Primer and History

Global Warming Explained

A brief explanation of global warming and its history.

To clarify

Unless noted, all of the temperatures given in this presentation will be in °C. Zero °C is the freezing point of water, and 100 °C is the boiling point. A °C is equal to about two °F (1.8X).

It’s all about energy

The earth receives almost all of its energy from the Sun in the form of infrared and visible light, which are kinds of radiation. Just as there are different frequencies of radio or TV broadcasts, the Sun emits radiation at different frequencies. Infrared radiation has a longer wavelength than we can see, but we can still feel its effects as heat.

The first graphic shows the radiation that the earth receives from the sun1.

The graph looks confusing, but most of the details aren’t important. From left to right indicates the different frequencies of the solar radiation, like a radio dial. High frequencies have short wavelengths, such as ultraviolet light on the left. Low frequencies have long wavelengths, like infrared on the right. From bottom to top represents the amount of solar radiation received by the earth at a given frequency.

The yellow area shows the amount of radiation the earth receives above the atmosphere, where there is nothing to block it. The red area shows the energy that the surface receives at sea level on a clear day. The difference between the red and yellow is the energy that is either absorbed, reflected, or scattered by the atmosphere before reaching the surface of the earth. The greenhouse gases, such as water vapor, carbon dioxide (CO2) and ozone, absorb infrared radiation, and this is shown by the gaps in the red area. The gaps are examples of absorption bands, and there are many more of these bands at even longer wavelengths of infrared radiation shown in the next graph2.

The absorption bands of various gases overlap. The total absorption and scattering is shown in the top pane.

All of the energy that enters the atmosphere must eventually leave. The third image describes a simplified path the energy might take3.

About 30% of the solar radiation is reflected by the atmosphere (A) and by snow, ice, and other parts of the surface (B). This is called the albedo, or the reflectivity of the earth. The brighter the object, the higher its albedo, and the more energy it reflects.

Another portion of the solar radiation is absorbed by the atmosphere (C). The remaining portion is absorbed by the oceans and the land surface (D). The radiation reaching the surface is mostly visible light which is unobstructed by the greenhouse gases. After the radiation is absorbed by the surface, it is then given off (emitted) as infrared radiation (E), which is why the ground feels warm on a sunny day.

As shown in the last graph, the infrared radiation emitted by the earth’s surface (blue) has a longer wavelength than what is emitted by the sun (red) and is more readily absorbed by the greenhouse gases4.

Only 15 to 30% of the outgoing infrared radiation is transmitted directly to space. The rest is absorbed by the greenhouse gases, which further warms the troposphere. The troposphere is the lowest layer of the atmosphere. When the infrared radiation is let go (emitted) by the greenhouse gases, the energy is released in a random direction, so the path that the energy follows isn’t always up. As it meanders throughout the troposphere, the infrared radiation is re-absorbed and re-emitted countless times by the greenhouse gases until finally, by chance, it reaches the top of the troposphere where it is free to escape into space. The top of the troposphere is called the tropopause, which is the boundary between the troposphere and the stratosphere.

The greenhouse effect: Bad name, good thing (but not too good)

This process of warming the troposphere from infrared radiation emitted by the surface is called the greenhouse effect, but it is not a perfect metaphor. An actual greenhouse works by physically trapping air within the glass, thus preventing convection to the outside. However, the metaphor has stuck.

The average surface air temperature of the earth is roughly 14 °C. Without the greenhouse effect, the temperature would be about 33 ° lower. Not only does the greenhouse effect warm the earth, it helps to smooth out the difference between the day and night.

Leave it to beavers

For the last 200 years or so, humans have been increasing the amount of greenhouse gases in the atmosphere, primarily through the burning of fossil fuels, but also due to forest clearing and other land use changes. That is why the current warming is called anthropogenic, or human caused, global warming. The result is an enhanced greenhouse effect.

The problem can be explained if we think of greenhouse gases as a beaver dam and the troposphere as a pond of energy. When the beaver decides he wants a bigger pond, he builds up the dam. Although gaps remain, the pond grows larger as the amount of water flowing through the dam is temporarily reduced due to the efforts of the beaver. However, once the beaver stops building up the dam, a new equilibrium is eventually reached. Thus, the amount of water leaving the pond once again matches the amount of water flowing into the pond (we are ignoring evaporation and water soaking into the ground), but the end result is a larger pond. As long as the amount of water entering the dam stays the same, and the dam doesn’t deteriorate, the pond will maintain its new size.

