John S. Daniel

Nitrous Oxide (N 2 O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century

Unwelcome Dominance Stratospheric ozone is depleted by many different chemicals; most prominently, chlorofluorocarbons (CFCs) responsible for causing the Antarctic ozone hole. Nitrous oxide is also an ozone-depleting substance that has natural sources in addition to anthropogenic ones. Moreover, unlike CFCs, its use and emission are not regulated by the Montreal Protocol, which has helped to reverse the rate of growth of the ozone hole. Surprisingly, Ravishankara et al. (p. 123, published online 27 August; see the Perspective by Wuebbles) now show that nitrous oxide is the single greatest ozone-depleting substance that, if its emissions are not controlled, is expected to remain the dominant ozone-depleting substance throughout the 21st century. Reducing nitrous oxide emissions would thus enhance the rate of recovery of the ozone hole and reduce the anthropogenic forcing of climate. Nitrous oxide causes more stratospheric ozone destruction than any other ozone-depleting substance. By comparing the ozone depletion potential–weighted anthropogenic emissions of N2O with those of other ozone-depleting substances, we show that N2O emission currently is the single most important ozone-depleting emission and is expected to remain the largest throughout the 21st century. N2O is unregulated by the Montreal Protocol. Limiting future N2O emissions would enhance the recovery of the ozone layer from its depleted state and would also reduce the anthropogenic forcing of the climate system, representing a win-win for both ozone and climate.

Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming

Dropping a Notch Between 2000 and 2001, the concentration of water vapor in the stratosphere dropped by about 10%. Water vapor is an important greenhouse gas, so did the decrease affect climate and slow global warming? Solomon et al. (p. 1219, published online 28 January) used a combination of data and models to show that lower stratospheric water vapor probably has contributed to the flattening of global average temperatures since 2000, by acting to slow the rate of warming by about 25%. Furthermore, the amount of water vapor in the stratosphere probably increased between 1980 and 2000, a period of more rapid warming, suggesting how important the concentration of stratospheric water vapor might be to climate. Decreases in stratospheric water vapor after the year 2000 slowed the rate of increase in global surface temperature. Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000–2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.

The Persistently Variable “Background” Stratospheric Aerosol Layer and Global Climate Change

An increase in the amount of aerosols in the stratosphere during the past decade has decreased the rate of global warming. Recent measurements demonstrate that the “background” stratospheric aerosol layer is persistently variable rather than constant, even in the absence of major volcanic eruptions. Several independent data sets show that stratospheric aerosols have increased in abundance since 2000. Near-global satellite aerosol data imply a negative radiative forcing due to stratospheric aerosol changes over this period of about –0.1 watt per square meter, reducing the recent global warming that would otherwise have occurred. Observations from earlier periods are limited but suggest an additional negative radiative forcing of about –0.1 watt per square meter from 1960 to 1990. Climate model projections neglecting these changes would continue to overestimate the radiative forcing and global warming in coming decades if these aerosols remain present at current values or increase.

The Mixed-Phase Arctic Cloud Experiment

The Mixed-Phase Arctic Cloud Experiment (M-PACE) was conducted from 27 September through 22 October 2004 over the Department of Energys Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) on the North Slope of Alaska. The primary objectives were to collect a dataset suitable to study interactions between microphysics, dynamics, and radiative transfer in mixed-phase Arctic clouds, and to develop/evaluate cloud property retrievals from surface-and satellite-based remote sensing instruments. Observations taken during the 1977/98 Surface Heat and Energy Budget of the Arctic (SHEBA) experiment revealed that Arctic clouds frequently consist of one (or more) liquid layers precipitating ice. M-PACE sought to investigate the physical processes of these clouds by utilizing two aircraft (an in situ aircraft to characterize the microphysical properties of the clouds and a remote sensing aircraft to constraint the upwelling radiation) over the ACRF site on the North Slope of Alaska. The measureme...

The importance of the Montreal Protocol in protecting climate

The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer is a landmark agreement that has successfully reduced the global production, consumption, and emissions of ozone-depleting substances (ODSs). ODSs are also greenhouse gases that contribute to the radiative forcing of climate change. Using historical ODSs emissions and scenarios of potential emissions, we show that the ODS contribution to radiative forcing most likely would have been much larger if the ODS link to stratospheric ozone depletion had not been recognized in 1974 and followed by a series of regulations. The climate protection already achieved by the Montreal Protocol alone is far larger than the reduction target of the first commitment period of the Kyoto Protocol. Additional climate benefits that are significant compared with the Kyoto Protocol reduction target could be achieved by actions under the Montreal Protocol, by managing the emissions of substitute fluorocarbon gases and/or implementing alternative gases with lower global warming potentials.

