National Aeronautics and Space Administration

How does the earth system respond to natural and human-induced changes?

Climate scientists have been monitoring Earth's energy budget since the 1978 launch of NASA's Nimbus-7 satellite. That mission carried a new instrument into space called the Earth Radiation Budget Experiment (or ERBE), designed to measure all of the energy leaving through the top of Earth's atmosphere. All of the incoming sunlight minus all of the reflected sunlight and emitted heat is our world's energy budget. The second law of thermodynamics compels Earth's climate system to seek equilibrium so that, over the course of a year the amount of energy received equals the amount of energy lost to space. So typically the global energy budget is in balance.

But the energy budget can tip out of balance in any of three ways:

1. a change in the amount of incoming solar radiation. Such a change can be either or both due to a change in the Sun's energy output, or the ongoing changes in Earth's orbital mechanics.
2. a change in the abundance of greenhouse gases in Earth's atmosphere. Increasing the concentration of gases like carbon dioxide and methane slows the rate at which Earth emits warmth back to space relative to the rate at which sunlight warms the surface.
3. a change in Earth's reflective features. Bright white objects reflect more sunlight than they absorb, whereas dark brown, dark green, and dark blue objects absorb more sunlight than they reflect. Thus, increasing the extent of reflective objects -- like clouds, aerosols, and ice sheets -- cools the Earth. Conversely, reducing the extent
   

Earth's climate system is "sensitive" to any of the aforementioned changes at a scale of 1 watt per square meter, or greater. That amount of energy equals one Christmas tree light bulb for every 3-foot by 3-foot square of our world's surface. Climate scientists found that adding to or reducing Earth's energy budget by 1 watt per square meter is enough to cause the globe to warm or cool, depending upon the direction and magnitude of the change.