The energy balance sets Earth's temperature. Anything that changes the energy coming in or going out, measured in watts per square meter, pushes climate warmer or cooler. These pushes are called forcings; the rising carbon dioxide is the biggest one today.
Key idea
A radiative forcing is a change in Earth's energy balance, in W/m²: positive warms, negative cools. Natural forcings include changes in sunlight and volcanic eruptions. Human forcings include greenhouse gases (positive), pollution particles called aerosols (mostly negative: they reflect sunlight and brighten clouds) and land-cover changes. By 2019 the total human forcing since 1750 was about +2.7 W/m²; changes in the Sun over that time add almost nothing (IPCC 2021).
Why it works
Think of Earth's energy budget as income (absorbed sunlight) and spending (infrared to space). More greenhouse gas lowers the spending until Earth warms enough to restore the balance. More reflective haze or brighter clouds cut the income. A big volcanic eruption that reaches the stratosphere makes a haze of sulfuric acid droplets that reflects sunlight for a year or two; then it settles out. That is why a volcano gives a short, sharp cooling, while long-lived carbon dioxide gives a lasting push.
| Forcing (1750 to 2019) | About (W/m²) |
|---|---|
| carbon dioxide | +2.2 |
| methane | +0.5 |
| nitrous oxide, halocarbons and ozone | +1.1 |
| aerosols (pollution particles) | −1.1 (very uncertain) |
| land use (brighter cleared land) | −0.2 |
| other small effects (contrails, soot on snow) | +0.1 |
| changes in the Sun | about 0.0 |
| total human | +2.7 |
Worked example: Adding forcings
Using the rounded table values, what is the total forcing from greenhouse gases (carbon dioxide, methane, and nitrous oxide, halocarbons and ozone)?
- $2.2 + 0.5 + 1.1 = 3.8$ W/m².
- Aerosols (−1.1), land use (−0.2) and the small effects (+0.1) bring the total to about +2.7 W/m² (the table's values are rounded).
Answer: About +3.8 W/m²
Worked example: The forcing from carbon dioxide
A standard formula for the forcing from carbon dioxide is $\Delta F = 5.35 \ln(C/C_0)$ W/m². Find the forcing for 420 ppm compared with 280 ppm.
- $C/C_0 = 420/280 = 1.5$, and $\ln 1.5 \approx 0.405$.
- $\Delta F = 5.35 \times 0.405 \approx 2.2$ W/m², matching the table.
Answer: About 2.2 W/m²
Common mistake
Thinking a big volcanic eruption warms the climate. Its gases do include carbon dioxide, but far less than people emit; the big effect is the sulfur haze in the stratosphere, which reflects sunlight and cools Earth for a year or two. Mount Pinatubo erupted in 1991 and cooled the global average by about half a degree Celsius in 1992. Check the sign: volcanic forcing is negative.