In much the same way, we are increasing the energy capacity of the troposphere. As long as greenhouse gases continue to increase, we are slowing the energy on its way back to space. More energy is entering the atmosphere than is leaving, which is why global warming is sometimes described in terms of the earth’s energy balance. The amount going out doesn’t balance the amount coming in. (For more on this imbalance, see section 7). In a simplified world, if we were to stop emitting greenhouse gases, a new equilibrium would eventually be reached, and temperatures would stabilize. However, that ignores the likely effect of amplifying feedbacks.

Forcing or feedback?

Outside influences on the climate system such as human emissions of greenhouse gases are called forcings, but there are natural forcings as well. A change in solar activity is a forcing, as is a large volcanic eruption that injects sunlight blocking debris into the stratosphere.

In addition to forcings are feedbacks. These act to amplify (positive feedback) or counteract (negative feedback) an initial forcing. The water vapor feedback is the strongest of these. Water vapor is the most powerful greenhouse gas, but the amount of water vapor in the atmosphere depends on its temperature. A warmer atmosphere holds more water vapor, which in turn increases the greenhouse effect. Take away the initial forcing, however, and the water vapor feedback cannot sustain itself and the additional water literally falls from the sky.

Another obvious positive feedback is the ice albedo feedback. For example, during ice ages, much of the world is covered with ice. Because the earth is much brighter, it reflects more energy. The colder it gets, the more ice is retained and more energy is reflected, causing even colder temperatures. Just as increased ice cover enhances cooling, decreased ice cover enhances warming. A melting Arctic makes the problem worse, and there are many other feedbacks that will be described in section 12.

Now that we have described the greenhouse effect, we will cover the history of the science, and its influence on policy.

Early pioneers5

It is thanks to the work of many scientists over nearly 200 years that our understanding of atmospheric physics has greatly improved.

In 1827 Jean Baptiste Joseph Fourier notes the similarity between the atmosphere and a greenhouse, thus “the greenhouse effect.”

In 1860 the Irish physicist John Tyndall measures the absorption of infrared radiation by CO2 and water vapor.

In 1896 the Swedish chemist Svante Arrhenius calculates an increase in global temperatures of 5 to 6 °C for doubled CO2 levels. Our current understanding gives about 3 °C of warming for doubled CO2. This is known as “climate sensitivity” (See section 7 for why we are confident that the correct value is likely 3 °C).

In 1922, the English mathematician Lewis Fry Richardson, publishes a book describing the first numerical model of the weather. He proposes a very large room filled with mathematicians carrying out laborious calculations. Although this wouldn’t be practical until digital computers became widely available, his methods became the basis for modern weather and climate models. (See section 8 and 9 for more on weather and climate models).

Laying the foundation6

Up to the 1950s, the mainstream belief among scientists was that adding CO2 would not have any effect because they believed the greenhouse effect was saturated by water vapor. In other words, they believed that water vapor was already blocking all of the infrared radiation that CO2 ever could. This was not the case, however. Affirming and building on the work of previous scientists such as Guy Callendar, the American physicists Lewis Kaplan and Gilbert Plass, publishing separately in 1952 and 1956 calculate that the CO2 increase is most important at high altitudes. (see section 7 for why this is important).

In 1957, Roger Revelle and Hans Suess write their famous paper discussing the changing CO2 levels in the atmosphere and ocean. Prior to their paper, many believed that the ocean would quickly absorb all of the CO2 emitted by humans, but Revelle and Suess found otherwise. In the paper, Revelle and Suess write:7

Human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in the sedimentary rocks over hundreds of millions of years. This experiment, if adequately documented, may yield a far-reaching insight into the processes determining the weather and climate.

Documenting CO2

Also in 1957, the Mauna Loa Observatory is established 11,000 feet above sea level on Hawaii’s Big Island. There, Revelle's colleague Charles David Keeling begins measuring CO2 in real time. This is the graph that his work produced, and it’s called the Keeling Curve8.

It shows the rising CO2 concentration in the atmosphere, starting at about 315 ppm in 1958 and rising to over 380 ppm today.

The models cometh

In 1967, Syukuro “Suki” Manabe and Richard Wetherald publish the results of most sophisticated model of the atmosphere up to that point and they calculate climate sensitivity of 2.3 °C for doubled CO2.

