Publications

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2012
Xu, YY, Ramanathan V.  2012.  Latitudinally asymmetric response of global surface temperature: Implications for regional climate change. Geophysical Research Letters. 39   10.1029/2012gl052116   AbstractWebsite

The Earth's climate system was subject to two multidecadal warming trends in the beginning (1910-1940) and end (1975-2005) of the 20th century, having been interrupted only by a cooling trend in mid-century (1940-1975). The spatio-temporal distribution of surface temperature during this time, especially the land-ocean warming contrast in recent decades, has been the subject of many climate change detection studies. The focus of this study is the south-to-north warming asymmetry and we observed a similar Latitudinal Asymmetry of Temperature Change (LATC) for the two warming sub-periods and the cooling sub-period. Basically, the temperature change was low in the Southern Hemisphere extra-tropics (60 degrees S) and increased monotonically to peak values (0.15 degrees C/decade for warming trends) in the Northern Hemisphere extra-tropics (60 degrees N). We hypothesized that the LATC is a fundamental characteristic of the planet's transient response to global forcing. We tested this hypothesis using climate model simulations of CO2 and aerosol forcing, and the simulations revealed very similar LATC as seen in the observations. In the simulations, the LATC did not depend on the asymmetry of the forcing and furthermore weakened significantly in equilibrium simulations, leading to the deduction that the LATC was caused by a corresponding asymmetry in the land-ocean fraction, i.e., the analyses of model simulations supported the hypothesis of LATC being a fundamental characteristic of the planet's transient response. If LATC is preserved as the planet warms beyond 2 degrees C, precipitation patterns can be drastically disrupted in the tropics and sub-tropics, with major implications for regional climate. Citation: Xu, Y., and V. Ramanathan (2012), Latitudinally asymmetric response of global surface temperature: Implications for regional climate change, Geophys. Res. Lett., 39, L13706, doi:10.1029/2012GL052116.

Kim, D, Ramanathan V.  2012.  Improved estimates and understanding of global albedo and atmospheric solar absorption. Geophysical Research Letters. 39   10.1029/2012gl053757   AbstractWebsite

This study integrates available surface-based and satellite observations of solar radiation at the surface and the top of the atmosphere (TOA) with a comprehensive set of satellite observations of atmospheric and surface optical properties and a Monte Carlo Aerosol-Cloud-Radiation (MACR) model to estimate the three fundamental components of the planetary solar radiation budget: Albedo at the TOA; atmospheric solar absorption; and surface solar absorption. The MACR incorporates most if not all of our current understanding of the theory of solar radiation physics including modern spectroscopic water vapor data, minor trace gases, absorbing aerosols including its effects inside cloud drops, 3-D cloud scattering effects. The model is subject to a severe test by comparing the simulated solar radiation budget with data from 34 globally distributed state-of-the art BSRN (Baseline Surface Radiation Network) land stations which began data collection in the mid 1990s. The TOA over these sites were obtained from the CERES (Cloud and Earth's Radiant Energy System) satellites. The simulated radiation budget was within 2 Wm(-2) for all three components over the BSRN sites. On the other hand, over these same sites, the IPCC-2007 simulation of atmospheric absorption is smaller by 7-8 Wm(-2). MACR was then used with a comprehensive set of model input from satellites to simulate global solar radiation budget. The simulated planetary albedo of 29.0% confirms the value (28.6%) observed by CERES. We estimate the atmospheric absorption to be 82 +/- 8 Wm(-2) to be compared with the 67Wm(-2) by IPCC models as of 2001 and updated to 76Wm(-2) by IPCC-2007. The primary reasons for the 6 Wm(-2) larger solar absorption in our estimates are: updated water vapor spectroscopic database (similar to 1 Wm(-2)), inclusion of minor gases (similar to 0.5 Wm(-2)), black and brown carbon aerosols (similar to 4Wm(-2)), the inclusion of black carbon in clouds (similar to 1 Wm(-2)) and 3-D effect of clouds (similar to 1 Wm(-2)). The fundamental deduction from our study is the remarkable consistency between satellite measurements of the radiation budget and the parameters (aerosols, clouds and surface reflectivity) which determine the radiation budget. Because of this consistency we can account for and explain the global solar radiation budget of the planet within few Wm(-2). Citation: Kim, D., and V. Ramanathan (2012), Improved estimates and understanding of global albedo and atmospheric solar absorption, Geophys. Res. Lett., 39, L24704, doi:10.1029/2012GL053757.

