Why Is Atmospheric Carbon Dioxide Concentration Higher in the Northern Hemisphere?

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1 374 TOMIZUKA Environmental Science 26(4): (2013) Article Why Is Atmospheric Carbon Dioxide Concentration Higher in the Northern Hemisphere? Akira TOMIZUKA Abstract The atmospheric carbon dioxide (CO 2 ) concentration is about 3 ppmv higher in the Northern Hemisphere than in the Southern Hemisphere. In this research, the reason for this difference is discussed by using a box model that facilitates the understanding of the carbon cycle. Simulation shows that the observed concentrations at Mauna Loa and the South Pole are reproduced with an exchange coefficient between hemispheres of 0.93 yr -1. This value is in close agreement with estimates from meteorological data and other models. Current atmospheric concentrations in each hemisphere are strongly dependent on the scarcity of industrial CO 2 emissions in the Southern Hemisphere (only 5% of the total amount globally). A certain amount of time is needed for CO 2 generated in the Northern Hemisphere to reach the South Pole, and the hemispherical difference in atmospheric concentration would be smaller if the ratio of CO 2 emissions in the Southern Hemisphere were larger. On the other hand, variations in past CO 2 emissions in both hemispheres resulting from changes in land use and absorption into the surface ocean and the biosphere have not been sufficiently large to affect the hemispherical difference. Specifically, the uneven hemispherical distribution of the two CO 2 sinks, namely, the surface ocean and the biosphere compensate for this small effect. Key Words: Atmospheric CO 2 concentration, hemispheric difference, carbon cycle, box model 1. Introduction Since the Industrial Revolution, humans have continued to generate large quantities of carbon dioxide (CO 2 ) through industrial activities and forest development. The gross weight of anthropogenic carbon released over this 150-year period ( ) amounts to 282 PgC from burned fossil fuel and 149 PgC from changes in land use, 1,2) where 1 PgC = g of carbon. This released carbon has circulated the Earth and has accumulated everywhere. As a result, the global atmospheric CO 2 concentration has increased from 280 ppmv in the preindustrial era to 379 ppmv in ) In actuality, the atmospheric CO 2 concentration is not uniform and is higher in the Northern Hemisphere than in the Southern Hemisphere. More precisely, atmospheric CO 2 concentrations change mainly with latitude. This change is often given as a three-dimensional representation of latitudinal distribution in the marine boundary layer. 4) Figure 1 shows CO 2 concentration trends for the period at eight sites (from north to south): (1) Barrow, AK, United States (71 N, 156 W); (2) La Jolla, CA, United States (32 N, 117 W); Received January 26, 2012; Accepted March 14, 2013 Graduate School of Fisheries Science and Environmental Studies, Nagasaki University 1-14 Bunkyou-machi, Nagasaki , Japan Corresponding author: tommy@nagasaki-u.ac.jp

2 Why is atmospheric CO2 concentration higher in the Northern Hemisphere? 375 (3) Mauna Loa, HI, United States (30 N, 155 W); (4) Christmas Island, Australian territory (2 N, 157 W); (5) American Samoa, U.S. territory (14 S, 170 W); (6) Kermadec Islands, New Zealand (29 S, 178 W); (7) Baring Head, New Zealand (41 S, 3, 5) 174 E); and (8) South Pole, Antarctica (90 S, 24 W). Because of the large fluctuations that occur from year to year, these plots show the 5-year moving average of the annual average of the monthly observed values. Except for the South Pole, these sites have comparable longitudes, but different latitudes. Furthermore, except for La Jolla and Baring Head, these sites are located in the ocean, away from CO 2 sources. Naturally, the sites are not considered representative of other sites at the same latitude; however, the minimum CO 2 concentration is found at high latitudes in the Southern Hemisphere, and the concentration increases toward the Equator, reaching a maximum at high latitudes in the Northern Hemisphere. What is the cause of this apparent correlation in CO 2 concentrations? Firstly, the scarcity of industrial CO 2 sources in the Southern Hemisphere results in lower atmospheric CO 2 concentration in the Southern Hemisphere in comparison with the Northern Hemisphere, and a certain amount of time is required for CO 2 generated in the Northern Hemisphere to reach Figure 1 Observed atmospheric CO 2 concentrations at eight sites of different latitude: (1) Barrow, AK, United States (71 N); (2) La Jolla, CA, United States (32 N); (3) Mauna Loa, HI, United States (30 N); (4) Christmas Island, Australian territory (2 N); (5) American Samoa, U.S. territory (14 S); (6) Kermadec Islands, New Zealand (29 S); (7) Baring Head, New Zealand (41 S); and (8) South Pole, Antarctica (90 S). Figure 2 Latitudinal distributions of CO 2 emissions from burned fossil fuel in 1950, 1970, and 1990.

