Time evolution of the CO2 concentration measured in the period 1992-2002
Ice core studies have shown that the atmospheric carbon dioxide concentration has varied roughly between 180 and 290 ppmv throughout the last four glacial cycles, has remained at approximately 280 ppmv during the last interglacial period, and has dramatically increased since the industrial revolution. The atmospheric CO2 concentration has been monitored at a few sites during the last 50 yr; new measurement stations have been progressively added, and high-quality observations are nowavailable at many sites throughout the world. The present observations show that the carbon dioxide concentration has reached a value of about 380 ppmv, a concentration unprecedented in the last 400 000 yr; an increase by more than 25% with respect to the pre-industrial period has thus occurred. This increase, and its possible influence on the Earth’s climate, has prompted the need for high-quality accurate measurements of the carbon dioxide concentration in the atmosphere. Accurate surface measurements of atmospheric CO2 are also used to constrain the global carbon budget and to identify carbon source and sink regions. Beside anthropogenic emissions, many natural phenomena influence the CO2 concentration in the atmosphere. Global-scale processes, like El Niño/Southern Oscillation, changes in global temperature, precipitation, ocean upwelling and others, appear to be related to the carbon dioxide behaviour. These relationships are believed to proceed through a variety of mechanisms, such as photosynthesis and respiration by the terrestrial vegetation, uptake and emission from oceanic waters, assimilation by marine micro-organisms, storage and release by terrestrial soils, etc. In the global carbon dioxide budget the tropical oceans seem to play a fundamental role, indicating that certain regions may intervene more efficiently than others in the CO2 balance. Little is known of the role on the Mediterranean in the global carbon dioxide budget. The Mediterranean sea has several peculiar characteristics that make this basin unique: it is the only large closed basin worldwide, and is characterized by a complex oceanic and atmospheric circulation. The Mediterranean is an oligotrophic basin, and CO2 exchange with the marine ecosystem is probably small; Mediterranean forests appear, however, to be an efficient CO2 sink. For a better understanding of the role of the Mediterranean, long-term accurate measurements of the atmospheric CO2 concentration are crucial, mainly in the marine environment.
The atmospheric concentration of carbon dioxide has been routinely measured at Lampedusa since 1992. Air samples have been collected on a weekly basis and the CO2 concentration has been determined using a NDIR analyzer (Siemens Ultramat 5E). The data of the period 1992-2002 show an average trend of +1.7 ppmv/yr; the average annual cycle has an amplitude of about 9 ppmv. In the period of investigation the annual growth rate varies between 0.5 and 4.5 ppmv/yr, and the amplitude of the annual cycle between 7 and 11 ppmv/yr. By comparing the observed growth rate with recent estimates of carbon dioxide emissions, it is calculated that 58–61% of the emitted CO2 remains in the atmosphere. The CO2 growth rate appears to be related to large-scale dynamic phenomena, primarily El Niño/Southern Oscillation (ENSO). An evident signature of the 1997–98 El Niño is apparent in the CO2 record, and corresponds to a weakening of the exchange with the biosphere. A high correlation between the global average temperature and the 12-month average carbon dioxide growth rate is also found. Wind direction displays a significant inter-annual variability throughout the measurement period, possibly influencing the observed evolution of the CO2 concentration. The CO2 weekly flask program is now performed in parallel with continuous measurements; the combined analysis of the two data series is in progress.