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Trends of column-averaged methane for the time period (1996, Sep 2011) arederived from the mid-infrared (mid-IR) solar FTIR time series at theZugspitze (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 thede-seasonalized time series along with bootstrap resampling for quantifyingtrend uncertainties. We find a positive trend during [1996, 1998] of 9.0[3.2, 14.7] ppb yr−1 for Zugspitze (95% confidence interval), aninsignificant growth during [1999, mid 2006] of 0.8 [−0.1, 1.7] ppb yr−1(Zugspitze), and a significant renewed increase during [mid 2006, Sep 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 sitesZugspitze and Garmisch within the uncertainties indicates a goodstation-to-station consistency as a basis for future trend analyses by theground-based mid-IR FTIR network on the global scale. Furthermore, theZugspitze FTIR trend for the time interval [Jul 2006, Jun 2009] is found toagree with the trend derived from SCIAMACHY (WFM-DOAS v2.0.2) data withinthe 95% confidence intervals. In case a 1000-km pixel selection radiusaround the Zugspitze is used, the confidence interval is narrower for theFTIR trend (6.9 [4.2, 9.5] ppb yr−1) compared to SCIAMACHY (7.1 [5.1,8.6] ppb yr−1). If, however, a loosened pixel selection is used (≈1000-kmhalf-width latitudinal band), the SCIAMACHY trend significance interval isnarrower (6.8 [5.1, 8.6] ppb yr−1) compared to Zugspitze FTIR (5.7 [3.0,8.3] ppb yr−1). While earlier studies using surface network data revealed changes of8.0 ± 0.6 ppb in 2007, 6.4 ± 0.6 ppb in 2008, and 4.7 ± 0.6 ppb in 2009(Dlugokencky et al., 2011), our updated result proves that the renewedmethane increase meanwhile has been persisting for >5 years [mid 2006, Sep 2011]. This is either the longest and largest positive trend anomaly sincethe beginning of systematic observations more than 25 years ago or the onsetof a new period of strongly increasing CH4 levels in the atmosphere.Several scenarios have been developed to explain the persistent increaseobserved, mainly invoking an increase in emissions from natural wetlands, anincrease in fossil fuel-related emissions or a decrease in OHconcentrations. However, more work is needed to fully attribute thisincrease to a particular source or sink

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