Publications

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2009
Ramanathan, V, Feng Y.  2009.  Air pollution, greenhouse gases and climate change: Global and regional perspectives. Atmospheric Environment. 43:37-50.   10.1016/j.atmosenv.2008.09.063   AbstractWebsite

Greenhouse gases (GHGs) warm the Surface and the atmosphere with significant implications for rainfall, retreat of glaciers and sea ice, sea level, among other factors. About 30 years ago, it was recognized that the increase in tropospheric ozone from air pollution (NO(x), CO and others) is an important greenhouse forcing term. In addition, the recognition of chlorofluorocarbons (CFCs) on stratospheric ozone and its climate effects linked chemistry and climate strongly. What is less recognized, however, is a comparably major global problem dealing with air pollution. Until about ten years ago, air pollution was thought to be just an urban or a local problem. But new data have revealed that air pollution is transported across continents and ocean basins due to fast long-range transport, resulting in trans-oceanic and trans-continental plumes of atmospheric brown Clouds (ABCs) containing sub micron size particles, i.e., aerosols. ABCs intercept Sunlight by absorbing as well as reflecting it, both of which lead to a large surface dimming. The dimming effect is enhanced further because aerosols may nucleate more cloud droplets, which makes the clouds reflect More solar radiation. The dimming has a Surface cooling effect and decreases evaporation of moisture from the surface, thus slows down the hydrological cycle. On the other hand, absorption of solar radiation by black carbon and some organics increase atmospheric hearing and tend to amplify greenhouse warming of the atmosphere. ABCs are concentrated in regional and mega-city hot spots. Long-range transport from these hot spots causes widespread plumes over the adjacent oceans. Such a pattern Of regionally concentrated Surface dimming and atmospheric Solar heating, accompanied by widespread dimming over the oceans, gives rise to large regional effects. Only during the last decade, we have begun to comprehend the surprisingly large regional impacts. In S. Asia and N. Africa, the large north-south gradient in the ABC dimming has altered both the north-south gradients in sea Surface temperatures and land-ocean contrast in surface temperatures, which in turn slow down the monsoon circulation and decrease rainfall over the continents. On the other hand, heating by black carbon warms the atmosphere at elevated levels from 2 to 6 kin, where most tropical glaciers are located, thus strengthening the effect of GHGs on retreat of snow packs and glaciers in the Hindu Kush-Himalaya-Tibetan glaciers. Globally, the surface cooling effect of ABCs may have masked as Much 47% of the global warming by greenhouse gases, with an uncertainty range of 20-80%. This presents a dilemma since efforts to curb air pollution may unmask the ABC cooling effect and enhance the surface warming. Thus efforts to reduce GHGs and air pollution should be done under one common framework. The uncertainties in our understanding of the ABC effects are large, but we are discovering new ways in which human activities are changing the climate and the environment. (C) 2008 Elsevier Ltd. All rights reserved.

2007
Hadley, OL, Ramanathan V, Carmichael GR, Tang Y, Corrigan CE, Roberts GC, Mauger GS.  2007.  Trans-Pacific transport of black carbon and fine aerosols (D < 2.5 ┬Ám) into North America. Journal of Geophysical Research-Atmospheres. 112   10.1029/2006jd007632   AbstractWebsite

[1] This study presents estimates of long-range transport of black carbon (BC) and aerosol fine mass (diameter less than 2.5 mm) across the Pacific Ocean into North America during April 2004. These transport estimates are based on simulations by the Chemical Weather Forecast System (CFORS) model and evaluated across 130 degrees W, (30 degrees N-60 degrees N) from 26 March through 25 April 2004. CFORS calculates BC transport into North America at 25-32 Gg of which over 75% originates from Asia. Modeled fine aerosol mass transport is between 900 and 1100 Gg. The BC transport amounts to about 77% of the published estimates of North American BC emissions. Approximately 78% of the BC and 82% of the fine aerosol mass transport occur in the midtroposphere above 2 km. Given the relatively large magnitude of the estimated BC transport, we undertake a detailed validation of the model simulations of fine aerosol mass and BC over the west coast of North America. In situ aircraft data were available for the month of April 2004 to assess the accuracy of model simulations of aerosols in the lower troposphere. Aircraft data for aerosol mass collected in the eastern Pacific Ocean during April 2004 as part of the Cloud Indirect Forcing Experiment, as well as surface measurements of fine mass and BC at 30 west coast locations, are compared to CFORS predictions. These surface sites are part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Both the aircraft and the IMPROVE data sets reveal similar patterns of good agreement near and above the boundary layer accompanied by large overprediction within the boundary layer. The observational data validate the CFORS simulations of BC and fine aerosol mass above the boundary layer. The near-surface overprediction does not impair the major conclusions of this study regarding long-range aerosol and BC transport, as most of the long-range transport occurs above 2 km. From this we conclude that the transport of BC from Asia and other regions west is a major source of BC at high elevations over North America. The simulated concentrations of BC between 1 and 3 km, as well as the measured BC concentrations over the elevated IMPROVE sites, range from 0.1 to 0.3 mu g/m(3). Direct radiative forcing over North America due to the modeled BC concentration between 1 and 15 km is estimated at an additional 2.04-2.55 W/m(2) absorbed in the atmosphere and a dimming of-1.45 to-1.47 W/m(2) at the surface. The impact of transported BC on the regional radiation budget through direct and indirect effects of the transported BC and other aerosols warrants further study.

