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Joe Tyndall, a member of the EARSI staff, was an undergraduate at MIT in the early 1970s.   At that time, preliminary scientific data concerning the greenhouse gas history of our planet was being analyzed.   Joe wrote a paper in 1979 for a group of legislators at the Massachusetts State House that is identical to what follows.   Nothing has changed, except that we are finally paying attention.

So, what is all this Greenhouse Gas nonsense?

Every second, ten times more energy passes through the atmosphere than the total used by human beings since we first harnessed fire.   Figure 1 shows where this energy goes.

Figure 1 - Energy Flows (1 PW = 1,000,000,000,000,000 Watts)

More than 99% of the incoming solar energy is reflected or reradiated back into space.   Of the small fraction that remains, plants use some for photosynthesis.   The rest causes an increase in the surface temperature of the Earth.   Contrary to widely held belief, the global warming now happening is not "normal."

Energy passing through the atmosphere causes weather.   Sunlight warms some places more than others.   This temperature differential causes air to flow and to absorb and release water, thereby moving energy from warmer places to cooler places, always trying to cancel the temperature differences.   Weather, averaged over long time periods, is climate.

All warm bodies radiate energy.   That is how objects cool.   The higher the surface temperature, the faster the object sheds energy.   The Earth is no exception.

Incoming solar energy that is absorbed must exactly balance the energy radiated back into space, or the surface temperature of the Earth will change.   By analogy, if you eat too much (incoming solar energy that is absorbed) and don't exercise (outgoing infrared heat energy that is radiated back into space), you will get fat (hotter).   The temperature of the Earth will rise (or fall) until equilibrium is reached where the absorbed solar energy exactly balances the infrared heat energy radiated back into space.   Because the Earth is large compared to these energy flows, the change in global surface temperature occurs over time periods of a century or longer.

The Carbon Cycle

Photosynthesis combines carbon dioxide (CO2) with water (H2O) to make sugar.

                [6]CO2 + [6]H2O + Energy from Light >>>> C6H12O6 + [3]O2

Free oxygen (O2) is left over after this reaction.   Sugar is then used as an energy source and as raw material to synthesize the components of cells, particularly cellulose (wood).   Free oxygen is not normal in planetary atmospheres and is the unambiguous signature of carbon-based life.   If plants were not continuously releasing oxygen into the air as a byproduct of photosynthesis, the free oxygen would soon be consumed in fires and other corrosive processes.

Photosynthesis removes carbon from the atmosphere.   As leaves fall and plants die, this material (mainly cellulose) is gradually covered over and buried in the ground.   Oil, coal, and natural gas are the remains of dead plants buried during the dim past.   Digging this stuff up and burning it puts the carbon back into the air.

Human Impact

Prior to 1800, the start of the industrial revolution, nearly all the energy used by human beings was from sources above ground.   We burned wood.   But, as we began to manufacture things on an industrial scale, so began the use of buried carbon (oil, coal, and natural gas).

Before this, human beings had a minor impact through deforestation.   By 1800, most of the dense forests that once covered Europe had been cut down to make way for farming and animal grazing. The same thing occurred elsewhere as population grew.   Deforestation slows the rate at which carbon is removed from the atmosphere and buried.   Population is the driving factor.   By 1800, world population had reached one billion people.   By 1930, it hit two billion.   By 1960, there were three billion.   Today, the population is seven billion and growing.

Temperature Regulation of the Earth

The surface temperature of the Earth will change to balance the incoming solar energy with the outgoing radiated heat.   What determines the temperature at which this equilibrium occurs?

The two major components of our atmosphere (nitrogen 80% and oxygen 20%) are transparent to visible light (solar radiation from the Sun) and to infrared radiation (heat from the surface of the Earth).   Because the major gasses do not absorb light or heat energy, trace gasses that do absorb light and heat determine the temperature of our planet.   These greenhouse gasses total less than one percent of the atmosphere and include CO2, methane, nitrous oxide, ozone, and water vapor. As the amount of these gasses in the atmosphere increases, more incoming solar radiation is absorbed (converted to heat) and more infrared radiation from the surface is absorbed and reradiated back to the ground.   This "trapped" energy raises the temperature.   Some of this "greenhouse effect" is normal - but not at the level happening today.

The other factor that sets temperature is the surface reflectivity of the Earth.   Things that are dark (water, trees, asphalt) absorb more light.   Things that are light (snow, sand, ice) reflect more light. The more light that is absorbed, the hotter the surface temperature.   The exception is photosynthesis, where the energy is stored in chemical bonds rather than being converted to heat.

Since greenhouse gases account for only a small portion of the atmosphere, it is comparatively easy for human activity to change their concentrations.   While other greenhouse gases also contribute, CO2 has driven most of the global warming we have seen thus far.

Figure 2 - Historic Atmospheric CO2 Concentration (ppm) for 1,000 Years

Back in 1800, at the start of the industrial revolution, the concentration of CO2 in the atmosphere was 280 parts per million (ppm) and had remained at roughly 280 ppm for a thousand years as shown in Figure 2.   Then we began to use coal and oil in large quantities.   280 ppm is 0.28 percent.   This data is derived from analysis of gas bubbles trapped within sediments of ice.

The sudden and rapid rise in atmospheric CO2 correlates exactly with the amount of coal and oil pulled from the ground and burned worldwide.   Figure 2 was copied from an article published in 1980.   The fact that human energy use has caused a significant rise in CO2 concentrations was well established 25 years ago.   New measurements have not changed this data.

