GEOL 1302, Week 5 Notes
GEOL 1302, Week 5 Notes GEOL 1302
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This 6 page Class Notes was uploaded by Theresa Nguyen on Saturday September 24, 2016. The Class Notes belongs to GEOL 1302 at University of Houston taught by yunsoo choi in Fall 2016. Since its upload, it has received 33 views. For similar materials see Intro To Global Climate Change in Geology at University of Houston.
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Date Created: 09/24/16
Chapter 5: The Carbon Cycle Simple Model In our “simple” model of the climate, the temperature was determined by: - the number of atmospheric layers - the albedo - solar constant with number of atmospheric layers corresponding to the abundance of greenhouse gases in our atmosphere. In this chapter we will: - define what a greenhouse gas is - discuss in detail carbon dioxide (CO2), one of our atmosphere’s most important GHGs. O 2 Carbon dioxide – C O C 2 is the primary greenhouse gas emitted by human activities. Policies to control modern climate change frequently focus on reducing C O2 emissions. O2 Understanding C is more than just measuring emissions, need to understand the carbon cycle. Carbon cycle includes how carbon moves between atmosphere, oceans, land biosphere, and rocks on Earth. O This will help us understand what happens to C 2 after it is emitted into the atmosphere, which will give us a better idea of the future trajectory of our climate. Combustion C 3 8 + O2 → C O2 + H2O C 3 8 S + O2 → C O 2 + H 2 + CO + S O 2 + NO + C H4 + soot C H O → O H O 3 8 + 5 2 3 C 2 + 4 2 “Dry” Atmospheric Composition 78% N2 O 2 21% ~ 1% Ar Total is more than 99.95% of our Dry Atmosphere. None of the big three absorb infrared photons, so they are NOT GHGs. And therefore, do not warm the surface of our planet. H2O Next is water vapor ( ) H O - 2 varies widely from 4% in the tropics to 0.2% in polar regions. To 0.0005% of atmosphere in the stratosphere. Why does the abundance of H2O vary so much? Water Vapor H O Water vapor ( 2 ) is the most abundant and important greenhouse gas in our atmosphere. Its main source is evaporation from the oceans, and is primarily removed from atmosphere when water forms raindrops which fall back to the surface. Human emissions of water vapor contribute essentially nothing to its atmospheric abundance. Why? In Chapter 6, we will discuss the role of water vapor plays in climate change and how humans are indirectly increasing its abundance. Greenhouse gases (GHGs) 99.95% of Atmosphere is N2 , O 2 , and Ar. How can the missing 0.05% be important? O 2 The largest fraction of the remaining 0.05% is carbon dioxide (C ) which in 2010 made up 0.039% of the atmosphere. Because C O2 absorbs IR photons, it is a greenhouse gas. Turns out to be the second most important GHG behind water. Since 0.039% is an awkwardly small number, scientists typically express the concentration of trace gases in a more convenient unit: parts per million In this case, 0.039% corresponds to 390 ppmv. Methane The next most important GHG in our atmosphere is Methane (C H 4 ) C H 4 currently has an atmospheric abundance of 1.8 ppm. What is methane? Methane is about 21 times more powerful at warming the atmosphere than carbon dioxide (C O 2 ) by weight (see box below). Methane's chemical lifetime in the atmosphere is approximately 12 years.* Methane’s relatively short atmospheric lifetime, coupled with its potency as a greenhouse gas, makes it a candidate for mitigating global warming over the near-term (i.e., next 25 years or so).* Nitrous Oxide N 2 The next most important GHG is nitrous oxide ( ) Currently about 0.3 ppm or approximately 300 parts per billion (ppb). N 2 is emitted into the atmosphere from nitrogen-based fertilizer and industrial processes, as well as from natural microbial activities in the soils. N2O “Nitrous oxide ( ) is a clear, colorless gas, with a slightly sweet odor. Due to its long atmospheric lifetime (approximately 120 years) and heat trapping effects —about 310 times more powerful than carbon dioxide on a per molecule basis — N 2 is an important greenhouse gas”. Halocarbons Halocarbons include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which are synthetic industrial chemicals used as refrigerants (e.g., in air conditioners and refrigerators). The category also includes natural chemicals methyl chloride (C H 3 Cl). - They are present in abundances of a few parts per billion (ppb). - All of them are potent GHGs. Contribution of GHGs to Climate Change In next chapter we will quantify the contribution of these greenhouse gases to climate change. Please note here that greenhouse gases are not equal in their ability to warm the planet. - C O2 vs. N 2 vs. CFCs. Global warming potential product of abundance and lifetime. These differences in warming potential among various gases have important implications when designing