3 376 TOMIZUKA the South Pole. Secondly, differences exist between the forest and ocean areas in the two hemispheres, which act as sinks of CO 2. Carbon cycle models include box models, box diffusion models (box models that consider vertical diffusion of carbon in the ocean), 6) and advective-diffusion models (box diffusion models that consider convection of carbon in the ocean). 7) Box models are widely utilized for studying the flow of pollutants and the global circulation of various materials, 8 ~ 10) where a box represents a well-mixed reservoir of a material, and the flux from each box is proportional to the amount of material (stock) in the reservoir. The change with time in the stock inside the box must be equal to the flow difference between sources and sinks. This mass budget equation can be used to investigate the distribution of material in each box. Bacastow and Keeling investigated the hemispherical exchange time of atmospheric CO 2 by using a four-box model in which the atmosphere and surface ocean were divided at the Equator, and the CO 2 emission rate from burned fossil fuel and the increase in CO 2 concentration at Mauna Loa and at the South Pole between 1959 and ) As their estimate was in close agreement with other estimates, they concluded that biospheric sources and sinks in the two hemispheres are equal in size. However, it is unclear how the difference in atmospheric CO 2 concentration can grow to about 3 ppmv during global carbon circulation, and the question remains whether CO 2 emission from changes in land use or CO 2 uptake into oceans and forests in fact affect the hemispherical difference. Previously, the author has shown the time dependence of the atmospheric CO 2 concentration for the past 250 yr, reproduced the projected atmospheric CO 2 concentrations based on various scenarios given in the Intergovernmental Panel on Climate Change report by using the sevenbox model, 12) and calculated the distribution of anthropogenic carbon within the total carbon contained in the atmosphere and the surface ocean. 13) The seven-box model considers the main carbon reservoirs: the atmosphere, the ocean (surface, intermediate, and deep), sediment, the biosphere, and soil. In the present paper, the hemispherical difference in atmospheric CO 2 concentration is discussed based on this model, with the atmosphere box divided into northern and southern atmosphere boxes and with consideration of the carbon flux between the hemispheric boxes across the Equator. In addition, the effect on concentration resulting from differences in ocean and forest areas between hemispheres is explored. However, simulating CO 2 concentrations at the respective latitudes by using the observed values of eight sites in Figure 1 is somewhat meaningless since they differ in altitude, climate, and scale of neighboring CO 2 sources. Therefore, an attempt is made instead to simulate CO 2 concentrations at Mauna Loa (3,397 m above mean sea level) and at the South Pole (2,810 m above mean sea level) as representative sites of each hemisphere. These sites are located away from carbon sources, and the weather at both sites is stable. Thus, these two sites are unlikely to be affected by other factors. 2. Indicators Characterizing the Northern and Southern Hemispheres In this section, we discuss two types of indicators characterizing the Northern and Southern hemispheres, which are necessary to perform carbon budget calculations based on the extended box model with the northern and southern atmosphere boxes. For each hemisphere, the respective rates of CO 2 emissions from burned fossil fuel and changes in land use constitute one indicator, and the other indicator is the area of oceans and forests (sinks of CO 2 ) Industrial Emissions of CO2 Most of the CO 2 from burned fossil fuel is released in the Northern Hemisphere. Figure 2 shows the latitudinal CO 2 emission distributions in 1950, 1970, and ) In 1950, the total emissions in the Northern Hemisphere were 1.53 PgC, which was 96.4% of the total emissions that year. Moreover, the total emissions in the Northern Hemisphere were 3.70 PgC (95.9%) in 1970 and 5.50 PgC (94.7%) in Therefore, the ratio of industrial CO 2 emissions between Northern and Southern hemispheres has remained roughly the same at 95% to 5%. The total CO 2 emissions from burned fossil fuel are available from an online database. 1) Figure 3 shows the estimated industrial

