Estimates Of Radiative Forcing Research Articles

Aerosol optical depths and direct radiative forcing for INDOEX derived from AVHRR: Theory

A scheme is presented for retrieving aerosol properties for ocean regions from reflected sunlight at both the visible and near infrared wavelengths measured by the NOAA advanced very high resolution radiometer (AVHRR). For the Indian Ocean Experiment (INDOEX), aerosols were presumed to be a mixture of a continental haze that had small particles, contained soot, and absorbed sunlight, and a marine haze that had large particles and absorbed practically no sunlight. Because of the difference in particle sizes, the two aerosols reflect sunlight differently at visible and near infrared wavelengths. Reflectances at visible and near infrared wavelengths were thus used to determine mixing fractions for the continental and marine aerosols and the optical depth of the aerosol mixture. The fractions and optical depths along with the optical properties of the aerosols were then used in radiative transfer calculations to estimate the diurnally averaged top of the atmosphere and surface aerosol direct radiative forcing for ocean regions. Comparison of retrieved optical depths at visible and near infrared wavelengths with surface measurements revealed that several different retrieval schemes employing a variety of aerosol types provided comparable levels of agreement, but none of the aerosol models or retrieval schemes produced ratios of the near infrared to visible optical depths that agreed with the ratios obtained with the surface measurements. In estimating the top of the atmosphere radiative forcing, errors in the retrieved optical depths were in some cases found to be partially compensated by the effect of the aerosol on the radiative flux. For example, different aerosol models led to retrieved optical depths that differed by as much as 60%, but the top of the atmosphere forcing obtained with the models differed by less than 35% for cloud‐free conditions. When aerosols absorb sunlight, there is no comparable compensation for the surface forcing. Cloud conditions contribute sizable uncertainties to estimates of the aerosol direct radiative forcing. For INDOEX, estimates of the aerosol direct radiative forcing for average cloud conditions were obtained by (1) setting the forcing to zero for all 1° × 1° latitude‐longitude boxes that contained any amount of upper‐level cloud; (2) ascribing to regions with upper‐level clouds the radiative forcing obtained for regions having only low‐level clouds and, (3) setting the forcing to zero for all regions containing upper‐level clouds and all portions of regions overcast by low‐level clouds. Relative differences in the extreme values for the top of the atmosphere aerosol direct radiative forcing were less than 50%, but for the surface, the relative differences of the extreme values reached 70%.

Towards a reliable GCM estimation of contrail radiative forcing

Selected results of the ECHAM4 general circulation model are compared with sophisticated radiative transfer model calculations in order to investigate the performance of the ECHAM4 radiation schemes with respect to the radiative forcing (RF) of contrails. The mean shortwave RF is found to be in accordance with radiative transfer model results. For the longwave part it is shown that the effective emissivity approach combined with the assumption of maximum‐random cloud overlap used in the standard ECHAM4 leads to a considerable underestimation of the longwave contrail RF. Hence, this approach cannot be recommended for calculating the longwave RF of optically thin clouds in general. As an alternative, more adequate results of a modified version of the ECHAM4 longwave scheme are discussed.

Role of Tropical Clouds in Surface and Atmospheric Energy Budget

In this paper diagnostic estimates of cloud radiative forcing (CRF) and clear-sky radiation budget at the surface and in the atmosphere, based on satellite-observed radiation budget at the top of the atmosphere (TOA) and empirical parameterizations derived from radiation models and field observations, are presented. This analysis is restricted to the tropical Pacific. High clouds over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), and the warm pool (WP) exert a positive CRF of about 70 W m22 within the atmosphere and a negative CRF of about 2 70 Wm 22 at the surface, although with a negligible net CRF at the TOA. On the other hand, low clouds over the eastern subtropical Pacific and the equatorial cold tongue exert a negative CRF of about 2 20 Wm 22 at the surface as well as in the atmosphere. The spatial gradients of the clear-sky radiation budget at the surface and in the atmosphere are small. In particular, it is shown that the clear-sky radiative cooling in the atmosphere is larger over the ITCZ, the SPCZ, and the WP, when compared with that over the subtropics and the cold tongue. Next, based on these diagnostic estimates and available surface turbulent heat flux data, the role of atmospheric CRF in the large-scale atmospheric moist static energy (MSE) transport is quantified. It is found that the atmospheric CRF provides the major energy source for balancing the divergence of MSE transport (from the ITCZ, the SPCZ, and the WP to the subtropics and the cold tongue) by the large-scale atmospheric circulation. On the other hand, the clear-sky radiative flux convergence and the surface turbulent heat fluxes have just the reverse spatial pattern and hence cannot satisfy the large-scale atmospheric MSE transport requirements.

