Around 1900, a respected Swedish scientist named Svante Arrhenius published a series of papers and a book that included a crazy-sounding prediction. He and a colleague were studying the carbon cycle by estimating the changes in carbon dioxide (or CO₂) produced by natural processes such as rock weathering, volcanic eruptions, and ocean absorption.

He also looked at a source no one had thought of before—humans. Was it possible that humans could change the climate? Arrhenius took his colleague's calculations of carbon emissions from human activities and crunched the numbers.

When atmospheric carbon doubled, he figured, it would be enough to raise Earth's temperature 9-11°F (5-6°C), but it would take thousands of years to do it at 1896 rates. By the time his book on the subject was published in 1908, so much more coal was being burned, Arrhenius revised his estimate to centuries. But he reasoned that a warmer climate would be a GOOD thing—understandable, perhaps, given his home in Stockholm, a few hundred miles shy of the Arctic Circle.

Although Arrhenius went on to win the Nobel Prize for Chemistry in 1903, his carbon calculations faded into obscurity. In the hundred years since Arrhenius made his estimate—remarkably close to today's best figure—scientists hotly debated the causes of past and current climate change. However today, most climate scientists agree that multiple lines of evidence clearly show that human-induced climate change is taking place.

Earth's climate shifts over time because so many different land, ocean, and space phenomena have a hand in it. The sun is the main driver of Earth's climate, as it provides most of the energy. The sun's energy output increased about a tenth of a percent from 1750 to 1950, which contributed about 0.2°F (0.1°C) warming in the first part of the 20th century. But since 1979, when we began taking measurements from space, the data show no long-term change in total solar energy, even though Earth has been warming.

Repetitive cycles in Earth's orbit can influence the angle and timing of sunlight. The tilt and wobble of Earth's axis and the degree to which its orbit is stretched produce the Milankovitch cycles, which scientists believe triggered and shut off ice ages for the last few million years. But these changes take thousands of years, and so cannot explain the warming in this century.

Click to open a drifting continents & ocean currents animation.

Drifting continents make a big difference in climate over millions of years by changing ice caps at the poles and by steering ocean currents, which transport heat and cold throughout the ocean depths. These currents in turn influence atmospheric processes. Snow and ice on Earth also affect climate because they reflect more solar energy than darker land cover or open water.

Huge volcanic eruptions can cool Earth by injecting ash and tiny particles into the stratosphere. The resulting haze shades the sun for a year or two after each major blast. Dust and tiny particles thrown into the air by both natural processes and human activities can have a similar effect, although some absorb sunlight and help heat the climate.

Greenhouse gases, which occur both naturally and as a result of human activities, also influence Earth's climate.

Earth's surface absorbs heat from the sun and then re-radiates it back into the atmosphere and to space.

Much of this heat is absorbed by greenhouse gases, which then send the heat back to the surface, to other greenhouse gas molecules, or out to space. This is commonly called "the Greenhouse Effect" but "the blanket effect" may be more appropriate. Though only 1% of atmospheric gases are greenhouse gases, they are extremely powerful heat trappers. By burning fossil fuels faster and faster, humans are effectively piling on more blankets, heating the planet so much and so quickly that it's hard for Mother Nature and human societies to adapt. 

Though carbon dioxide gets the most press, it's certainly not the only greenhouse gas, nor even the most powerful. However, humans produce more of it than any other greenhouse gas, and it's very long-lasting (50-100+ years). In the United States, CO₂ comprises more than 80% of total greenhouse gas emissions.

Example of Human-Produced Greenhouse Gases
		Global Warming Potential (Relative to CO2)
Species	Lifetime (years)	20 years	100 years	500 years
Methane	12+/- 3	56	21	6.5
Nitrous Oxide	120	280	310	170
Sulfur hexafluoride	3,200	16,300	23,900	34,900
Carbon tetrafluoride	50,000	4,400	6,500	10,000

The other greenhouse gases are both natural and human-made. The most common are methane, nitrous oxide, ozone, fluorinated gases, and water vapor. Methane, for example, is only about 8% of U.S. greenhouse gas output, but is 21 times more powerful than carbon dioxide per molecule, although it does not stay in the atmosphere as long. It is produced naturally in wetlands, melting permafrost, termites, belching cows, and by human activities, such as fossil fuel production, landfills, and rice cultivation.

Water vapor is by far the most important gas in the natural greenhouse effect, contributing 60% of the effect to carbon dioxide's 26%. Human activities don't directly increase water vapor. Instead, warming produced by other gases, such as CO₂, increases evaporation and allows the atmosphere to hold more water vapor. And in fact, satellites have detected an increase in atmospheric moisture over the oceans at a rate of 4% per degree F of warming (7% per degree C) since 1988. This additional water vapor then adds to the warming because water vapor is a greenhouse gas. More water vapor can also produce more clouds, which have a complicated effect of both cooling the atmosphere by reflecting light and warming it by trapping heat below the clouds.

The main factors that determine the effect of greenhouse gases on climate are:

• The amount and rate of greenhouse gas emissions
• The effectiveness of each gas in trapping heat, and
• The length of time each gas stays in the atmosphere (for example, CO₂ lingers in  the atmosphere for hundreds of years)

Other greenhouse gases like water vapor are more powerful on a molecule for molecule basis, but the flood of manmade carbon dioxide emissions into the atmosphere this century and its atmospheric staying power are why carbon dioxide is the focus of concerns.

Carbon dioxide isn't stuck in the atmosphere once it gets there. It moves into and out of living organisms, soil, rock, and water. For example, plants take up carbon dioxide in the air to make wood, stems, and leaves, and then release it back into the air when the leaves fall or the plants die. Forest fires release large amounts of CO₂, providing an important reason to preserve forests. Animals, including humans, take up carbon when they eat plants, and then release CO₂ back into the atmosphere via respiration.

Over very long time frames, the weathering of rocks can add carbon to surface waters that run into the ocean. Eventually, this carbon is removed from the water and forms limestone. It can later be released back into the atmosphere from volcanoes when the rock is melted.

In ancient times when Earth had a much warmer climate, huge swamps buried plant material faster than it could decay, and when the buried remains were subjected to heat and pressure, they became coal. In similar ways, microorganisms buried on lake and sea bottoms throughout Earth's history produced oil. These processes locked up lots of carbon as oil, gas, and coal. By burning these fuels in the last 150 years, we have suddenly released into the atmosphere carbon that took hundreds of millions of years to store.

Certainly, temperatures in Earth's past have been higher (and lower) than today, and
CO₂ concentrations have varied considerably. At certain times, changes in the Earth's orbit caused warmer temperatures, which increased CO₂ and produced additional warming in a feedback process. But today, the CO₂ released by human activities is triggering the increase in temperatures.

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