4 Why is atmospheric CO2 concentration higher in the Northern Hemisphere? 377 Figure 3 Estimated industrial CO 2 emission rates over a period of 250 yr in both hemispheres for a constant emissions ratio of 95:5 between hemispheres. The total emission rate is taken from Ref. 1. emissions of CO 2 in both hemispheres over a period of 250 yr, assuming a constant emission ratio of 95: CO2 Emissions from Changes in Land Use The forest area in each country has been listed in a report by the Food and Agriculture Organization of the United Nations. 15) The total forest area globally was ha in 2005, with ha in the Northern Hemisphere and ha in the Southern Hemisphere. Therefore, the ratio between forest areas in the Northern and Southern hemispheres in 2005 was 65% to 35%. The difference in emissions between hemispheres caused by changes in land use can be simply estimated by assuming that the difference corresponds to this forest area ratio. Another estimation method is to sum emissions data for different regions within each hemisphere and scale the values by the respective total land areas of the hemispheres. We estimate the hemispherical emissions by this method as follows. Variations in the CO 2 emissions resulting from changes in land use have been estimated for nine regions. 2,16) Five of these regions are located entirely in the Northern Hemisphere: North America, Europe, North Africa and the Middle East, the post-soviet states, and China. The remaining four sites are located in both hemispheres: South and Central America (where 74.2% of the total forest area is in the Southern Hemisphere), tropical Africa (64.8%), South and Southeast Asia (31.4%), and the developed Pacific region (84.2%). These percentages were calculated from forest area data for each country. 15) Table 1 shows estimated CO 2 emissions in units of gc/yr for 1850, 1900, 1950, and 2000 as a total for the five regions located entirely in the Northern Hemisphere, for each of the four regions located in both hemispheres, and as a total for each hemisphere. The values in each hemisphere are calculated by summing the emissions in the regions with a weight amounting to the percentage of the forest belonging to the respective hemisphere. The global total net flux from changes in land use increased from 0.5 PgC/yr in 1850 to 1.4 PgC/yr in Between 1950 and 2000, the flux from five regions located entirely in the Northern Hemisphere, where many industrialized countries are located, decreased rapidly to zero. However, the increase in flux continued from South Asia and Southeast Asia, which extend widely across the Northern Hemisphere. As a result, total flux for the Northern Hemisphere decreased by 0.1 PgC/yr for this most recent 50 year period. On the other hand, the flux from the regions extending widely across the Southern Hemisphere, especially, that from South America and Central America increased rapidly. Therefore, the total flux for the Southern

5 378 TOMIZUKA Table 1 Estimated net CO 2 emission from changes in land use for each region in the Northern and Southern hemispheres (in units of gc/yr) for 1850, 1900, 1950, and For the four regions located in both hemispheres, the percentage of the forest area belonging to the Southern Hemisphere is shown in parentheses. Negative values indicate that absorption due to forestation exceeds emission due to deforestation. Year Five regions located entirely in the Northern Hemisphere South and Central America (74.2%) Tropical Africa (64.8%) South and Southeast Asia (31.4%) Developed Pacific region (84.2%) Total for the Northern Hemisphere Total for the Southern Hemisphere Figure 4 (a) Estimated CO 2 emissions in each hemisphere resulting from changes in land use over a period of 250 yr. The flux in each region is summed and multiplied by the fraction of the total flux in each hemisphere. The trend before 1850 is obtained by linear interpolation between 0.2 PgC/yr in 1750 and 0.5 PgC/yr in (b) CO 2 emissions in the Southern Hemisphere as a percentage of the total emissions globally.