On summing the components of radiative forcing of climate change

Radiative forcing is a useful concept in determining the potential influence of a particular mechanism of climate change. However, due to the increased number of forcing agents identified over the past decade, the total radiative forcing is difficult to assess. By assigning a range of probability distribution functions to the individual radiative forcings and using a Monte-Carlo approach, we estimate the total radiative forcing since pre-industrial times including all quantitative radiative forcing estimates to date. The resulting total radiative forcing has a 75–97% probability of being positive (or similarly a 3–25% probability of being negative), with mean radiative forcing ranging from +0.68 to +1.34 W m−2, and median radiative forcing ranging from +0.94 to +1.39 W m−2.

Updated radiative forcing estimates of 65 halocarbons and nonmethane hydrocarbons

The direct radiative forcing of 65 chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, hydrofluoroethers, halons, iodoalkanes, chloroalkanes, bromoalkanes, perfluorocarbons and nonmethane hydrocarbons has been evaluated using a consistent set of infrared absorption cross sections. For the radiative transfer models, both line‐by‐line and random band model approaches were employed for each gas. The line‐by‐line model was first validated against measurements taken by the Airborne Research Interferometer Evaluation System (ARIES) of the U.K. Meteorological Office; the computed spectrally integrated radiance of agreed to within 2% with experimental measurements. Three model atmospheres, derived from a three‐dimensional climatology, were used in the radiative forcing calculations to more accurately represent hemispheric differences in water vapor, ozone concentrations, and cloud cover. Instantaneous, clear‐sky radiative forcing values calculated by the line‐by‐line and band models were in close agreement. The band model values were subsequently modified to ensure exact agreement with the line‐by‐line model values. Calibrated band model radiative forcing values, for atmospheric profiles with clouds and using stratospheric adjustment, are reported and compared with previous literature values. Fourteen of the 65 molecules have forcings that differ by more than 15% from those in the World Meteorological Organization [1999] compilation. Eleven of the molecules have not been reported previously. The 65‐molecule data set reported here is the most comprehensive and consistent database yet available to evaluate the relative impact of halocarbons and hydrocarbons on climate change.

A physically based estimate of radiative forcing by anthropogenic sulfate aerosol

Estimates of direct and indirect radiative forcing by anthropogenic sulfate aerosols from an integrated global aerosol and climate modeling system are presented. A detailed global tropospheric chemistry and aerosol model that predicts concentrations of oxidants as well as aerosols and aerosol precursors, is coupled to a general circulation model that predicts both cloud water mass and cloud droplet number. Both number and mass of several externally mixed aerosol size modes are predicted, with internal mixing assumed for the different aerosol components within each mode. Predicted aerosol species include sulfate, organic and black carbon, soil dust, and sea salt. The models use physically based treatments of aerosol radiative properties (including dependence on relative humidity) and aerosol activation as cloud condensation nuclei. Parallel simulations with and without anthropogenic sulfate aerosol are performed for a global domain. The global and annual mean direct and indirect radiative forcing due to anthropogenic sulfate are estimated to be −0.3 to −0.5 and −1.5 to −3.0 W m−2, respectively. The radiative forcing is sensitive to the model's horizontal resolution, the use of predicted versus analyzed relative humidity, the prediction versus diagnosis of aerosol number and droplet number, and the parameterization of droplet collision/coalescence. About half of the indirect radiative forcing is due to changes in droplet radius and half to increased cloud liquid water.