policies to address climate change. Because of the importance of carbon dioxide to the problem of modern climate change, today we will mainly focus on it and the processes that regulate its atmospheric abundance, which are collectively known as the carbon cycle. Atmosphere – land biosphere exchange Photosynthesis C O 2 + H2O + sunlight → C H 2 O + O 2 H2 O 2 O2 H 2 Respiration C O + → C + O + energy Please note, previous equations were not balanced chemical reactions; rather, they represent the net of a large number of complex individual reactions. Also, should be apparent that respiration is the reverse of photosynthesis. The production of a carbohydrate through photosynthesis followed by its consumption during respiration therefore produces no net change in either carbon dioxide or molecular oxygen. Instead, the net effect is the conversion of sunlight into energy that powers living creatures Atmospheric Carbon Reservoir The atmosphere contains approximately 740 gigatonnes of carbon (GtC). A gigatonne is 1 billion metric tons, where 1 metric ton is 1,000 kg or 2,200 lbs. - Note that this is just the mass of the carbon atoms in the atmosphere – although the carbon dioxide molecule also contains two oxygen atoms, their mass is not included. Unfortunately, you will sometimes see the mass expressed as the mass of carbon dioxide, which does include the mass of the two oxygen atoms. O - In that case, the atmosphere contains roughly 2,700 GtC 2 . You can convert between these units by using the fact that 1 GtC = 3.67 GtC O2 . - In this course, we will use GtC exclusively, but you must be very O careful to recognize whether the mass is given in GtC or GtC 2 when you read anything about climate change. How much does a gallon of gasoline weight? Land Biosphere Carbon Reservoir The land biosphere contains ~2000 GtC in living plants, animals, and as organic material in the soils (e.g., leaves). During a given year, photosynthesis removes about 100 GtC from the atmosphere. Respiration closely matches this, transferring 100 billion tons C back to O2 atmosphere as C . That fact that PSN and RESP are balanced over a year does not mean that are balanced for a given season. Atmosphere – ocean carbon exchange One of carbon dioxide’s most important properties is that it readily dissolves in water. Once it has dissolved in water, carbon dioxide can be converted to H O carbonic acid ( 2 C 3 ) by means of this reaction: O H H O C 2 + 2 O → 2 C 3 The carbonic acid can then be converted into other forms of carbon. - Consequently, the ocean can absorb an enormous amount of carbon dioxide from the atmosphere. Carbon is returned to the atmosphere in a reaction that is the reverse rxn: H 2 C O 3 → C O 2 + H2 O This is followed by the escape of carbon dioxide back into the atmosphere. This is similar chemistry to that used to make fizzy soft drinks. - The manufacturer dissolves large amounts of carbon dioxide into water, thereby producing carbonated water. This explains why soft drinks tend to be highly acidic. We can therefore expect that, as the oceans absorb carbon dioxide, they will become more acidic, a topic that we will return to a later chapter. As a result of this chemistry, carbon cycles easily between the atmosphere and ocean. To fully understand this exchange, however, we must think of the ocean as being split into two parts. - The first part is the top 100 m or so of the ocean, which exchanges carbon very rapidly with the atmosphere. This part of the ocean is sometimes referred to as the mixed layer because it is well mixed by winds and strong weather events, such as hurricanes. This layer contains approximately 1,000 GtC. - Below this lies the vast majority of the ocean, and this deep ocean exchanges carbon with the mixed layer. The deep ocean also contains most of the ocean’s carbon, approximately 38,000 GtC, or 50 times or so more carbon than is in the atmosphere. Combined Atm – Land – Ocean System This means that a carbon atom will stay in the atmosphere for only 4 years or so before it is transferred into the land biosphere or ocean. Remember that this is an average value – an individual molecule of carbon dioxide may remain in the atmosphere for a shorter or longer time. Another way to think about a turnover time is that, over a period of 4 years, enough exchange will have taken place to replace much of the carbon that is in the atmosphere with carbon from the land biosphere or ocean. The turnover times for ocean mixed layer: 1,000 GtC ÷ 200 GtC/yr = 5 years. The turnover time for the deep ocean; 38,000 GtC ÷ 100 GtC/yr = 380 years. - Thus, it takes several centuries for a carbon atom to make a round trip from the atmosphere through the mixed layer to the deep ocean, and back. The atmosphere exchanges carbon rapidly (time scale of years) with the land biosphere and mixed layer, and much more slowly (time scale of centuries) with the deep ocean.