6 Why is atmospheric CO2 concentration higher in the Northern Hemisphere? 379 Hemisphere increased by 0.47 PgC/yr and exceeded the flux from the Northern Hemisphere. Figure 4(a) shows the estimated trends in CO 2 emissions caused by changes in land use in both hemispheres. The trend before 1850 is found by linear interpolation between the values of 0.2 PgC/yr in 1750 and 0.5 PgC/yr in ) Figure 4(b) shows CO 2 emissions in the Southern Hemisphere as a percentage of the total amount of emissions globally. The total emission ratio due to changes in land use between the Northern and Southern hemispheres is 71% to 29%. However, the percentage of annual emissions in the Southern Hemisphere exceeded 40% after 1962 and 50% after Ocean and Forest Areas The major CO 2 sinks of the Earth are the oceans and the terrestrial biosphere. The global annual CO 2 emission rate from fossil fuels is estimated as 7.2 PgC/yr for the period between 2000 and ) About 57% of this carbon remains in the atmosphere, about 30% is absorbed by the oceans, and the remainder is absorbed by the terrestrial biosphere. Oceans cover about 71% of the surface of the Earth, and the continents are disproportionately located in the Northern Hemisphere. In fact, the ratio between land and ocean is 19% to 81% in the Southern Hemisphere and 39% to 61% in the Northern Hemisphere. Therefore, if the quantity of CO 2 uptake by the ocean is simply proportional to the ocean area, the uptake ratio between the Northern and Southern hemispheres is 43% to 57%. In contrast, estimating the ratio of CO 2 uptake by the terrestrial biosphere between both hemispheres is more challenging. The net primary productivity (NPP), which is the difference between the CO 2 uptake rate by photosynthesis and the CO 2 emission rate by respiration, is different for each biome type. The NPP in tropical forests and savannas is 36.8 PgC/yr, which is approximately 59% of the total NPP globally (62.6 PgC/yr). 19) Although these areas are located roughly around the Equator, their distribution is skewed heavily toward the Southern Hemisphere. Although the NPPs in temperate forests and boreal forests constitute about 28% of the total NPP globally and the NPP in croplands adds a further 7%, these areas are mainly located in the Northern Hemisphere. Therefore, Table 2 Four different biome areas (in units of 10 6 ha) for the nine regions and estimated total NPP (in units of PgC/yr) in each hemisphere. The total NPP values for the four biomes are as follows: tropical forests, savannas and grasslands, 36.8 PgC/yr; temperate forests and grasslands, 15.1 PgC/yr; boreal forests, 2.6 PgC/yr; and croplands, 4.1 PgC/yr. Biome Five regions located entirely in the Northern Hemisphere South and Central America Tropical Africa South and Southeast Asia Developed Pacific region Total NPP in the Northern Hemisphere Total NPP in the Southern Hemisphere Tropical Temperate Boreal Croplands Table 3 Indicators characterizing Northern and Southern hemispheres. Ocean area NPP Forest area Industrial CO 2 emissions Northern 43% 56% 65% 95% Southern 57% 44% 35% 5%

7 380 TOMIZUKA the difference in net carbon flux to the terrestrial biosphere in both hemispheres is not sufficiently large to create a notable hemispherical difference in CO 2 concentration. Table 2 shows the estimated NPP in each hemisphere for the four biomes using the forest areas 16) and cropland areas 20) in nine regions. The total NPP of the four biomes in the Northern and Southern hemispheres are 32.7 PgC/yr and 25.9 PgC/yr, respectively, which forms a ratio of 56% to 44% Summary of Hemispherical Indicators and Modeling The hemispherical indicators necessary for calculating the carbon budget are summarized in Table 3 and in Figures 3 and 4. On the basis of these results, we investigate the growth in the difference in atmospheric CO 2 concentration between hemispheres under the following assumptions. 1) The atmosphere is divided into northern and southern boxes. The exchange coefficient between the boxes is used as a control parameter which changes its value so that the difference between calculated and observed values is minimal. 2) CO 2 emissions from burned fossil fuel in the Northern Hemisphere constitute 95% of the total emissions globally. 3) CO 2 exchange between the atmosphere and the ocean is equal in both hemispheres at the first approximation. In considering small differences in ocean area, these fluxes are proportional to the respective ocean areas in each hemisphere. 4) The NPP flux from the atmosphere to the terrestrial biosphere is equal in both hemispheres at the first approximation. In considering small differences in NPP flux, the ratio of the fluxes from the soil to the atmosphere for each hemisphere is the same as the ratio of the NPP fluxes. 3. Extended Box Model and Carbon Budget Equations We introduce an extended box model in which the atmosphere reservoir is divided into two boxes, Figure 5 Extended box model of the carbon cycle. The atmosphere is divided into Northern and Southern boxes such that the flux between each atmospheric box and other boxes interacting with it (surface ocean, biosphere, and soil) is half of that in the original model. 21) Assumed values of the carbon stock and flux in the preindustrial era are expressed in units of PgC and PgC/yr, respectively (revised from Fig in Ref. 21).