In 1969, Manabe and Kirk Bryan collaborate to create the first atmosphere-ocean general circulation model (AOGCM), which is what we usually think of as climate models today.

By 1975, Manabe and Bryan’s model is refined to approximate the Earth’s geography.

Global warming goes mainstream

In 1979, the Carter Administration requests the National Academy of Sciences assess previous reports on increasing CO2 in the atmosphere. The National Academy of Sciences was set up by Abraham Lincoln to answer any scientific questions asked of it by the president or congress. It is made up of the top scientists in their fields and is like the “supreme court of science.”

The document that they present is now called the Charney Report for its chairman, Jule Charney. The best models at that time concluded that a doubling of CO2 would result in “a global surface warming of between 2 °C and 3.5 °C with greater increases at high latitudes.”9 Taking into account additional uncertainty, the report concluded that the best figure was likely between 1.5 and 4.5 °C.

Based on the latest calculations, these figures from 1979 are spot on.

They are unable to find any negative feedback that might reduce or reverse the warming. They also note that because of the slow response of the ocean, which is quite massive, the total effect of increasing CO2 levels will be delayed several decades. This is an important point because all of the warming that we have observed to date is roughly half of what we are locked into due to this thermal inertia of the ocean.

Doctor Hansen goes to Washington

In 1988, James Hansen, who we talked about in the Introduction, testifies before Congress and declares with 99% confidence that global warming is here. Due to the publicity surrounding this hearing, this is probably the first time that many people have heard about global warming.

In his testimony, Hansen provides this chart with three different future scenarios based on rising concentrations of greenhouse gases10 (See section 9 for more on Hansen’s model).

The IPCC is formed

Also in 1988, the Intergovernmental Panel on Climate Change (IPCC) is established to periodically evaluate the state of climate science and inform policymakers. The IPCC has since issued four reports:

  • 1990 – First Assessment Report (FAR)

  • 1995 – Second Assessment Report (SAR)

  • 2001 – Third Assessment Report (TAR)

  • 2007 – Fourth Assessment Report (AR4)

The Fourth Assessment Report cannot be abbreviated “FAR” again, so from now on they will be abbreviated AR4, AR5, etc. Please take special note of these abbreviations as they will be referred to numerous times over the course of this presentation.

Measuring the past

Building on past work, the Greenland Ice Core Project (GRIP) drills the first reliable ice core record of substantial length. Started in 1989 and finished in 1991, the finished core is 3000 meters long and records climatic data of the past 200,000 years. Subsequent ice cores from Antarctica extend this record back 850,000 years.

Blame it on Rio

In 1992, the first Bush Administration attends the Earth Summit in Rio De Janeiro and agrees to the UN Framework Convention on Climate Change (UNFCCC) with the stated goal to achieve “stabilization of greenhouse gas concentrations in the atmosphere at a low enough level to prevent dangerous anthropogenic interference with the climate system.”11

The emissions targets that they establish are voluntary.

The upper limit to prevent “dangerous anthropogenic interference” remains undefined although the current best estimate is usually assumed to be 450 ppm CO2.

The Kyoto Protocol

In 1997, the signatories of the UN convention came together in Kyoto, Japan to establish mandatory emissions targets. The voluntary targets originally negotiated were not working, since no one was paying any attention to them.

The signatories agree to reduce their combined greenhouse gas emissions to 5% below 1990 levels.

Some countries have smaller targets while others have larger targets depending on the circumstances of that country. France for example, already has a large portion of its electricity generated by nuclear power, so it would be more difficult for them to reduce their emissions, since they are already relatively low.

In what would become a sticking point to this day, developing countries are exempt, but may participate in carbon trading that we will talk about shortly.

US Senate rejects Kyoto

In 1997, and before Kyoto was finalized, the US Senate declares in a 95-0 vote that it will reject the Kyoto protocol if it exempts developing nations. This was out of fear that it would place the US economy at a disadvantage. So, not all of the blame can be laid at Bush’s feet.

National Research Council report

In 2001, the second Bush administration requests the National Research Council, the research arm of the National Academy of Sciences, evaluate the Third Assessment Report from the IPCC.

In its first two sentences, the report states, “Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising.”12

Despite this key finding, the Bush administration focuses on the uncertainties listed by the report and continues to stress the exemption of developing countries from Kyoto to justify a policy that avoids mandatory reductions in greenhouse gases13.