Estimates of cloud radiative forcing in contrail clusters using GOES imagery

Using data from the Geostationary Operational Environmental Satellite (GOES), the evolution of solar and longwave radiative forcing in contrail clusters is presented in several case studies. The first study examines contrails developing over the midwestern United States in a region of upper tropospheric moisture enhanced by the remnants of Hurricane Nora on September 26, 1997. Two other cases involve contrail clusters that formed over the Chesapeake Bay and the Atlantic Ocean on February 11 and March 5, 1999, respectively. The last study includes contrails forming over the tropical Pacific near Hawaii. Observations of tropical contrails near Hawaii show that the contrail optical properties are similar to those measured from satellite in the midlatitudes, with visible optical depths between 0.3 and 0.5 and particle sizes between 30 and 60 μm as the contrails mature into diffuse cloudiness. Radiative transfer model simulations of the tropical contrail case suggest that ice crystal shape may have an important effect on radiative forcing in contrails. The magnitudes of the observed solar and longwave radiative forcings were 5.6 and 3.2 W m−2 less than those from the corresponding model simulations, and these differences are attributed to the subpixel scale low clouds and uncertainties in the anisotropic reflectance and limb‐darkening models used to estimate the observed forcing. Since the broadband radiative forcing in contrails often changes rapidly, contrail forcing estimates based only on the polar orbiting advanced very high resolution radiometer (AVHRR) data could be inaccurate due to the lack of sufficient temporal sampling.

Estimation of Shortwave Direct Radiative Forcing of Biomass-Burning Aerosols Using New Angular Models

Using a new angular distribution model (ADM) for smoke aerosols, the instantaneous top-of-atmosphere (TOA) shortwave aerosol radiative forcing (SWARF) is calculated for selected days over biomass-burning regions in South America. The visible and infrared scanner data are used to detect smoke aerosols and the Clouds and the Earth’s Radiant Energy System (CERES) scanner data from the Tropical Rainfall Measuring Mission are used to obtain the broadband radiances. First, the ADM for smoke aerosols is calculated over land surfaces using a discrete-ordinate radiative transfer model. The instantaneous TOA shortwave (SW) fluxes are estimated using the new smoke ADM and are compared with the SW fluxes from the CERES product. The rms error between the CERES SW fluxes and fluxes using the smoke ADM is 13 W m−2. The TOA SWARFs per unit optical thickness for the six surface types range from −29 to −57 W m−2, showing that smoke aerosols have a distinct cooling effect. The new smoke ADM developed as part of this study could be used to estimate radiative impact of biomass-burning aerosols.

Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review

This paper reviews the many developments in estimates of the direct and indirect global annual mean radiative forcing due to present‐day concentrations of anthropogenic tropospheric aerosols since Intergovernmental Panel on Climate Change [1996]. The range of estimates of the global mean direct radiative forcing due to six distinct aerosol types is presented. Additionally, the indirect effect is split into two components corresponding to the radiative forcing due to modification of the radiative properties of clouds (cloud albedo effect) and the effects of anthropogenic aerosols upon the lifetime of clouds (cloud lifetime effect). The radiative forcing for anthropogenic sulphate aerosol ranges from −0.26 to −0.82 W m−2. For fossil fuel black carbon the radiative forcing ranges from +0.16 W m−2 for an external mixture to +0.42 W m−2 for where the black carbon is modeled as internally mixed with sulphate aerosol. For fossil fuel organic carbon the two estimates of the likely weakest limit of the direct radiative forcing are −0.02 and −0.04 W m−2. For biomass‐burning sources of black carbon and organic carbon the combined radiative forcing ranges from −0.14 to −0.74 W m−2. Estimates of the radiative forcing due to mineral dust vary widely from +0.09 to −0.46 W m−2; even the sign of the radiative forcing is not well established due to the competing effects of solar and terrestrial radiative forcings. A single study provides a very tentative estimate of the radiative forcing of nitrates to be −0.03 W m−2. Estimates of the cloud albedo indirect radiative forcing range from −0.3 to approximately −1.8 W m−2. Although the cloud lifetime effect is identified as a potentially important climate forcing mechanism, it is difficult to quantify in the context of the present definition of radiative forcing of climate change and current model simulations. This is because its estimation by general circulation models necessarily includes some level of cloud and water vapor feedbacks, which affect the hydrological cycle and the dynamics of the atmosphere. Available models predict that the radiative flux perturbation associated with the cloud lifetime effect is of a magnitude similar to that of the cloud albedo effect.

