Climate forcings are factors that push Earth’s energy balance out of equilibrium, causing the planet to warm or cool. The concept is straightforward: Earth constantly absorbs energy from the sun and radiates energy back to space. When something disrupts that balance so that more energy comes in than goes out, temperatures rise. When more energy leaves than arrives, temperatures drop. That disruption is a climate forcing.
How Forcings Work
Think of Earth’s climate as a bank account where deposits are sunlight and withdrawals are heat radiating back to space. When the account is balanced, the climate stays stable. A forcing is anything that adds or removes money from the account without the system adjusting on its own.
Scientists measure forcings in watts per square meter (W/m²), which represents the change in energy over every square meter of Earth’s surface at the top of the atmosphere. A positive number means extra energy is being trapped (warming). A negative number means energy is being reflected or lost (cooling). As of 2019, the total human-caused forcing since the start of the industrial era in 1750 was about 2.72 W/m², according to the IPCC’s Sixth Assessment Report. That might sound small, but spread across the entire planet, it represents an enormous amount of extra energy.
Forcings vs. Feedbacks
One important distinction: a forcing is the initial push, not the chain reaction that follows. When carbon dioxide traps heat, that’s a forcing. When the resulting warmth melts ice, exposing darker ocean water that absorbs even more heat, that’s a feedback. Feedbacks amplify or dampen the original forcing, but they don’t start the process. Scientists keep these categories separate because forcings are things we can directly measure and, in many cases, control, while feedbacks are the climate system’s own response.
Cloud behavior is where this gets complicated. Clouds can act as both a forcing and a feedback depending on the situation. Aerosol particles from pollution can seed new clouds or make existing ones more reflective, which is a forcing. But when rising temperatures change cloud patterns on their own, that’s a feedback. The cloud feedback is currently the single largest source of uncertainty in projecting future warming.
The Major Human-Caused Forcings
Carbon dioxide is the dominant forcing agent. It enters the atmosphere through burning fossil fuels, deforestation, and industrial processes like cement production. Its forcing value since 1750 is about 2.16 W/m², meaning CO₂ alone accounts for roughly 80% of the total warming push from greenhouse gases. It also lingers in the atmosphere for centuries, so its effects accumulate over time.
Methane is a more potent heat-trapper molecule for molecule, though it breaks down faster, persisting for about a decade. It comes from natural gas and coal production, livestock, rice paddies, landfills, and wetlands. Fluorinated gases, including hydrofluorocarbons and perfluorocarbons, are synthetic chemicals used in refrigeration, air conditioning, and manufacturing. They exist in tiny concentrations but trap thousands of times more heat per molecule than CO₂, and some persist in the atmosphere for tens of thousands of years.
Not all human-caused forcings warm the planet. Aerosols, tiny particles released by burning fossil fuels and biomass, have a cooling effect. They scatter sunlight directly back to space and also make clouds brighter and longer-lasting, which reflects additional solar energy. This cooling partially offsets greenhouse gas warming, though the exact magnitude remains uncertain. Research shows that aerosol cooling through cloud interactions extends well beyond the areas where pollution is emitted, reaching remote oceans at mid and high latitudes, while the direct scattering effect stays more localized near emission sources.
Land use changes also act as a forcing. Since 1750, widespread deforestation in temperate regions has exposed more reflective surfaces (especially snow-covered ground in winter), which pushes slightly toward cooling. But in the tropics, removing forests reduces evaporation, which has a warming effect. The net result depends on latitude and season.
Natural Forcings
The sun’s energy output isn’t perfectly constant. It fluctuates on an 11-year cycle and over longer periods, and those variations count as natural forcings. However, solar changes over the industrial era have been small compared to the greenhouse gas buildup. Measurements show the sun’s contribution to recent warming is minor.
Volcanic eruptions are the most dramatic natural forcing. Large eruptions inject sulfur dioxide into the stratosphere, where it forms reflective aerosol particles that can cool the planet by several tenths of a degree for one to three years. The 1991 eruption of Mount Pinatubo, for example, temporarily cooled global temperatures by about 0.5°C. But volcanic cooling is short-lived because the particles settle out of the atmosphere relatively quickly.
Over much longer timescales, slow shifts in Earth’s orbit around the sun (known as Milankovitch cycles) change how much solar energy reaches different parts of the planet at different times of year. These orbital forcings operate over tens of thousands of years and are the primary driver of ice age cycles, but they change too slowly to matter on human timescales.
Why Scientists Use Effective Radiative Forcing
The standard metric has evolved over time. Early climate science used “instantaneous radiative forcing,” which simply measured the immediate energy change when you add a greenhouse gas or aerosol to the atmosphere. But this ignored rapid adjustments: clouds shifting, the stratosphere warming or cooling, and other atmospheric responses that happen within days or weeks before surface temperatures have time to change.
The preferred metric now is effective radiative forcing (ERF), which accounts for all of these fast atmospheric and surface adjustments. ERF gives a more accurate picture of how much a given forcing will actually change global temperature. One practical advantage is that different forcing agents (CO₂, methane, aerosols) behave more consistently under ERF. Using the older metric, scientists had to apply correction factors because a watt of forcing from aerosols didn’t produce the same temperature change as a watt from CO₂. ERF largely eliminates that problem.
How Forcings Add Up
The climate doesn’t respond to any single forcing in isolation. What matters is the net forcing: all the warming influences minus all the cooling ones. Since 1750, the greenhouse gas warming push has been partially offset by aerosol cooling, leading to that net value of about 2.72 W/m². The range of uncertainty (1.96 to 3.48 W/m²) is wide largely because of the difficulty in pinning down aerosol effects.
This net forcing doesn’t translate instantly into temperature change. The oceans absorb enormous amounts of heat, which creates a lag between when a forcing is applied and when the full warming shows up in surface temperatures. Even if all emissions stopped today, temperatures would continue rising for years as the climate system catches up with the energy already in the pipeline. This committed warming is one reason climate scientists emphasize that earlier emissions reductions produce disproportionately better outcomes: every year of delay adds forcing that the planet will spend decades responding to.