Ramanathan has contributed to many areas of the atmospheric sciences. His first major findings were in the mid-1970s and were related to the greenhouse effect of CFCs and other trace gases Until that time, carbon dioxide was thought to be the sole greenhouse gas responsible for global warming. He also contributed to the early development of global circulation models and the detecting and attribution of climate change.
His focus then shifted to the radiative effects of clouds on the climate. This was done using the Earth Radiation Budget Experiment (ERBE), which showed that clouds have a large cooling effect on the planet. ERBE was also able to measure the greenhouse effect without the use of climate models.
Recently, he has published on the aerosol radiative properties. His work has shown that aerosols have a cooling effect on the surface of the planet, and at the top of the atmosphere, but the forcing at the top of the atmosphere was only one-third the magnitude as the surface forcing. This has implications for the hydrologic cycle. While working on the Central Equatorial Pacific Experiment, he discovered that absorbing black carbonaceous aerosols have a larger influence on climate than previously thought, which led to the development of the Indian Ocean Experiment (INDOEX). In the 1990s, he led the Indian Ocean Experiment with Paul Crutzen and discovered the widespread existence of atmospheric brown clouds covering much of the Indian Ocean region. They found that the vast majority of the aerosols were anthropogenic in origin, and that the surface cooling caused by the aerosols is more important than the atmospheric heating. These atmospheric brown clouds may have masked as much as 50% of the surface heating caused by the increase in carbon dioxide, and caused reduced precipitation during the Indian monsoon.
Ramanathan is also interested in the impact of climate change on agriculture in India. While atmospheric brown clouds partially offset the warming due from carbon dioxide, their effect on agriculture has been less certain. A statistical rice model couple to a regional climate model has shown that reductions of both carbon dioxide and atmospheric brown clouds will increase crop yield.
He has also written on avoiding dangerous anthropogenic climate change. Ramanathan writes that there are several tipping points in the climate system, and that they do not all occur at the same temperature threshold; the tipping point for the arctic summer sea ice is likely to be smaller than that for the West Antarctic Ice Sheet. While the planet has seen an observed warming of 0.6 °C since pre-industrial times, it has already most likely committed itself to 2.4 °C (1.4 °C to 4.3 °C) of warming. These values surpass several of the tipping point thresholds. In a 2014 paper, Ramanathan and co-authors suggested that mitigating methane, soot, ozone and hydrofluorocarbons in the atmosphere could reduce the expected sea level rise due to climate change.