Radiative forcing is the change in the balance of energy entering and leaving Earth’s atmosphere. When something adds more energy than escapes, the planet warms. When more energy escapes than arrives, it cools. Climate scientists measure this imbalance in watts per square meter (W/m²), the same unit used for any energy flow across a surface. A positive number means warming; a negative number means cooling.
The concept is central to understanding climate change because it lets scientists compare wildly different influences on climate, from carbon dioxide to volcanic eruptions to changes in the sun’s brightness, on a single common scale.
How Radiative Forcing Works
Earth receives energy from the sun as visible light and ultraviolet radiation. The planet absorbs some of that energy and radiates it back toward space as infrared heat. In a stable climate, the energy coming in roughly equals the energy going out. Radiative forcing measures any disturbance to that balance.
When you add a heat-trapping gas like carbon dioxide to the atmosphere, it absorbs outgoing infrared radiation and re-emits some of it back toward the surface. Less energy escapes to space, so the system gains heat. That extra retained energy, spread across every square meter of the atmosphere at the boundary between the lower and upper atmosphere (the tropopause), is the radiative forcing of that gas. A forcing of +1 W/m² might sound small, but multiplied across the entire surface of the planet it represents a massive amount of added energy.
The Biggest Contributors
Carbon dioxide is by far the dominant driver of warming. As of 2024, CO₂ alone contributes about 2.33 W/m² of radiative forcing, accounting for roughly 66% of the total forcing from long-lived greenhouse gases. Methane is the second largest contributor at about 0.57 W/m², or 16% of the total. Nitrous oxide adds another 0.23 W/m². These figures, tracked annually by NOAA’s Global Monitoring Laboratory, represent changes relative to pre-industrial levels around 1750.
Recent research has revised some of these numbers upward. Calculated methane forcing is now estimated to be about 25% higher than earlier estimates, and forcing at very high CO₂ concentrations is around 9% higher than previous formulas suggested. These updates matter for projecting how much warming to expect as greenhouse gas levels continue rising.
Factors That Cool the Planet
Not everything pushes the energy balance toward warming. Tiny airborne particles called aerosols, many of them produced by burning fossil fuels and biomass, reflect sunlight back to space before it can warm the surface. Sulfate aerosols from industrial pollution produce a cooling effect estimated between −0.26 and −0.82 W/m². Aerosols also change the properties of clouds, making them brighter and more reflective, with this indirect effect estimated between −0.3 and −1.8 W/m².
Changes in land surface reflectivity also play a role. Over the past two decades, snow-free land surfaces became about 2.2% more reflective globally, producing a negative forcing of roughly −0.16 W/m². That cooling effect was equivalent to about 60% of the warming caused by CO₂ emissions over a similar period. Mineral dust in the atmosphere has uncertain effects: depending on the type and location, it can either warm or cool, with estimates ranging from +0.09 to −0.46 W/m².
These cooling factors partially offset greenhouse gas warming, which is why scientists distinguish between gross greenhouse gas forcing and the net total forcing that actually drives temperature change.
Natural vs. Human-Caused Forcing
Solar variability and volcanic eruptions are the two main natural sources of radiative forcing. Changes in the sun’s energy output have fluctuated slowly over the past thousand years, with rates of change that are small compared to the pace of recent greenhouse gas increases. The average rate of increase in human-caused forcing from 1960 to 2000 was roughly 0.035 W/m² per year, faster than the rate of increase in natural forcing during any comparable 40-year window in the past millennium.
Volcanic eruptions can produce sharp, short-lived cooling by injecting sulfur compounds into the upper atmosphere, but these effects typically fade within a few years. The sustained, cumulative nature of greenhouse gas forcing is what sets it apart: CO₂ persists in the atmosphere for centuries, so its forcing builds over time rather than dissipating.
From Forcing to Temperature
Radiative forcing translates to temperature change through a relationship that climate scientists express simply: the change in global average surface temperature equals the forcing multiplied by a climate sensitivity parameter. This parameter captures how responsive the entire climate system is to an energy imbalance, including feedbacks like melting ice (which exposes darker surfaces that absorb more heat) and increased water vapor (which traps more infrared radiation).
The sensitivity parameter is not a fixed, perfectly known number, which is one reason projections of future warming come as ranges rather than single values. But the linear relationship holds well enough that radiative forcing remains the standard way scientists quantify and compare the strength of different climate influences.
Standard vs. Effective Radiative Forcing
Earlier climate assessments used a version called “stratospherically adjusted radiative forcing,” which accounted for temperature changes in the upper atmosphere but held everything else fixed. More recent work has shifted to a newer measure called effective radiative forcing (ERF), which also captures rapid changes in the lower atmosphere and land surface that happen within weeks to months of a disturbance.
For example, adding CO₂ to the atmosphere doesn’t just trap heat. It also cools the stratosphere, changes the temperature structure of the lower atmosphere, alters cloud patterns, and affects how plants open and close their pores, which changes how much water vapor they release. All of these responses happen quickly and modify the energy balance before the slower warming of the oceans even begins. ERF captures these fast-acting adjustments, making it a better predictor of how much the surface will eventually warm. This distinction is especially important for aerosols, which have complex interactions with clouds that the older measure handled poorly. The IPCC’s most recent assessment reports its headline numbers as effective radiative forcing from 1750 to 2019.