It starts as a reasonable question: If the Earth's climate changed before humans existed, how can we be so sure the current change is due to us and not something natural?

To answer that question, we need to understand what caused the natural changes of the past. Fortunately, science has a good handle on the causes of Earth’s natural climate changes going back hundreds of millions of years. Some were cyclical; others were gradual shifts or abrupt events, but none explain our changing climate today.

A zombie claim

With energy policy and elections in the news, the claim by some politicians that climate change is natural is once again bubbling up from the disinformation swamp. So I asked some scientists a very unscientific question: What would they buy if they had a dollar for every time they heard it?

“A heat pump for my house,” said professor Mathew Owens of the University of Reading. “A time machine to… convince policymakers to act on climate decades ago,” said Professor Michael Mann of the University of Pennsylvania. Professor Anja Schmidt of the German Aerospace Center and the Universities of Munich and Cambridge would make a movie to explain that “volcanoes are not to blame,” while Professor Tim Lenton of the University of Exeter would “lobby governments to teach this stuff in school.”

“I love cycling, so I’d probably buy another bike,” Professor Michel Crucifix of University of Louvain in Belgium told me. “I would probably buy some solar panels,” said Professor Jeremy Caves Rugenstein of Colorado State University.

Fortunately, these scientists also had a lot to say about the natural forces of climate change and their non-role in global warming today.

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.