1. National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan
2. Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-8506, Japan
3. Tsukuba Botanical Garden, National Science Museum, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan

Correspondence to: Yoko Yokouchi¹ Correspondence and requests for materials should be addressed to Y.Y. (e-mail: Email: yokouchi@nies.go.jp).

Methyl chloride is the largest natural source of ozone-depleting chlorine compounds, and accounts for about 15 per cent of the present atmospheric chlorine content¹. This contribution was likely to have been relatively greater in pre-industrial times², when additional anthropogenic sources—such as chlorofluorocarbons—were absent. Although it has been shown that there are large emissions of methyl chloride from coastal lands in the tropics^{3, 4}, there remains a substantial shortfall in the overall methyl chloride budget. Here we present observations of large emissions of methyl chloride from some common tropical plants (certain types of ferns and Dipterocarpaceae), ranging from 0.1 to 3.7 g per gram of dry leaf per hour. On the basis of these preliminary measurements, the methyl chloride flux from Dipterocarpaceae in southeast Asia alone is estimated at 0.91 Tg yr^-1, which could explain a large portion of missing methyl chloride sources. With continuing tropical deforestation, natural sources of chlorine compounds may accordingly decrease in the future. Conversely, the abundance of massive ferns in the Carboniferous period⁵ may have created an atmosphere rich in methyl chloride.