Long-term variation of atmospheric methyl iodide and its link to global environmental change

[1] It has been suggested that the emissions of volatile organic compounds (VOCs) from the ocean could be affected by global warming, with feedback effects on the climate. In order to detect changes in their emissions as a response to global environmental change, long-term observations are required. Here we report for the first time long-term variations of atmospheric methyl iodide (CH₃I), the most abundant iodine-containing compound predominantly emitted from the ocean. We monitored its concentration periodically at five remote sites covering 82.5°N–40.4°S and over the western and northern Pacific Ocean from the late 1990s to 2011. At most observation sites, CH₃I increased from 2003/2004 to 2009/2010 by several tens of per cent, with a decreasing trend before 2003. The inter-annual variation pattern is well approximated by a sine curve with a period of 11 years and showed a good correlation with the Pacific Decadal Oscillation (PDO), suggesting that CH₃I emissions are affected by global-scale, sea surface temperature (SST)-related, decadal anomalies. The influence of natural oscillations or environmental change on trace gas emissions from the ocean may be greater than has been thought previously, and these emissions may thus be sensitive to future climate change.