Visible light from the sun penetrates to warm the Earth's surface. The energy is radiated back as infrared light (heat rays). These are absorbed by carbon dioxide, water vapor etc. in the atmosphere, causing the Earth to get warmer.

This effect was first explained by the French physicist and mathematician Jean Fourier in 1827.

Naturally occurring global warming raises the Earth's temperature nearly 30 degrees C -- the Earth would be mostly frozen if not for this effect.

Human activity has increased CO2 concentrations by 30% since pre-industrial times. Earth is about 1 degree warmer since 1900. This amount of warming is about equal to the average global cooling during the "Little Ice Age" 1450 - 1850.

In the chart at left, the little up-and-down wiggles represent annual fluctuations. During the Northern Hemisphere summer, plants take up CO₂ from the atmosphere; in winter, they put it back.

The Swedish chemist Svente Arrhenius was the first scientist to suggest that human burning of fossil fuels could possibly alter the Earth's climate. In 1896 he predicted that doubling the Earth's atmospheric CO₂ content might raise the average global temperature by 10 degrees F -- not a bad estimate for the 19th century. Current models predict a range of 3 to 8 degrees F.

The Kyoto protocol is a treaty signed in 1998 but not yet ratified which aims to control global CO2 emissions. Under the treaty each nation has a different CO2 reduction quota. The US must reduce to 5% less than its 1990 emissions; Europe must reduce to 8% less; many developing countries are not covered.

The Kyoto Protocol will take effect when at least 55 countries accounting for at least 55% of developed countries' CO2 emissions. In 2002, the European Union and Japan ratified the protocol. Russia is expected to ratify later this year, which will bring the protocol into effect.

Greenhouse gasses include carbon dioxide (CO2), methane, clorofluorocarbons and water vapor. Water vapor is actually the No. 1. greenhouse gas because there is so much of it.

Values in this diagram are in units of gigatons (billions of tons) per year. Note that human activities -- fossil fuel burning and deforestation -- are minor compared to natural flows.

Plant growth is important because plants suck CO₂ out of the air. Hopefully increased CO₂ should stimulate plant growth -- the CO₂ fertilization effect -- but this does not seem to be established as yet.

When the estimated values for these flows are compared with the observed increase in atmospheric CO₂ concentrations, there is a "missing sink" that is sucking out 1.5 gigatons of carbon per year from the atmosphere. Is this the carbon fertilization effect? No-one really knows.

For a glimpse into the complexities involved in this scientific question, check out this article from Climate Change, a scientific journal dedicated to this topic. Note: "C3" and "C4" represent two different types of plants which differ in the way they metabolize carbon dioxide.

Here is a simple version of the RC model with two layers. The layers are so thick that infrared cannot penetrate through a layer. So while visible light from the sun can directly affect the lower layer, infrared rays from the lower layer cannot directly radiate into space.

Here is what this model looks like in Stella.

Now we try to include the effect of clouds in the model. There are two competing effects:

• The whiteness of the clouds raises the albedo or reflectiveness of the earth. This tends to make the atmosphere cooler ("sunscreen effect").

• The clouds trap infrared from the lower layer and prevent it from radiating out to the upper layer ("blanket effect").

It appears that for upper-atmospheric clouds the sunscreen effect is larger than the blanket effect. (This is why volcanic eruptions tend to cool the planet). But for lower-atmospheric clouds the blanket effect is apparently larger. However the physics of how the clouds form is not well understood and very hard to model.

Terrestrial Vegetation

600

Atmosphere

750

Surface Ocean

1000

Soil & Detritus

1600

Marine Biota

3

Dissolved Organic Carbon

700

Deep Ocean

38,000

Here is an image, from , representing possible positive and negative feedbacks involving clouds.

Here is a possible positive feedback loop involving the oceans. Although warm water can dissolve more solids than cold water (think about stirring sugar into your hot or iced tea), warm water holds less dissolved gas than cold water. Thus, increasing the temperature of the oceans could force more CO₂ into the atmosphere.