Keith P. Shine

On the downward control of extratropical diabatic circulations by eddy-induced mean zonal forces

Abstract The situation considered is that of a zonally symmetric model of the middle atmosphere subject to a given quasi-steady zonal force F, conceived to be the result of irreversible angular momentum transfer due to the upward propagation and breaking of Rossby and gravity waves together with any other dissipative eddy effects that may be relevant. The models diabatic heating is assumed to have the qualitative character of a relaxation toward some radiatively determined temperature field. To the extent that the force F may be regarded as given, and the extratropical angular momentum distribution is realistic, the extratropical diabatic mass flow across a given isentropic surface may be regarded as controlled exclusively by the F distribution above that surface (implying control by the eddy dissipation above that surface and not, for instance, by the frequency of tropopause folding below). This “downward control” principle expresses a critical part of the dynamical chain of cause and effect governin...

The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary radiation budget

Carbonaceous soot within the troposphere can significantly modify the clear-sky radiative forcing. Using an extension to a simple radiation calculation and two model-derived sulfate aerosol data sets, the impact of an assumed soot/sulfate mass ratio of between 0.05 and 0.1 is examined. Fossil fuel derived soot causes a positive global-mean radiative forcing which for one data set ranges from +0.03 to +0.24Wm−2; the lower estimate is for an external mixture with a soot/sulfate ratio of 0.05 and the upper estimate is for an internal mixture and a soot/sulfate ratio of 0.10. These values compare to a global-mean radiative forcing of −0.34Wm−2 due to sulfate aerosol. Soot also significantly reduces the interhemispherical difference in the radiative forcing due to sulfate aerosol. The nature and amount of soot must be well established if the climatic role of tropospheric aerosols is to be fully understood.

New estimates of radiative forcing due to well mixed greenhouse gases

We have performed new calculations of the radiative forcing due to changes in the concentrations of the most important well mixed greenhouse gases (WMGG) since pre-industrial time. Three radiative transfer models are used. The radiative forcing due to CO2, including shortwave absorption, is 15% lower than the previous IPCC estimate. The radiative forcing due to all the WMGG is calculated to 2.25 Wm−2, which we estimate to be accurate to within about 5%. The importance of the CFCs is increased by about 20% relative to the total effect of all WMGG compared to previous estimates. We present updates to simple forcing-concentration relationships previously used by IPCC.

Stratospheric water vapour changes as a possible contributor to observed stratospheric cooling

The observed cooling of the lower stratosphere over the last two decades has been attributed, in previous studies, largely to a combination of stratospheric ozone loss and carbon dioxide increase, and as such it is meant to provide one of the best pieces of evidence for an anthropogenic cause to climate change. This study shows how increases in stratospheric water vapour, inferred from available observations, may be capable of causing as much of the observed cooling as ozone loss does; as the reasons for the stratospheric water vapour increase are neither fully understood nor well characterized, it shows that it remains uncertain whether the cooling of the lower stratosphere can yet be fully attributable to human influences. In addition, the changes in stratospheric water vapour may have contributed, since 1980, a radiative forcing which enhances that due to carbon dioxide alone by 40%.

Radiative forcing and temperature trends from stratospheric ozone changes

Detailed shortwave and longwave radiative transfer models are used to calculate the radiative forcing and temperature trends due to stratospheric ozone depletion. These were calculated using the fixed dynamical heating approximation to adjust the stratospheric temperatures. Recent estimates of stratospheric ozone loss between 1979 and 1991 (from solar backscattered ultraviolet (SBUV) and stratospheric aerosol and gas experiment (SAGE) instruments) and updated radiative transfer schemes are used to obtain improved estimates of the radiative forcings. An annually and globally averaged radiative forcing of −0.13±0.02 W m−2 decade−1 (−0.22±0.03 W m−2 for the 1979–1996 period) was found from SBUV total column ozone trends, applying a constant percentage ozone depletion to a 7 km thick layer directly above the tropopause. SAGE ozone trends gave forcing estimates of −0.10±0.02 W m−2 decade−1 (−0.17±0.03 W m−2 for the 1979–1996 period), although assumptions needed to be made about the choice of the vertical profile of the ozone depletion below 17 km. Using Dobson instrument trends from 1964 to 1996, the total ozone forcing could be as negative as −0.26±0.05 W m−2. The quoted error bars derive from uncertainties in the total ozone trends. Using these values, the stratospheric ozone change may have offset about 30% of the forcing due to increases in well-mixed greenhouse gases since 1979, and about 15% of the forcing since 1964, at least on a global and annual mean. The ozone forcings are shown to be nearly a linear function of the ozone amount depleted from the atmospheric column, provided the vertical profile of the depletion remains constant. As in previous studies, it was found that stratospheric adjustment altered the sign of the ozone forcing by strongly cooling the lower stratosphere. It is shown that (depending on the vertical structure of ozone depletion) coolings of up to 0.4 K decade−1 can be found at altitudes of 35 km; this provides a mechanism for cooling the stratosphere nearly as large as that from well-mixed greenhouse gas increases, at altitudes where ozone changes were previously thought not to strongly affect stratospheric temperature trends. Previous studies have examined the effect of ozone trends by looking at the response of the surface temperature to a fixed absolute ozone change at different heights in the atmosphere. We argue that using absolute ozone perturbations places an unrealistically large emphasis on ozone changes near the tropopause. As the vertical profile of ozone change is more often reported as a percentage change, we present the sensitivity of the surface temperature to constant percentage, rather than absolute, changes in ozone.

