“Warming of climate system is unequivocal, and since the 1950s, many of the observed changes are unprecendeted over dedades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased” [IPCC, 2013].

The continuous, consistent, and high quality long-term monitoring of the atmospheric composition is indispensable to understand the physical and chemical processes that control global atmosphere.

Although scientific consensus recognises that the main driver of climate change is being the increase in the amospheric concentration of the well-mixed greenhouse gases (GHGs), with our current knowledge it is hard to exactly predict their impact or give precise recommendations about the reductions of GHGs emissions needed to limit the current and future global warming. Identifying and quantifying the global GHGs sources/sinks and an improved understanding of the complex climate feedback processes are, thus, major challenges of current climate research. After carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are currently the most important well-mixed GHGs. Although they are much less abundant than CO in the atmosphere, their warming potential is significantly larger: CH4 and N2O are about 20 and 30 times, respectively, more efficient than CO2 to trap outgoint long wave radiation, on a 50-yr timescale. Their atmospheric concentrations have increased to levels unprecendented in at least the last 800.000 years, and exceeded the pre-industrial levels (1750) by about 150% and 20% for CH4 and N2O, respectively.

It is well recognized that the imbalance between their sources and sinks has unquestionably increased during the last centuries. However, the exact location, intensity and nature of CH4 and N2O sources and sinks are not as well uncerstood as those for CO2. Since the late 1970s, surface in-situ measurements of these GHGs gases are systematically taken within the GAW-WMO programme (Global Atmospheric Watch-World Meteorological Organisation, www.wmo.int). More recently, ground-based remote sensing FTIR (Fourier Transform Infrared Spectrometer) experiments also routinely provided high-quality CH4 and N2O concentrations in the framework of the international networks NDACC (Network for Detection of Atmospheric Composition Change, www.adc.ucar.edu/irwg) and TCCON (Total Carbon Column Observing Network, www.tccon.caltech.edu). However, both surface in-situ and ground-based remote sensing measurements are only representative for local and regional scales. In this context, space-based remote sensing instruments have an outstanding importance, since their global coverage allows for a better monitoring of the GHGs sources/sinks and their heterogenous distributions as well as for a better understanding of the atmospheric processes affecting their flux variations.

IASI (Infrared Atmospheric Sounding Interferometer) onboard the MetOp satellites has special relevance, among other space-based sensors, since it successfully combines the meteorology requirements to weather forecasting (high spatial coverage and a relatively good temporal resolution) and the atmospheric chemistry needs (high spectral resolution thereby allowing for trace gases retrievals), with a long-term data availability. Its mission is guaranteed until 2022 through the meteorological satellites MetOp series (MetOp-A launched in Oct-2006; MetOp-B launched in Sep-2012; and MetOp-C expected launched in Oct-2018). MetOp is the space component of the EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites, www.eumetsat.int) Polar System (EPS) programme. Furthermore, the IASI mission will be continued by the new generation of IASI-NG (New Generation), with improved spectral resolution and radiometric performance, to be flown on three successive Metop-Second Generation (SG) satellites of the EPS-SG,extending the IASI data record by two decades in the 2021-2040 timeframe.

However, for scientific purposes, it is essential that the accuracy and long-term consistency of these observational space-based data sets is precisely known. Otherwise, their interpretation and/or inclusion in chemistry-climate models might lead to misleading conclusions.