Global Warming, Part 1: Recent and Future Climate

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1 C H A P T E R 15 Global Warming, Part 1: Recent and Future Climate Learning Objectives After reading this chapter, students should be able to: Know that the global climate fluctuates on small timescales of years to decades, and that scientists are still not sure about the causes of these short-term fluctuations Know the basic shifts in climate during the last 10,000 years, including the Holocene Climatic Optimum, the Little Ice Age, and the Medieval Warm Period Be aware of the short-term influence of volcanoes on climate Understand the evidence for solar variability, its connection to the 14 C record, and its possible connection with climate Understand the implications these past climate fluctuations have on our analysis of changes that could occur as a result of anthropogenic global warming Know the relative sizes of different carbon reservoirs and fluxes, in particular the size of anthropogenic fluxes compared to natural fluxes Know the timescales of the various CO 2 removal processes Understand the chemistry of CO 2 uptake in the oceans Know some of the projections for future CO 2 concentrations and the predicted climatic implications of global warming Realize the level of uncertainty that exists in these models, and the sources of these uncertainties Realize that anthropogenic gases other than CO 2 contribute to the greenhouse effect Understand the long-term implications of global warming, i.e., on time scales of centuries to thousands of years, and its potential for causing changes in deep ocean circulation patterns Review Questions 1.) What is the Holocene epoch? The Holocene is the geologic interval from the last glacial retreat to the present. It has lasted for the past 10,000 years. 2.) What are proxy data? Describe several examples of proxy climate data. Proxy data are data used to determine what a measurable quantity was at a time or place for which these measurements are unavailable. Examples include pollen analysis and tree ring thickness. 117

2 3.) Briefly describe the Younger Dryas, the Holocene Climate Optimum, the European Medieval Warm Period, and the Little Ice Age. The Younger Dryas Event and the Little Ice Age are both periods of rapid cooling. The Younger Dryas Event occurred around 10,500 years ago, and the Little Ice Age occurred in the late 1500s. The Holocene Climate Optimum and the European Medieval Warm Period are both periods of relative warmth in the Holocene. The Holocene Climate Optimum was a period of stable and mild climate conditions that began around 6,000 years ago and lasted for about 1,000 years. The Medieval Warm Period allowed agriculture on Greenland, with peak temperatures occurring 900 years ago (~1100 A.D.). (Note: The discussion of the Younger Dryas has been moved to Chapter 14.) 4.) How do volcanoes affect climate? Volcanoes release gases into the atmosphere which have varying effects on the climate system. On timescales of a few years, the most important impact volcanoes have on climate is the release of SO 2, which forms aerosols in the atmosphere. These aerosols reflect incoming solar radiation, reducing the amount of radiation that reaches the surface. 5.) What are sunspots? Why are they thought to have a possible effect on climate? Sunspots are relatively cool, dark blotches that appear on the surface on the Sun. They are thought to be caused by changes in the Sun s magnetic field and are surrounded by plages, areas of above average solar temperatures. The plages are larger than the sunspots, and the net effect of sunspots is a sun with an above-average surface temperature. Cyclic variations in the amount of sunspots lead to cyclic variations in the amount of incoming solar radiation reaching the Earth. These cycles match up with changes in the Earth s climate on a number of timescales. However, the magnitude of the forcing is much less than the observed changes in climate, so if sunspot cycles are driving climate, it must be with the help of a positive feedback system that has not yet been identified. 6.) How does the amount of CO 2 produced by fossil fuel consumption compare to the natural flux of CO 2 in the carbon cycle? The CO 2 produced by fossil fuel consumption is ~ 7.5 Gton(C)/yr, while the flux from global respiration alone is ~60 Gton(C)/yr, and the flux from volcanism is about 0.06 Gton(C)/yr. 7. What are the major processes that can remove CO 2 form the atmosphere? What are the approximate time scales for these processes to be effective? The major loss processes for atmospheric CO 2 are (with their effective timescales in parentheses): reforestation (10 s 100 s of years), fertilization of forests (10 s 118

