Greenhouse gas molecules in the atmosphere absorb light, preventing some of it from escaping the Earth. This heats up the atmosphere and raises the planet’s average temperature. February 19, 2021 Show
What do CO2, methane, and water vapor have in common? If your first thought was “greenhouse gases,” you’d be correct! Greenhouse gases trap heat in the atmosphere, in a process called the “greenhouse effect.”1 But how do these molecules actually warm our planet? We’ll start our exploration of greenhouse gases with a single carbon dioxide (CO2) molecule. Let’s say this CO2 molecule came from the exhaust in your car. From your tailpipe, it drifts up into the atmosphere, diffusing among the other gases. There, particles of light—photons—hit our molecule. So what happens to those photons? “Greenhouse gas molecules will absorb that light, causing the bonds between atoms to vibrate,” says Jesse Kroll, Professor of Civil and Environmental Engineering and Chemical Engineering at MIT. “This traps the energy, which would otherwise go back into space, and so has the effect of heating up the atmosphere.” Basically, the bonds between the carbon and oxygen atoms in our CO2 molecule bend and stretch to absorb photons. (With other greenhouse gases, the molecular bonds are different, but in all cases, they absorb photons, stopping them from leaving the atmosphere.) Eventually, our CO2 molecule will release these photons. Sometimes, the photons continue out into space. But other times, they rebound back into the Earth’s atmosphere, where their heat remains trapped. And importantly, greenhouse gases don’t absorb all photons that cross their paths. Instead, they mostly take in photons leaving the Earth for space. “CO2 molecules absorb infrared light at a few wavelengths, but the most important absorption is light of about 15 microns,” says Kroll. Incoming light from the sun tends to have much shorter wavelengths than this, so CO2 doesn’t stop this sunlight from warming the Earth in the first place. But when the Earth re-emits this light,2 it has a longer wavelength, in the infrared spectrum. And the range of wavelengths around 15 microns is a particularly crucial window. The most common greenhouse gas, water vapor, doesn’t efficiently absorb photons in this range. So when CO2 grabs photons with wavelengths around 15 microns, it’s selecting for the same light that normally has the easiest time escaping Earth’s atmosphere. There’s another reason why CO2 is such an important greenhouse gas: it has a long atmospheric lifetime. This has to do with the way CO2 reacts (or rather, doesn’t react) with the atmosphere. “The atmosphere is a very oxidative environment due to the presence of oxygen and ultraviolet radiation,” says Kroll. Oxidation occurs when oxygen steals electrons from another atom—it’s the same chemical reaction that causes iron to rust. Methane, another greenhouse gas, reacts easily with oxygen, which removes it from the atmosphere within around 12 years. That’s long enough to affect the climate, but nowhere near the lifetime of CO2, which does not react with oxygen and can last over a century. CO2’s long lifespan is the key reason that human activities are leading to climate change. As we keep taking carbon-based compounds like coal and oil out of the ground, and put that carbon in the atmosphere in the form of CO2, the added CO2 piles up much faster than it can be naturally removed. Thank you to Brittney Andrews of Clearlake, California, for the question. You can submit your own question to Ask MIT Climate here. Read more Ask MIT Climate Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license (CC BY-NC-SA 4.0). ‘Greenhouse gases’ are crucial to keeping our planet at a suitable temperature for life. Without the natural greenhouse effect, the heat emitted by the Earth would simply pass outwards from the Earth’s surface into space and the Earth would have an average temperature of about -20°C. Greenhouse gases The greenhouse effect: some of the infrared radiation from the Sun passes through the atmosphere, but most is absorbed and re-emitted in all directions by greenhouse gas molecules and clouds. The effect of this is to warm the Earth’s surface and the lower atmosphere. A greenhouse gas is called that because it absorbs infrared radiation from the Sun in the form of heat, which is circulated in the atmosphere and eventually lost to space. Greenhouse gases also increase the rate at which the atmosphere can absorb short-wave radiation from the Sun, but this has a much weaker effect on global temperatures. The CO2 released from the burning of fossil fuels is accumulating as an insulating blanket around the Earth, trapping more of the Sun’s heat in our atmosphere. Actions carried out by humans are called anthropogenic actions; the anthropogenic release of CO2 contributes to the current enhanced greenhouse effect. Which gases cause the greenhouse effect?The contribution that a greenhouse gas makes to the greenhouse effect depends on how much heat it absorbs, how much it re-radiates and how much of it is in the atmosphere. In descending order, the gases that contribute most to the Earth’s greenhouse effect are:
