Few people in the late 20th century realized that humans have never known Earth's typical climate, or even had the slightest notion that the climate they were accustomed to was sharply colder than Earth’s typical climate. It would have shocked most people at that time to learn that the entirety of human civilization was embedded in an Ice Era that began roughly 60 million years earlier and that Earth's typical climate is considerably warmer than anything humans have ever experienced.
The early 20th century experienced cooling temperatures that had some scientists concerned about a return to ice age conditions. Then, from about 1920 through 1940, a shift to warmer weather had scientists fretting about the dramatic warming. The decade of the 1930s remains the warmest in the relatively short US temperature history as does the year 1934. Then concerns about global warming faded as patterns again shifted around 1940 and global cooling prevailed. As glaciers and polar ice expanded, concerns grew among some scientists that coal burning added particulates to the atmosphere that blocked incoming solar radiation to reduce global temperatures.
However, just as concerns peaked about a possible human-induced new ice age, another natural warm cycle began. During the last few decades of the 20th century, Earth experienced a natural warming cycle related to normal shifts in oceanic and atmospheric circulations and solar cycles. Nevertheless, some scientists became alarmed about increasing global temperatures, dubbing this warming episode "global warming" and, noting an upward trend in atmospheric carbon dioxide, presumed it to come from human activity. These scientists ignored historic climate trends and leapt at a theory - “anthropogenic global warming” (AGW), that tried to explain global warming as catastrophic to the planet’s future and caused by humans burning fossil fuels!
Ironically, in the entire reconstructed history of Earth’s climate, there has never been a time when climate change was driven by changes in atmospheric greenhouse gases! Then as the 21st century began, Earth’s climate managed to grab the headlines from the scientists. Once again the forces that create short-term climate variability worked to switch back to a cold phase. Ocean and atmospheric circulations moved to cold phase and solar activity dropped to an unusually low level that promised to be of extended duration. These factors led to a prolonged cooling trend whose effects are still being played out at this date! As global cooling replaced warming, the flawed AGW theory was rejected in favor of more research into natural climate variability and planning so humans could adapt to climate change.
During concerns about late 20th century “global warming” few asked the questions, “What should Earth’s global temperature be?” and “How do you measure global temperature with sufficient accuracy to know what it is?” Had these two questions been seriously considered, it is unlikely that efforts to postulate a human cause for such shifts would have succeeded. Humanity learned a valuable lesson in humility as it’s understanding of natural climate change matured and we became better prepared for inevitable climate changes.