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Wednesday
Mar012017

Greenhouses Need Carbon Dioxide

Carbon dioxide (CO2) is a plant food.  The CO2 concentration in the atmosphere has been increasing for the last 150+ years thanks to human activity, due to extracting and consuming fossil fuels, as well as land use changes. The added CO2 has benefited and continues to benefit natural systems and agriculture.  Scientists estimate that 15% of the current agricultural production comes from elevated CO2.For most of Earth’s evolutionary history, the CO2 concentrations were much higher than they are today or that they can reach in the foreseeable future. For example, the CO2 concentration in the Jurassic period was 3-6 times higher than it is today.

The so-called “pre-industrial atmosphere” was anomalously impoverished in CO2. Our sun is the main source of energy on the Earth’s surface. Heat is removed from the Earth’s surface and lower atmosphere through multiple mechanisms, including convection and radiation.  Earth radiates in the infrared spectrum.  Gases with molecules having three or more atoms are infrared-active: their molecules can absorb and emit radiation in specific parts of the infrared spectrum.  The main infrared-active gas in the Earth atmosphere is water vapor.  CO2 plays a very minor role, except during ice ages.  Further, it has a very narrow absorption/emission spectrum, and it is almost saturated even at low CO2  The amount of infrared radiation, temporarily absorbed by CO2 molecules in the atmosphere, grows nearly logarithmically (i.e., very slowly) with increases in CO2 concentrations.

Thus, the anthropogenic increase in atmospheric CO2 concentration has an extremely small warming effect, which is negligible compared with natural temperature fluctuations in any region. Even larger mean warming would be beneficial for human societies and the biosphere, especially as a buffer against natural temperature drops.

A greenhouse is a man-made construction, minimizing air convection that removes heat from the ground and thus increasing the inside air temperature. Most greenhouses are artificially enriched with CO2 to facilitate plants’ growth.  CO2 plays no role in heat retention in a greenhouse!

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