The greenhouse effect got its name from the phenomenon of building heat in the atmosphere and surroundings within a closed space where sunlight can get in through windows. Most people see this effect on clear day where the sun is shining through the closed windows of a car. Once you open the door, you immediately feel a rush of heat and the seats are almost impossible to sit on because they are so hot. The name comes from greenhouses where nurseries of tropical and early starting plants get heat from sunlight coming in through the windows of the greenhouse. Some of the sunlight gets converted to infra-red waves which cannot pass through the glass. The energy builds up and heats the greenhouse on the cheap. This is especially good for starting plants early. Simpler greenhouses are called cold frames and cloches which capture and convert sunlight to warm young plants and help to extend the growing season.
Take this idea and extend it to the whole world. Carbon dioxide and methane act similarly to glass in that incoming sunlight can readily bounce back into space, but sunlight converted into infra-red electromagnetic radiation cannot go through carbon dioxide or methane. It is reflected back to the earth while more sunlight is converted into more heat. The result is that heat gets trapped and builds up in the lower atmosphere where we spend most of our time. The shape of the Earth and its orientation in space helps to dissipate and spread the excess heat around, but by adding more carbon dioxide and methane, the heating up increases and heat dissipation cannot keep up. The result is something called the "run away greenhouse effect" or in other words, an infra-red catastrophe.
Heat accumulation in the lower atmosphere does not end there. The greenhouse effect can take off quite dramatically. Heat accumulates in the ground and more importantly in the surface layers of the ocean. Once the temperature of the surface reaches and passes 79 degrees Fahrenheit, evaporation increases dramatically and there is a build up of hot moisture that goes into feeding tropical depressions which can evolve to severe hurricanes when other conditions are right. Excess carbon filters into the oceans as well, acidifying the water and contributing to the escalation of carbon dioxide build up as calcium carbonate in shells and coral is dissolved and escapes into the water and atmosphere.
As water evaporates, it also contributes to the greenhouse effect as water vapor is a greenhouse gas as well. The one advantage to water vapor is that it condenses into clouds and creates more reflectivity into space. We know this, because on a hot summer day, the shade offered by passing clouds offers us relief from the heat.
The planet Venus has been called the run away greenhouse planet. Temperatures on Venus are typically 800 degrees Fahrenheit. Venus is the hottest planet in our solar system, even though Mercury is much closer to the sun. This is due to the atmosphere of Venus being predominantly carbon dioxide. The atmosphere is also much denser than that of Earth; so much more incoming sunlight converted to infra-red radiation gets trapped in the atmosphere. The result is that Venus is so hot that it literally glows a dull red hue at the surface of the planet.
It is doubtful that the Earth will ever get that hot, even with our overactive consumption of carbon. Earth contains regulatory systems that helps keep excess carbon in check. However, even a modest increase in the greenhouse effect can have major consequences for humanity with shifting climate zones, polar and glacial melt down, more droughts and flooding.
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