Stratospheric water vapor feedback

We show here that stratospheric water vapor variations play an important role in the evolution of our climate. This comes from analysis of observations showing that stratospheric water vapor increases with tropospheric temperature, implying the existence of a stratospheric water vapor feedback. We estimate the strength of this feedback in a chemistry–climate model to be +0.3 W/(m²⋅K), which would be a significant contributor to the overall climate sensitivity. One-third of this feedback comes from increases in water vapor entering the stratosphere through the tropical tropopause layer, with the rest coming from increases in water vapor entering through the extratropical tropopause.Doubling carbon dioxide in our atmosphere by itself leads to a global average warming of ∼1.2 °C. However, this direct warming from carbon dioxide drives other changes, known as feedbacks, that increase the eventual warming to 2.0–4.5 °C. Thus, much of the warming predicted for the next century comes not from direct warming by carbon dioxide but from feedbacks.The strongest climate feedback is the tropospheric water vapor feedback (1, 2). The troposphere is the bottom 10–15 km of the atmosphere, and there are physical reasons to expect it to become moister as the surface warms (3)—and, indeed, both observations (4–6) and climate models (7, 8) verify this. Because water vapor is itself a greenhouse gas, tropospheric moistening more than doubles the direct warming from carbon dioxide.Stratospheric water vapor is also a greenhouse gas (9) whose interannual variations may have had important climatic consequences (10). This opens the possibility of a stratospheric water vapor feedback (11, 12) whereby a warming climate increases stratospheric water vapor, leading to additional warming. In this paper, we investigate this possibility.     

From the Cover: Dynamic model constraints on oxygen-17 depletion in atmospheric O2 after a snowball Earth

A large perturbation in atmospheric CO₂ and O₂ or bioproductivity will result in a drastic pulse of ¹⁷O change in atmospheric O₂, as seen in the Marinoan Oxygen-17 Depletion (MOSD) event in the immediate aftermath of a global deglaciation 635 Mya. The exact nature of the perturbation, however, is debated. Here we constructed a coupled, four-box, and quick-response biosphere–atmosphere model to examine both the steady state and dynamics of the MOSD event. Our model shows that the ultra-high CO₂ concentrations proposed by the “snowball’ Earth hypothesis produce a typical MOSD duration of less than 10⁶ y and a magnitude of ¹⁷O depletion reaching approximately −35‰. Both numbers are in remarkable agreement with geological constraints from South China and Svalbard. Moderate CO₂ and low O₂ concentration (e.g., 3,200 parts per million by volume and 0.01 bar, respectively) could produce distinct sulfate ¹⁷O depletion only if postglacial marine bioproductivity was impossibly low. Our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean–atmosphere exchange only if partial pressure of atmospheric O₂ is larger than 0.02 bar during the snowball period and records of weathering-derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown. In any case, a snowball Earth is a precondition for the observed MOSD event.     

