While other planets in Earth's solar system are either scorching hot or bitterly cold, Earth's surface has relatively mild, stable temperatures. Earth enjoys these temperatures because of its atmosphere, which is the thin layer of gases that cloak and protect the planet. Show However, 97 percent of climate scientists agree that humans have changed Earth's atmosphere in dramatic ways over the past two centuries, resulting in global warming. To understand global warming, it's first necessary to become familiar with the greenhouse effect, though. Energy in, energy outThere's a delicate balancing act occurring every day all across the Earth, involving the radiation the planet receives from space and the radiation that's reflected back out to space. Earth is constantly bombarded with enormous amounts of radiation, primarily from the sun. This solar radiation strikes the Earth's atmosphere in the form of visible light, plus ultraviolet (UV), infrared (IR) and other types of radiation that are invisible to the human eye. UV radiation has a shorter wavelength and a higher energy level than visible light, while IR radiation has a longer wavelength and a weaker energy level. About 30 percent of the radiation striking Earth's atmosphere is immediately reflected back out to space by clouds, ice, snow, sand and other reflective surfaces, according to NASA. The remaining 70 percent of incoming solar radiation is absorbed by the oceans, the land and the atmosphere. As they heat up, the oceans, land and atmosphere release heat in the form of IR thermal radiation, which passes out of the atmosphere and into space. It's this equilibrium of incoming and outgoing radiation that makes the Earth habitable, with an average temperature of about 59 degrees Fahrenheit (15 degrees Celsius), according to NASA. Without this atmospheric equilibrium, Earth would be as cold and lifeless as its moon, or as blazing hot as Venus. The moon, which has almost no atmosphere, is about minus 243 F (minus 153 C) on its dark side. Venus, on the other hand, has a very dense atmosphere that traps solar radiation; the average temperature on Venus is about 864 F (462 C). The greenhouse effectThe exchange of incoming and outgoing radiation that warms the Earth is often referred to as the greenhouse effect because a greenhouse works in much the same way. Incoming UV radiation easily passes through the glass walls of a greenhouse and is absorbed by the plants and hard surfaces inside. Weaker IR radiation, however, has difficulty passing through the glass walls and is trapped inside, thus warming the greenhouse. This effect lets tropical plants thrive inside a greenhouse, even during a cold winter. A similar phenomenon takes place in a car parked outside on a cold, sunny day. Incoming solar radiation warms the car's interior, but outgoing thermal radiation is trapped inside the car's closed windows.  Greenhouse gases and global warming"Gas molecules that absorb thermal infrared radiation, and are in significant enough quantity, can force the climate system. These type of gas molecules are called greenhouse gases," Michael Daley, an associate professor of Environmental Science at Lasell College told Live Science. Carbon dioxide (CO2) and other greenhouse gases act like a blanket, absorbing IR radiation and preventing it from escaping into outer space. The net effect is the gradual heating of Earth's atmosphere and surface, a process known as global warming. These greenhouse gases include water vapor, CO2, methane, nitrous oxide (N2O) and other gases, according to the Environmental Protection Agency (EPA). Since the dawn of the Industrial Revolution in the early 1800s, the burning of fossil fuels like coal, oil and gasoline have greatly increased the concentration of greenhouse gases in the atmosphere, especially CO2, National Oceanic and Atmospheric Administration (NOAA). "Deforestation is the second largest anthropogenic source of carbon dioxide to the atmosphere ranging between 6 percent and 17 percent," said Daley. Atmospheric CO2 levels have increased by more than 40 percent since the beginning of the Industrial Revolution, from about 280 parts per million (ppm) in the 1800s to 400 ppm today. The last time Earth's atmospheric levels of CO2 reached 400 ppm was during the Pliocene Epoch, between 5 million and 3 million years ago, according to the University of California, San Diego's Scripps Institution of Oceanography. The greenhouse effect, combined with increasing levels of greenhouse gases and the resulting global warming, is expected to have profound implications, according to the near-universal consensus of scientists. If global warming continues unchecked, it will cause significant climate change, a rise in sea levels, increasing ocean acidification, extreme weather events and other severe natural and societal impacts, according to NASA, the EPA and other scientific and governmental bodies. There are those that say that gases are not the cause of global warming, though that goes against the opinion of the global scientific community. "I think that measuring with precision human activity on the climate is something very challenging to do, and there's tremendous disagreement about the degree of impact. So no, I would not agree that it's a primary contributor to the global warming that we see," EPA chief Scott Pruitt told CNBC's morning news show "Squawk Box" March 9, 2017. [Carbon Dioxide Is Warming the Planet (Here's How)] Can the greenhouse effect be reversed?Many scientists agree that the damage to the Earth's atmosphere and climate is past the point of no return or that the damage is near the point of no return. "I agree that we have passed the point of avoiding climate change," Josef Werne, an associate professor at the department of geology & planetary science at the University of Pittsburgh told Live Science. In Werne's opinion, there are three options from this point forward:
Keith Peterman, a professor of chemistry at York College of Pennsylvania, and Gregory Foy, an associate professor of chemistry at York College of Pennsylvania, think that the damage isn't to that point yet, and that international agreements and action can save the planet's atmosphere. Currently, some scientists are investigating how to re-engineer the atmosphere to reverse global warming. For example, theories published in the journal Science in July 2017 by lrike Lohmann and Blaž Gasparini, researchers at the Institute of Atmospheric and Climate Science at ETH Zurich in Switzerland, proposed reducing cirrus clouds that trap heat. "If cirrus clouds behave like a blanket around the Earth, you're trying to get rid of that blanket," Lohmann, a professor of experimental atmospheric physics at ETH Zurich, told Live Science. [Cool the Planet? Geoengineering Is Easier Said Than Done] "You remove the water vapor, you remove the humidity and you prevent the normal cirrus cloud formation," Lohmann said.
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‘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 gasesThe 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 gases
Carbon 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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