Most graphs of 20th-century temperatures show a warming period starting in the mid-1970s. One reason for this rise in temperatures might be the onset of a “warm” phase of the roughly 30-year cycle known as the Pacific Decadal Oscillation (PDO).
However, climate alarmists assume that any warming seen since that time has been caused by increasing emissions of carbon dioxide (CO2). Climate critics disagree, though, and believe that a strong increase in solar output since the late 1800s is responsible for progressively rising temperatures.
While there are various reasons to believe that both increasing solar activity and a fluctuating PDO helped to drive the late-20th-century surge in temperatures, there’s also another factor that may also explain some of this increase—and also help to explain the subsequent observed “pause” in surface warming.
Since 2000 or so, there has been a net flatlining of global temperatures. Despite strong El Niño patterns in 1997, 2010, and 2015, global temperatures have remained within a stable band—and show essentially no warming over that time.
Intriguingly, this “hiatus” in warming is also apparent in stratospheric temperature measurements. As reported in this 2015 article, the flatlining of stratospheric temperatures occurred after the successful implementation of the Montreal Protocol—a global effort to eliminate Chlorofluorocarbons (CFCs). By international agreement, CFCs were banned after they were shown to break down ozone concentrations in the stratosphere.
The relevant point is that declining ozone in the stratosphere allowed progressively greater levels of ultraviolet radiation to reach the Earth’s surface. In fact, there is a clear correlation between accelerating CFC concentrations in the late 1970s and declining ozone levels. The resulting increase in ultraviolet penetration also corresponds with a discernible increase in global temperatures starting at the same time.
Ultraviolet radiation helps to warm the Earth in what is commonly known as the “greenhouse effect.” Solar ultraviolet radiation strikes the planet’s surface and is re-emitted as infrared radiation. Atmospheric gases like water vapor and CO2 subsequently “trap” some of this infrared radiation as heat.
Overall, an increase in ultraviolet radiation reaching the Earth’s surface would drive an increase in observed temperatures. Once the Montreal Protocol took full effect in 1996, CFC levels began to fall and ozone levels in the stratosphere stabilized. At the same time, temperatures in the stratosphere and troposphere stabilized as well—leading to the current “pause” in observed temperatures.
In short, mankind may have contributed to rising temperatures in the late 20th century due to the release of CFC gases into the atmosphere. But the global effort to eliminate CFCs subsequently brought both declining ozone and rising temperatures under control. This may help explain why the current “pause” has continued, even after the latest, strong El Niño occurrence.