It has now been an astonishing 60 years, to the day, since the richest country on the planet issued its first high-level, governmental warning about the dangers of increasing greenhouse gas concentrations in the atmosphere. The authors made clear that the details were still hazy, in the sense that the precise response of the climate to specific amounts of increased CO2 at that point had a relatively wide range. Exactly how much warming was on its way was nebulous; that it was, in fact, on its way, was not.
They estimated that a 25-percent increase in atmospheric CO2 "compared to the amount present during the 19th century" could yield a temperature increase between 0.6° Celsius and 4.0° C (1.1° F to 7.2° F). They guessed that milestone would arrive around the turn of the century — they were wrong. In November 1965, the concentration of atmospheric CO2 had risen from that pre-industrial level of about 280 parts-per-million to 319 ppm; it crossed 350 ppm, that 25 percent threshold, in 1988. Today it sits at 425 ppm, more than 50 percent higher than before humans got busy burning things.
Though they emphasized the need for better modeling and measurement to nail town the temperature increase, the authors did write that even that 350 ppm mark would produce "measurable and perhaps marked changes in climate, and will almost certainly cause significant changes in the temperature and other properties of the stratosphere."
The report went on to describe some of the potential effects of this "unwitting" experiment, including the melting of ice caps and subsequent sea level rise, acidification of water sources, and more that has become familiar in the years since. And in the end, a technocratically worded but doom-tinged conclusion: "The climatic changes that may be produced by the increased CO2 content could be deleterious from the point of view of human beings."
The blame, throughout appendix Y4, was put squarely on its proper target: "We can conclude with fair assurance that at the present time, fossil fuels are the only source of CO2 being added to the ocean-atmosphere-biosphere system." It even offered estimates of the excess carbon that might be added in the future based on recoverable reserves of coal, oil, and gas — and we have found a lot more of the stuff since then. I think, more often than is healthy, about a line from yet another decades-old climate change report, from EPA authors in 1983: "[T]he shift away from fossil fuels perhaps could be instituted more gradually and therefore less expensively if energy policies were adopted now rather than several decades later."