8 Why is atmospheric CO2 concentration higher in the Northern Hemisphere? 381 as shown in Figure 5. The stock and flux values in Figure 5 are assumed to be the same as those in Here,γ N and γ S are the emission rates due to burned fossil fuel in each hemisphere,δ N andδ S are the emission rates resulting from changes in land use in each hemisphere, and δ=δ N +δ S. Furthermore, αis the hemispherical exchange coefficient. The flux between each atmospheric box and the other boxes interacting with it (surface ocean, biosphere, sediments, and soil) is half of that in the original model, which was used for reproducing the atmospheric CO 2 concentration for the past 250 yr in a previous paper by the author. 12) The carbon budget equations are as follows: Furthermore, N 1 and S 1 denote the stocks in the northern and southern atmosphere boxes, respectively. The factor of 0.5 in Eqs. (1) and (2) signifies that the fluxes are equal in the two hemispheres. The transfer coefficients are provided as follows by dividing flux values by stock values in Figure 5: 21). (9), (1), (2) Here, ξ is a buffer factor that accounts for the suppre ssion of atmospheric CO 2 uptake into the ocean, and this factor is dependent on the atmospheric CO 2 concentration. 12) Moreover, M 0 2 is the stock of the surface ocean in the preindustrial era, and f is the NPP in plants given by 22), (10), (3), (4), (5), (6), (7), (8) where M i denotes the stock in reservoir i, and k ij is the transfer coefficient of carbon flux from reservoir i to reservoir j. Subscripts 1 to 7 denote the atmosphere, the surface ocean, the intermediate ocean, the deep ocean, sediment, the terrestrial biosphere, and soil and detritus, respectively. where f 0 = 62 PgC/yr represents the preindustrial value of f, P is the total atmospheric CO 2 concentration in both hemispheres, and P 0 is the preindustrial value of P. The fertilization factor β takes a value of around ) Time (denoted as t) is expressed in years, and the equations are solved numerically as difference equations, with Δt=1. Here, we solve the carbon budget equations based on the indicators discussed in Section 2 for the following four cases. Case1: Emissions from changes in land use are proportional to the forest area Emission rates resulting from changes in land use are proportional to the respective percentages of forest area in each hemisphere (65% and 35%). Thus, Eqs. (1) and (2) are modified as follows:, (11)

9 382 TOMIZUKA factors are the same as in Case 2, and thus Eqs. (1) and (2) are modified as. (12) Case 2: Emissions from changes in land use are estimated from regional emission data Equations (1) and (2) are modified as follows:, (18). (19),, (13) (14) where δ N and δ S are the emission rate from changes in land use shown in Figure 4. Case 3: Considering the small difference in ocean area The exchange between the atmosphere and surface ocean in the Northern and Southern hemispheres is proportional to the distribution of ocean areas, which is 43% and 57% respectively. Furthermore, the emission rates resulting from changes in land use are the same as in Case 2. Equations (1)-(3) are modified as,(15),(16). (17) Case 4: Considering the small difference in NPP The exchange between the atmosphere and the terrestrial biosphere in the Northern and Southern hemispheres represented by the NPP flux ratio, which is 56% and 44%, respectively. The other 4. Results and Discussion The author s previous study 12) using a simple seven-box model shows that the calculation starting from 1750 reproduces the observed atmospheric CO 2 trends, and its reproducibility is especially good after the 1960s. Similarly, each hemispherical CO 2 trend is calculated using the extended box model for the periods in which emission data are available and the hemispherical differences from 1970 to 2005 are considered. Figure 6 shows the calculated trends in CO 2 concentration for each hemisphere, with a hemispheric exchange coefficient α for Case 1. At low values of α, the calculated CO 2 concentration in the Northern Hemisphere is substantially higher than the observed concentration due to the large number of CO 2 sources and the small amount of CO 2 transferred to the Southern Hemisphere. Accordingly, the calculated CO 2 concentration in the Southern Hemisphere is considerably lower than the observed concentration, because of the scarcity of CO 2 sources and the small amount of CO 2 transferred from the Northern Hemisphere. CO 2 concentrations in each hemisphere approach the observed values as α increases. However, when α> 0.93 yr -1, the concentrations become unstable and oscillate as a result of excessive transfer in the extended box model. Nevertheless, this ultimate value of α is in close agreement with the estimated values of yr -1 found by using meteorological data 24) 11, 25) or by other methods. For each value of α, Table 4 lists the calculated atmospheric CO 2 concentrations (ppmv) for 2000 (averaged from 1988 to 2002, hereafter) in the Northern and Southern hemispheres, as well as the difference between these hemispherical

