Did Climate Scientists Flip Flop?

In the mid-20th century global warming took a short vacation and temperatures cooled. But did the scientific community really warn of an imminent ice age?

Since the beginning of the 20th century, global temperatures have increased by about 0.8 degrees centigrade or a little less than 1 1⁄2 degrees Fahrenheit. But the increase was not constant, and from the late 1940s to the mid-1960s temperatures actually cooled (see graphic).

Skeptics claim that climate scientists of the time sounded a false alarm: "An ice age is around the corner!" Today, the skeptics' claim goes, these same scientists warn of a global warming. The implication: it's a huge scientific flip flop and the scientific warnings cannot be taken seriously.

Did scientists back then really think an ice age cometh? And what caused that mid-century global cooling?

Climate Science 101

The Earth's climate for the past 2 million years has been characterized by ice ages lasting 100,000 years or so, punctuated by warm periods (or "interglacials") lasting tens of thousands of years. We have a pretty good understanding of the causes of these climate swings: changes in the Earth's orbit around the sun amplified by natural feedbacks involving greenhouse gases.

The Earth entered the present interglacial about 10,000 years ago. All things being equal (i.e., in the absence of a large human-produced source of the heat-trapping gas carbon dioxide [CO]), it is almost certain that the Earth will swing back into another ice age. But this will not occur for tens of thousands of years.

Popular Press vs. Science Journals

Around 1950 the upward temperature trend unexpectedly halted and temperatures declined somewhat. Some scientists became concerned about this trend, prompting news headlines about an imminent ice age. Skeptics like to point to a 1975 Newsweek article entitled "The Cooling World" [pdf] as evidence of the science community's consensus that an ice age was around the corner.

First off, Newsweek may be a great weekly news magazine (or not), but it is not by any stretch a scientific journal. The fact is that within the scientific community there was no consensus on where the climate was headed. And you don't have to take my word for it; you just have to read the Newsweek article itself. If you do, you will find the following quote from a report of the U.S. National Academy of Sciences on the state of scientific understanding of climate change at that time:

"Our knowledge of ... climatic change is ... fragmentary. Not only are the basic scientific questions largely unanswered but in many cases we don't yet know enough to pose the questions."

I would hardly characterize that as a scientific consensus.

Even during this period, global warming remained a serious concern. Many scientists of the time argued that whatever the cause of the cooling, natural or otherwise, it would eventually be overshadowed by the warming from CO. And, in the late 1970s global temperatures began again to inch upward.

Our Understanding of Climate Continues to Improve

Since the 1975 Newsweek article, our understanding of the climate system has grown considerably as a result of an intense and international research program using satellites to gather data on a global scale and modern computers to analyze these data. We now know that the mid-20th century pause in global warming was caused by pollution from burning coal, which produces tiny particles (or aerosols) that scatter and reflect the energy from the sun back out to space. (Read more about aerosols.) As the United States and Western Europe cut down on its use of coal, aerosol emissions were reduced and the cooling effect lessened, but since greenhouse gas pollution continued to increase, global warming resumed.

In 2005, the U.S. National Academy of Sciences weighed in again, reflecting on the great strides made in climate science:

"The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action ... to contribute to substantial and long-term reductions in net global greenhouse gas emissions."

A scientific flip flop? Not at all, just scientists doing their job moving up the learning curve.

Dr. Bill Chameides is the dean of Duke University's Nicholas School of the Environment. He blogs regularly at www.thegreengrok.com.