DEAR SIR: A very recent scientific publication by Robert W Howarth of Cornell University (Biogeosciences, 16, 3033–3046,) has commented on the rapid rise in atmospheric methane emissions since 2008.

Since then, atmospheric methane has risen rapidly, but it has become more depleted in 13C (the heavier isotope of non-radioactive carbon) in comparison with its greater enrichments in the late 20th century.