Methane and nitrous oxide from Iberian inland waters: novel overall equations and a preliminary assessment of emissions

Estimations of gas emissions that impinge on global warming are growing worldwide as concern for this problem widens. Most are devoted to anthropogenic sources, but there is an increasing number dealing with natural sources. We offer here a preliminary assessment of methane and nitrous oxide emissions from Iberian freshwaters, mostly ascertained from reservoir data, which is probably the ecosystem type encompassing the higher fraction of inland aquatic cover in the Iberian Peninsula. Novel linear equations are produced using literature data and relating ecosystem area and annual emissions of CH4 and N2O at the ecosystem level. They enable us to estimate annual Iberian emissions of those gases which may be considered as a high limit because of many still unresolved uncertainties. Such equations could also be helpful to make estimations in other territories worldwide. Annual methane emissions by Iberian reservoirs would attain 19.45 Gg CH4/y as a high extreme, that of nitrous oxide accounting for 0.43 Gg N2O/y. As a result, Iberian reservoirs emit 541 Gg [CO2-equivalent] per year of both gases together. The percentage of their inland water emissions is lower than 1 % of joint Portugal and Spain overall emissions in 2015. Interestingly, these estimations of methane and nitrous oxide emissions from freshwaters represent 71 % and 19 %, respectively, of non-anthropogenic emissions from Iberian Peninsula. The obviously interesting topic of ascertaining CO2 emissions by Iberian freshwaters is impaired by the fact of lacking data on the surface area of streams (which might be the main source on account of their more frequent heterotrophy) for the whole territory. Clearly, simple databases on Iberian inland waters (streams, wetlands, natural and man-made lakes, ponds...) including surface area, volume, depth and shape must be compiled to enable a more accurate description of their future changes, partly arising from global change. This morphometric database would also be very helpful to improve CH4 and N2O estimations. Non-anthropogenic emissions from the Iberian Peninsula appear to be biased by IPCC estimation procedures, which are employed uncritically by Spanish and Portuguese governments. There is an urgent need to improve them regionally if the share of inland waters in gas emissions is to be accurately assessed.