In terms of the amount of heat these gases can absorb and re-radiate (known as their global warming potential or GWP), CH4 is 23 times more effective and N2O is 296 times more effective than CO2. However, there is much more CO2 in the Earth’s atmosphere than there is CH4 or N2O. Not all the greenhouse gas that we emit to the atmosphere remains there indefinitely. For example, the amount of CO2 in the atmosphere and the amount of CO2 dissolved in surface waters of the oceans stay in equilibrium, because the air and water mix well at the sea surface. When we add more CO2 to the atmosphere, a proportion of it dissolves into the oceans. Anthropogenic greenhouse gasesSince the start of the Industrial Revolution in the mid-18th century, human activities have greatly increased the concentrations of greenhouse gases in the atmosphere. Consequently, measured atmospheric concentrations of CO2 are many times higher than pre-industrial levels. Overview of global anthropogenic greenhouse gas emissions in 2017; figures here are expressed in CO2-equivalents. Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2015 (EPA, 2017). Main sources of anthropogenic greenhouse gasesCarbon dioxide levels are substantially higher now than at any time in the last 750 000 years. The burning of fossil fuels has elevated CO2 levels from an atmospheric concentration of approximately 280 parts per million (ppm) in pre-industrial times to over 400 ppm in 2018. This is a 40 per cent increase since the start of the Industrial Revolution. CO2 concentrations are increasing at a rate of about 2–3 ppm/year and are expected to exceed 900 ppm by the end of the 21st century. If this continues, together with rising emissions of CH4 and other greenhouse gases, by 2100 the global average surface temperature could have increased by up to 4.8°C compared to pre-industrial levels. Consequently, some scientists suggest goals to limit concentrations to keep temperature change below +2°C. This would include substantial cuts in anthropogenic greenhouse gas emissions by the middle of the 21st century through large-scale changes in energy systems and land use. In 2010, the burning of coal, natural gas and oil for electricity and heat was the largest single source of global greenhouse gas emissions (25 per cent). By comparison, in 2010, 14 per cent of global greenhouse gas emissions came from fossil fuels burned for road, rail, air and marine transportation. Agriculture, deforestation and other changes in land use account for one quarter of net anthropogenic greenhouse gas emissions. According to a United Nations report, livestock is responsible for about 14.5 per cent of this. The main sources of emissions are:
The rest is attributable to the processing and transportation of animal products. Higher concentrations of atmospheric CH4 are also caused by changes in land and wetland use, pipeline losses and landfill emissions. The use of fertilisers can also lead to higher N2O concentrations. Agriculture is estimated to be the main driver for around 80 per cent of deforestation worldwide. Source: Pixabay. Cement manufacture contributes CO2 to the atmosphere when calcium carbonate is heated, producing lime and CO2. Estimates vary, but it is widely accepted that the cement industry produces between five and eight per cent of global anthropogenic CO2 emissions, of which 50 per cent is produced from the chemical process itself and 40 per cent from burning fuel to power that process. The amount of CO2 emitted by the cement industry is more than 900 kg of CO2 for every 1000 kg of cement produced. Aerosols are small particles suspended in the atmosphere that can be produced when we burn fossil fuels. Other anthropogenic sources of aerosols include pollution from cars and factories, chlorofluorocarbons (CFCs) used in refrigeration systems and CFCs and halons used in fire suppression systems and manufacturing processes. Aerosols can also be produced naturally from a number of natural processes e.g. forest fires, volcanoes and isoprene emitted from plants. We know that greenhouse gases provide a warming effect to Earth’s surface, but aerosol pollution in the atmosphere can counteract this warming effect. For example, sulphate aerosols from fossil fuel combustion exert a cooling influence by reducing the amount of sunlight that reaches the Earth. Aerosols also have a detrimental impact on human health and affect other parts of the climate system, such as rainfall.
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Footnotes1. Enhanced Greenhouse effect 'Greenhouse gases' are actually crucial to keeping our planet at a habitable temperature, without them the Earth would be about minus 17 degrees! Anthropogenic or human release of carbon dioxide is what is contributing to an additional or enhanced greenhouse effect.
What is mechanism of atmospheric greenhouse effect?The greenhouse effect is a process that occurs when gases in Earth's atmosphere trap the Sun's heat. This process makes Earth much warmer than it would be without an atmosphere. The greenhouse effect is one of the things that makes Earth a comfortable place to live.
Which of the following best describes the greenhouse effect on Earth?The greenhouse effect happens when heat trapped by gases in the atmosphere warms the Earth. The best description of this process is found in answer choice (E). Atmospheric gases trapping heat and warming the planet's surface is the correct answer.
Which of the following best describes the cause of the greenhouse effect on Earth quizlet?Which of the following best describes the cause of the greenhouse effect on Earth? Gases in the atmosphere absorb infrared radiation and radiate the energy back toward Earth's surface.
How does Earth's atmosphere produce the greenhouse effect quizlet?The greenhouse effect is a natural process that warms the Earth's surface. When the Sun's energy reaches the Earth's atmosphere, some of it is reflected back to space and the rest is absorbed and re-radiated by greenhouse gases.
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Which of the following best describes the mechanism of the greenhouse effect in earths atmosphere?
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