Why the Earth is heating up: the delicate physics of incoming and outgoing energy

Nature · Published 3 October 2026 Swipe more like this →

In short: Radiative forcing is the fundamental metric scientists use to measure the Earth's energy imbalance. When the energy entering our atmosphere from the sun exceeds the heat escaping back into space, the planet warms. This simple concept is the key to understanding how human activity is actively reshaping our global climate.

At its core, radiative forcing describes the difference between incoming solar radiation and outgoing infrared heat. In a stable climate, these two flows are in equilibrium. However, since the dawn of the industrial era, this balance has been disrupted. Researchers typically use 1750 as a baseline year to calculate how much human activity has altered the planet’s energy budget, as this period predates the rapid industrialization that significantly increased greenhouse gas concentrations.

The primary driver of current warming is the accumulation of greenhouse gases, which trap heat that would otherwise escape into the cold vacuum of space. Yet, the picture is complicated by other factors. Land-use changes, such as deforestation, alter the Earth's surface reflectivity, or albedo, changing how much sunlight is absorbed versus reflected. Furthermore, human-produced aerosols add a layer of complexity; bright particles like sulfates can reflect sunlight and cool the atmosphere, while dark particles like black carbon absorb heat and contribute to warming.

Quantifying these effects is a monumental challenge. Radiative forcing is not measured by a single instrument but is instead estimated through complex physical modeling that accounts for clouds, ice cover, and atmospheric gas properties. According to the Intergovernmental Panel on Climate Change, human-caused radiative forcing reached 2.72 watts per square meter by 2019 compared to 1750. While natural variations—such as fluctuations in solar output or volcanic eruptions—also influence the energy balance, the current net gain in energy is primarily driven by anthropogenic emissions.

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