Short answers, pulled from the story.
Climate variability includes all variations in climate that last longer than individual weather events, while climate change refers specifically to variations that persist for decades or more. Climate change is now commonly used to describe contemporary, often human-driven changes, frequently called global warming.
Earth's energy budget is the balance between the energy received from the Sun and the energy radiated back to outer space. When incoming energy exceeds outgoing energy, the budget is positive and the climate system warms; when more energy leaves than arrives, the budget is negative and the planet cools.
El Nino is part of the El Nino-Southern Oscillation (ENSO), described as the most prominent known source of year-to-year variability in weather and climate worldwide. The full cycle recurs every two to seven years, with the El Nino phase lasting nine months to two years within that longer pattern.
The 1991 eruption of Mount Pinatubo lowered global temperatures by about 0.5 degrees Celsius for up to three years. It did so by injecting sulfur dioxide into the stratosphere, which formed a global layer of sulfuric acid haze that scattered and absorbed incoming solar radiation.
Three to four billion years ago the Sun emitted only 75 percent of its current energy output, which should have prevented liquid water from existing on Earth's surface. Yet evidence from the Hadean and Archean eons confirms water was present; scientists hypothesize that greenhouse gas concentrations in the early atmosphere were far higher than today.
Paleoclimatologists use proxy methods including ice cores, tree rings, pollen, beetle remains, marine sediments, cave stalagmites, and historical documents such as chronicles and maps. Ice cores from Antarctica preserve ancient air bubbles that record past carbon dioxide concentrations, while tree ring growth rates reflect past precipitation and temperature stress.