The Greenhouse Effect

Cliff Mitchell - 29/12/2019

Earth as a greenhouse

The Greenhouse Effect - What it is and why it's so important?

We all know that we use greenhouses to grow things (plants, tomatoes, vegetables, etc). That's because a greenhouse stays warm inside, even during the winter. During the day the sunlight shines into the greenghouse and warms the plants and air inside. At night, even though it's colder outside, the greenhouse stays pretty warm inside because the glass walls and roof trap the sun's heat. This is known as the greenhouse effect and you can read more about the physics behind the greenhouse effect here or here.

This is important for life on earth because there are gases in our upper atmosphere, such as carbon dioxide and methane, that act like the glass in a greenhouse, trapping heat generated by the sun and warming our planet. Without this natural greenhouse effect the earth's temperature would average around -18°C rather than a comfortable +15°C, and without which life on earth would be very different.

There is also a natural equilibrium in play where the heat coming into the earth's atmospher is balanced by that going out into space. Roughly 30 percent of incoming sunlight is reflected back into space by bright surfaces like clouds and ice. Of the remaining 70 percent, most is absorbed by the land and ocean, and the rest is absorbed by the atmosphere. The absorbed solar energy heats our planet. As the rocks, the air, and the seas warm, they radiate “heat” energy (thermal infrared radiation). From the surface, this energy travels into the atmosphere where much of it is absorbed by water vapor and long-lived greenhouse gases such as carbon dioxide and methane.

When they absorb the energy radiating from Earth’s surface, microscopic water or greenhouse gas molecules turn into tiny heaters — like the bricks in a fireplace, they radiate heat even after the fire goes out. They radiate in all directions. The energy that radiates back toward Earth heats both the lower atmosphere and the surface, enhancing the heating they get from direct sunlight.

This absorption and radiation of heat by the atmosphere is the natural greenhouse effect and its equilibrium is what is so beneficial to life on earth.

Unfortunately that natural equilibrium in the earth's greenhouse effect has now been destroyed - by human activity! Since the start of the industrial revolution (about 1760) humans have been burning fossil fuels (coal, oil, natural gas in increasing quantities in order to power industry and transport. Fossil fuels contain high percentages of carbon and release carbon dioxide, a potent greenhouse gas, when burnt. As a consequence the level of carbon dioxide in the earths atmosphere has been rapidly increasing to the point where it is now the highest in human history - at least 14 million years!

You can see this increase in global carbon dioxide levels over time in this animation. It shows the rapid increase in carbon dioxide (CO2) levels since 1979 and then goes back in time over 800 thousand years (kyBCE = thousands of years Before Common Era).

As you would expect, increasing the levels of greenhouse gases in our atmosphere has led to an increase in global average temperatures - the more greenhouse gases we have in our atmosphere, the more exaggerated the greenhouse effect becomes.

Global average temperatures have increased by 1.1°C since the pre-industrial period. This may not sound much but a one-degree global change is significant because it takes a vast amount of heat to warm all the oceans, atmosphere, and land by that much. In the past, a one- to two-degree drop was all it took to plunge the Earth into the Little Ice Age. A five-degree drop was enough to bury a large part of North America under a towering mass of ice 20,000 years ago.

The effects of global warming include rising sea levels, regional changes in precipitation, more frequent extreme weather events such as heat waves, and expansion of deserts. Ocean acidification is also caused by greenhouse gas emissions and is commonly grouped with these effects even though it is not driven by temperature. Surface temperature increases are greatest in the Arctic, which has contributed to the retreat of glaciers, permafrost, and sea ice. Overall, higher temperatures bring more rain and snowfall, but for some regions droughts and wildfires increase instead. Climate change threatens to diminish crop yields, harming food security, and rising sea levels may flood coastal infrastructure and force the abandonment of many coastal cities. Environmental impacts include the extinction or relocation of many species as their ecosystems change, most immediately the environments of coral reefs, mountains, and the Arctic.


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Cliff Mitchell
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