Jeffery B. Greenblatt

Chemical depletion of Arctic ozone in winter 1999/2000

During Arctic winters with a cold, stable stratospheric circulation, reactions on the surface of polar stratospheric clouds (PSCs) lead to elevated abundances of chlorine monoxide (ClO) that, in the presence of sunlight, destroy ozone. Here we show that PSCs were more widespread during the 1999/2000 Arctic winter than for any other Arctic winter in the past two decades. We have used three fundamentally different approaches to derive the degree of chemical ozone loss from ozonesonde, balloon, aircraft, and satellite instruments. We show that the ozone losses derived from these different instruments and approaches agree very well, resulting in a high level of confidence in the results. Chemical processes led to a 70% reduction of ozone for a region ∼1 km thick of the lower stratosphere, the largest degree of local loss ever reported for the Arctic. The Match analysis of ozonesonde data shows that the accumulated chemical loss of ozone inside the Arctic vortex totaled 117 ± 14 Dobson units (DU) by the end of winter. This loss, combined with dynamical redistribution of air parcels, resulted in a 88 ± 13 DU reduction in total column ozone compared to the amount that would have been present in the absence of any chemical loss. The chemical loss of ozone throughout the winter was nearly balanced by dynamical resupply of ozone to the vortex, resulting in a relatively constant value of total ozone of 340 ± 50 DU between early January and late March. This observation of nearly constant total ozone in the Arctic vortex is in contrast to the increase of total column ozone between January and March that is observed during most years.

Observational evidence for the role of denitrification in Arctic stratospheric ozone loss

Severe and extensive denitrification, chlorine activation, and photochemical ozone loss were observed throughout the lower stratosphere in the 1999–2000 Arctic vortex. A large number of air parcels sampled between late February and mid-March, 2000, were photochemically intercomparable for chemical O3 loss rates. In these air parcels, the temporal evolution of the correlations of O3 with the NOy remaining after denitrification provides strong evidence for the role of NOy in moderating O3 destruction. In 71%-denitrified air parcels, a chemical O3 destruction rate of 63 ppbv/day was calculated, while in 43%-denitrified air parcels the destruction rate was only 43 ppbv/day. These observational results show that representative denitrification models will be required for accurate prediction of future Arctic O3 changes.

“Wedges”: Early mitigation with familiar technology

Publisher Summary This chapter focuses on the Stabilization Triangle with respect to global carbon management. The Stabilization Triangle pays attention to the choice between two paths for the next fifty years: a path consistent with stabilization at less than double the pre-industrial CO2 concentration (500 ppm), and a path that is likely to lead to tripling of that concentration (850 ppm). If the world is willing to accept a tripling of the pre-industrial atmospheric CO2 concentration, significant carbon mitigation can be delayed for most part of the next fifty years. If the world is to be put on a path to avoid a doubling, however, a monumental mitigation effort needs to be initiated now. To convey the magnitude of the effort, the chapter introduces the “wedge” as the unit of mitigation. A wedge is an activity that would create 1 GtC/y of carbon emission reductions in 2054, relative to a world unconcerned about global carbon emissions. To pursue 500 ppm stabilization, the task for the next fifty years is to achieve about seven wedges by avoiding about 175 billion tons of carbon emissions.