10 Why is atmospheric CO2 concentration higher in the Northern Hemisphere? 383 concentrations, for the four cases described above. The difference between CO 2 concentrations in the hemispheres asymptotically decreases to about 3 ppmv, and in all cases the concentration in each hemisphere approaches that observed whenα is increased. The difference between Cases 1 and 2 is the difference in estimation of emission rate from changes in land use, however, the hemispheric difference between the two cases is about 10%, or 0.3 ppmv with α = 0.93yr -1 in In reality, the result for Case 2, which reflects the regional emission rates in both hemispheres, is still close to the actual value of 2.63 ppmv. Case 3 considers the difference in CO 2 uptake into the surface ocean in both hemispheres in comparison to Case 2. Since the CO 2 uptake into the surface ocean area in the Northern Hemisphere is slightly smaller than that in the Southern Hemisphere due to the ocean area ratio, the difference in hemispherical CO 2 concentration in Case 3 is greater than in Case 2. Case 4 considers Figure 6 Comparison between simulated CO 2 concentrations in the Northern Hemisphere (black dots) and the Southern Hemisphere (gray dots) with observed values for Mauna Loa (black line) and the South Pole (gray line) for Case 1 with a hemispheric exchange parameter of (a) α = 0.1, (b)α = 0.3, (c) α = 0.5, (d) α = 0.7, and (e) α = 0.93; for α > 0.93 yr 1, the concentrations become unstable.

11 384 TOMIZUKA the difference in CO 2 uptake into the terrestrial biospheres as the NPP flux in both hemispheres as compared to Case 2. Since the NPP flux into the terrestrial biosphere in the Northern Hemisphere is slightly greater than that in the Southern Hemisphere, conversely the hemispherical CO 2 difference in Case 4 is smaller than that in Case 2. Table 5 shows the simulated atmospheric CO 2 concentrations for Case 2 with α= 0.93 yr -1 in 2000 when the ocean area ratio between the hemispheres is varied between 35:65 and 65:35. In fact, while the hemispherical difference decreases with the increase in ocean area in the Northern Hemisphere, this change is small, whereby for a change of 10% in the ocean area ratio the change in the difference in hemispherical CO 2 concentration is only 0.15 ppmv. Table 6 shows simulated atmospheric CO 2 concentrations for Case 2 with α= 0.93 yr -1 in 2000 when the NPP flux ratio between the hemispheres is changed from 35:65 to 65:35. While the hemispherical difference decreases with the increase in ocean area in the Northern Hemisphere, the change is not substantial, whereby for a change of 10% in the NPP flux ratio the change in the difference in hemispherical CO 2 concentration is 0.35 ppmv. In this regard, the mean (2.70 ppmv) of the values for Cases 3 and 4 with α = 0.93 yr -1 in Table 4 is extremely close to the observed value of 2.63 ppmv, and the result of a calculation considering the ocean area ratio and the NPP ratio in Table 3 for Case 2 is 2.64 ppmv. Therefore, variations in past CO 2 emission rates in each hemisphere caused by changes in land use and changes in CO 2 uptake from the atmosphere into the surface ocean and the biosphere do not exert a notable influence on the difference between hemispheric concentrations. The main reason for the difference in CO 2 Table 4 Calculated atmospheric CO 2 concentrations (ppmv) in the Northern and Southern hemispheres in 2000, with hemispheric transfer coefficient α, and the difference between concentrations in the two hemispheres. The observed values (averaged from 1998 to 2002) were and ppmv in the two hemispheres, 3,5) amounting to a difference of 2.63 ppmv. 3,5) Difference Northern Southern α Case Case Case Case Case Case Case Case Case Case Case Case Table 5 Simulated atmospheric CO 2 concentrations (ppmv) in each hemisphere in 2000 with different ocean ratios for Case 2 with α = 0.93 yr -1. Ratio 35:65 40:60 45:55 50:50 55:45 60:40 65:35 Difference Northern Southern

