Renewed methane increase for five years (2007–2011) observed by solar FTIR spectrometry 1Karlsruhe Institute of Technology, IMK-IFU, Garmisch-Partenkirchen, Germany
04 Jun 2012
2Laboratoire des Sciences du Climat et de l'Environnement, IPSL-LSCE, CEA-CNRS-UVSQ, UMR8212 91191, France
3Université de Versailles Saint Quentin en Yvelines, France
Received: 11 Oct 2011 – Published in Atmos. Chem. Phys. Discuss.: 16 Nov 2011 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.
Revised: 16 May 2012 – Accepted: 21 May 2012 – Published: 04 Jun 2012
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.
Citation: Sussmann, R., Forster, F., Rettinger, M., and Bousquet, P.: Renewed methane increase for five years (2007–2011) observed by solar FTIR spectrometry, Atmos. Chem. Phys., 12, 4885-4891, doi:10.5194/acp-12-4885-2012, 2012.