The Advances of Satellite Remote Sensing for the Earth Environment–Determination of CO2 Distribution in the North Pacific Ocean

Yasuhiro Sugimori    School of Marine Science and Technology, Tokai University Orido 3-20-1, Shimizu, Shizuoka, Japan 424

GREEN HOUSING EFFECT AND GLOBAL CO2 DISTRIBUTION

It is growing for an international consensus among scientists that the warming of the earth's climate is realizing and has been caused by an increase in emissions of Carbon Dioxide (CO2) and other greenhouse gases. All available geological and atmospheric evidence shows that the earth's average annual temperature has raised a half degree in this centenary. In a new IPCC (the Intergovernmental Panel on Climate Change) report, it suggests that global warming is unlikely to be caused by natural cause entirely, and predicts previously that population growth and increased use of carbon fuels can cause a doubling or tripling in atmospheric carbon dioxide levels in the next 100 years, with an average global temperature rise of 1 to 4 Celsius degree. Meanwhile, the world ocean plays an important role in the Earth's climate: not only absorbs heat from the sun, transports in thousands of miles away, but plays a major role in carbon cycle processes because it contains 50 times more carbon than that in the atmosphere (Fig. 1). For long-term climate forecasts, knowledge of the heat, momentum and substance exchange between the atmosphere and the ocean is essential because the time constants and capacities of the ocean are much larger than those of the atmosphere. The greenhouse phenomenon, for example, is very serious for the global environment and caused mainly by Carbon Dioxide (CO2) in the atmosphere. The exchange rate of CO2 between the atmosphere and the ocean is therefore of great importance. Variation in physical and chemical properties of the ocean, especially in the upper mixed layer where ocean-atmospheric interaction is very active, should be researched in detail.

Carbon Dioxide (CO2), as the main greenhouse gas in the atmosphere, has been studied for many years. A dramatic increase in the atmospheric concentration of carbon dioxide (CO2) has been directly observed over the passed 30 years in Hawaiian Island from 315 ppmv (parts per million in volume) to 358 ppmv in 1994 (Keeling et al., 1995) (Figure 2). Pre-industrial concentration was estimated at 270 - 280 ppmv and there is no doubt that the CO2 concentration in the atmosphere will continue to increase in the future as shown in the Fig. 2. Some numerical models have been developed to describe the climate response as CO2 concentration increases, and the results show that the globally average temperature on the earth surface would increase from 1.5 to 5 Celsius degree due to different models, if the CO2 concentration will be produced to be double of the recent concentration (Lindzen et al., 1990). The CO2 average increase rate in atmosphere is about 1.56 ppmv per year (Nakazawa et al 1991, CO2 Trends Report, 1993), which means about 3.4 gigatons (10l5g) of carbon is released to atmosphere in every year. However, the present estimate of anthropogenic CO2 source to the atmosphere is much larger (approximately 6-7 gigatons per year; IPCC 1990, Table 1). Since the growth rate of CO2 in atmosphere is less than the rate of carbon release, some released carbon dioxide must be absorbed by either the terrestrial biosphere or the oceans. Furthermore, the distribution of carbon is uneven in the oceans due to the influence of biological process, chemical process and physical process, which control the partial pressure of CO2 in surface water, the exchange rate of CO2 between the atmosphere and surface water, and the exchange between surface and deep waters shown in Fig. 3. However all these processes are not well understood up to now.

Generally, the flux of CO2 exchange between the atmosphere and the ocean can be determined from a bulk formula, i.e., the product of CO2 gas exchange coefficient and the CO2 partial pressure difference (∆Pco2) between the ocean and the atmosphere. Some researches (Etcheto and Merlivat, 1988; Tans et al., 1990) show that ocean acts as a main sinking source in regulating CO2 concentration in the atmosphere and absorbs 1-2 Gt y-1 Fig. 3 shows the distribution of total carbon concentration in the Pacific and the Atlantic ocean in the vertical profile and shows the presence of biological pumping that transports CO2 from surface to the deep ocean. The distribution of the CO2 partial pressure difference (∆Pco2) is shown in Fig. 4 and shows that the most intense CO2 source lies in the equatorial Pacific and the main sink area is around 55 degree to subantarctic belt. The distribution of ∆Pco2 is not only influenced by temperature, biological activities in the ocean surface, but by ocean circulation especially equatorial upwelling. There is still some debate about the absorption of CO2 amount due to the measurement difficulty of CO2 exchange coefficient and concentration, and the ∆Pco2 data sparse in time and space scale.

CO2 GAS TRANSFER THROUGH THE SEA SURFACE

CO2 exchange coefficient between ocean and atmosphere is related to physical process and chemical process. Liss (1983) gave a review of CO2 gas exchange between air and water. Physical transfer processes generally include wind stress, waves, bubbles and turbulence Theoretical model and laboratory experiments have been continued for many years. Deacon (1977) applied boundary-layer theory to describe air-water gas transfer and obtained that the gas transfer velocity is proportional to wind stress and Schmidt number with the power of -2/3 for smooth surface. Several different wind tunnel studies show the gas exchange coefficient increases with wind speed (Broecker et al 1978, Hoover and Bershire 1969, Liss 1973). With wavy surface in wave tank, the rapid increase of gas exchange velocity was observed by Broecker et al (1983) and Jahne et al (1979) because of the surface area increase. However in Kanwiser (1963) experiment, a decrease trend in gas exchange velocity was measured. It is assumed to be the wave being sheltered from the wind. Although there are considerable theoretical and experimental results about the role of bubbles on gas exchange, generally the tunnel experiments ignored this effect due to strong nonlinear effects in wave tank than in the ocean. Chemical transfer process is another effect related to gas exchange because CO2 can react rapidly in the aqueous phase. Hoover and Berkshire (1969) firstly described CO2 chemical enhancement theoretically and verified it in water tank. Later, Liss (1973) gave similar result after measuring oxygen and CO2 gas exchange velocity in wind tunnel and found the exchange velocity of CO2 seems to be proportional to the square of wind speed and chemical enhancement effect should be considered in calm conditions (wind speed less than 5 m/s) and pH value greater than 5.

In recent years, some new progresses in gas exchange have been made based on experimental and field experiments. Liss and Merlivat (1986) have suggested a synthesis relation between CO2 transfer velocity and wind speed based on wind tunnel and the Rockland lake experiment, which consists of three different linear relations related to smooth, rough surface and wave breaking region (Fig. 5). Tans et al (1990) used another relation to estimate CO2 budget between atmosphere and ocean, which is originally proposed by Smethie et al (1985) (Fig. 6a). In this relationship, it is assumed no gas exchange takes place if wind speed is less than 3 m/s (at 10 m above ocean). Wanninkhof (1992) reviewed the relationship between wind speed and gas transfer velocity over the ocean and proposed gas exchange velocity should be proportional to wind speed square, U102, based on water tank experiments (Fig. 6b). Especially, Waninkhof (1992) noted chemical enhancement factor due to CO2 chemical properties. Wind speed is not only the factor influencing the gas...