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.

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.

1997
Ramanathan, V, Vogelmann AM.  1997.  Greenhouse effect, atmospheric solar absorption and the Earth's radiation budget: From the Arrhenius-Langley era to the 1990s. Ambio. 26:38-46. AbstractWebsite

In his 1896 paper, Svante Arrhenius laid the foundation for the modern theory of the greenhouse effect and climate change. The paper is required reading for anyone attempting to model the greenhouse effect of the atmosphere and to estimate the resulting temperature change. Arrhenius demonstrates how to build a radiation and an energy balance model directly from observations. Arrhenius was fortunate to have access to Langley's data, which are some of the best radiometric observations ever undertaken from the surface. The successes of Arrhenius' model are many, even when judged by modern-day data and computer simulations: the suggestion of the diffusivity factor including its correct numerical value; the remarkably accurate simulation of the total emissivity of the atmosphere which seem to agree within 5% of modern-day values; the logarithmic dependence of the CO2 radiative heating effect; and others documented in the text. We uncover two aspects of the model which, most likely, were not recognized by earlier studies: First, Arrhenius included the water vapor feedback by introducing the fixed relative humidity assumption, which amplified the surface warming in his model by about 30%. Second, his model overestimated the surface warming, primarily because the radiation model overestimates the opacity of the CO2 bands in the 6 to 8 mu m region. In constructing his model, Arrhenius had to account for the magnitude of the solar radiation absorbed within the atmosphere-a topic that is currently pursued with renewed vigor. The second part of the paper addresses this topic, including the controversy that surrounds it. Observed values of solar absorption, since the 1950s, have almost always exceeded theoretical and model values. The magnitude of this excess absorption, i.e., observed-theoretical absorption, on climatologically relevant time and spatial scales was quantified recently (1990s) by six independent studies, to be about 25 W m(-2) or larger. We review some of the key results published between the 1950s to 1990s to clarify the central issues and offer suggestions for resolving the discrepancy between observations and models.