Climate change happens when Earth’s energy balance is altered—more heat is kept in the climate system than escapes to space. Today’s rapid warming is dominated by human activities. Key reasons:

Main human causes
- Greenhouse gases (GHGs) increase the greenhouse effect
  - Carbon dioxide (CO2): from burning coal, oil, and gas for electricity, heat, transport, and industry; also from cement production and deforestation. CO2 is the largest driver, responsible for about two-thirds of human-caused warming.
  - Methane (CH4): leaks from oil and gas systems and coal mines; livestock digestion and manure; rice paddies; landfills. Short-lived but very potent; contributes roughly a quarter of warming.
  - Nitrous oxide (N2O): mainly from agricultural fertilizers and manure management; some industrial sources.
  - Fluorinated gases: refrigerants and industrial gases; very strong warmers in small amounts.
- Land-use change and deforestation: release CO2 and reduce the land’s capacity to absorb it; also alter surface reflectivity and moisture exchange.
- Air pollution particles (aerosols) and soot
  - Sulfate and other reflective aerosols cool by reflecting sunlight and brightening clouds, masking some GHG warming.
  - Black carbon (soot) warms by absorbing sunlight and darkening ice/snow.

Amplifying feedbacks in the climate system
- More water vapor in a warmer atmosphere increases warming (water vapor feedback).
- Loss of snow and ice reduces reflection of sunlight (albedo feedback).
- Thawing permafrost can release CO2 and methane.
- Changes in clouds likely add net warming.
- Warmer oceans absorb heat and CO2 less efficiently over time, reinforcing warming.

Natural influences (not responsible for recent warming trend)
- Solar variability has been roughly flat since the 1970s.
- Large volcanic eruptions cause short-term cooling via stratospheric aerosols.
- Ocean–atmosphere cycles (e.g., El Niño/La Niña) shift heat around, driving year-to-year ups and downs, not the long-term rise.
- Slow orbital cycles (Milankovitch) drive ice ages over tens of thousands of years, not recent decades.

How we know it’s human-caused
- “Fingerprints” match GHG warming: troposphere warming with stratosphere cooling; nights and winters warming faster; more downward longwave radiation; ocean heat content rising.
- Carbon isotopes and falling atmospheric oxygen show added CO2 comes from fossil fuel burning.
- Climate models only reproduce observed warming when human GHGs are included.

In short: human emissions of greenhouse gases—primarily from fossil fuels, agriculture, and land-use change—are the main reason for today’s climate change, partially offset by cooling aerosols, and amplified by feedbacks in the Earth system. Natural factors affect variability but do not explain the sustained modern warming.
