Interannual variability of atmospheric CO₂ in the Mediterranean: measurements at the island of Lampedusa

Abstract

The atmospheric concentration of carbon dioxide and other greenhouse gases has been measured weekly since 1992 at the island of Lampedusa, in the Mediterranean sea. Lampedusa is a small island located approximately 100 km east of Tunisia, and 250 km south of Sicily. The 10-yr CO₂ data set has been analysed to quantify trends, and characterize semi-annual, annual and inter-annual variability. The data show an average trend of ×1.7 ppmv yr⁻¹; 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⁻¹, and the amplitude of the annual cycle between 7 and 11 ppmv yr⁻¹. 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.