An effective absorber of infrared radiation has a broader absorption profile, which means that it can absorb a wider spectrum of wavelengths. Water vapor and carbon dioxide can absorb radiation wavelengths in the range of 4 μm to 80 μm, except those between 8 μm and 12 μm. Ozone can absorb wavelengths between 9 μm and 10 μm, but as you have learned, it is found in low concentrations. The sun's ultraviolet wavelengths are strongly absorbed by ozone in the stratosphere.

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III. Much of this infrared radiation does not reach space, however, because it is absorbed by greenhouse gases in atmosphere, and is then emitted as infrared radiation back toward the Earth's surface. This process is known as the greenhouse effect.

Frequency combs -- a type of laser that emits thousands of regularly spaced colors, or frequencies -- have been revolutionary for atomic clocks and timekeeping. In the Optica Publishing Group journal Optics Letters, Jones and colleagues describe an optical atomic clock that uses a frequency comb to directly excite a two-photon transition in rubidium-87 atoms. They show that this new design achieves the same performance as a traditional optical atomic clock with two lasers.

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Halocarbons are composed of carbon, chlorine, fluorine, and hydrogen. They include chlorofluorocarbons (CFCs), which are man-made gases commonly used in refrigerators and air conditioners. Concentrations of CFC gases in the atmosphere are the highest of any of the halocarbons, and they can absorb more infrared radiation than any other greenhouse gas. The impact of 1 molecule of a CFC gas is equivalent to 10,000 molecules of carbon dioxide.

Water vapor (H2O) is the strongest greenhouse gas, and the concentration of this gas is largely controlled by the temperature of the atmosphere. As air becomes warmer, it can hold more moisture or water vapor. When the air becomes saturated (or holds as much moisture as the air can at that temperature), the excess moisture will condense into cloud droplets. And if these droplets are large enough, they will fall as precipitation.

Carbon dioxide (CO2) is also an important greenhouse gas. It has a long lifetime in Earth's atmosphere. Carbon dioxide strongly absorbs energy with a wavelength of 15 μm (micrometers). This makes carbon dioxide a good absorber of wavelengths falling in the infrared radiation region of the spectrum.

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The sun's visible wavelengths of radiation pass easily through the atmosphere and reach Earth. Approximately 51% of this sunlight is absorbed at Earth's surface by the land, water, and vegetation. Some of this energy is emitted back from the Earth's surface in the form of infrared radiation.

Carbon dioxide constantly moves into and out of the atmosphere through four major processes: photosynthesis, respiration, organic decomposition or decay, and combustion or the burning of organic material. You will learn more about carbon dioxide and the carbon cycle in Module 4.

"Over the last two decades, many great advances have been made in the performance of next generation atomic clocks," said research team leader Jason Jones from the University of Arizona. "However, many of these systems are not suitable for use in real world applications. To take this advanced technology out of the lab, we use a simplified design in which a single frequency comb laser acts as both the clock's pendulum, or ticking mechanism, and as the gearwork that tracks time."

Water vapor, carbon dioxide, methane, and other trace gases in Earth's atmosphere absorb the longer wavelengths of outgoing infrared radiation from Earth's surface. These gases then emit the infrared radiation in all directions, both outward toward space and downward toward Earth. This process creates a second source of radiation to warm to surface – visible radiation from the sun and infrared radiation from the atmosphere – which causes Earth to be warmer than it otherwise would be. This process is known as the natural greenhouse effect and keeps Earth's average global temperature at approximately 15°C (59°F).

To avoid the need for such extreme cooling, Jones and colleagues used atomic energy levels that require absorption of two photons -- instead of one photon -- to move to a higher energy level. When photons are sent from opposite directions at the atom, motion effects on one of these photons cancels any motion effect on the other photon. This allows the use of hot (100°C) atoms and a significantly simpler clock design.

Because the two processes are similar, the name “greenhouse effect” was coined to describe Fourier's explanation. However, part of a greenhouse's warmth results from the physical barrier of the glass, which prevents the warmer air from flowing outward. So despite the fact that the atmospheric greenhouse effect has some processes in common with an actual greenhouse, the overall mechanisms driving the greenhouse effect are different and more complex.

If the concentration of greenhouse gases increases, then more infrared radiation will be absorbed and emitted back toward Earth's surface, creating an enhanced or amplified greenhouse effect.

"This advance could also help enhance the GPS network -- which relies on satellite-based atomic clocks -- by improving performance and making backup or alternative clocks more accessible," said the paper's first author Seth Erickson. "It is also a first step toward bringing high-performing atomic clocks into everyday applications and even people's homes, which could, for example, allow the telecommunications network to switch between different conversations very quickly. This could make it possible for many people to simultaneously communicate over the same telecom channels and increase data rates."

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The researchers are now working to improve their optical atomic clock design by making it smaller and more stable over the long term as well as incorporating new advances in laser technology. The direct frequency comb approach could also be used with other two-photon atomic transitions, including ones for which low-noise single frequency lasers are not currently available.

