The United Nation's Intergovernmental Panel on Climate Change (IPCC) defines climate change as "a change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer." What this means is that we know that climate change is happening when the 30 year average shows a statistical, persistent change over several decades. Climate change has happened throughout the Earth's history - as an example: around 20,000 years ago, the climate of Western Washington allowed a mile-high glacier to cover Puget Sound for about 3,000 years.

Climate change is triggered by a disruption to the Earth's energy balance. Causes of this disruption include changes in the energy:

• Coming into the Earth's atmosphere
• Factors: amount of incoming solar radiation from the Sun; scattering in the atmosphere
• When it hits the Earth
• Factors: energy absorption and reflection due to land and ice cover
• Leaving the Earth
• Factors: atmospheric concentrations of greenhouse gases and aerosols

Greenhouse gases are essential to maintaining a livable on the Earth. However, the IPCC and other scientists have shown that carbon dioxide, a type of greenhouse gas, has increased sharply since the beginning of the Industrial Revolution in the 1800's due mostly to the burning of fossil fuels by humans. The amount of carbon dioxide in the air has increased from 280 parts per million (ppm) in 1750 to 388 ppm in 2010, the highest level it's ever been in the past 650,000 years. This, along with similar increases in other greenhouse gases and changes to land cover, has contributed to the warming of air and ocean temperatures over the past several decades. One of the impacts of this warming is that most major ice sheets and glaciers, including on Greenland and Antarctica, are melting and causing a global rise in sea level.