developing nations as the primary reasons for pulling out.

A more detailed explanation of the Kyoto Protocol can be found at http://www.cnie.org/nle/clim-3.html. This document is a Congressional Research Service Report for Congress, and was written by Susan R. Fletcher, Senior Analyst in International Environmental Policy. It was last updated March 2000.

 

 

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  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.