2002
Tian, BJ, Ramanathan V.  2002.  Role of tropical clouds in surface and atmospheric energy budget. Journal of Climate. 15:296-305.   10.1175/1520-0442(2002)015<0296:rotcis>2.0.co;2   AbstractWebsite

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 m(-2) within the atmosphere and a negative CRF of about 270 W m(-2) 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 220 W m(-2) 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.

1998
Ramanathan, V.  1998.  Trace-gas greenhouse effect and global warming - Underlying principles and outstanding issues - Volvo Environmental Prize Lecture - 1997. Ambio. 27:187-197. AbstractWebsite

This paper describes the developments that transformed the global warming problem from that arising solely from CO2 increase to the trace-gas greenhouse effect problem in which several non-CO2 gases, CFCs, CH4, N2O, O-3 and others contribute as much as CO2. Observed trace-gas increases, including CO2 increase, since the mid-19th century have enhanced the atmospheric greenhouse effect, G(a), (approximate to 130 +/- 5 W m(-2)) by about 2%. Without other competing factors, this heating should have committed the planet to a warming of about 1 to 1.5 K. The added radiative energy is maximum in the low latitudes and about a factor of two smaller in the polar regions. The largest effect of the warming is increased back radiation at the surface by as much as 6 to 8 W m(-2) per degree warming. Not all of this increased energy is balanced by surface emission; evaporation (and hence precipitation) increases to restore surface energy balance, by as much as 2 to 4% per degree warming. The increase in evaporation along with the increase in saturation vapor pressure of the warmer troposphere, contributes through the atmospheric dynamics to an increase in water vapor. This water vapor feedback enhances G(a) by another 1% per degree warming. Our ability to predict regional and transient effects, depends critically on resolving a number of outstanding issues, including: i) Aerosol and stratospheric ozone effects; ii) Response of the tropical convective-cirrus clouds, the extra-tropical storm-track systems and persistent coastal stratus to both global warming and to regional emissions of aerosols; iii) The causes of excess solar absorption in clouds; and iv) Upper troposphere water vapor feedback effects.

Wielicki, BA, Barkstrom BR, Baum BA, Charlock TP, Green RN, Kratz DP, Lee RB, Minnis P, Smith GL, Wong TM, Young DF, Cess RD, Coakley JA, Crommelynck DAH, Donner L, Kandel R, King MD, Miller AJ, Ramanathan V, Randall DA, Stowe LL, Welch RM.  1998.  Clouds and the Earth's Radiant Energy System (CERES): Algorithm overview. IEEE Transactions on Geoscience and Remote Sensing. 36:1127-1141.   10.1109/36.701020   AbstractWebsite

The Clouds and the Earth's Radiant Energy System (CERES) is part of NASA's Earth Observing System (EOS), CERES objectives include the following. 1) For climate change analysis, provide a continuation of the Earth Radiation Budget Experiment (ERBE) record of radiative fluxes at the top-of-the-atmosphere (TOA), analyzed using the same techniques as the existing ERBE data. 2) Double the accuracy of estimates of radiative fluxes at TOA and the earth's surface; 3) Provide the first long-term global estimates of the radiative fluxes within the earth's atmosphere. 4) Provide cloud property estimates collocated in space and time that are consistent with the radiative fluxes from surface to TOA, In order to accomplish these goals, CERES uses data from a combination of spaceborne instruments: CERES scanners, which are an improved version of the ERBE broadband radiometers, and collocated cloud spectral imager data on the same spacecraft. The CERES cloud and radiative flux data products should prove extremely useful in advancing the understanding of cloud-radiation interactions, particularly cloud feedback effects on the earth's radiation balance, For this reason, the CERES data should be fundamental to our ability to understand, detect, and predict global climate change, CERES results should also be very useful for studying regional climate changes associated with deforestation, desertification anthropogenic aerosols, and El Nino/Southern Oscillation events. This overview summarizes the Release 2 version of the planned CERES data products and data analysis algorithms. These algorithms are a prototype for the system that will produce the scientific data required for studying the role of clouds and radiation in the earth's climate system. This release will produce a data processing system designed to analyze the first CERES data, planned for launch on Tropical Rainfall Measuring Mission (TRMM) in November 1997, followed by the EOS morning (EOS-AM1) platform in 1998.