3 100 s of years), dissolution of CO 2 in the oceans (10 s 1000 s of years), dissolution of deep-sea carbonates (100s 1000s of years), and the weathering of continental rocks ( years). 8. What is the size of the fossil fuel reservoir compared with the atmospheric CO 2 reservoir? The atmosphere currently contains ~760 Gton(C), whereas the fossil fuel reservoir represents ~5000 Gton(C). 9. Why does the ocean have a limited capacity for CO 2 uptake? The ocean s capacity for CO 2 uptake is greatly enhanced by the fact that it is buffered by dissolved carbonate and borate species. Once the buffering capacity of the ocean is exceeded by CO 2 dissolution, the ability of the oceans to take in CO 2 will be greatly diminished. Therefore, oceanic CO 2 uptake is limited by its buffering capacity. 10. By how much is global temperature predicted to rise over the next century? As shown in figure 15-2 the predicted change in surface temperature ranges from 1.4ºC to 4.0ºC. Critical-Thinking Problems 1.) a. The present atmosphere contains approximately 700 Gton(C) in the form of CO 2. Earth s total recoverable fossil fuel reserves contain at least 4200 Gton(C), mostly in the form of coal. (We shall use the value 4200 Gton(C) to be specific.) At present, about half the CO 2 produced by the burning of fossil fuels stays in the atmosphere. The other half dissolves in the oceans or is taken up by the terrestrial biosphere. If this ratio remained constant and we burned up all of our fossil fuels instantaneously, by how much would atmospheric CO 2 concentrations rise? (Express your answer in terms of the new CO 2 level divided by the old one.) The CO 2 level would increase by 4200 Gton(C) / 2 = 2100 Gton(C). The new atmospheric concentration would be: 700 Gton(C) Gton(C) = 2800 Gton(C) Gton(C) / 700 Gton(C) = 4, so atmospheric CO 2 levels would be 4 times the present-day levels after such a change. 119

4 b. Climate models predict that each doubling of the atmospheric CO 2 concentration will cause the mean global temperature to increase by 1.5ºC-4.5ºC. (The range is due largely to uncertainties about how clouds will respond.) By how much would the mean temperature increase for the scenario described in part (a)? Express your answer as a temperature range in degrees Celsius and in degrees Fahrenheit. Increasing the CO 2 concentrations to 4 times their present-day levels would represent two doublings. According to these models, this would lead to two increases of 1.5ºC 4.5ºC, a total increase of 3.0ºC 9.0ºC. To convert this to Fahrenheit, multiply by (9ºF / 5ºC). When this is done, we find that the temperature increase would be 5.4ºF-16.2ºF. c. The actual problem of global warming could be more severe than we have just calculated. Forests and soils together contain an additional 2100 Gton(C) of carbon that might go into the atmosphere if deforestation is not prevented. The ocean becomes more acidic as it absorbs CO 2, so it might not be able to continue taking up as much CO 2 as it has been until now. If we burned up all our fossil fuels and deforested one-third of the globe without losing any CO 2 to the ocean (or to CO 2 fertilization) by how much would atmospheric CO 2 and temperature increase? The increase in CO 2 levels would be (1/3) 2100 Gton(C) Gton(C) = 4900 Gton(C). The new atmospheric CO 2 concentration would be: 4900 Gton(C) Gton(C) = 5600 Gton(C). Thus, atmospheric CO 2 concentrations would be 5600 Gton(C)/700 Gton(C) = 8 times higher than present-day levels. This represents 3 doublings of CO2 concentrations, because 8 = 2 3 ) The corresponding temperature increase would be 4.5 C 13.5 C, or 8.1 F 24.3 F. 2.) The atmospheric CO 2 concentration is currently increasing by about 1.9 ppm/yr. How many gigatons of carbon are being added to the atmosphere each year? (Hint: The total mass of the atmosphere is kg, and its mean molecular weight is about 29. You will need to do the calculation in moles and then convert back to mass units.) The mass of the atmosphere is kg = g. There are ( kg) / (29 mol/g) = moles of air in the atmosphere. For a 1.9 ppm increase in CO 2 levels, the number of moles in the atmosphere increases by ( ) ( ) = moles. This is equivalent to ( moles) (12 g/mol) = g(c) = 3.8 Gton(C). 120

5 3.) The surface ocean contains about liters of water with a carbonate ion content of about mol/l. The deep ocean contains about L of water with a carbonate ion content of roughly mol/l. If each mole of carbonate reacts with 1 mole of CO 2 according to the reaction CO 2 + CO 3 = + H 2 O 2 HCO 3 - what percentage of the fossil fuel reservoir, 4200 Gton(C), can be neutralized by the surface ocean? By the deep ocean? The fossil fuel reservoir contains 4200 Gton(C) = g(c). This is equivalent to ( ) / (12 g/mol) = moles C. The number of moles of CO 2 that can be neutralized by the either part of the ocean should be equal to the number of moles of CO 3 = dissolved in that part of the ocean. Thus, the surface ocean can dissolve ( L) ( mol/l) = mol CO 2, whereas the deep ocean can dissolve ( L) ( mol/l) = mol CO 2. The surface ocean can absorb 100% ( mol CO 2 ) / ( moles CO 2 ) = 1.5% of the carbon in the fossil fuel reservoir, and the deep ocean can absorb 100% ( mol CO 2 ) / ( moles CO 2 ) = 37% of the carbon in the fossil fuel reservoir. Resource Guide Video/Film: An Inconvenient Truth Al Gore (2 hours, color) This is by far the most up-to-date and powerful movie about the perils of global warming. Although Gore himself is politically controversial, the science in this movie is for the most part very good. A must show if you are teaching this course. Show it here if you did not show it while doing Chapter 1. Watch for an upcoming book and film series starring Richard Alley, our Penn State glaciologist colleague. It should be out in either 2010 or