Methane and nitrous oxide in the ice core record

Abstract

Polar ice cores contain, in trapped air bubbles, an archive of the concentrations of stable atmospheric gases. Of the major non-CO 2 greenhouse gases, methane is measured quite routinely, while nitrous oxide is more challenging, with some artefacts occurring in the ice and so far limited interpretation. In the recent past, the ice cores provide the only direct measure of the changes that have occurred during the industrial period; they show that the current concentration of methane in the atmosphere is far outside the range experienced in the last 650 000 years; nitrous oxide is also elevated above its natural levels. There is controversy about whether changes in the pre-industrial Holocene are natural or anthropogenic in origin. Changes in wetland emissions are generally cited as the main cause of the large glacial-interglacial change in methane. However, changing sinks must also be considered, and the impact of possible newly described sources evaluated. Recent isotopic data appear to finally rule out any major impact of clathrate releases on methane at these time-scales. Any explanation must take into account that, at the rapid Dansgaard-Oeschger warmings of the last glacial period, methane rose by around half its glacial-interglacial range in only a few decades. The recent EPICA Dome C (Antarctica) record shows that methane tracked climate over the last 650 000 years, with lower methane concentrations in glacials than interglacials, and lower concentrations in cooler interglacials than in warmer ones. Nitrous oxide also shows DansgaardOeschger and glacial-interglacial periodicity, but the pattern is less clear.

Response of methane emissions from wetlands to the Last Glacial Maximum and an idealized Dansgaard–Oeschger climate event: insights from two models of different complexity

“…As suggested by Bock et al (2010) and discussed above, a change in boreal wetland CH 4 emissions appears to be required during some D-O events. In these regions, slow processes such as the exposure of land surface as the ice sheet retreated are clearly not capable of producing such fast variations (Wolff and Spahni, 2007). CH 4 emissions associated with permafrost destabilization need to be incorporated into paleo-modelling studies such as the one performed here.…”

Critical issues in trace gas biogeochemistry and global change

“…The nature of this feedback can be assessed directly for the Quaternary by using archives of variations in stable GHGs recovered by analysing ancient samples of air closed-off from the atmosphere in ice (Spahni et al 2005;Wolff & Spahni 2007). Remarkable new records of N 2 O and CH 4 from polar cores extend back over the past 650 000 years and illustrate unequivocally that the current concentrations of both trace gases are unprecedented in recent Earth history.…”

“…400-500 ppbv), while the modern N 2 O concentration of 319 ppbv (Jiang et al 2007) exceeds a typical interglacial value by approximately 100 ppbv. Analyses of the covariation of GHGs and palaeoclimate from the EPICA Dome C record shows methane variations have tracked glacial-interglacial climate change for the last eight glacial cycles (Spahni et al 2005;Wolff & Spahni 2007). The response of N 2 O is more complex but exhibits a periodicity tracking millennialscale Dansgaard-Oeschger warming events (Flückiger et al 2004;Spahni et al 2005;Wolff & Spahni 2007).…”

Response of terrestrial N2O and NOxemissions to abrupt climate change

“…The ice core record of atmospheric N 2 O concentrations shows considerable temporal variability that closely parallels polar temperature records [2], [3], [4], [5], indicating that nitrogen trace gas emissions may be very sensitive to climate change. Variability in atmospheric N 2 O and NO x concentrations is linked to nitrogen cycling in the oceans, e.g., [6] and the terrestrial biosphere [1].…”

“…Over the last decade, a number of studies have attempted to quantify N 2 O emissions both for the present-day and the past based on measurement-based inventories of N 2 O concentrations in ice cores [2], [3], [4], [5]. In addition, several research groups have attempted to model regional and global source emissions of N 2 O and NO x , e.g [9], [10], [11].…”