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.

2008
Ramanathan, V, Feng Y.  2008.  On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead. Proceedings of the National Academy of Sciences of the United States of America. 105:14245-14250.   10.1073/pnas.0803838105   AbstractWebsite

The observed increase in the concentration of greenhouse gases (GHGs) since the preindustrial era has most likely committed the world to a warming of 2.4 degrees C (1.4 degrees C to 4.3 degrees C) above the preindustrial surface temperatures. The committed warming is inferred from the most recent Intergovernmental Panel on Climate Change (IPCC) estimates of the greenhouse forcing and climate sensitivity. The estimated warming of 2.4 degrees C is the equilibrium warming above preindustrial temperatures that the world will observe even if GHG concentrations are held fixed at their 2005 concentration levels but without any other anthropogenic forcing such as the cooling effect of aerosols. The range of 1.4 degrees C to 4.3 degrees C in the committed warming overlaps and surpasses the currently perceived threshold range of 1 degrees C to 3 degrees C for dangerous anthropogenic interference with many of the climate-tipping elements such as the summer arctic sea ice, Himalayan-Tibetan glaciers, and the Greenland Ice Sheet. IPCC models suggest that approximate to 25% (0.6 degrees C) of the committed warming has been realized as of now. About 90% or more of the rest of the committed warming of 1.6 degrees C will unfold during the 21st century, determined by the rate of the unmasking of the aerosol cooling effect by air pollution abatement laws and by the rate of release of the GHIGs-forcing stored in the oceans. The accompanying sea-level rise can continue for more than several centuries. Lastly, even the most aggressive CO2 mitigation steps as envisioned now can only limit further additions to the committed warming, but not reduce the already committed GHGs warming of 2.4 degrees C.

2005
Ramanathan, V, Chung C, Kim D, Bettge T, Buja L, Kiehl JT, Washington WM, Fu Q, Sikka DR, Wild M.  2005.  Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle. Proceedings of the National Academy of Sciences of the United States of America. 102:5326-5333.   10.1073/pnas.0500656102   AbstractWebsite

South Asian emissions of fossil fuel SO2 and black carbon increased approximate to 6-fold since 1930, resulting in large atmospheric concentrations of black carbon and other aerosols. This period also witnessed strong negative trends of surface solar radiation, surface evaporation, and summer monsoon rainfall. These changes over India were accompanied by an increase in atmospheric stability and a decrease in sea surface temperature gradients in the Northern Indian Ocean. We conducted an ensemble of coupled ocean-atmosphere simulations from 1930 to 2000 to understand the role of atmospheric brown clouds in the observed trends. The simulations adopt the aerosol radiative forcing from the Indian Ocean experiment observations and also account for global increases in greenhouse gases and sulfate aerosols. The simulated decreases in surface solar radiation, changes in surface and atmospheric temperatures over land and sea, and decreases in monsoon rainfall are similar to the observed trends. We also show that greenhouse gases and sulfates, by themselves, do not account for the magnitude or even the sign in many instances, of the observed trends. Thus, our simulations suggest that absorbing aerosols in atmospheric brown clouds may have played a major role in the observed regional climate and hydrological cycle changes and have masked as much as 50% of the surface warming due to the global increase in greenhouse gases. The simulations also raise the possibility that, if current trends in emissions continue, the subcontinent may experience a doubling of the drought frequency in the coming decades.

2003
Novakov, T, Ramanathan V, Hansen JE, Kirchstetter TW, Sato M, Sinton JE, Sathaye JA.  2003.  Large historical changes of fossil-fuel black carbon aerosols. Geophysical Research Letters. 30   10.1029/2002gl016345   AbstractWebsite

Anthropogenic emissions of fine black carbon (BC) particles, the principal light-absorbing atmospheric aerosol, have varied during the past century in response to changes of fossil-fuel utilization, technology developments, and emission controls. We estimate historical trends of fossil-fuel BC emissions in six regions that represent about two-thirds of present day emissions and extrapolate these to global emissions from 1875 onward. Qualitative features in these trends show rapid increase in the latter part of the 1800s, the leveling off in the first half of the 1900s, and the re-acceleration in the past 50 years as China and India developed. We find that historical changes of fuel utilization have caused large temporal change in aerosol absorption, and thus substantial change of aerosol single scatter albedo in some regions, which suggests that BC may have contributed to global temperature changes in the past century. This implies that the BC history needs to be represented realistically in climate change assessments.