Indirect Aerosol Forcing

Crowley ([1][1]) used estimates of radiative forcing, together with an energy balance model, to estimate the temperature response to variations in volcanic emissions, solar irradiance, increases in greenhouse gases, and aerosol forcing. He did not account for the indirect effects of aerosols,

Shortwave direct radiative forcing of biomass burning aerosols estimated using VIRS and CERES data

Using collocated data from the Visible Infrared Scanner (VIRS) and the Clouds and the Earth's Radiant Energy Budget Scanner (CERES) from the Tropical Rainfall Measuring (TRMM) satellite, observational estimates of the instantaneous Shortwave Aerosol Radiative Forcing (SWARF) of smoke aerosols at the top‐of‐atmosphere (TOA) are obtained for four days in May 1998 during a biomass‐burning episode in Central America. The detection of smoke aerosols is demonstrated using VIRS imagery. Assuming a single scattering albedo (ωo) of 0.86 (at 0.63 µm) that is representative of absorbing aerosols, smoke optical thickness (τ0.63) is retrieved over ocean areas. The average τ0.63 for these four days was 1.2 corresponding to a SWARF value of −68 Wm−2. The SWARF changes from −24 to −99 Wm−2 as τ0.63 changes from 0.2 to 2.2. Global observational estimates of biomass burning aerosol radiative forcing can be obtained by combining data sets from TRMM and Terra satellites.

Future estimates of tropospheric ozone radiative forcing and methane turnover — The impact of climate change

We present a range of estimates for future radiative forcings due to changes in tropospheric ozone (O3T). Ozone distributions were generated by the UKMO 3‐D chemistry‐transport model for 1990, 2030, 2060, and 2100, using four sets of boundary conditions. Anthropogenic emissions evolved following either the IPCC SRES “high” (A2) or “central” (B2) case. Each scenario was run with both a fixed (1990) climate, and with a changing climate, as generated by a coupled ocean‐atmosphere GCM, forced with IS92a emissions. Calculated global mean O3T radiative forcings for the A2 (B2) cases for 1990–2100 were +0.43 (+0.22) W m−2 when climate change was ignored; these fell to +0.27 (+0.09) W m−2 when climate change was included. Without climate change, CH4 lifetimes (τCH4) lengthened by 7–12% between 1990 and 2100; however, when climate change was included, τCH4 fell by 0–5%. Hence climate warming exerts a negative feedback on itself by enhancing O3T and CH4 destruction.

A model estimate of cooling in the mesosphere and lower thermosphere due to the CO2 Increase over the last 3–4 decades

Long‐term observations indicate a substantial cooling in the mesosphere and lower thermosphere (MLT) over the last 3–4 decades. Available model studies have primarily considered the effects of CO2 doubling expected to occur in the future. We present a benchmark estimate of radiative forcing in the MLT due to the increase of CO2 mixing ratio from 313 ppm to about 360 ppm (or by 15%) observed over the last four decades. The Spectral Mesosphere/Lower Thermosphere Model is employed for “retrocasting” the atmospheric response. As expected, the thermal response is predominantly negative. As a function of altitude, the cooling maximizes in the mesosphere at about 3 K, practically vanishes at 100–120 km, and grows to 10–15 K in the thermosphere. Although this vertical shape is remarkably consistent with various sets of observations, the magnitude of the cooling rate is smaller by about a factor of 2–10. This suggests that other mechanisms, e.g., the ozone depletion, might have contributed substantially to the negative temperature trend.

Anthropogenic sulphate aerosol from India: estimates of burden and direct radiative forcing

A one-box chemical-meteorological model had been formulated to make preliminary estimates of sulphate aerosol formation and direct radiative forcing over India. Anthropogenic SO 2 emissions from India, from industrial fuel use and biomass burning, were estimated at 2.0 Tg S yr -1 for 1990 in the range of previous estimates of 1.54 and 2.55 Tg S yr -1 for 1987. Meteorological parameters for 1990 from 18 Indian Meteorological Department stations were used to estimate spatial average sulphate burdens through formation from SO 2 reactions in gas and aqueous phase and removal by dry and wet deposition. The hydrogen peroxide reaction was found dominating for undepleted oxidant-rich conditions. Monthly mean sulphate burdens ranged from 2–10 mg m -2 with a seasonal variation of winter–spring highs and summer lows in agreement with previous GCM studies. The sulphate burdens are dominated by sulphate removal rates by wet deposition, which are high in the monsoon period from June–November. Monthly mean direct radiative forcing from sulphate aerosols is high (−3.5 and −2.3 W m -2) in December and January, is moderate (−1.3 to −1.5 W m -2) during February to April and November and low (−0.4 to −0.6 W m -2) during May to October also in general agreement with previous GCM estimates. This model, in reasonable agreement with detailed GCM results, gives us a simple tool to make preliminary estimates of sulphate burdens and direct radiative forcing.

