[ Science · Reflectivity & Albedo ]

Why Cooling
Matters.

As the Earth loses its natural reflectivity, it absorbs more of the Sun's energy. Restoring reflectivity may become an increasingly important complement to emissions reduction in maintaining Earth's energy balance.

Albedo trend: −0.5 W/m²
CERES corroboration: Match
Meaning: More heat in

Restoring Earth's reflectivity

[ [ Section 1 · Foundations ] ]

Earth's energy balance

Climate depends on two fundamental processes: how much solar energy enters the Earth system, and how much heat escapes back into space. When these are equal, temperatures are stable. When they diverge, the planet warms or cools.

Roughly 340 W/m² of sunlight reaches the top of Earth's atmosphere on average. Of that, about 30% is reflected straight back to space by clouds, aerosols, snow, ice and bright land surfaces. The remaining 70% is absorbed — mostly by the oceans and land — and then re-emitted as infrared radiation.

Greenhouse gases in the atmosphere selectively absorb some of that outgoing infrared and re-emit it in all directions, including back toward the surface. The balance between what enters, what is reflected, and what escapes determines the planet's equilibrium temperature.

Earthshine: the planet's natural cooling system

Earthshine is simply sunlight reflected from Earth back into space — a portion of which grazes the dark side of the Moon and returns to us as a faint glow. Measuring it is one of the oldest ways humans have quantified Earth's albedo.

Albedo is the fraction of incoming solar radiation reflected back into space by a surface. It ranges from 0 (perfectly absorbing) to 1 (perfectly reflecting).

Fresh snow0.80–0.90
Sea ice0.50–0.70
Clouds0.40–0.80
Desert sand0.30–0.40
Forest0.10–0.20
Open ocean0.06–0.10
Asphalt0.04–0.12
Global avg.≈ 0.30

Even a small change in the global average — a few thousandths of an albedo unit — is climatologically significant, because it modifies hundreds of terawatts of solar power at the top of the atmosphere.

[ [ Section 3 · Observations ] ]

We are losing our Earthshine

Two independent measurements — ground-based Earthshine photometry and NASA's CERES satellite instruments — agree: Earth has become measurably less reflective over the past two decades. The planet is absorbing more solar energy every year.

Schematic reconstruction of shortwave anomaly relative to 1998, based on Goode et al. (2021) and CERES EBAF. Negative values mean more solar energy is being absorbed by Earth. Hover the chart to inspect each year.

Earthshine

≈ −0.5 W/m²

Additional shortwave energy absorbed by Earth over 1998–2017, as measured from the ashen light on the Moon (Goode et al., 2021).

CERES

Similar sign, larger magnitude

NASA's Clouds and Earth's Radiant Energy System satellite record independently corroborates the decline in reflected shortwave, with the largest change concentrated in the most recent years.

What it means

More energy retained

A darker Earth absorbs more sunlight. That extra energy warms the oceans, land and atmosphere, and adds to the warming from greenhouse gases.

Note: The two records use very different methods and are not expected to match exactly; what is striking is that they agree on the sign, the approximate timing, and the general magnitude of the change.

[ [ Section 4 · Drivers ] ]

Why the Earth is becoming darker

The decline in albedo is not a single phenomenon. It is the sum of many bright-to-dark transitions unfolding across the cryosphere, the atmosphere and the land surface — each individually small, collectively significant.