The Greenhouse Effect and Climate Change
This image sequence shows the dramatic retreat of the Franz Josef glacier (New Zealand) from 1951 to 1964. There are several natural causes of climate change and it has been well documented that the Earth cycles in and out of glacial periods. QuickTime and a Photo - JPEG decompressor are needed to see this picture. However, recent evidence indicates that human activity may be contributing to global climate change. To understand the greenhouse effect, we must first understand how energy is exchanged in the Earth system.
1941 2004 Muir Glacier, Alaska (USGS) 1973 2006 Whitechuck Glacier, North Cascades National Park retreated 1.9 km (Wikipedia) 3
Heat Exchange Mechanisms There are several important heat transfer mechanisms that operate in the Earth's atmosphere. Conduction transfer of heat from molecule to molecule within a substance. Heat transfer occurs from warmer to colder regions. Air is a poor conduction and thus, conduction is not too important in meteorology. Convection transfer of heat by mass movement of fluid. Important for liquids and gases (not solids). Radiation heat transferred as electromagnetic radiation
Radiation Electromagnetic waves transfer energy as radiation. We are all familiar with the electromagnetic spectrum. Note that longer wavelength photons have less energy than short wavelength photons. All objects emit electromagnetic radiation. Hot objects emit radiation at shorter wavelengths than cooler objects. As the temperature increases, the radiation shifts to shorter wavelengths. In addition, hotter objects emit radiation at a higher rate (higher intensity).
Radiation The Sun emits radiation at almost all wavelengths, but its maximum output is at relatively short wavelengths. Note that the Sun emits its maximum amount of radiation at ~500 nm in the visible region of the electromagnetic spectrum - greenish light. Note the strong absorption bands due to H2O, CO2, O2 and O3 atmosphere.
This figure shows the emission spectrum of the Earth obtained from satellite measurements. The jagged profile shows the actual data which are the result of absorption of different components in the Earth s atmosphere (mostly H2O and CO2). The smoothed curve represents the theoretical blackbody emission spectrum at 280 K.
Absorption of shortwave solar radiation heats up our planet s surface. As the surface and the atmosphere warm, they emit thermal longwave radiation, some of which escapes into space and allows the Earth to cool. This image shows the emission of thermal infrared radiation from our planet. This false-color image of the Earth was produced on September 30, 2001, by the Clouds and the Earth s Radiant Energy System (CERES) instrument flying aboard NASA s Terra spacecraft.
The thermal radiation leaving the oceans is fairly uniform. The blue swaths across the central Pacific represent cold cloud tops. In the American Southwest and Mexico, there is often little cloud cover to block outgoing radiation and relatively little water to absorb solar energy. Consequently, the amount of outgoing radiation exceeds that of the oceans. This false-color image of the Earth was produced on September 30, 2001, by the Clouds and the Earth s Radiant Energy System (CERES) instrument flying aboard NASA s Terra spacecraft.
Radiation The hotter Sun radiates more energy at shorter wavelength than the cooler Earth. The maximum solar output is in the visible region (~0.5 m) and the peak of the Earth's emission is in the longer wavelength infrared region(5-25 m).
Consider the hypothetical case where we do not have an atmosphere containing greenhouse gases. Incoming shortwave solar radiation would heat the Earth s surface. Hong Kong Observatory The surface would heat up and reradiate the energy as longer wavelength infrared radiation. Without greenhouse gases, the incoming solar radiation would be equal to the outgoing IR radiation from the Earth's surface. Without the greenhouse effect, the Earth would be ~30 C cooler (-18 C) than it is currently and would not support life as we know it.
Mars is an example of a planet with an atmosphere that does not significantly enhance the greenhouse effect; the average temperature is ~-55 C. This image and many others of the Martian surface suggest that Mars may have been very different in the past - much warmer and possessing surface waters. However, the low gravitational field of the planet has allowed much of its atmosphere to bleed to interplanetary space. This has slowly reduced the greenhouse capacity of the Martian atmosphere.
Selective Absorbers and the Greenhouse Effect Water is the most important/potent greenhouse gas. Water vapor is transparent to short wavelength radiation and transmits solar radiation to the Earth's surface with little attenuation. However, water absorbs longer wavelength radiation (IR) from the Earth.
Selective Absorbers and the Greenhouse Effect CO 2 is not very abundant but it is a potent greenhouse gas that we are able to increase in the atmosphere. By contrast, H 2 O is already saturated and its amount in the atmosphere cannot be increased. Increasing CO 2 content in the atmosphere by burning fossil fuels is thought to result in global warming. Global warming results in more atmospheric water thus increasing the greenhouse effect feedback.
Although H 2 O and CO 2 are the most important greenhouse gases, other gases such as methane (CH 4 ), nitrous oxide (N 2 O), CFC s and ozone (O 3 ) absorb IR radiation. Besides being selective IR absorbers, these gases also selectively emit radiation at IR wavelengths as well. The percentages show the relative importance of these anthropogenic greenhouse gases. When these gases in the atmosphere emit IR radiation, it is emitted in all directions - some being radiated towards the Earth s surface.
With insulating greenhouse gases in the atmosphere, the average global temperature is ~15 C. Greenhouse gases allow most of the Sun's radiation to reach and heat the Earth's surface. Hong Kong Observatory However greenhouse gases absorb a portion of the outgoing IR radiation, preventing it from escaping into space. Greenhouse gases reradiate IR further warming the Earth s surface - it s like a tennis match where IR photons are the balls. This provides an insulating blanket around the Earth.
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The Earth s atmosphere is heated from the bottom up. This is evident by the heat distortion that occurs in the atmosphere above pavement on a hot day. We are all familiar with the idea that the atmosphere gets colder as we go to higher elevation. The thick atmosphere of Venus is ~95% CO 2. It is an example of a planet with a run-away greenhouse effect; the average temperature at ~480 C.
Most scientists agree that CO 2 plays a role in warming the Earth through the greenhouse effect. However, the somewhat controversial hypothesis is that increased (anthropogenic?) CO 2 in the atmosphere may enhance the warming of our planet.
The figure shows that the concentration of CO 2 in the atmosphere has been steadily increasing since the mid-1700's. This corresponds with the Industrial Revolution and the increased use of fossil fuels (especially coal). These data were obtained from trapped air bubbles in ice layers from Siple Station (Antarctica).
These data represent direct sampling of the atmosphere at four stations around the world. They show the same trend at each station and with the previous ice core data.
This plot shows data from the two sources: 1. trapped air bubbles in ice layers from a variety of Antarctic sites 2. direct sampling of the atmosphere during the last 50 years (black dots) Source: ITER It is striking that the data line up and agree so well even though they are very different samples and methodologies.
Source: NOAA This plot shows current trends in anthropogenic greenhouse gases.
This figure shows how temperature and atmospheric CO2 levels have covaried for the last 400,000 years. Note that anthropogenic CO2 has caused the largest change during this time period. 24
This figure indicates that the global average temperature appears to be rising. It shows the deviation of the estimated global temperature (relative to the mean during 1961-90). Source: NOAA Taken at face value, it indicates that the Earth's average temperature may have risen as much as 0.8 C over the last century.
Earth's Climate in the Past The Earth's climate was much different in the past - just 18,000 years ago, we were in an ice age. In the last 2 m.y., glaciers have advanced ~10 times. Glaciers advance during periods when the average temperature of the Earth decreases. In warmer periods, between glacier advances, the average global temperature is slightly higher than at present. During the last glacial period, ~30% of the Earth s surface was covered in ice. Today, glaciers cover only 10% of the Earth's surface mostly in Antarctica and Greenland. 3 miles Kalstenius Ice Field, Canada.
How do we determine the Earth's climate in the past? When we look at climate, we must investigate not only the atmosphere, but also the hydrosphere, solid Earth, biosphere, and cryosphere. Climate involves the exchange of energy (and water) between the different spheres of our planet. Meteorology short-term weather Climatology long-term weather In order to reconstruct and investigate the past climate on Earth, we look at how climate affected the Earth in the past (indirect evidence): seafloor sediment cores ice cores from glaciers and ice sheets biologic effects (shells, tree rings, pollen)
Seafloor Sediment Cores Sediment contains the remains (shells) of marine organisms such as the microscopic foraminifera pictured. Certain organisms live within a narrow range of temperature and the type of organisms in the sediment may indicate the temperature of the water. The shells are composed of CaCO 3 oxygen has two stable isotopes: 16 O 99.999% 18 O one out of every thousand oxygen atoms When water evaporates, "light water" containing 16 O vaporizes and leaves the remaining liquid water enriched in 18 O. Thus shells that grow in 18 O-enriched water will themselves be 18 O- enriched.
Ice Cores As glaciers form, more snow falls than melts each year. Successive snowfall accumulations over many years compact the snow into ice forming a glacier. We can estimate the temperature by looking at the oxygen isotopes in water. In addition, bubbles of ancient air are trapped in ice and we can analyze them as samples of the Earth's past atmosphere. CO 2 data come from bubbles in ice and are correlated with 18 O temperature estimates from ice core recovered at the Vostok station in Antarctica representing ~160,000 years.
Other types of climatic data 1. palynology environmental reconstruction for 100,000's years. 2. dendrochronoloy tree rings may give record of precipitation and temperature for a period of hundreds of years. 3. soil studies 4. hydrology changes in water levels for lakes, etc. 5. glaciology 6. oxygen isotope studies of coral reefs and cave formations
Possible Causes of Climatic Change Why the Earth's climate changes is not well understood there are very complex interactions between different systems and spheres. There have been a number of mechanisms that have been proposed to account for changes in climate: 1. Anthropogenic 2. Plate Tectonics - apparent effects 3. Milankovitch cycles (Earth's orbit) 4. Feedback Mechanisms 5. Volcanic activity - short term 6. Variations in solar output - very active area of research
Possible Causes of Climatic Change: Milankovitch Cycles This theory ascribes climatic changes to variation in the Earth's orientation to the Sun. As the Earth moves through space, there are 3 separate cyclic movements that can produce changes in the Earth's climate: 1. eccentricity 2. precession 3. obliquity They are named after a Serbian mathematician, Milutin Milankovitch, who explained how these orbital cycles cause the advance and retreat of the polar ice caps.
Eccentricity changes in the shape of the Earth's orbit around the sun. The Earth's orbit changes from being nearly circular to more elliptical over a 100,000 year cycle. The greater the eccentricity, the greater the variation in the solar radiation received by the Earth during its closest and farthest point from the sun. NASA The top image shows an orbit with an eccentricity = 0 (circular). The eccentricity factor of the Earth s orbit varies from 0 to 0.07. The bottom image shows an exaggerated orbit with an eccentricity = 0.5 for comparison. NASA
Today, the difference during a one-year period is only ~3% at the closest approach to the Sun (perihelion), the Earth is ~5 million kilometers closer than at its furthest departure (aphelion). This difference results in a difference of ~6% in the amount of radiation received by the Earth. Currently, the Earth is closer to the sun in January and farther away in July. When the Earth's orbit is very eccentric, the difference in distance is ~9% and that results in a 20% difference in the amount of energy that it receives between perihelion and aphelion. NASA NASA
Obliquity the Earth's axial tilt changes over a 41,000 year cycle. Currently, the Earth's axial tilt is ~23.5 the total range is 22.1-24.5. A smaller tilt will result in less seasonal variation this may result in enhanced snows in polar regions and the development of a glacial period. NASA A larger tilt will result in more seasonal variation.
Precession as the Earth rotates on its axis, it is spinning like a top during a period that lasts for ~23,000 years. Currently, the Earth's axis points at Polaris; in ~11,000 years, Vega will be our new North Star. As we mentioned earlier, the Earth is closer to the Sun in January and further in July in ~11,000 years the opposite will be true. NASA QuickTime and a Photo - JPEG decompressor are needed to see this picture. NASA Current orientation gives the N. hemisphere a milder winter and summer. However, in the S. hemisphere, the Earth is closer to the sun during the summer and further during the winter. In 11,000 years, the opposite will be true.
Milankovitch calculated the interaction of these three orbital parameters and their effects on solar insolation. The figure shows the change in the three orbital parameters for the last 1 million years - line titled "solar forcing," adds the effects of the three orbital parameters together. The bottom curve indicates glacial cycles - glacialinterglacial spikes are in sync with the pulses of insolation in the Milankovitch cycles. Eccentricity appears to be the dominant factor. NOAA 37