It’s not the Sun
The Sun is the source of energy on the surface of our planet, so it stands to reason that variations in solar activity might cause climate changes. But solar activity has been declining over the past few decades as our planet warmed, so there’s no link. Although solar energy is immense, its variations are tiny.

“It was called the solar ‘constant’ for a long time because you need extremely sensitive instruments to see any variation in the Sun's energy output,” said Owens. Over an 11-year sunspot cycle, the solar energy reaching the top of the atmosphere varies by about 0.15 percent, but it rises and falls every cycle, so it can’t drive climate trends like ours. //

In addition to these 11-year cycles, the Sun also goes through “grand solar minima” and “grand solar maxima” of activity that last decades. One of those, called the “Maunder Minimum,” was once thought to be the cause of a cold period between about 1300 and 1850, called the Little Ice Age.” But “it just doesn't add up,” Owens told me. “The temperature starts to drop long before the Maunder Minimum happened.”

The Maunder Minimum may have contributed a fraction of a degree to the cooling during the Little Ice Age, which evidence has since indicated was mostly the result of volcanic eruptions and human land use changes.

The Sun also regulates the dose of cosmic rays inflicted on our atmosphere. These are mostly protons that originate in space from things like supernovae, and there was an idea in the late 1990s that they might affect climate by seeding cloud formation. But the data shows no correlation, Owens told me, and experiments with the CERN particle accelerator show that cloud seeding by cosmic rays is weak. “The growth rate of droplets is just too small to really do anything in the atmosphere,” said Owens, so it can’t explain the Little Ice Age or modern climate change.

Owens is underwhelmed by the Sun’s current activity: “We're ramping up into solar cycle 25. It's looking very, very average!” he said. //

Mann and others have found no discernible climate oscillation in the last thousand years that lasts as long as our climate has been warming, so the warming has outlasted all of these natural oscillations.

It turns out that some apparently natural cycles are illusions. The 40-60-year-long “Atlantic Multidecadal Oscillation” is one of several that are really just echoes of decades-long cooling caused by explosive volcanic eruptions in the preindustrial era. More recently, competition between human-caused warming and human-caused cooling resulting from sulfurous pollution has also left its imprint on the oscillation. Consequently, “key trends, such as the warming of the tropical Atlantic and the increase in hurricane activity associated with it cannot, as some researchers have claimed, be blamed on an internal oscillation,” said Mann. They are instead the result of human-caused warming. //

If an eruption is explosive enough to loft material into the stratosphere and if that material includes a lot of sulfur dioxide gas, the gas forms tiny droplets of sulfuric acid in the stratosphere. These “act like a shiny mirror,” Schmidt said, which reflect some sunlight back into space and cool Earth’s surface.

Eventually the droplets “sediment out of the atmosphere,” as Schmidt put it, and temperatures recover. The 1991 Pinatubo eruption cooled the climate by up to 0.5°C for nearly three years, but bigger historical eruptions had stronger impacts. The eruption of Tambora in 1815 caused 1816 to be “The Year Without a Summer,” and eruptions in 1257, 1452, and 1600 were probably the main causes of the “Little Ice Age.”

“The ocean has a long memory of any changes in temperature,” Schmidt told me, so cooling by past eruptions, like the enormous 1883 eruption of Krakatau, still slosh back and forth in climate variations today.

Ironically, human-caused warming will raise the altitude of the stratosphere, making it harder for eruption plumes to reach it, and will also speed up a stratospheric wind known as the “Brewer-Dobson Circulation,” which will enhance the cooling by those fewer eruptions that manage to reach the higher stratosphere. //

We can rule out the usual natural suspects people often bring up to sow doubt about our role in climate change, and we can rule in humans because multiple lines of evidence prove our role. As the IPCC and agencies in the US, UK, Europe, Japan, China, and others have documented in exhaustive detail, global warming is unequivocally driven by emissions from human activities.

As sure as sure can be, it’s not natural—it’s us.