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Methane (CH4) is 30 times stronger than carbon dioxide as an absorber of infrared radiation. Methane, however, is present in smaller concentrations than carbon dioxide, so its net contribution to the greenhouse effect is not as large. Methane is also relatively short-lived (lasting approximately 8 years) in the atmosphere. Methane is produced when bacteria decompose organic plant and animal matter in such places as wetlands (e.g., marshes, mudflats, flooded rice fields), sewage treatment plants, landfills, and the guts of cattle and termites. Scientists are concerned about the concentration of methane increasing in regions where the Arctic and alpine permafrost is thawing and releasing methane as it warms.

You have already learned that Earth's atmosphere is composed primarily of nitrogen and oxygen. These gases are transparent to incoming solar radiation. They are also transparent to outgoing infrared radiation, which means that they do not absorb or emit solar or infrared radiation. However, there are other gases in Earth's atmosphere that do absorb infrared radiation. These gases are known as greenhouse gases. Below are the most important greenhouse gases that influence Earth's climate system.

"A major innovation of this work is that instead of using a single-color laser to send photons at the atom from each direction, we send a broad range of colors from a frequency comb," said Jones. "Using the correct pairs of photons with different colors from the frequency comb allows them to add together in the same way as two photons from a single-color laser would, thus exciting the atom in similar way. This eliminates the need for a single-color laser, further simplifying the atomic clock."

The researchers say that the widespread availability of commercial frequency combs and robust fiber components, such as Bragg gratings, at telecommunication wavelengths greatly facilitated the development of this new design. They used fiber Bragg gratings to narrow the broadband frequency comb spectrum to less than 100 GHz, centered at the atomic transition of rubidium-87. This narrowly filtered spectrum increased the overlap between the frequency comb output and the excitation spectrum for the rubidium-87 atoms.

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In an optical clock, exciting atomic energy levels with a laser causes atoms to transition between specific energy levels. The precise frequency of these transitions serves as the "tick" of the clock, allowing the measurement of time with high precision. Although portable chip-scale optical atomic clocks have been developed, the most accurate and stable optical clocks use atoms trapped at temperatures near absolute zero to minimize atomic motion, which can change the laser light frequencies experienced by the atoms.

In the 1860s, John Tyndall, an Irish scientist who was fascinated by the growth and formation of glaciers, wanted to test his ideas explaining how Earth maintained a fairly constant temperature. He began a series of experiments to measure the amount of radiant heat (infrared radiation) that certain gases could absorb and transmit. Tyndall found that water vapor and carbon dioxide were good absorbers and emitters of infrared radiation.

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Researchers have demonstrated a new optical atomic clock that uses a single laser and doesn't require cryogenic temperatures. By greatly reducing the size and complexity of atomic clocks without sacrificing accuracy and stability, this advance could lead to high-performance atomic clocks that are compact and portable.

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Nitrous oxide (N2O), a relatively long-lived gas, has increased in atmospheric concentration due mainly to agriculture. Nitrate (NO3-) and ammonia (NH4+) are used as fertilizers. Bacteria convert a small amount of this nitrate and ammonia into the form of nitrous oxide. Internal combustion engines also produce nitrous oxide.

To have an average temperature of 15°C (59°F), Fourier knew that there had to be another process occurring in the atmosphere –– something similar to the way a greenhouse retains heat. A greenhouse's glass enclosure allows visible light to enter and be absorbed by the plants and soil. The plants and soil then emit the absorbed heat energy as infrared radiation. The glass of the greenhouse then absorbs that infrared radiation, emitting some of it back into the greenhouse and thus keeping the greenhouse warm even when the temperature outside is lower.

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To test the new approach, the researchers compared two almost identical versions of the new direct frequency comb clock with a traditional clock that included the use of an additional single frequency laser. The new clocks showed consistent performance with instabilities of 1.9×10−13 at 1 second and averaging down to 7.8(38)×10−15 at 2600 seconds. This performance was similar to that of the traditional clock and other published results using a single frequency laser architecture.

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Ozone (O3) is also a relatively minor greenhouse gas because it is found in relatively low concentrations in the troposphere (the lowest layer of the atmosphere). In the troposphere, it is produced by a combination of pollutants — mostly hydrocarbons and nitrogen oxide compounds.

The relative importance of a greenhouse gas depends on its abundance in Earth's atmosphere and how much the gas can absorb specific wavelengths of energy.

When averaged over the course of a year, the amount of incoming solar radiation received from the sun has balanced the amount of outgoing energy emitted from Earth. This equilibrium is called Earth's energy or radiation balance. Relatively small changes in the amounts of greenhouse gases in Earth's atmosphere can greatly alter that balance between incoming and outgoing radiation. Earth then warms or cools in order to restore the radiative balance at the top of the atmosphere.

In 1827, Joseph Fourier, a French mathematician and physicist, wondered why Earth's average temperature is approximately 15°C (59°F). He reasoned that there must be some type of balance between the incoming energy and the outgoing energy to maintain this fairly constant temperature. His calculations indicated that Earth should actually be much colder (-18°C or 0°F).

I. The sun's visible wavelengths of radiation pass easily through the atmosphere and reach Earth. Approximately 51% of this sunlight is absorbed at Earth's surface by the land, water, and vegetation.