Abstract
Abstract. In the 1990s the rates of increase of greenhouse gas concentrations, most notably of methane, were observed to change, for reasons that have yet to be fully determined. This period included the eruption of Mt. Pinatubo and an El Niño warm event, both of which affect biogeochemical processes, by changes in temperature, precipitation and radiation. We examine the impact of these changes in climate on global isoprene emissions and the effect these climate dependent emissions have on the hydroxy radical, OH, the dominant sink for methane. We model a reduction of isoprene emissions in the early 1990s, with a maximum decrease of 40 Tg(C)/yr in late 1992 and early 1993, a change of 9%. This reduction is caused by the cooler, drier conditions following the eruption of Mt. Pinatubo. Isoprene emissions are reduced both directly, by changes in temperature and a soil moisture dependent suppression factor, and indirectly, through reductions in the total biomass. The reduction in isoprene emissions causes increases of tropospheric OH which lead to an increased sink for methane of up to 5 Tg(CH4)/year, comparable to estimated source changes over the time period studied. There remain many uncertainties in the emission and oxidation of isoprene which may affect the exact size of this effect, but its magnitude is large enough that it should remain important.
Highlights
In the early 1990s the rate of increase in the concentrations of several greenhouse gases, including methane and carbon dioxide, dropped significantly (Dlugokencky et al, 2003; Denman et al, 2007)
Barkley et al (2008) infer isoprene emissions from South America from HCHO column measurements later in the decade, and see increases in emissions in 1997 in the western Amazon related to an El Nino Southern Oscillation (ENSO) warm phase
The impact of the El Nino event can be seen in the western Amazon where warmer conditions produce increases in isoprene emissions
Summary
In the early 1990s the rate of increase in the concentrations of several greenhouse gases, including methane and carbon dioxide, dropped significantly (Dlugokencky et al, 2003; Denman et al, 2007) These changes occurred close to the eruption of Mt. Pinatubo (June 1991) and a strong warm phase of the El Nino Southern Oscillation (ENSO) (1992– 1993). Some processes are ignored, such as changes in methane emissions, but this simplified modelling framework allows the effect of other processes, like climate related biogenic emissions, to be considered We believe that this is an important step in developing process understanding and towards developing more sophisticated models
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