Abstract
Abstract. Trends of column-averaged methane for the time period (1996, Sep 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] of 9.0 [3.2, 14.7] ppb yr−1 for Zugspitze (95% confidence interval), an insignificant 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 sites Zugspitze and Garmisch within the uncertainties indicates a good station-to-station consistency as a basis for future trend analyses by the ground-based mid-IR FTIR network on the global scale. Furthermore, the Zugspitze FTIR trend for the time interval [Jul 2006, Jun 2009] is found to agree with the trend derived from SCIAMACHY (WFM-DOAS v2.0.2) data within the 95% confidence intervals. In case a 1000-km pixel selection radius around the Zugspitze is used, the confidence interval is narrower for the FTIR 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-km half-width latitudinal band), the SCIAMACHY trend significance interval is narrower (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 of 8.0 ± 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 renewed methane increase meanwhile has been persisting for >5 years [mid 2006, Sep 2011]. This is either the longest and largest positive trend anomaly since the beginning of systematic observations more than 25 years ago or the onset of a new period of strongly increasing CH4 levels in the atmosphere. Several scenarios have been developed to explain the persistent increase observed, mainly invoking an increase in emissions from natural wetlands, an increase in fossil fuel-related emissions or a decrease in OH concentrations. However, more work is needed to fully attribute this increase to a particular source or sink.
Highlights
The molecular symmetry of methane (CH4) allows for highly active vibration-rotation excitation by infrared absorption
The FTIR trends retrieved for the neighboring sites of Zugspitze and Garmisch (Table 2) are in very good agreement: For the [mid 2006–2008] period, a trend of 6.6 ppb yr−1 with a 95 % confidence interval of [3.5, 9.8] ppb yr−1 is found above Zugspitze, which agrees with 5.1 [2.0, 8.3] ppb yr−1 retrieved from Garmisch measurements for the same period
A positive anomaly of CH4 emissions from natural wetlands in 2007–2008 is suggested to be the cause by several authors (Duglokencky et al, 2009; Bloom et al, 2010; Bousquet et al, 2011), with the contribution from boreal regions in 2007 being estimated to amount about 25 % (Bousquet et al, 2011)
Summary
The molecular symmetry of methane (CH4) allows for highly active vibration-rotation excitation by infrared absorption. After a period of near-zero growth at the beginning of this century (Dlugokencky et al, 2003; Bousquet et al, 2006), the growth rate of atmospheric methane started to increase strongly again after 2006 (Rigby et al, 2008; Dlugokencky et al, 2009). Columnar methane retrieved from ground-based Fourier-transform-infrared (FTIR) spectrometry in the midinfrared (mid-IR) provides a column view of atmospheric methane at a high precision close to that attainable by surface networks (≈0.3 %, Sussmann et al, 2011). Such ground-based observations are very useful to document atmospheric methane changes.
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