CO2 causes traps heat. Here is the general reason why:
Sunlight is caused by certain wavelengths of light from the sun. These wavelengths of light contain energy in the form of heat. This energy is partially absorbed by the earths surface, warming it, and partially reflected by the earths surface as a longer wavelength (the amount absorbed depends on the color of that surface). Some of that energy escapes out into space but some is absorbed by carbon dioxide molecules and reflected back to the earths surface, further warming the surface of the planet.
CO2 spends a great deal of time in the atmosphere (on the scale of years). In comparison, water vapor exits the atmosphere quickly as rainfall or snow.
CO2 also affects the ocean. Some of the CO2 is absorbed by the ocean. That absorbed CO2 causes a chemical reaction with dissolved calcium carbonates in the water. As a result, there are more H+ ions. The H+ causes and overall drop in pH (of about .1 pH units since preindustrial times). In addition, ocean acidification causes a decrease in the calcium carbonate ions corals and other calcifying animals use to build their skeletons. This leads to slower growth rates and a less dense dense coral skeletons.
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