Are humans responsible for climate change?

Yes, humans are unequivocally responsible for modern climate change.

An overwhelming scientific consensus—comprising 97% to over 99% of actively publishing climate scientists, alongside virtually every major scientific academy and organization in the world (including NASA, NOAA, and the Intergovernmental Panel on Climate Change)—confirms that human activities are the primary driver of the rapid global warming observed since the mid-20th century.

The Primary Cause: The Greenhouse Effect

The fundamental mechanism of modern climate change is the human-driven expansion of the greenhouse effect.
When we burn fossil fuels like coal, oil, and gas for energy, we release heat-trapping greenhouse gases—primarily carbon dioxide ($\text{CO}_2$), methane ($\text{CH}_4$), and nitrous oxide ($\text{N}_2\text{O}$)—into the atmosphere.

Other human actions, such as deforestation (clearing forests, which naturally absorb $\text{CO}_2$) and industrial agriculture, further exacerbate the problem. These greenhouse gases act like a thickening blanket wrapped around the Earth, trapping heat that would otherwise escape into space.

The "Smoking Gun" Human Fingerprints

Scientists do not merely guess that humans are responsible; they have identified distinct physical fingerprints in the climate system that point directly to human activity.

1. A Cooling Stratosphere and a Warming Troposphere

If the warming were caused by a hotter Sun, we would expect the entire atmosphere to warm up. However, satellites and weather balloons show that only the lower atmosphere (the troposphere, where we live) is warming, while the upper atmosphere (the stratosphere) is actually cooling. This is exactly what we expect from a thickening blanket of greenhouse gases: they trap heat close to Earth's surface, preventing it from reaching and warming the stratosphere.

2. The Isotopic Carbon "Signature"

Not all carbon is the same. Carbon from fossil fuels has a unique chemical signature. Because fossil fuels are millions of years old, they contain no radioactive Carbon-14, and they have a lower ratio of Carbon-13 to Carbon-12 compared to carbon from natural, current sources. By measuring the isotopes of carbon in the atmosphere, scientists have shown that the surging level of atmospheric $\text{CO}_2$ matches the exact signature of burned fossil fuels.

3. Less Heat Escaping into Space

Satellites measuring infrared radiation leaving Earth have recorded a direct, physical trend: less and less heat is escaping into space. Crucially, the missing heat is escaping in smaller amounts at the exact wavelengths that are absorbed by human-emitted gases like carbon dioxide and methane.

4. Unprecedented Pace of Warming

While Earth's climate has changed naturally over millions of years (due to shifts in Earth's orbit or volcanic activity), those changes occurred over thousands or tens of thousands of years. Today's warming is occurring roughly 10 times faster than the average rate of natural warming following past ice ages, and atmospheric carbon dioxide is increasing 250 times faster than it did from natural sources after the last ice age.

Why Natural Factors are Ruled Out

Scientists routinely test natural culprits—such as changes in the Sun's energy, orbital cycles (Milankovitch cycles), and volcanic eruptions—to see if they could explain the current warming.

Summary

The scientific evidence is incontrovertible. Earth is warming, and the "fingerprints" left behind by this warming point directly and undeniably to human greenhouse gas emissions.