12 Why is atmospheric CO2 concentration higher in the Northern Hemisphere? 385 Table 6 Simulated atmospheric CO 2 concentrations (ppmv) in each hemisphere in 2000 with different NPP ratios for Case 2 with α = 0.93 yr -1. Ratio 35:65 40:60 45:55 50:50 55:45 60:40 65:35 Difference Northern Southern Table 7 Simulated atmospheric CO 2 concentrations (ppmv) in each hemisphere in 2000 with different industrial emissions ratios for Case 2 with α= 0.93 yr -1 for an ocean ratio of 43:57 and an NPP ratio of 56:44. Ratio 95:5 90:10 85:15 80:20 75:25 70:30 Difference Northern Southern concentration between hemispheres is the highly uneven distribution of industrial emission sources. Table 7 shows simulated atmospheric CO 2 concentrations for Case 2 considering the ocean area ratio and the NPP ratio with α = 0.93 yr -1 in 2000 when the industrial emissions ratio between the hemispheres is changed from 95:5 to 70:30. The calculations clearly demonstrate that by increasing the proportion of emissions in the Southern Hemisphere, the concentration in the Northern Hemisphere decreases and that in the Southern Hemisphere increases. Consequently, the hemispheric difference decreases. 5. Conclusion The hemispherical difference of about 3 ppmv observed in current atmospheric CO 2 concentrations is heavily dependent on the scarcity of industrial CO 2 emission sources in the Southern Hemisphere. Furthermore, this difference corresponds to an exchange coefficient between hemispheres of 0.93 yr -1. On the other hand, differences in past CO 2 emissions in both hemispheres resulting from changes in land use or absorption into the surface ocean and the biosphere are not sufficiently large to affect the hemispherical difference. Acknowledgments The author would like to acknowledge the referees for many constructive suggestions. References 1 ) Boden, T.A., G. Marland, and R.J. Andres (2010) Global, Regional, and National Fossil-Fuel CO2 Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi: /CDIAC/00001_V ornl.gov/trends/emis/tre_glob.html (accessed ). 2 ) Houghton, R.A. and J.L. Hackler (2008) Carbon Flux to the Atmosphere from Land-Use Changes: In TRENDS: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. houghton/houghton.html (accessed ). 3 ) Keeling, R.F., S.C. Piper, A.F. Bollenbacher, and J.S. Walker (2009) Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi: /CDIAC/ atg html (accessed ). 4 ) Tans, P. and T. Conway (2012) Global Distribution of Atmospheric Carbon Dioxide. Earth System Research Laboratory. gmd/photo_gallery/gmd_figures/ccgg_figures/ tn/co2_surface_color.png.html (accessed ).

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14 Why is atmospheric CO2 concentration higher in the Northern Hemisphere? 387 冨塚 明 ( 長崎大学大学院水産 環境科学総合研究科, 長崎市文教町 1-14) 現在, 北半球の二酸化炭素濃度は南半球と比較して約 3ppmv 高くなっている 産業革命以降, この差がどのようにして生じてきたのか, 炭素循環のボックスモデルを用いて検討した 南北大気間の交換係数を 0.93/ 年としたとき, マウナロア及び南極点での測定値に近い値が得られることがわかった この交換係数値は濃度の実測値や他のモデルでの算出結果にほぼ一致した また現在の南北の濃度差は主に北半球に強く偏在 ( 約 95%) している化石燃料からの二酸化炭素発生量によるものであることが明らかとなった 仮に南半球での放出割合がもっと多ければ濃度差は現在よりも小さいものとなったであろう 一方, これまでの森林伐採などによる二酸化炭素放出量, 海洋や森林への二酸化炭素取り込み量の南北での差は現在の大気濃度差に影響を与えるほど大きなものではなかった さらに海洋と森林への小さな取り込みの差も実際にはお互い相殺しているものと思われる 大気中の二酸化炭素濃度, 南北差, 炭素循環, ボックスモデル