The atmosphere, biosphere, hydrosphere, and lithosphere are the four physical systems. These are the basic building blocks of the planet's physical systems. The Earth-Sun connections must be synced for the planet to be livable and capable of maintaining life as we know it. The Earth's environmental conditions have changed over time due to these physical systems interacting with each other. The atmosphere is the layer of gases that surrounds and protects the biosphere. The atmosphere works with the ocean system to regulate temperature and water vapor content on the planet. Climate change occurs when the amount of heat trapped by carbon dioxide in the atmosphere changes, causing temperatures to rise. Greenhouse gases allow more sunlight into the atmosphere than would otherwise occur with only ozone and clouds blocking some of this light energy. But too much of this heating can be dangerous: a doubling of the concentration of carbon dioxide in the atmosphere will increase global average temperatures by about 6 degrees Celsius (10 degrees Fahrenheit). The biosphere is the part of the Earth that supports life. It is made up of the oceans, landmasses, ice caps, and forests, all of which play a role in determining our climate. Humans have become a significant force in changing the environment through deforestation, soil degradation, and greenhouse gas emissions. The hydrosphere is the portion of the Earth that contains water.