The US withdraws from Kyoto

Later in 2001, the Bush Administration withdraws from the Kyoto protocol despite the conclusions of the TAR and the concurrence of the National Academy of Sciences. It was never submitted to the Senate for ratification, and no attempt to fix lingering concerns was attempted.

After Australia finally ratifies it in 2007, the United States is the only developed country to reject Kyoto.

Europe begins emissions trading

In 2005, Europe begins its carbon trading system.

This is modeled after the US sulfur dioxide trading system, which was very successful at reducing emissions from coal power plants. It worked a lot better and a lot cheaper than anyone had thought possible. So it is determined that the cap and trade system might be a way to reduce greenhouse gas emissions as well.

For certain industries, companies are given an emission cap. Any company emitting more than their cap must buy carbon credits and any company emitting less than their cap have carbon credits to sell. The caps are periodically reduced so it becomes more and more difficult for a company to come in under their cap. This creates a market based solution for reducing whatever pollution you are trying to reduce.

Unfortunately, the Europeans misallocate carbon credits leading to greatly reduced carbon prices. The price of carbon plunges resulting in very little incentive to reduce emissions. No one pays any attention to their cap since the penalty for emitting is so low.

Prices are reset in 2008 when phase 2 begins.

The end of Kyoto

Kyoto expires in 2012. Assuming one can be agreed upon, a new treaty negotiated in Copenhagen, Denmark (December, 2009) will take its place.


  1. (Rohde, 2007, Image: Solar Spectrum.png) Online here

  2. (Rohde, 2007, Image: Atmospheric Absorption Bands.png) Online here

  3. (Kunzig, 2008)

  4. (Rohde, 2007, Image: Atmosphere Transmission Blackbody Only.png) Online here

  5. (Houghton, 2004)

  6. (Weart) The Discovery of Global Warming, online here, is an invaluable resource documenting the history and science of global warming.

  7. (Revelle & Suess, 1957)

  8. (NOAA) Online here

  9. (Charney, et al., 1979) Online here. Carl Wunsch, the swindled scientist in The Great Global Warming Swindle (see Introduction), is a co-author of the report.

  10. (Hansen, et al., 1988) Online here, (Hansen J. , 1988) Online here

  11. (UN, 1992) Online here

  12. (Committee on the Science of Climate Change, National Research Council, 2001) Online here

  13. (Bush, 2001) Online here

Sources cited in Primer and History

American Society for Testing and Materials Terrestrial Reference Spectra. (n.d.). Solar Spectrum. Retrieved May 25, 2008, from Wikipedia:

Bush, G. W. (2001, June 11). President Bush Discusses Global Climate Change. Retrieved May 25, 2008, from The White House: President George W. Bush:

Charney, J. G., Arakawa, A., James, B. D., Bolin, B., Dickinson, R. E., Goody, C. E., et al. (1979). Ad Hoc Study Group on Carbon Dioxide and Climate. Washington D.C.: National Academy of Sciences.

Committee on the Science of Climate Change, National Research Council. (2001). Climate Change Science: An Analysis of Some Key Questions. Washington DC: National Academy of Sciences.

Hansen, J. (1988, June 23). Greenhouse Effect and Global Climate Change: Oral Testimony of James Hansen. Hearing Before the Committe on Energy and Natural Resources, United States Senate, One Hundredth Congress: First Session on the Greenhouse Effect and Global Climate Change, Part 2 . Washington DC: U.S. Government Printing Office.

Hansen, J., I., F., Lacis, A., Rind, D., Lebedeff, S., Ruedy, R., et al. (1988). Global Climate Changes as Forecast by Goddard Institute for Space Studies Three-Dimensional Model. Journal of Geophysical Research , 9341-9364.

Houghton, J. (2004). Global Warming: The Complete Briefing, third edition. New York: Cambridge University Press.

Kunzig, R. (2008, June 22). Proof Positive. National Geographic Special Report: Changing Climate , p. 30.

NOAA. (n.d.). Atmospheric Carbon Dioxide - Mauna Loa. Retrieved May 25, 2008, from NOAA Earth System Research Laboratory:

Revelle, R., & Suess, H. (1957). Carbon dioxide exchange between atmosphere and ocean and teh question of an increase of atmospheric CO2 during the past decades. Tellus , 18-27.

UN. (1992). United Nations Framework Convenction on Climate Change.

Weart, S. (n.d.). The Discovery of Global Warming. Retrieved May 25, 2008, from The American Institute of Physics: