The Greenhouse Effect on Earth

Transcription

1 The Greenhouse Effect on Earth Earth s atmosphere is slightly warmer than what it should be due to direct solar heating because of a mild case of greenhouse effect The ground is heated by visible and (some) infrared light from the Sun. The heated surface emits infrared light. The majority of Earth s atmosphere (N 2 and O 2 ) are not good greenhouse gas. The small amount of greenhouse gases (H 2 O, CO 2 ) traps (absorb and re-emit) the infrared radiation, increasing the temperature of the atmosphere Click on image to start animation Water On Earth The condition is just right! The combination of three factors: Distance to the Sun, the albedo, and the greenhouse effect, make it possible for water to stay on Earth. N 2 and O 2 are not greenhouse gas. Not much CO 2 in the atmosphere. Variable amount of H 2 O in the atmosphere regulated by the temperature. The result is a mild greenhouse effect not too hot, and not too cold, just the right temperature for most of the water to stay in liquid phase, and some to stay in gas phase in the atmosphere on the surface of the Earth Other planets also have Greenhouse Effects, but these are unsuitable for life Sun Venus Earth Mars Atmosphere: mass 90x earth s GH Gases: >90% CO 2 Sfc. Temp.: 477 C GH Effect: 523 C FAR TOO HOT! GH Gases: ~0.04% CO 2 ~ 1% H2O Sfc. Temp.: 15 C GH Effect: 33 C NOT BAD! Atmosphere: mass <1% earth s GH Gases: >80% CO 2 Sfc. Temp.: -47 C GH Effect: 10 C FAR TOO COLD! Greenhouse Gases The primary components of Earth s atmosphere, N 2 and O 2 do not have absorption in the IR wavelength range, therefore, do not have a significant role in setting the surface temperature of the planet Greenhouse gas are efficient in absorbing IR light The most important greenhouse gases are: H 2 O Water vapor. CO 2 Carbon Dioxide CH 4 methane The most abundant greenhouse gas in Earth s atmosphere is water vapor. Most of the greenhouse heating of Earth s atmosphere is due to Water vapor absorption of IR radiation emitted by Earth, and then transferring the energy to the surrounding air molecule Source of Water The terrestrial planets were built from rock and planetesimals. No gases or water can condense at the high temperature near the Sun. So, where did the water on Earth come from? The water on Earth (and other terrestrial worlds) most likely was brought over by the comets during the period of heavy bombardment about 4 billion years ago These water (and other gases) were trapped in the interior, and released by volcanic activities by Outgassing Mt. St Helen eruption, 2004! 1

2 The Atmosphere of Earth The atmosphere of Earth contains primarily N 2 (77%) and O 2 (21%). What happened to all the CO 2? Where did all the O 2 come from? The CO 2 Cycle The CO2 cycle acts as a thermostat that regulates the temperature of the Earth If Earth warms up a bit, then carbonate minerals form in the oceans at a higher rate. The rate at which the oceans dissolve CO 2 gas increases, pulling CO 2 out of the atmosphere. The reduced atmospheric CO 2 concentration leads to a weakened greenhouse effect that counteracts the initial warming and cools the planet back down. If Earth cools a bit, carbonate minerals form more slowly in the oceans. The rate at which the oceans dissolve CO 2 gas decreases, allowing the CO 2 released by volcanism to build back up in the atmosphere. The increased CO 2 concentration strengthens the greenhouse effect and warms the planet back up CO 2 CO 2 is a colorless gas condenses into solid form (dry ice) at -78 C in atmospheric pressure. condenses into liquid at -57 C at pressure above 5.1 atmospheric pressure. Atmospheric CO 2 is derived from (The sources ) Volcanic outgassing burning of organic matter Respiration of living organisms CO 2 can be stored in (The Sinks ) Highly soluble in water: forms H 2 CO 3 Dissolved CO 2 in water can interact with silicate minerals to form carbonated minerals Feedback Loop Positive Feedback Mechanisms that make things worse e.g., Increasing CO 2 in the atmosphere leading to the release of more CO 2 Negative Feedback Mechanisms that are self-correcting e.g., Increasing CO 2 in the atmosphere leading to higher rate of CO 2 removal, such as our CO 2 cycle. Carbon Dioxide Cycle The mechanism by which Earth self-regulates its temperature is called the carbon dioxide cycle, or the CO2 cycle for short. Starting with the carbon dioxide in the atmosphere: Volcanoes outgas CO 2 into the atmosphere. Atmospheric carbon dioxide dissolves in the oceans. At the same time, rainfall erodes rocks on Earth s continents and rivers carry the eroded minerals to the oceans. In the oceans, the eroded minerals combine with dissolved carbon dioxide and fall to the ocean floor, making carbonate rocks such as limestone. Over millions of years, the conveyor belt of plate tectonics carries the carbonate rocks to subduction zones, and subduction carries them down into the mantle. As they are pushed deeper into the mantle, some of the subducted carbonate rock melts and releases its carbon dioxide, which then outgasses back into the atmosphere through volcanoes. Plate Tectonics Plate tectonics plays an important role in the CO 2 cycle in that it helps to carry the carbonate rocks into the mantle, which are then released again by volcanic activities. Earth s lithosphere is broken into pieces (the plates). These plates float on top of the mantle, interacting with each other to produce the geological features we see and feel today. Click on image to start animation 2

3 Where Did O 2 Come From? The most important source of O 2 on Earth is Life and Photosynthesis. Photosynthesis converts CO 2 to O 2, and incorporates carbon into amino acids, proteins, and other components of living organisms. O 2 will be depleted from the atmosphere very rapidly without a source. O 2 is a very reactive chemical that likes to be combined with other elements through oxidation. For examples, CO 2, H 2 O, FeO (rust) That s how we make fire! O 2 Absorbs UV, which also transform some of the O 2 into O 3, which absorbs even more UV O 2 not only supports life, it also protect life! Comparative Planetology Mars and Venus are very similar to Earth in their size and location to the solar system. However, their surface environments are drastically different from that of the Earth today. By understanding how Mars and Venus end up with their current state, we may be able to better understand our Earth UV light can break the water molecules to release oxygen, but the contribution is small. The Role of the Magnetic Field of Earth Another important characteristics of the Earth is its magnetic fields, which shield us from the bombardment of the high-energy charged particles, mostly from the Sun. Without magnetic field, the high energy particles of solar wind can strip much of the Earth s atmosphere by breaking the bounds between the atoms in the air molecules N 2 N + N O 2 O + O H 2 O H + H + O The lighter gases then have higher probability of acquiring velocity higher than escape velocity and escape from Earth! Mars Mars image from Hubble Space Telescope Water On Earth in the Past Was it always like this on Earth? Yes. Water was plentiful throughout most of Earth s history, for about three billion years. No! Geological evidences suggest that Earth used to be covered by ice about million years ago Snowball Phase. How did Earth recover from the snowball phase? Once the water was frozen, CO 2 can no longer be removed from the atmosphere by dissolving in water interruption of the CO2 cycle. Increased CO2 level in the atmosphere leads to stronger greenhouse effect, which warms the atmosphere. Higher temperature melt the ice restoration of the CO2 cycle. Martian Season The tilt of Mar s rotation axis with respect to its ecliptic plane is The eccentricity of Mar s orbit around the Sun is The seasons on Mars are affected by both its orbital distance and its axis tilt. Mars is closer to the Sun during the southern hemisphere summer, and farther away from the Sun during its winter Mars therefore has more extreme seasons in its southern hemisphere that is, shorter, hotter summers and longer, colder winters than in its northern hemisphere. 3

4 Martian Weather Even though Mars only has a very thin atmosphere, it still has a weather system Martian weather are due to its extreme seasonal changes. Polar temperatures at the winter pole drop so low (about 130 C) that carbon dioxide condenses into dry ice at the polar cap. frozen carbon dioxide at the summer pole sublimates into carbon dioxide gas. The atmospheric pressure therefore increases at the summer pole and decreases at the winter pole, driving strong pole-to-pole winds. Storms on Mars can engulf the entire planet. Ancient Martian Ocean? Mars may once have an ocean. The smoother surface in the low lying areas in the northern hemisphere (blue areas in the image on the right) may once hold an ocean Martian surface is similar to Earth s desert and volcanic plane High elevation and numerous large impact craters in the southern hemisphere Lower elevation and few impact craters in the northern hemisphere Volcanism is the most likely mechanism responsible for changing the surface features of Mars. Many geological features suggest past water flows Geology of Mars Dry Ice (frozen CO 2 ) in the north and south poles Water on Mars Today? The gullies form when snow accumulates on the crater walls in winter and melts away in spring. Because the gullies are relatively small (note the scale bar in Figure 7.26), they should be gradually covered over by blowing sand during Martian dust storms. Thus, gullies that are still clearly visible must be no more than a few million years old. Geologically speaking, this time is short enough to make it quite likely that water flows are still forming gullies today Water on Mars in the Past? Many geological features of Mars suggest that it had a lot of water about 3 billion years ago. It may even have a pleasant, hospitable environment. Dried up riverbeds Gullies? Lake bottom Images from Mars Rover Spirit at a suspected ancient lake site showed rock structures consistent with those formed from sediments in standing water Riverbed? Gullies Lake Bottom? Why doesn t Mars have water today? If Mars used to hold a large amount of water, then why is Mars so different today? What caused it to lose its water? We don t know exactly what happened, but one likely explanation was because of the relatively small size of Mars: The smaller size of Mars means that it cools off faster. Once it cools, volcanic activities stop, halting the release of gases into the atmosphere. The cool interior temperature may means that Mars does not have a fluid metallic core to generate magnetic fields anymore. Without a magnetosphere, the atmosphere is exposed to the bombardment of high energy charged particles of solar wind, which break the air molecules, making them easier to escape. As Mars cools, the remaining CO 2 gases are frozen in the north and south pole, forming the ice cap. The remaining oxygen are trapped on surface rock, making it look red 4

5 We cannot see the rocky surface of Venus due to its thick atmosphere... Venus Runaway Greenhouse Effect If we were to move the Earth closer to the Sun, like where Venus is now, then we would suffer the runaway greenhouse effect as well, lose all the water, and become hot like Venus. NASA Image of Venus Geology of Venus Venus surface is similar to Earth and Mars few impact craters, volcanoes, and evidence of tectonics activities But no plate tectonics The volcanoes of Venus is most likely still active today few impact craters, sulfuric acid cloud (the volcanoes are still outgasing) However, there is no sign of erosion No liquid water? No wind, due to its slow rotation (243 Earth days per rotation). Venus dos not have a magnetic field! This is quite surprising given that most of the ingredients required for the dynamo are all present Click on the image to see image obtained by Venera 14 spacecraft What makes a planet habitable? Two important factors determine whether a planet is habitable Size: Need substantial mass to maintain an atmosphere Small planets cool off faster than large ones. Without the volcanic outgasing and a hot, fluid metallic core to generate magnetic field, atmospheric gas are easily depleted. Distance to the Sun the distance to the Sun determine the energy input to the planet: Too close too hot water evaporates. Too far too cold water freeze. Why doesn t Venus have water? Given the similarities between Earth and Venus, why is the atmosphere of Venus so different from Earth s? Venus is too hot! The proximity to the Sun keep the temperature on Venus high, even without greenhouse effect. Any water on Venus (from out-gassing of water trapped inside the planet) are vaporized into gaseous phases (water vapor). Water vapor and CO 2 are both greenhouse gas, causing the atmosphere to warm up more runaway greenhouse effect T = 740 ºK At 740 ºK, the molecules of gases has much higher average kinetic energy (recall the definition of temperature) higher average velocity. If the velocity of the gas molecules exceed the escape velocity, then they can escape into space Light gases (H, H 2 O, O 2, N 2 ) escape, heavy gases (CO 2 ) stay. Why? Without liquid water, CO 2 doesn t have a place to go, except to stay in the atmosphere in comparison, most of the CO 2 on Earth are locked in rock or liquid water... Internal Structure Surface Features Atmosphere What makes the Earth hospitable to life? Global Warming? 5

6 Global Warming, A Quick Poll Is global warming real? a) Yes b) No How much has the average temperature of the Earth risen in the last 100 years? a) ~ 0.5 C b) ~ 1.0 C c) ~ 5.0 C d) ~ 10.0 C This is the highest temperature ever recorded in Earth s history. a) Yes. b) No. Who is to blame? a) Human activities b) Nature causes c) ET d) NoCanTell! What is to blame? a) Ozone b) CO 2 c) H 2 O d) O 2 Earth s Temperature Variation in the past 1,100 years Reconstructions of (Northern Hemisphere average or global average) surface temperature variations from six research teams (in different color shades) along with the instrumental record of global average surface temperature (in black). Each curve illustrates a somewhat different history of temperature changes, with a range of uncertainties that tend to increase backward in time (as indicated by the shading). Reference: NRC, (Figure reprinted with permission from Surface Temperature Reconstructions (2006) by the National Academy of Sciences, Courtesy of the National Academies Press22 18, Washington, D.C.). Reproduced from EPA Climate Change Website. Global Warming, A Quick Poll Is global warming real? a) Yes b) No How much has the average temperature of the Earth risen in the last 100 years? a) ~ 0.5 C b) ~ 1.0 C c) ~ 5.0 C d) ~ 10.0 C As far as we know, this is the highest temperature ever recorded in Earth s history. a) Yes. b) No. Who is to blame? a) Human activities b) Nature causes c) ET d) NoCanTell! What is to blame? a) Ozone b) CO 2 c) H 2 O d) O 2 e) NoCanTell. It is real complicated! The Long-Term Stability of Earth s Climate 400,000 years The atmospheric concentration of CO 2 measured from Antarctic ice core data implies that Earth s climate has being pretty stable over the past 400,000 years It also shows a rapid increase of about 30% in the past few centuries 270 ppm (parts per million) to 370 ppm Fluctuations in temperature (blue) and in the atmospheric concentration of carbon dioxide (red) over the past 400,000 years as inferred from Antarctic ice-core records. The vertical red bar is the increase in atmospheric carbon dioxide levels over the past two centuries and before From A. V. Fedorov et al. Science 312, 1485 (2006) Reproduced from EPA Climate Change Website. There is a gradual increase in the average temperature of the Earth s atmosphere in the last 100 years It has risen about 1 C since 1900 Are human activities causing global warming? What other (non-human) factors can cause global warming? How does global warming affect our life? Just watch the movies Global Warming How do we measure atmospheric CO 2 concentration in the past? Precise measurements of atmospheric CO 2 concentration is available only in the last few decades Information about atmospheric CO 2 concentration and temperatures in the past can be inferred by several different methods, such as Tree-ring Deep ocean sediment Ice core records Coral Link to NOAA Paleoclimatology Website Paleoclimatology is the study of climate prior to the widespread availability of records of temperature, precipitation and other instrumental data. 6

7 Antarctic Ice Core Located high in mountains and in polar ice caps, ice has accumulated from snowfall over many millenia. Scientists drill through the deep ice to collect ice cores. These cores contain dust, air bubbles, or isotopes of oxygen, that can be used to interpret the past climate of that area. From NOAA Paleoclimatology Website. So, what s the big deal if human CO 2 causes 1 C temperature increase? An increase in atmospheric temperature (human or natural origin) will lead to the increase in the water vapor content of the troposphere. Because water vapor is a strong greenhouse gas, the increase in H 2 O vapor in turn causes enhanced greenhouse effect, raising the temperature more. Higher atmospheric temperature will cause more evaporation of water Which leads to even higher temperature Runaway Green House Effect! This figures shows estimates of the changes in carbon dioxide concentrations during the Phanerozoic. Three estimates are based on geochemical modeling: GEOCARB III (Berner and Kothavala 2001), COPSE (Bergmann et al. 2004) and Rothman (2001). These are compared to the carbon dioxide measurement database of Royer et al. (2004) and a 30 Myr filtered average of those data. Error envelopes are shown when they were available. The right hand scale shows the ratio of these measurements to the estimated average for the last several million years (the Quaternary). Customary labels for the periods of geologic time appear at the bottom. Direct determination of past carbon dioxide levels relies primarily on the interpretation of carbon isotopic ratios in fossilized soils (paleosols) or the shells of phytoplankton and through interpretation of stomatal density in fossil plants. Each of these is subject to substantial systematic uncertainty. Estimates of carbon dioxide changes through geochemical modeling instead rely on quantifying the geological sources and sinks for carbon dioxide over long time scales particularly: volcanic inputs, erosion and carbonate deposition. As such, these models are largely independent of direct measurements of carbon dioxide. Both measurements and models show considerable uncertainty and variation; however, all point to carbon dioxide levels in CO 2 over 500 million years From: hanerozoic_carbon_dioxide.png How about Clouds and Ice? Water vapor (water in gaseous phase) is one of the most potent and abundant greenhouse gas but Clouds (water in liquid form) reflect sunlight, decreasing the solar energy input into Earth s atmosphere during the day, but they trap IR radiation from the Earth during the night. It s net effect is not well know so far Albedo of clouds range from close to 0 to 70%. Testing climate impact of clouds after Sept. 11, 2001 Ice has a very high albedo, ~ 80 to 90%. Thus, reduction of the polar ice cap can cause more heating Which gas is keeping the Earth warm? A. N 2? B. O 2? C. CO 2? D. H 2 O? The major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes 9-26%; methane, which causes 4-9%, and ozone, which causes 3-7%. Note that it is not really possible to assert that a certain gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. (The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.)[3] [4] From Contrails and Climate Contrails are artificial clouds made by the exhaust of the aircraft engines, or the wingtip vortices ( ). Contrails produced by the heavy air traffic over the US may have noticeable influences on the weather Commercial air traffic were suspended for three days after the Sept. 11, 2001 attack. This provided a rare chance for the climate scientist to test their theory Measurements show that without contrails the local difference of day and night-time temperatures was about 1 degree Celsius higher than immediately before the attack 7

8 How About The CO 2 Cycle? The CO2 cycle acts as a thermostat that regulates the temperature of the Earth If Earth warms up a bit, then carbonate minerals form in the oceans at a higher rate. The rate at which the oceans dissolve CO 2 gas increases, pulling CO 2 out of the atmosphere. The reduced atmospheric CO 2 concentration leads to a weakened greenhouse effect that counteracts the initial warming and cools the planet back down. If Earth cools a bit, carbonate minerals form more slowly in the oceans. The rate at which the oceans dissolve CO 2 gas decreases, allowing the CO 2 released by volcanism to build back up in the atmosphere. The increased CO 2 concentration strengthens the greenhouse effect and warms the planet back up Feedback Loop Positive Feedback Mechanisms that make things worse e.g., Increasing CO 2 in the atmosphere leading to the release of more CO 2 Negative Feedback Mechanisms that are self-correcting e.g., Increasing CO 2 in the atmosphere leading to higher rate of CO 2 removal, such as our CO 2 cycle. My Two Cents It looks like most of the scientists agree that the global warming observed in the last century were caused by human activity. However, as we tried to demonstrate here, the global climate is a very complicated system. We understand the basic principle of the climate system, but we still don t understand how nature regulates Earth s climate over the long run, nor do we have the capability to create a realistic climate model and be able to predict with any certainty the effects of human activities on our climate system. My advices Keep an open mind. Read, and think for yourself! Do not rush into judgment (especially after you watch the movies). Please trust the scientific community to come up with an honest answer There are enough check and balance in the scientific community to weed out the bad theories 8