Renewed methane increase for five years (2007–2011) observed by solar FTIR spectrometry

Abstract

Abstract. Trends of column-averaged methane for the time period [1996, September 2011] are derived from the mid-infrared (mid-IR) solar FTIR time series at the Zugspitze (47.42° N, 10.98° E, 2964 m a.s.l.) and Garmisch (47.48° N, 11.06° E, 743 m a.s.l.). Trend analysis comprises a fit to the de-seasonalized time series along with bootstrap resampling for quantifying trend uncertainties. We find a positive trend during [1996, 1998] (9.0 [3.2, 14.7] ppb yr−1, Zugspitze, 95 % confidence interval), a non-significant growth during [1999, mid 2006] (0.8 [−0.1, 1.7] ppb yr−1, Zugspitze), and a significant renewed increase during [mid 2006, September 2011] of 5.1 [4.2, 6.0] ppb yr−1 for Garmisch, which is in agreement with 4.8 [3.8, 5.9] ppb yr−1 for Zugspitze. The agreement of methane trends at the two closely neighboring FTIR sites with strongly differing levels of integrated water vapor (min/max = 0.2 mm/12.7 mm for Zugspitze, 1.9 mm/34.9 mm for Garmisch) proves that potentially significant water-vapor-methane interference errors do not affect the trend results, if the updated mid-IR retrieval strategy MIR-GBM v1.0 is used. Furthermore, agreement of the trend of 6.6 ppb yr−1 derived from SCIAMACHY (WFMD v2.0) data for the time period [mid 2006, mid 2009] is found within the 95 % confidence interval of the ground-based FTIR result. While earlier studies using surface network data revealed changes of 8.0±0.6 ppb in 2007 and 6.4±0.6 ppb in 2008 (update from Dlugokencky et al., 2009), our updated result proves that meanwhile, the renewed methane increase has been persisting for >5 yr [mid 2006, September 2011]. This is either the longest and largest positive trend anomaly since >25 yr when systematic observations began or the onset of a new period of strongly increasing CH4 levels in the atmosphere. The 2007–2008 part of the anomaly was previously attributed to increased natural wetland emissions. For the full period from 2007 to 2011, our analysis of ECMWF ERA-INTERIM precipitations and 2-m temperatures shows that precipitations above tropical wetland areas increased in 2007–2008, decreased in 2009, and have been increasing again since 2010, while tropical land temperatures increased only slightly. As recent estimates of anthropogenic emissions are not yet available, it is not possible to finally conclude that the 2009–2011 period of methane increase was dominated by natural wetland emissions, although they probably play a significant role.