Fundamental differences between Arctic and Antarctic ozone depletion

Antarctic ozone depletion is associated with enhanced chlorine from anthropogenic chlorofluorocarbons and heterogeneous chemistry under cold conditions. The deep Antarctic “hole” contrasts with the generally weaker depletions observed in the warmer Arctic. An unusually cold Arctic stratospheric season occurred in 2011, raising the question of how the Arctic ozone chemistry in that year compares with others. We show that the averaged depletions near 20 km across the cold part of each pole are deeper in Antarctica than in the Arctic for all years, although 2011 Arctic values do rival those seen in less-depleted years in Antarctica. We focus not only on averages but also on extremes, to address whether or not Arctic ozone depletion can be as extreme as that observed in the Antarctic. This information provides unique insights into the contrasts between Arctic and Antarctic ozone chemistry. We show that extreme Antarctic ozone minima fall to or below 0.1 parts per million by volume (ppmv) at 18 and 20 km (about 70 and 50 mbar) whereas the lowest Arctic ozone values are about 0.5 ppmv at these altitudes. At a higher altitude of 24 km (30-mbar level), no Arctic data below about 2 ppmv have been observed, including in 2011, in contrast to values more than an order of magnitude lower in Antarctica. The data show that the lowest ozone values are associated with temperatures below −80 °C to −85 °C depending upon altitude, and are closely associated with reduced gaseous nitric acid concentrations due to uptake and/or sedimentation in polar stratospheric cloud particles.The extensive springtime depletion of Antarctic ozone has attracted both public and scientific interest since its discovery (1) and explanation in the 1980s. The ozone hole has been linked to the coupling of human-made chlorofluorocarbons with surface chemistry on and in polar stratospheric clouds (PSCs) that form during extreme cold conditions (2). Polar stratospheric clouds are composed of nitric acid hydrates, liquid solutions of sulfuric acid, water, and nitric acid, and (under very cold conditions) water ice (e.g., ref. 3 and citations therein). Some of the key reactions are photochemical, so that the ozone hole does not form during midwinter when the polar cap is dark, but rather in late winter/spring as sunlight returns, provided that temperatures remain low. Although the same basic chemical mechanisms operate in both hemispheres, the Arctic winter stratosphere is generally warmer than the Antarctic, and it warms up earlier in the spring. These two factors taken together explain why ozone depletion in the Arctic is generally much smaller than in the Antarctic. A particularly cold Arctic stratospheric winter and spring in 2010/2011 displayed much larger ozone depletion than typical years, as highlighted by Manney et al. (4). This noteworthy geophysical event has intrigued scientists and raised several important questions: Could this be the first Arctic ozone hole? Are Arctic ozone losses ever observed to be as extreme as those in the Antarctic? Some authors have variously characterized Arctic ozone loss in 2011 as unprecedented, an echo of the Antarctic, or on the brink of an Antarctic ozone hole (e.g., refs. 4 and 5). The unusual meteorology of this year has been explored by several studies (6–8), and a rich suite of observations of stratospheric chemical composition has been presented from both ground-based and satellite methods (e.g., refs. 4, 7, and 9–11). Understanding how the ozone losses of the two polar regions compare is important not only to ensure a clear understanding of ozone depletion chemistry but also to accurately communicate the state of the science to the public.Solomon et al. (12) presented ozonesonde and total ozone column data up to 2006 from stations in the Arctic and Antarctic. Here we update and expand the comparison of ozone and related chemistry over the two polar regions. Our goal is to present the observations in a manner that readily shows similarities and differences, and provides insights into chemical processes, particularly the role of polar stratospheric cloud chemistry. The data presentation should also be useful for future studies testing the ability of numerical models to fully simulate ozone depletion. Both average changes and the range of extreme values are presented, because these each provide important and distinct tests for physical and chemical understanding (just as in, for example, climate change studies).We first examine in situ ozone observations obtained by balloonsondes at ground stations. Although limited to a few sites in each hemisphere, these are the only data that extend from the 1960s onward, before the satellite era. We next present microwave limb sounder (MLS) satellite observations (available from 2004 to present), to probe the consistency between the limited spatial sampling of the balloons from a few surface sites to the extensive coverage of the satellite and to examine how data from the MLS platform compare with the most extreme local depletions observed in situ. We will present information on the range of extreme ozone observations in individual air parcels as observed by MLS, as well as averages of the measurements over the cold polar regions. There are important limitations of such simple comparisons, and these are noted where appropriate. Stratospheric temperatures, gaseous nitric acid concentrations, and their relationship to ozone losses in the two hemispheres are also discussed to provide insights to chemical processes. We include ozonesonde data for the region near 20 km altitude (50-mbar pressure), where the largest local Arctic losses are typically found, but also present results for other pressure levels. Antarctic depletion is typically somewhat greater at lower altitudes near 15–18 km (higher pressures) but the 50-mbar level provides the best chance for the Arctic to mirror Antarctica. We also present MLS data near 18 km (70 mbar) and 24 km (30 mbar), and show that comparison of different levels aids in understanding differences in chemical processes in the two hemispheres.     

Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation

The stratospheric CO₂ oxygen isotope budget is thought to be governed primarily by the O(¹D)+CO₂ isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO₂ isotopologue ¹⁶O¹³C¹⁸O, in concert with ¹⁸O and ¹⁷O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric ¹⁶O¹³C¹⁸O, observed as proportions in the polar vortex that are higher than in any naturally derived CO₂ sample to date. We show, through photochemical experiments, that lower ¹⁶O¹³C¹⁸O proportions observed in the midlatitudes are determined primarily by the O(¹D)+CO₂ isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher ¹⁶O¹³C¹⁸O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O(¹D)+CO₂. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO₂ or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric ¹⁶O¹³C¹⁸O enrichments may impose additional isotopic constraints on biosphere–atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.