A 156 kyr smoothed history of the atmospheric greenhouse gases CO₂, CH4, and N₂O and their radiative forcing

Köhler, Peter; Nehrbass-Ahles, Christoph; Schmitt, Jochen; Stocker, Thomas F.; Fischer, Hubertus (2017). A 156 kyr smoothed history of the atmospheric greenhouse gases CO₂, CH4, and N₂O and their radiative forcing. Earth System Science Data, 9(1), pp. 363-387. Copernicus Publications 10.5194/essd-9-363-2017

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Continuous records of the atmospheric greenhouse gases (GHGs) CO₂, CH₄, and N₂O are necessary input data for transient climate simulations, and their associated radiative forcing represents important components in analyses of climate sensitivity and feedbacks. Since the available data from ice cores are discontinuous and partly ambiguous, a well-documented decision process during data compilation followed by some interpolating post-processing is necessary to obtain those desired time series. Here, we document our best possible data compilation of published ice core records and recent measurements on firn air and atmospheric samples spanning the interval from the penultimate glacial maximum ( ~156 kyr BP) to the beginning of the year 2016 CE. We use the most recent age scales for the ice core data and apply a smoothing spline method to translate the discrete and irregularly spaced data points into continuous time series. These splines are then used to compute the radiative forcing for each GHG using well-established, simple formulations. We compile only a Southern Hemisphere record of CH₄ and discuss how much larger a Northern Hemisphere or global CH₄ record might have been due to its interpolar difference. The uncertainties of the individual data points are considered in the spline procedure. Based on the given data resolution, time-dependent cutoff periods of the spline, defining the degree of smoothing, are prescribed, ranging from 5000 years for the less resolved older parts of the records to 4 years for the densely sampled recent years. The computed splines seamlessly describe the GHG evolution on orbital and millennial timescales for glacial and glacial–interglacial variations and on centennial and decadal timescales for anthropogenic times.

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