Figure 3 started the controversy about carbon loading in the atmosphere and global warming.   In 1958, Dr. Charles Keeling began monitoring CO2 concentrations.   By the mid-60s, the upward trend was clear.   We now have 40 years more data.

Figure 3 - Historic Atmospheric CO2 Concentration (ppm) for 50 Years

The zigzag pattern is the seasons.   There is more land area on the northern hemisphere.   In the spring and the summer, the greater number of plants north of the equator breath in the CO2, so the amount of CO2 in the atmosphere drops.   In autumn, as leaves lose their chlorophyll and fall, the amount of CO2 in the air goes back up.   The net exhaling of CO2 continues during the winter. This illustrates two major points.   First, weather patterns are very efficient at mixing atmospheric gasses worldwide.   Second, foliage cover has a huge and rapid impact on CO2 concentrations.

The data in the above chart is not in question.   Still, many people claim that the facts are not all in. Aren't volcanoes the source of the CO2?   Mount Saint Helens erupted in 1980.   Pinatubo went off in 1991.   Neither of these events caused any noticeable effect on the curve.   The minor flattening from 1991 through 1993 corresponds to the worldwide recession.   We burned less coal and oil in those years.   So, economic events do show up.   None of the other objections I have heard, including sunspot cycles, wobble in the Earth's orbit, natural short-term climate cycles, and so on, appears in the data.   Again, this data correlates perfectly with the history of worldwide fossil fuel use.   The rise in atmospheric CO2 levels is caused by the burning of oil, coal, and natural gas and by other human activities including deforestation and slash-and-burn agriculture.

So, what is the link between CO2 and climate change?

Figure 4 - Historic Atmospheric CO2 Concentration (ppm) for 400,000 Years

Average global temperature correlates well with CO2 concentration and lags in time indicating that CO2 concentration drives the temperature changes.   Four ice age cycles can be seen.   When CO2 concentration drops to 200 PPM, there is a mile-thick sheet of ice sitting on top of Chicago. At 280 ppm of CO2, the climate is what we experienced two hundred years ago.   Back then, the glaciers were not melting and sea level was not rising.   Since 1900, sea level has risen roughly eight inches.   Today, sea level is rising nearly 0.2 inches per year.   That rate will grow quickly as ice packs worldwide melt faster as the planet warms.

Keep in mind that half of the excess CO2 in the atmosphere was put there during the last 30 years (see Figure 3).   Given the nature of exponential growth, and if we allow it to continue unabated, 30 years from today, the CO2 concentration will be 490 ppm.   The process of global warming, and all the effects that come with it, are just beginning to pick up steam, literally.

Regarding climate change, the significant thing to be learned from Figure 4 is that for 400,000 years, CO2 concentration has not risen above 300 ppm.   Today, we are at 385 ppm.   This is new territory and we are still dumping carbon into the atmosphere at an ever increasing rate.

So, where is the uncertainty?

Centuries in the future, what will the Earth look like as a result of the high CO2 concentration in the air?   The controversy is not that something will happen, but what that something will be.   Given the complexity of the global climate system, our current understanding, and the millions of small effects that combine to make the whole, reliably predicting the future is impossible.

Historically, when CO2 concentrations are high, we enter an ice age.   As temperature rises, the amount of moisture in the atmosphere also rises.   This results in a major snow out, where large areas of land are brightened dramatically.   This increases the amount of incoming solar radiation that is reflected back into space, thereby cooling the planet.   Brightening of the Earth's surface dominates over the increased energy absorption by the atmosphere due to greenhouse gases.

OR, because we are now well above the equilibrium range, could we be locked in heating mode? Sea level would continue to rise as all of the ice on the planet melts.   This would darken the Earth causing the amount of solar radiation absorbed to increase, further raising the temperature.

Melting the ice on Greenland would raise sea level by 20 feet.   Melting the ice sheet in Antarctica sitting over open water would also raise sea level by 20 feet.   These glaciers are crumbling rapidly. Melting all the ice on the planet would raise sea level by more than 260 feet.   Then, as the water in the oceans continues to warm, it expands, raising sea level further.

OR, will some intermediate scenario prevail?   We simply do not know and cannot reliably predict.
The Short Term

We do know that, because of excess greenhouse gas in the atmosphere, energy flows are out of balance.   Surface temperature is rising to increase the rate at which we radiate energy back into space.   Because some areas are warming faster than others, there is more energy available to drive the weather system.   Events like Katrina, and even more severe, are likely to occur, and with greater frequency, in the future.   The planet will find a new equilibrium.   Climate will change.

Have we passed the "tipping point" to a future catastrophe?   We don't know.   Do we care?   If we do care, the first goal is to decrease our emissions to the point where plants can reabsorb the carbon we put into the atmosphere as fast as we put it there.   That would cause the rising curve on Figure 3 to level off.   Then we must reduce our emissions a bit further, so the excess carbon in the atmosphere can be reabsorbed.   Under the most optimistic scenario, it will take a century to drop atmospheric carbon levels back to pre-industrial levels (280 ppm).

More Information

In February 2007, the Intergovernmental Panel on Climate Change released its Summary for Policymakers.   This report states, "Warming of the climate system is unequivocal, as it is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level."   The report indicates a 90% certainty that these effects are the result of human activity.   It originally stated that the certainty was 99%, but this number was reduced due to pressure from the Chinese government, the second largest greenhouse gas polluter, following the United States.   The 18-page Summary Report can be found at http://www.ipcc.ch.  

Regardless of your opinion about Al Gore, An Inconvenient Truth is a brief, yet comprehensive introduction to global warming, how we got here, and the implications for life on this planet.