Step outside on a sunny afternoon and you can feel the sun's energy on your skin. Step out again after dark and you can feel the pavement giving some of that warmth back. Every day, the whole planet does the same thing. It takes in sunlight and releases heat into space.

For most of human history, those two flows stayed close to even. Today they don't. Earth is absorbing slightly more energy than it sends away, and scientists can now measure that gap directly. It's called Earth's energy imbalance. It may be the most fundamental number in climate science: the planet's heat budget, measured in watts.

Think of Earth's climate as a bathtub with the tap running and the drain open. When water flows in as fast as it drains out, the level stays steady. Sunlight is the tap. Heat escaping to space as invisible infrared energy is the drain. A stable climate is a tub where the two match.

Averaged over the whole globe, the sun delivers about 340 watts to every square metre at the top of the atmosphere. Clouds, ice and other bright surfaces reflect about 100 of those watts straight back. The rest is absorbed, and normally an equal amount escapes as heat. Greenhouse gases like carbon dioxide partly block that escape route. In effect, they narrow the drain.

The current imbalance is roughly 1 watt per square metre. That sounds trivial, because it is less than half a percent of incoming sunlight. But spread across Earth's entire surface, it adds up to a continuous flow of about 500 trillion watts. That is more than twenty times all the energy humanity uses. The rate is small, but the surplus keeps accumulating, day after day and year after year.

Takeaway

Climate change is less about how hot any single day feels and more about energy piling up. A tiny surplus, sustained year after year, is enough to reshape a planet.

The first method looks down from orbit. NASA's CERES instruments, flying on several satellites since the late 1990s, measure both the sunlight Earth reflects and the infrared heat it emits. Comparing those figures with the sun's known output shows how much energy stays behind. Satellites are excellent at tracking changes from month to month. Pinning down the exact total precisely is harder.

The second method follows the heat to where it ends up. About 90 percent of the extra energy goes into the oceans. Since the early 2000s, a fleet of roughly 4,000 robotic Argo floats has drifted through the world's seas. Every ten days, each float sinks to about 2,000 metres, then rises, recording temperature along the way. Warmer water means stored energy, so scientists can calculate how much heat the ocean has gained.

These two approaches are completely independent. One counts energy at the edge of space. The other measures temperature in the deep ocean. Scientists combine them: the ocean data anchor the overall size of the imbalance, while the satellites reveal how it shifts over time. When researchers compared the trends from each method, they matched closely. Two very different instruments told the same story.

Takeaway

Confidence in science often comes from agreement between unrelated methods. When a satellite in orbit and a float in the deep sea reach the same answer, that answer is hard to dismiss.

In the mid-2000s, the imbalance was about 0.5 watts per square metre. By around 2020, both satellites and ocean floats showed it had grown to roughly 1 watt. That is approximately double in fifteen years, and some more recent estimates suggest it has climbed further. The planet isn't just gaining heat. It's gaining heat faster.

What's driving the increase? Satellite data show that most of the change comes from more sunlight being absorbed, rather than less heat escaping. In other words, Earth has become slightly darker. Melting sea ice and snow expose darker ocean and land beneath. Low, bright clouds appear to have thinned in some regions. Cleaner air plays a role too. As countries cut sulphur pollution, there are fewer tiny reflective particles to bounce sunlight back to space.

Rising greenhouse gases continue to narrow the drain. Natural ocean cycles, such as shifting temperature patterns in the Pacific, also affect clouds. Scientists are still working out how much each factor contributes, and whether some of these changes are feedbacks, meaning effects caused by warming itself. That question matters, because feedbacks could keep widening the gap on their own.

Takeaway

Cleaning the air saves lives, yet it also removes a haze of pollution that had been hiding some of the warming. The climate responds to everything we put into the sky, not just carbon dioxide.

Earth's energy imbalance cuts through daily weather and year-to-year noise. It answers a simple question: is the planet still gaining heat? Measured from orbit and from the deep ocean, the answer right now is yes, and the gain is faster than before.

Because the gap can be measured, so can progress. As emissions fall, this number is one of the places where a slowdown should show up. The heat budget gives us a clear yardstick for judging whether our climate choices are working.