Earth's climate runs on an energy budget with sunlight as income
Nearly all the energy in Earth's climate arrives as sunlight, and some leaks back out to space. When more comes in than goes out, the planet warms; when the balance tips the other way, it cools. Everything from volcanic winters to ocean cycles can be read as shifts in that energy budget.
Weather is the day-to-day expression of energy moving through the climate system. A region's climate is the long-run average and spread of its weather. Climate variability covers any variation lasting longer than a single weather event, while climate change refers to shifts that persist for decades or more. Some variability looks random and is called noise; other variability recurs in recognisable patterns, such as the Pacific decadal oscillation or the Atlantic multidecadal oscillation, both driven by processes inside the system itself.
External forcings push from outside. They include changes in the sun's output, wobbles in Earth's orbit, volcanic eruptions, the reflectivity of land, ice and ocean, mountain-building and drifting continents, and greenhouse gas concentrations. Humans add their own forcing through emissions of greenhouse gases and dust. Feedbacks can then strengthen or dampen the initial push, and some thresholds, once crossed, bring rapid or irreversible change.
Parts of the system respond at very different speeds. Volcanic ash can cool the air within a short time by reflecting sunlight, whereas the oceans take far longer: water expanding as it warms can take thousands of years to catch up. Arctic sea ice shows both at once, as melting ice suddenly darkens the surface and the water then warms and expands gradually. Winds and ocean currents spread incoming energy around the globe, shaping regional climates.
The terminology has a history. In 1966 the World Meteorological Organization proposed climatic change for any variation over more than ten years, whatever the cause. In the 1970s, as it became clear that people could drastically alter the climate, climate change took its place with a focus on human causes, later appearing in the names of the IPCC and the UN climate convention. Global warming became the dominant popular phrase in 1988, though scientists use it for surface temperature alone.
Source: Climate variability and change