Estimation of direct radiative forcing due to non-methane hydrocarbons

The direct radiative forcing due to non-methane hydrocarbons (NMHCs) has not previously been quantified. We use new measurements of infrared absorption cross-sections and a narrow band radiative transfer model to estimate a forcing. An upper limit to the global mean anthropogenic forcing is likely to be in the region of 0.015 W m -2, less than 1% of the forcing due to other greenhouse gases. However, taking account of the natural NMHC loading and the vertical profile of these gases the actual radiative forcing is likely to be somewhat less than this.

Changes in heat index associated with CO2-induced global warming

Changes in Heat Index (a combined measure of temperature and humidity) associated with global warming are evaluated based on the output from four extended integrations of the GFDL coupled ocean-atmosphere climate model. The four integrations are: a control with constant levels of atmospheric carbon dioxide (CO2), a second integration in which an estimate of the combined radiative forcing of greenhouse gases and sulfate aerosols over the period 1765–2065 is used to force the model, and a third (fourth) integration in which atmospheric CO2$ increases at the rate of 1% per year to double (quadruple) its initial value, and is held constant thereafter. While the spatial patterns of the changes in Heat Index are largely determined by the changes in surface air temperature, increases in atmospheric moisture can substantially amplify the changes in Heat Index over regions which are warm and humid in the Control integration. The regions most prone to this effect include humid regions of the Tropics and summer hemisphere extra-tropics, including the southeastern United States, India, southeast Asia and northern Australia.

Atmospheric distributions of soot particles by current and future aircraft fleets and resulting radiative forcing on climate

One of the least understood of the possible environmental effects from trace constituents emitted by aircraft is that of elemental carbon, soot. Distributions of soot in the troposphere and stratosphere based on emissions from current and projected fleets of aircraft are determined in this study; these distributions are then used in evaluating the resulting effects on radiative forcing on climate. The University of Illinois two‐dimensional chemical transport model of the troposphere and stratosphere is used in deriving the distributions of soot. Model results for 1992 aircraft emissions generally compare well with the upper tropospheric and lower stratospheric measurements for the same year. Estimates of atmospheric soot distributions due to aircraft are also derived for roughly the year 2015, when a fleet of high‐speed civil transports is assumed to be fully operational. Using the present and future distributions of atmospheric soot, direct radiative forcing estimates on climate were made with a modified version of the National Center for Atmospheric Research Community Climate Model 3 column radiation model. When compared with other forcings on climate, the determined soot distributions were found to have negligible impact.

Estimation of the direct radiative forcing due to sulfate and soot aerosols

Estimation of the direct radiative forcing due to sulfate and soot aerosols

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.

The 1985 Biomass Burning Season in South America: Satellite Remote Sensing of Fires, Smoke, and Regional Radiative Energy Budgets

Using satellite imagery, more than five million square kilometers of the forest and cerrado regions over South America are extensively studied to monitor fires and smoke during the 1985 biomass burning season. The results are characterized for four major ecosystems, namely, 1) tropical rain forest, 2) tropical broadleaf seasonal, 3) savanna/grass and seasonal woods (SGW), and 4) mild/warm/hot grass/shrub (MGS). The spatial and temporal distribution of fires are examined from two different methods using the multispectral Advanced Very High Resolution Radiometer Local Area Coverage data. Using collocated measurements from the instantaneous scanner Earth Radiation Budget Experiment data, the direct regional radiative forcing of biomass burning aerosols is computed. The results show that more than 70% of the fires occur in the MGS and SGW ecosystems due to agricultural practices. The smoke generated from biomass burning has negative instantaneous net radiative forcing values for all four major ecosystems within South America. The smoke found directly over the fires has mean net radiative forcing values ranging from 225.6 to 233.9 W m22. These results confirm that the regional net radiative impact of biomass burning is one of cooling. The spectral and broadband properties for clear-sky and smoke regions are also presented that could be used as input and/or validation for other studies attempting to model the impact of aerosols on the earth‐atmosphere system. These results have important applications for future instruments from the Earth Observing System (EOS) program. Specifically, the combination of the Visible Infrared Scanner and Clouds and the Earth’s Radiant Energy System (CERES) instruments from the Tropical Rainfall Measuring Mission and the combination of Moderate Resolution Imaging Spectrometer and CERES instruments from the EOS morning crossing mission could provide reliable estimates of the direct radiative forcing of aerosols on a global scale, thereby reducing the uncertainties in current global aerosol radiative forcing values.