The large contribution of projected HFC emissions to future climate forcing

The consumption and emissions of hydrofluorocarbons (HFCs) are projected to increase substantially in the coming decades in response to regulation of ozone depleting gases under the Montreal Protocol. The projected increases result primarily from sustained growth in demand for refrigeration, air-conditioning (AC) and insulating foam products in developing countries assuming no new regulation of HFC consumption or emissions. New HFC scenarios are presented based on current hydrochlorofluorocarbon (HCFC) consumption in leading applications, patterns of replacements of HCFCs by HFCs in developed countries, and gross domestic product (GDP) growth. Global HFC emissions significantly exceed previous estimates after 2025 with developing country emissions as much as 800% greater than in developed countries in 2050. Global HFC emissions in 2050 are equivalent to 9–19% (CO2-eq. basis) of projected global CO2 emissions in business-as-usual scenarios and contribute a radiative forcing equivalent to that from 6–13 years of CO2 emissions near 2050. This percentage increases to 28–45% compared with projected CO2 emissions in a 450-ppm CO2 stabilization scenario. In a hypothetical scenario based on a global cap followed by 4% annual reductions in consumption, HFC radiative forcing is shown to peak and begin to decline before 2050.

On the climate forcing of carbon monoxide

Carbon monoxide plays a primary role in governing OH abundances in the troposphere. It is likely that through this chemical interaction, CO plays an important role in climate forcing by affecting CH 4 concentrations. We use a photochemical box model to estimate the indirect global warming potential (GWP) of tropospheric carbon monoxide resulting from its effect on methane abundances. We also consider indirect GWPs of CO due to carbon dioxide production and possible CO GWP ranges due to ozone production to estimate a total CO indirect GWP range over various time horizons. We estimate that for current emission levels, the short-term (<15 years) cumulative radiative forcing due to the direct anthropogenic emission of CO may be larger than the cumulative forcing due to anthropogenically emitted N 2 O.

On the role of nitrogen dioxide in the absorption of solar radiation

Direct measurements of the absorption of downwelling visible radiation by nitrogen dioxide are presented. The data show that this gas can contribute significantly to local radiative forcing under certain conditions. The observed enhancements in nitrogen dioxide absorption are likely to be due both to pollution and to production by lightning in convective clouds. Case studies of several days of observations in Colorado reveal peak absorption of downwelling radiation by NO2 of up to 5–12%, corresponding to an estimated local radiative forcing that is likely to be in the range of 5–30 W/m2. The amount of local forcing associated with thunderstorm activity depends strongly upon the cloud optical depth and on where the NO2 resides within the clouds. These case studies suggest that NO2 can play a significant role in the absorption of radiation (including but not limited to anomalous cloud absorption) either under polluted conditions or when electrically active storms are considered.

On the evaluation of halocarbon radiative forcing and global warming potentials

Net global warming potentials and instantaneous radiative forcing values that include the cooling from halocarbon-induced ozone destruction have been calculated for 14 of the most significant halocarbons. These calculations were performed by incorporating knowledge of direct global warming potentials with an evaluation of the relationship between tropospheric cooling from stratospheric ozone loss and tropospheric halocarbon mixing ratios. The indirect cooling effect is strongly dependent upon the effectiveness of each halocarbon for ozone destruction. Strong net cooling is ascribed to additions of bromocarbon gases, while methyl chloroform and carbon tetrachloride are more nearly climate-neutral, and the CFCs and HCFCs display strong net warming. Consideration of indirect cooling also has important implications for the expected future net halocarbon forcing of the climate system: in the next 20 years, halocarbon radiative forcing is not predicted to decrease as mixing ratios of strongly ozone-depleting gases decline because of faster decreases in radiative cooling than in radiative warming. Furthermore, continuing production of HFCs as substitutes for CFCs could result in sharply increasing halocarbon radiative heating in the latter part of the 21st century because of the increasing atmospheric burden of these compounds.

A Focus on Mixed-Phase Clouds: The Status of Ground-Based Observational Methods

The phase composition and microphysical structure of clouds define the manner in which they modulate atmospheric radiation and contribute to the hydrologic cycle. Issues regarding cloud phase partitioning and transformation come to bear directly in mixed-phase clouds, and have been difficult to address within current modeling frameworks. Ground-based, remote-sensing observations of mixed-phase clouds can contribute a significant body of knowledge with which to better understand, and thereby more accurately model, clouds and their phase-defining processes. Utilizing example observations from the Mixed-Phase Arctic Cloud Experiment (M-PACE), which occurred at the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Programs Climate Research Facility in Barrow, Alaska, during autumn 2004, we review the current status of ground-based observation and retrieval methods used in characterizing the macrophysical, microphysical, radiative, and dynamical properties of stratiform mixed-phase clouds. In...