We've looked at natural cycles and causes. None of them can produce this warming.
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
“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.
It’s not natural variation
You're probably familiar with the El Niño and La Niña
cycles that influence our weather. These repeat irregularly every two to seven years, affecting rainfall and drought across America and even altering Atlantic hurricane activity. The cycles are the strongest of several oscillations that alter how ocean heat is distributed over time and place. Mann describes them as the “random sloshing back and forth of the climate.”
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.
Combined, all the events that are currently influencing the climate create a lot of year-to-year noise in temperatures. But a clear signal of human-caused climate change emerged back in the 1950s
above the random “sloshing back and forth” variability.
It’s not volcanoes
Volcanoes have a split personality when it comes to climate—they cool it temporarily, yet they also release CO2 that keeps Earth from freezing solid. Volcanic CO2 is the main source of geological carbon emissions that kept our planet habitable for billions of years. Without its “greenhouse effect
,” the planet’s average temperature would be an icy -18° C
compared to about +14° C
, where it is today. And yet, “the amount of CO2 emitted from volcanoes is tiny compared to human activities,” Schmidt told me.
Geological processes emit CO2 from volcanoes, mid-ocean ridges, rift valleys, geothermal systems, and from heat and pressure on rocks at depth. Combined, these release about 0.148 billion tons
of CO2 per year—just 0.4 percent of the 36.3 billion tons
of human emissions in 2021. To put that in perspective, it would take 1,650 eruptions
as big as the huge Pinatubo eruption in 1991, every year, to match human CO2 emissions. Even geological methane from sources such as mud volcanoes is much less
than methane from human activity.
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.
It’s not Earth’s orbit
Wobbles in Earth’s orbit around the Sun are actually cyclic and can affect climate. Called “Milankovitch Cycles
,” after the scientist who discovered them, they're the reason the climate has alternated between cold “glacial” times, when ice sheets covered large parts of the northern hemisphere, and less cold “interglacial” times, when those ice sheets melted away. These cycles happened some 50 times in the last 2.6 million years
, but they operate over 23,000-, 41,000-, or 100,000-year and longer timeframes, so they're far more gradual than modern warming.
It’s not plate tectonics
It’s true that dinosaurs thrived in a warm climate
, and the Arctic was fringed with palm trees
50 million years before the Pleistocene Ice Age. These multimillion-year shifts
between “greenhouse” and “icehouse” climates were the result of plate tectonics, which sometimes breaks out in more volcanoes than usual, constructs huge mountain chains, or lets those mountains erode away
These tectonic changes affect the balance between the CO2 emitted by geological processes (mainly volcanoes) and the CO2 removed by geological processes, mainly the chemical reaction of CO2 with water and silicate minerals, known as “silicate weathering.”
Our eroding landscape today is about 50 percent more efficient
at removing CO2 than it was 16 million years ago, and CO2 levels in the atmosphere have dropped and the climate has cooled since that time. But silicate weathering is much too slow to make a difference in our time. It’s like an ant eating an elephant: it will get there eventually, but at the slow pace of plate tectonics—in hundreds of thousands of years. In the meantime, half our emissions
are absorbed by plants and ocean water, and the rest is building up in the atmosphere, warming the climate.
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
, 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.