Identifying the mechanisms driving interannual fluctuations of atmospheric carbon dioxide is necessary for predicting future CO2 concentrations and climate change . A possible clue comes from a well-established positive correlation between atmospheric CO2 growth rates and the El Ni¿o-Southern Oscillation phenomenon . Most tropical droughts are also linked to El Ni¿o , suggesting carbon losses from drought as a major cause for interannual CO2 variations. A lag correlation between 7-month running means (see Figure 1) of monthly atmospheric CO2 concentrations and Ni¿o 3 sea surface temperatures (available at httpc//www.cpc.noaa.gov/data/indices/) for the period 1979-2003 peaks at a lag of four months with a correlation of 0.49, and a significance level above 99.9% (assuming 42 independent measurements).