Climate Histories: Glen T. Trewartha and “the so-called greenhouse effect of the earth’s atmosphere”

It is mid-thirties, and the United States is in the middle of the Great Depression and the Dust Bowl. The drought that has struck Texas and Oklahoma is wreaking havoc in the American prairies. Lacking the strong root system of grass, the winds easily pick the loose topsoil and swirl it into dense dust clouds, known as the black blizzards. The dust chokes thousands of cattle and drives 60 percent of the population, later they called them exodusters, first in the cities, and later in the agricultural regions in the Far West.

At the same time at the University of Wisconsin, the geographer Glen Thomas Trewartha keeps himself busy investigating the weather and the climate elements, such as temperature, precipitation, and storms and their relative significance in contributing to an understanding of regional climates.

In 1937, Trewartha publishes an obscure textbook on meteorology “designed to provide an outline of content for a general introductory course on the atmosphere.” It is titled Introduction to Weather and Climate. The sun energy, he says, enters our atmosphere as shortwave radiation in the form of ultraviolet (UV) rays and visible light.  Once in the Earth’s atmosphere, clouds and ozone absorb a part of it (today we know it is about 23 percent), while about half (48%) of this energy, passes through the atmosphere and is absorbed by the Earth’s surface. But the earth also re-emits energy back to space as longwave radiation in the form of infrared rays.

The troposphere (the lowest region of the atmosphere) is almost transparent to sunlight says Trewartha,  however, there are gases in the troposphere that are efficient absorbers of the Earth’s emitted infrared energy. The two most effective are water vapour and carbon dioxide. These absorbing gases and their surrounding air warm up, emitting radiation downward, towards the Earth’s surface, as well as upward, towards space.  “Thus, the troposphere of the Earth is warmed by the surface of the Earth, not by the Sun directly.”

“Obviously”, Trewartha argues,  “the effect of the atmosphere is analogous to that of a pane of glass, which lets through most of the incoming short-wave solar energy but greatly retards the outgoing long-wave earth radiation, thus maintaining surface temperatures considerably higher than they otherwise would be. This is the so-called greenhouse effect of the earth’s atmosphere.”

It is the first time to call this phenomenon by its now familiar name. [1] Yet global warming, emerged in the public sphere, as a social and political issue, much later in late 1960s and 1970s, when the atmospheric modelling community began to provide corroborating evidence on global warming.

In 1968, Glenn T. Trewartha published a modified version  of Wladimir Köppen’s climate classification, published in the early 20th century, based on temperature and rainfall.  The agreed classification that followed the Köppen-Trewartha map  (Figure 1) incorporates five major domains and subdivisions. The five major domains were Tropical and the Subtropical, Temperate, Boreal and Polar. These were considered to divide the globe into five subzones provide more detailed categories where required within a broad zone.

Global distribution of Köppen-Trewartha climatic groups and types (from Trewartha 1968) Image Credit: FAO

[1] Glen T. Trewartha, An Introduction to Weather and Climate” (New York: Mc Graw-Hill, 1937)

Post image: Dust Bowl, Texas, 1935 Image Credit: Wikimedia