 Imagine you're a climate scientist in the 1970s. At the time, global temperature hadn't changed much for several decades. In fact, it had even cooled slightly. We'll look at why in a moment, but what's interesting is what climate scientists were expecting to happen next. Most research in the 1970s projecting future climate expected that global temperatures should soon start to increase. In other words, these papers predicted imminent global warming. Why? Because humans were burning huge amounts of fossil fuel. We were emitting heat-trapping greenhouse gases into the atmosphere. In fact, among climate papers published between 1965 and 1979, the number predicting global warming was accelerating. The scientific community was becoming more convinced that global warming was just around the corner. And what happened next? Global temperatures did start to increase, just as climate scientists expected. However, a small number of papers in the 1970s speculated that under certain conditions global cooling might happen. This is the basis for one persistent myth about climate change. That in the 1970s, the scientific community was predicting global cooling, maybe even a new ice age in the next few decades. This myth is a misrepresentation. It distorts the scientific position in the 1970s and to understand how we need to look at why that small number of papers were speculating about global cooling. After World War II, the global economy was growing fast. Burning of fossil fuels, like oil and coal, grew rapidly. When we burned fossil fuels, besides releasing carbon dioxide pollution, we also released other byproducts into the atmosphere. Among these byproducts are tiny particles called sulfate aerosols. As fossil fuel burning increased in the mid-20th century, we emitted more sulfate aerosols into the atmosphere. We know that carbon dioxide is a heat-trapping gas causing global warming. Sulfate aerosols have the opposite effect. They reflect sunlight, which leads to an overall cooling. With both carbon dioxide and sulfate aerosols increasing at the same time, climate scientists were faced with a challenging question. Which would win out? The warming from carbon dioxide or the cooling from sulfate aerosols? Because humans are unpredictable, climate scientists don't make predictions. Instead, they make projections. A projection is when we estimate what will happen as a result of a certain scenario. Each projection represents a different path that humanity could take. How much more global warming we'll see depends on which path we choose. The same was true in the 1970s. Scientists didn't know exactly how much fuel we'd burn or whether we'd start installing filters to reduce aerosol pollution. What happened next would determine how Earth's temperature would change. Like they do today, climate scientists considered different scenarios. Then they made global temperature projections for those scenarios. In one scenario, NASA scientists considered what would happen if sulfate aerosol pollution accelerated rapidly. Specifically, they examined what would happen if sulfate aerosols in the atmosphere quadrupled due to fossil fuel burning. They calculated that this would block enough sunlight to cool global temperatures by 3.5 degrees Celsius or 6 degrees Fahrenheit. They further concluded that if that kind of cooling continued for several years, it might even trigger a new ice age. Despite this projection, climate scientists in the 1970s generally expected to see global warming from increased greenhouse gases. So how did a handful of papers turn into a myth about the scientific community predicting an imminent ice age? In 1974 and 1975, Time and Newsweek magazines published articles warning about a possible oncoming ice age. This was partly based on NASA's research. But Time and Newsweek are news magazines, not scientific journals. No one is surprised when a news magazine publishes a sensationalised headline. But in scientific papers, which take a more reasoned evidence-based approach, the research indicated oncoming global warming. Moreover, those NASA scientists weren't predicting global cooling or an ice age. Their paper projected that if sulfate aerosol pollution quadrupled, then we might trigger an ice age. But the opposite happened. In the 1970s, a number of countries enacted regulations to reduce sulfate aerosol pollution. One example of this legislation is the passing of the Clean Air Act in the United States. As a result, sulfate pollution began declining in the late 1970s. The scenario where sulfate aerosol emissions quadrupled just didn't happen. This example of sulfate pollution is instructive for today. Scientists published research explaining the effects of sulfate pollution. In response, governments enacted regulation to reduce the pollution. In the same way, we can avoid the worst impact of global warming if we act to reduce carbon dioxide pollution today.