Because science is often about trying to disprove theories, our understanding of natural phenomena is constantly being tested. As the great physicist Albert Einstein pointed out, “No amount of experimentation can ever prove me right; a single experiment can prove me wrong.”
This is especially true of a complex field like climate science. With so many variables, conditions, effects, hypotheses, and predictions, it is impossible to be 100 percent certain about any of it. But scientists are now about as certain as they ever get that the earth is warming at an unusually rapid pace and that humans are largely responsible. For the IPCC’s Fifth Assessment Report, released in four chapters in 2013–14, hundreds of scientists and experts worldwide combed through the most up-to-date peer-reviewed scientific literature and other relevant materials to assess “the state of scientific, technical and socio-economic knowledge on climate change, its causes, potential impacts and response strategies.”
They determined that it is “extremely likely,” or 95 percent certain, that humans are a major factor in rapid global warming and that evidence for climate change itself is “unequivocal.” Science rarely gets more certain than that, and the uncertainty only lies in the understanding that there may be undetermined factors or that natural factors could play a larger or smaller role than experiments and observation have illuminated. And, because a large part of climate science is predictive, there is room for variation. But all of the theories surrounding climate change have been and are being constantly tested, with scientists looking for flaws as well as ways that the theories can be confirmed. The overwhelming evidence shows that although the earth’s climate constantly changes, it is now changing, warming, more rapidly than ever, and although natural phenomena such as solar and volcanic activity play a role in climatic changes, this rapid warming can only be explained by considering the major contribution of human activity. Increasingly sophisticated predictive models and observation also show that the extreme weather and other consequences we’re experiencing now will only get worse if we continue to emit greenhouse gases into the atmosphere and damage or destroy the natural systems that absorb and store carbon.
As I’ll show in the next section, this evidence has been building for much longer than many people realize.
Ice Age Studies, Feedback Loops, and the Greenhouse Effect
SCIENTIFIC UNDERSTANDING OF the greenhouse effect isn’t new. French mathematician and natural philosopher Joseph Fourier discovered in 1824 that the earth’s atmosphere retains heat that would otherwise be emitted back into space by infrared radiation. Although he didn’t call it the greenhouse effect, he explained his concept by comparing the earth and its atmosphere to a box with a glass cover. It’s a simplistic comparison, and as the American Institute of Physics points out in an excellent summary of the history on its website (from which some of this section is drawn), a glass box or greenhouse does not function in entirely the same way as the earth and its atmosphere.1
Fourier’s research inspired other scientists to consider the phenomenon. In 1859, Irish-English scientist John Tyndall began studying the ability of gases such as water vapor, carbon dioxide (then known as carbonic acid), ozone, and hydrocarbons to absorb and transmit radiant heat.2 On finding that water vapor, ozone, and carbon dioxide, or CO2, absorbed heat radiation better than gases such as oxygen, hydrogen, and nitrogen, he theorized that fluctuations in water vapor and carbon dioxide could affect global climate. He also discovered the idea of heat islands, by noting that the city of London was warmer than its surroundings.
Some years later, self-taught British scientist James Croll observed that dark surfaces such as soil, rock, and trees hold heat from the sun, whereas snow and ice remain cool, and that as a region cools, wind patterns change, which could affect ocean currents.
Much of the research to this time was aimed at understanding the causes of ice ages. A major breakthrough in our understanding of the effect of greenhouse gases occurred in 1896. Croll’s ideas led Swedish scientist Svante Arrhenius to surmise that a drop in Arctic temperatures could cause land that had been bare in summer to remain covered in ice year round.3 This ice would reflect more of the sun’s heat back into space, lowering the temperature even more, thus creating a positive feedback cycle. He then observed that water vapor could also cause a feedback loop, as warmer air puts more water vapor into the atmosphere, and because water vapor holds heat in, more warm air is created. Because CO2 also absorbs heat radiation, Arrhenius concluded that adding CO2 to the atmosphere would contribute to this feedback cycle. Thus, burning fossil fuels and increasing CO2 emissions into the atmosphere could increase water vapor, causing global average temperatures to rise.
Arrhenius wanted to understand what could cause an ice age, and his studies led him to conclude that cutting CO2 in the atmosphere by half could cause one. But he also calculated what would happen if the amount was doubled by burning fossil fuels, and concluded that this would cause a 5-or-6-degree-Celsius (9-or-10.8-degree-Fahrenheit) increase in global average temperatures—an estimate surprisingly close to the one climate scientists came up with using much better computer models one hundred years later.
A year after Arrhenius published his findings, American geologist Thomas Chamberlin examined the earth’s carbon cycles more deeply, and according to the American Institute of Physics, wrote that ice ages are “intimately associated with a long chain of other phenomena to which at first they appeared to have no relationship.” It’s a concept that indigenous peoples have taught me, and one that I often talk and write about: Everything is interconnected.
In his “very speculative” paper, published in 1897, Chamberlin hypothesized that CO2 could affect feedback cycles that bring about ice ages. The complexity of his ideas involved looking at the effect of volcanoes as they spew CO2 into the air, and what happens when volcanic activity is lower and carbon is absorbed and stored by minerals, plants, and oceans, called carbon sinks. Because the atmosphere contains only a small fraction of the earth’s carbon compared to these carbon sinks, and carbon cycles through the atmosphere every few thousand years, Chamberlin proposed that climate conditions “congenial to life” are in a delicate balance.
At the time, however, it was believed that natural forces, such as solar activity and the ability of oceans to absorb and store carbon, were far more important factors and that CO2 had an insignificant influence compared to water vapor. Many scientists believed climate was self-regulating and that small changes to atmospheric composition could not alter climate over brief time periods. Any CO2 that human activity did emit into the air would be absorbed quickly by oceans (and, to some extent, forests and peat bogs)—which was mostly true at the time, when far smaller amounts of fossil fuels were being burned. Some also argued that excess atmospheric CO2 would fertilize plants and create more lush life—which is also true, to a point. Although the notion that human activity, such as burning ever-increasing amounts of fossil fuels, could not affect a self-regulating climate has been thoroughly disproven by modern science, many people still make the same outdated arguments today.
As with earlier scientific investigations, most climate science in the first half of the twentieth century was driven by a desire to explain the causes of ice ages. In the 1950s, scientists started to get an idea of the bigger picture. In 1956, Maurice Ewing and William Donn, at New York’s Lamont Geological Observatory, were also trying to explain ice ages, in particular the abrupt end of the most recent one. In looking at feedback cycles in the Arctic, they speculated that a complex set of circumstances could lead to rapid climate change over the next few hundred or thousand years. But the change they saw was the coming of another ice age.
Their theories were controversial and often criticized, but they did serve to spark a renewed interest in climate science, more testing of theories, and wider acceptance of the idea that changes in Arctic ice sheets and snow cover could cause rapid changes in planetary surface conditions.
By the 1950s, researchers in the Soviet Union were using this growing scientific knowledge to consider ways to deliberately alter local climatic conditions, by “making Siberia