METRICS OF CLIMATE CHANGE: ASSESSING RADIATIVE FORCING AND EMISSION INDICES

In this paper, we review existing and alternative metrics of climate change, with particular emphasis on radiative forcing and global warming potentials (GWPs), in terms of their scientific performance. Radiative forcing is assessed in terms of questions such as the utility of the concept, uncertainties and sensitivity to key assumptions. The assessment of emission indices focuses on the climate and other resulting impacts (end points) against which emissions are weighted; the extent to which (and how) time dependence is included, with regard to both emission control and impact; how cost issues are dealt with; and the sensitivity of the metrics to various assumptions. It is concluded that the radiative forcing concept is a robust and useful metric of the potential climatic impact of various agents and that there are prospects for improvement by weighing different forcings according to their effectiveness. We also find that although the GWP concept is associated with serious shortcomings, it retains advantages over any of the proposed alternatives in terms of political feasibility. Alternative metrics, however, make a significant contribution to addressing important issues, and this contribution should be taken into account in the further development of refined metrics of climate change.

An update of observed stratospheric temperature trends

An updated analysis of observed stratospheric temperature variability and trends is presented on the basis of satellite, radiosonde, and lidar observations. Satellite data include measurements from the series of NOAA operational instruments, including the Microwave Sounding Unit covering 1979–2007 and the Stratospheric Sounding Unit (SSU) covering 1979–2005. Radiosonde results are compared for six different data sets, incorporating a variety of homogeneity adjustments to account for changes in instrumentation and observational practices. Temperature changes in the lower stratosphere show cooling of ∼0.5 K/decade over much of the globe for 1979–2007, with some differences in detail among the different radiosonde and satellite data sets. Substantially larger cooling trends are observed in the Antarctic lower stratosphere during spring and summer, in association with development of the Antarctic ozone hole. Trends in the lower stratosphere derived from radiosonde data are also analyzed for a longer record (back to 1958); trends for the presatellite era (1958–1978) have a large range among the different homogenized data sets, implying large trend uncertainties. Trends in the middle and upper stratosphere have been derived from updated SSU data, taking into account changes in the SSU weighting functions due to observed atmospheric CO2 increases. The results show mean cooling of 0.5–1.5 K/decade during 1979–2005, with the greatest cooling in the upper stratosphere near 40–50 km. Temperature anomalies throughout the stratosphere were relatively constant during the decade 1995–2005. Long records of lidar temperature measurements at a few locations show reasonable agreement with SSU trends, although sampling uncertainties are large in the localized lidar measurements. Updated estimates of the solar cycle influence on stratospheric temperatures show a statistically significant signal in the tropics (∼30°N–S), with an amplitude (solar maximum minus solar minimum) of ∼0.5 K (lower stratosphere) to ∼1.0 K (upper stratosphere).

General circulation model calculations of the direct radiative forcing by anthropogenic sulfate and fossil-fuel soot aerosol

Abstract A new radiation code within a general circulation model is used to assess the direct solar and thermal radiative forcing by sulfate aerosol of anthropogenic origin and soot aerosol from fossil-fuel burning. The radiative effects of different aerosol profiles, relative humidity parameterizations, chemical compositions, and internal and external mixtures of the two aerosol types are investigated. The contribution to the radiative forcing from cloudy sky regions is found to be negligible for sulfate aerosol; this is in contrast to recent studies where the cloudy sky contribution was estimated using a method in which the spatial correlation between cloud amount and sulfate burden was ignored. However, the radiative forcing due to fossil-fuel soot aerosol is enhanced in cloudy regions if soot aerosol exists within or above the cloud. The global solar radiative forcing due to sulfate aerosol is estimated to be −0.38 W m−2 and the global thermal radiative forcing is estimated to be +0.01 W m−2. The hemi...

Radiative forcing of climate by hydrochlorofluorocarbons and hydrofluorocarbons

We measure infrared absorption spectra of 18 hydrochlorofluorocarbons and hydrofluorocarbons, seven of which do not yet appear in the literature. The spectra are used in a narrowband model of the terrestrial infrared radiation to calculate radiative forcing and global warming potentials. We investigate the sensitivity of the radiative forcing to the absorption spectrum temperature dependence, halocarbon vertical profile, stratospheric adjustment, cloudiness, spectral overlap, and latitude, and we make some recommendations for the reporting of radiative forcings that would help to resolve discrepancies between assessments. We investigate simple methods of estimating instantaneous radiative forcing directly from a molecules absorption spectrum and we present a new method that agrees to within 0.3% with our narrowband model results.

The effect of human activity on radiative forcing of climate change: a review of recent developments

Abstract Human activity has perturbed the Earths energy balance by altering the properties of the atmosphere and the surface. This perturbation is of a size that would be expected to lead to significant changes in climate. In recent years, an increasing number of possible human-related climate change mechanisms have begun to be quantified. This paper reviews developments in radiative forcing that have occurred since the second assessment report of the Intergovernmental Panel on Climate Change (IPCC), and proposes modifications to the values of global-mean radiative forcings since pre-industrial times given by IPCC. The forcing mechanisms which are considered here include those due to changes in concentrations of well-mixed greenhouse gases, tropospheric and stratospheric ozone, aerosols composed of sulphate, soot, organics and mineral dust (including their direct and indirect effects), and surface albedo. For many of these mechanisms, the size, spatial pattern and, for some, even the sign of their effect remain uncertain. Studies which have attributed observed climate change to human activity have considered only a subset of these mechanisms; their conclusions may not prove to be robust when a broader set is included.