A common refrain in the climate change literature, both popular and professional, is the impending doom of human society by human-induced global warming. A temperature threshold is even suggested that will tip the world into “dangerous climate change”: 2°C (3.2°F) over pre-industrial temperature, “an upper limit beyond which the risks of grave damage to ecosystems, and of non-linear responses, are expected to increase rapidly”. The U.S. National Research Council claims that such a threshold has been passed and “animals and other species are already struggling to keep up with rapid climate shifts, increasing the risk of mass extinction that would rival the end of the dinosaurs.” But, this has not always been the view of scientists working on the climate. Not long ago geologists thought that the Earth was headed into another ice age, and that very soon humans might be in danger from advancing ice, not thawing ice. (Some researchers still think this).
In the mid 20th Century, there was renewed interest in the origin of the Pleistocene “ice ages”, which waxed and waned between about 2.6 million and 12,000 years ago. During the Pleistocene large, miles-deep ice sheets covered most of northern Europe and northern North America (see figure below).
Deposits left from these glaciers, recorded several ice advances and retreats. But the exact number and timing of these cycles were poorly known. Then in the 1950s and 1960s new tools were discovered that unlocked the details of the ice ages. One scientist who’s research fundamentally changed how we think about ice ages, was Cesare Emiliani. Emiliani examined tiny fossils called foraminifera (termed “forams” for short). Forams construct a tiny shell out of calcium carbonate (similar to chalk) by extracting calcium and carbonate from seawater. So, their shells mirror the composition of seawater at the time they lived. When forams die and settle to the ocean bottom they record seawater composition over time as sediment builds up. Emilani discovered that he could use the signature of oxygen isotopes in forams to measure past ocean temperatures. (I will leave the details of this for a later post, it is somewhat complicated.)
Using these tiny “paleothermometers”, Emiliani was able to construct a much more detailed picture of Pleistocene ice sheet advance and retreat. (link to paper). He found numerous glacial advances and retreats over hundreds of thousands of years. He also correlated advances and retreats to the amount of the sun‘s radiation hitting the Earth (termed, insolation) in the Northern Hemisphere. Insolation is fundamentally controlled by changes in the tilt of the Earth’s axis, the shape of its orbit, and the precession of the equinoxes, what are termed “orbital parameters”. In 1966, Emiliani used these relationships to predict a coming glacial event: “…it is to be expected that a new glaciation will begin within a few thousand years and reach its peak about 15,000 years from now.” (figure below left; link).
During the late 1960s and early 1970s other geologists looked at the newly-precise Pleistocene record and came up with similar predictions. In 1972, George Kukla summarized an international workshop on the subject: “It is likely that the presentday warm epoch will terminate relatively soon if man does not intervene”. And, the National Science Board found: “Judging from the record of the past interglacial ages, the present time of high temperatures should be drawing to an end, to be followed by a long period of considerably colder temperatures leading into the next glacial age some 20,000 years from now.” These views were based on the new understanding at the time of glacial cycles but were also spurred on by new global records of climate becoming available through better climate instrumentation. One of these instrumental records was for temperature. What it showed in the mid 1970s was a strong decrease in temperature over the last 40 years.
This trend combined with the new insights about the ice ages, lead to cautious predictions of continued cooling by many scientists, including those working for the NOAA. This research first made widespread public attention when it was summarized in a story in Time on June 24, 1974 titled “Another Ice Age?”. This article presented a map of increasing ice and snow cover in the Arctic, some of the first data from newly launched environmental satellites. Then a year later, “The Cooling World” was published in Newsweek on April 28, 1975. This article showed a global map of mostly cooling temperatures and a time series of global temperature showing the sharp drop in temperature after 1938 (as shown in the figure below).
Although these articles are now commonly presented as poor journalism because they do not fit our current view of global warming, they were not. They were mostly accurate accounts of what many scientists thought and had published at the time.
Now let’s fast forward to 2014. Forty years after the Time article on the coming of another ice age, we have a much deeper understanding of the length, magnitude and duration of Pleistocene glacial cycles. Let’s look at a couple of relative new articles to see what has changed. First, a paper in 2012 by P.C. Tzedakis from University College London and four co-authors (link here). Tzedakis and his co-authors found that if CO2 concentrations did not exceed 240 ppmV (they are now about 390 ppmV) our current interglacial would end in about 1500 years, followed by another glacial advance. They used both patterns of past interglacials matched to the isolation cycles for our present interglacial and adding ocean processes transporting heat to make their comparisons. Their conclusion was that humans have now added too much CO2 to the atmosphere for the next glaciation to occur if glacials and interglacials followed the same patterns and were controlled by the same processes in the late Pleistocene. But how long would that hold up? To start to answer that question we need to look at a paper by Archer and Ganopolski published in 2005 (link here).
Archer and Ganopolski used a climate model to simulate future climates and ice sheet conditions at different concentrations of CO2. They found that the next glaciation could be postponed a very long time if enough CO2 was added to the atmosphere. Since the start of the industrial revolution, humans have released a bit more than 300 gigatons (Gt) of excess carbon into the atmosphere. Their model predicted that those concentrations would postpone the start of the next glaciation by about 1500 years. But they found that if we had added about 1000 Gt of carbon (about three times what we have added so far), the next glaciation would be put off for 140,000 year. And if we had burned all the known reserves of fossil fuels, there would not be another glaciation for at least another 500,000 years—the length of their modeling experiments. If these modeling simulations are correct, humans could completely negate the effects of the global processes that have controlled ice sheet advance and retreat for at least the last 2.5 million years by burning all available fossil fuels.
This is a sobering example of how humans have become a major geologic force, rivaling the controls on Earth’s climate originating at a cosmic scale (orbit of the planet)!