Midcentury Global Cooling

What explains the 1940–1975 cooling trend?

Photo by Atle Mo on Unsplash

In one of my first articles, I addressed the change in ice cover observed on lakes as a result of a warming climate. In response to that article, I received a disparaging email claiming that scientists can’t make up their minds, and once thought the planet was cooling and heading into an ice age.

Climate Change in Madison, WI

Since this isn’t the first time I have heard, “scientists changed their minds”, I thought it was important to explain what actually happened during this time to try and dispel this myth.

The global temperature was declining from 1940–1975 (Figure 1); this is a fact. The drop was slight, about 0.1°C over the 35 year period, and was sandwiched between two warming periods. However, this cooling was only observed in the Northern Hemisphere (Damon and Kunen, 1976). The Southern Hemisphere was actually warming. This is an important piece of evidence that I’ll circle back to.

Figure 1: Global average temperature with 95% confidence intervals. (Image courtesy of Berkley Earth)

The increase in industrial activities following World War II was the main driver of this mid-century global cooling. An increasing amount of sulfate aerosols was being released into the lower atmosphere and these emissions were rising over these decades (Figure 2). Sulfate aerosols in particular effectively act like a mirror to the sun’s incoming radiation, reflecting some of the radiation back into space and cooling the planet (Andreae et al., 2005). This leaves less to reach the surface and warm the Earth. This phenomenon is called global dimming.

Figure 2: Sulfur dioxide (SO₂) emissions from fossil fuel combustion and processing. The black line is the central value and dotted lines indicated uncertainty bounds. Sulfur dioxide reacts with water to form sulfuric acid, which dissociates into hydrogen ions and sulfate. (source: Smith et al. 2011. license: CC Attribution 3.0)

There are also natural sources of these aerosols, such as large volcanic eruptions. Unlike industrial emissions, which hang in the air for a few months. Volcanos shoot them high in the atmosphere where they hang…