40's to 70's cooling, CO2 rising?
It's important to understand that there are positive and negative forcing components in the climate system that are natural cycle, or natural variability, and some forcing caused by human industrial output to the atmosphere. This human-induced forcing is both positive (greenhouse gases) and negative (sulfates and aerosols).
Mankind was in the midst of a large amount of production post WWII. During that time we added large amounts of greenhouse gases and aerosols to the atmosphere. Thus increasing positive and negative forcings.
After World War II, the industrial economies of Europe and the United States were revving up to a level of productivity the world had never seen before. To power this large-scale expansion of industry, Europeans and Americans burned an enormous quantity of fossil fuels (coal, oil, and natural gas). In addition to carbon dioxide, burning fossil fuel produces particulate matter—including soot and light-colored sulfate aerosols. Dr. James Hansen suspects the relatively sudden, massive output of aerosols from industries and power plants contributed to the global cooling trend from 1940-1970.
The result was the ozone layer was being destroyed increasing the risk of skin cancer, acid rain, and smog so thick, you could not see from one side of a city to the other.
“That’s my suggestion, though it’s still not proven,” Hansen said. “There is a nice record of sulfates in Greenland ice cores that shows this type of particle was peaking in the atmosphere around 1970. And then the ice core record shows a rapid decline in sulfates, right about the time nations began regulating their emission.” (Sulfates cause acid rain and other health and environmental problems.)
Here is the most reasonable explanation of what happened based on what we know:
The greenhouse gases increased the positive forcing (warming) and the aerosols increased the negative forcing (cooling). During the time between the mid 40's to the 70's, the negative forcing was strong enough to override the positive forcing of the additional greenhouse gases in the atmosphere.
The aerosol pollution got so bad that we had acid rain beginning to destroy crops and destroy the protective ozone layer in our upper atmosphere. So while we had the advantage of the aerosol pollution to offset global warming, the pollution was killing our food supply and causing cancer.
The result was the Montreal Protocol to reduce atmospheric pollutants and aerosols. Everyone remembers CFC's, skin cancer, and the Styrofoam problem.
So we cleaned up to save our food, air & skin..., it worked!
We removed the negative forcing that those pollutants contributed to the climate system we removed an impediment to the positive forcing, and then the warming trend returned.
Does this mean we should add CFC's to the atmosphere? No, that would only contribute to the damage to the agricultural and even damage the carbon sink as well as increase the incidence of skin cancer through further damage to the ozone protection in the upper atmosphere.
- 2005 RealClimate - The Global Cooling Myth