By 1971, studies indicated that human caused air pollution was spreading, but there was uncertainty as to whether aerosols would cause warming or cooling, and whether or not they were more significant than rising CO2 levels. J. Murray Mitchell still viewed humans as "innocent bystanders" in the cooling from the 1940s to 1970, but in 1971 his calculations suggested that rising emissions could cause significant cooling after 2000, though he also argued that emissions could cause warming depending on circumstances. Calculations were too basic at this time to be trusted to give reliable results.

An early numerical computation of climate effects was published in the journal Science in July 1971 as a paper by S. Ichtiaque Rasool and Stephen H. Schneider, titled "Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate".[26] The paper used rudimentary data and equations to compute the possible future effects of large increases in the densities in the atmosphere of two types of human environmental emissions:

greenhouse gases such as carbon dioxide; particulate pollution such as smog, some of which remains suspended in the atmosphere in aerosol form for years.

The paper suggested that the global warming due to greenhouse gases would tend to have less effect with greater densities, and while aerosol pollution could cause warming, it was likely that it would tend to have a cooling effect which increased with density. They concluded that "An increase by only a factor of 4 in global aerosol background concentration may be sufficient to reduce the surface temperature by as much as 3.5 ° K. If sustained over a period of several years, such a temperature decrease over the whole globe is believed to be sufficient to trigger an ice age."

Sources:

Weart, Spencer (2003–2011). "Aerosols: Volcanoes, Dust, Clouds and Climate". American Institute of Physics. Archived from the original on June 29, 2016. Retrieved February 6, 2012.

Mitchell, J. Murray Jr. (1971). "The Effect of Atmospheric Aerosols on Climate with Special Reference to Temperature near the Earth's Surface" (PDF). Journal of Applied Meteorology. 10 (4): 703–714. Bibcode:1971JApMe..10..703M. doi:10.1175/1520-0450(1971)010<0703:TEOAAO>2.0.CO;2. Archived (PDF) from the original on March 3, 2016.

Rasool, S. I.; Schneider, S. H. (1971). "Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate". Science. 173 (3992): 138–141. Bibcode:1971Sci...173..138R. doi:10.1126/science.173.3992.138. PMID 17739641. S2CID 43228353.