What Is Ozone?
Ozone is a naturally occurring gas that resides in our atmosphere. Each molecule of ozone contains three atoms of oxygen and has the chemical symbol O3. Ozone is found mainly in two regions of the atmosphere: about 10 percent is in the troposphere, the layer closest to the surface of the Earth. The rest of the ozone (90 percent) is in the lower stratosphere – the layer just above the troposphere – between about 20 and 30 km (12-19 miles) high.
The mass of ozone in the stratosphere is commonly called the “ozone layer.” 1
Stratospheric ozone absorbs harmful ultraviolet (UV) radiation from the sun, notably UVB-type rays. Exposure to UVB radiation is associated with an increased risk of sunburn, skin cancer and cataracts. Because of its protective role, higher-altitude ozone is sometimes dubbed “good” ozone.
It should not be confused with the “bad” ground level ozone (also called tropospheric ozone) found near the surface of the earth, which is produced mainly as a result of the chemical reaction between UV radiation and combustion gases from vehicle exhausts. 2
Tropospheric ozone is a key component of photochemical smog, a type of urban air pollution that causes a range of serious respiratory complaints. Please note that this article deals exclusively with stratospheric ozone.
There is no direct link between global warming and ozone depletion. However, there are some indirect links. For example, UVB radiation accelerates polar ice melt and permafrost thaw, which would result in extra warming from the reduced ice albedo and the release of carbon dioxide (CO2) and methane (CH4) respectively.
- Why Is the Ozone Layer Important?
- What Is Ozone Depletion?
- Who Discovered Ozone Depletion?
- The Rowland-Molina Hypothesis
- What Was The Reaction Of The Aerosol Propellant Industry?
- When Did Ozone Depletion Start?
- Ozone Depletion In 4 Simple Steps
- What Is The Ozone Hole?
- Why Isn’t There An Ozone Hole Above The Arctic?
- How Is Ozone Measured?
- How Bad Is Ozone Depletion In The Antarctic?
- How Bad Is Ozone Depletion In The Arctic?
- How Bad Is Ozone Depletion Globally?
- Do Volcanoes Affect The Ozone Layer?
- What Are The Effects Of Ozone Depletion?
- Hiperion Report Highlights Global Health Risks Of Ozone Depletion
- Does Ozone Depletion Affect Animals?
- Are Crops Affected?
- What Was The Global Response To Ozone Depletion?
- Are Ozone Levels In The Atmosphere Improving?
- How Long Before The Ozone Layer Recovers Completely?
- Does Ozone Depletion Have Anything To Do With Global Warming?
Why Is the Ozone Layer Important?
As mentioned, ozone is important because it shields us from high-energy, ultraviolet solar radiation. If there was no ozone layer to protect us, UV rays would break up biological molecules in our bodies, causing DNA damage, skin cancers, eye damage and more.
In addition, UV rays would cause reduced photosynthesis and crop damage, as well as damage to marine life including phytoplankton and corals. They would also speed up the thawing of permafrost in the arctic bioregion.
A recent study indicates that sunlight boosts bacterial conversion of soil carbon into carbon dioxide gas by at least 40 percent. 3 UV rays would also lead to increased polar ice-melt with a corresponding fall in the beneficial albedo effect. 4
NOTE: For more about the timeline of our planet, its oxygenation and the development of its atmosphere, as well as a historical summary of global warming, see: History of Earth in One Year (Cosmic Calendar).
What Is Ozone Depletion?
Ozone depletion is the erosion or thinning of the stratospheric ozone layer caused by the release of gaseous chlorine or bromine compounds from industrial and other human activities.
The thinning is predominantly confined to polar regions (but see HIPERION report, below), especially over the continent of Antarctica. For the health reasons listed above, ozone depletion is a serious environmental problem because it allows more ultraviolet (UV) radiation to reach Planet Earth. It’s worth emphasizing that these chlorine and bromine gases – known as ozone-depleting substances (ODSs) – do not occur naturally and therefore ozone depletion is entirely a man-made problem.
The sulfur cycle – specifically, sulfur dioxide emissions from volcanic activity – is also responsible for ozone depletion, albeit, indirectly. During a major volcanic eruption, such as the 1991 eruption at Mount Pinatubo, a huge amount of sulfur dioxide is released into the stratosphere. This outgassed sulfur dioxide is gradually (over 2 months) converted to sulfuric acid by reaction with hydroxyl radicals (OH). This condenses into aerosols, whereupon nitrogen oxides (NOx) react with the surface of the aerosols to form nitric acid (HNO3). The absence of NOx leads to increased depletion of the ozone layer.
Fortunately, due to an unusually high level of international cooperation, the ozone layer is expected to recover during the course of the 21st century.
Where is the Ozone Hole?
The ozone layer is located in the stratosphere 10-40 kilometers (6-25 miles) above the Earth’s surface. The ozone hole is the gap created in the ozone layer above Antarctica each year. It is caused by a combination of winter winds and very low temperatures, which facilitates the release of ozone-destroying CFCs. Between Sept and Nov (the Antarctic Spring), roughly half the ozone in the atmosphere above Antarctica is destroyed. At some altitudes, the loss can be as high as 90 percent. In November, as temperatures rise and the winds die down, the ozone layer starts to recover.
Who Discovered Ozone Depletion?
In 1951, U.S. manufacturers unveiled a promising new product – spray cans containing a chemical propellant called Freon-11. Freon-11 and its sister compound Freon-12 (used as a coolant in refrigerators, freezers and air-conditioners) belong to a family of chlorofluorocarbons (CFCs). CFCs rapidly became popular in a wide variety of industries – including the foam-blowing, electronics, ball-bearing and medical industries – not least because they were thought to be chemically inert and therefore safe. Researchers assumed that any CFCs that escaped into the air would diffuse into the upper atmosphere and would be broken down by sunlight. They knew that this mechanism would result in the release of some chlorine (chlorine free radicals), but assumed it would have little or no effect on the upper layers of the atmosphere. To begin with, it looked as though they were right. When global measurements began in 1957, ozone levels were still stable.
The Rowland-Molina Hypothesis
Then in 1970, a Dutch chemist named Paul Crutzen published a paper in the Quarterly Journal of the Royal Meteorological Society, explaining his theory that when nitrous oxide enters the stratosphere, it reacts with solar energy and eventually catalyzes the destruction of any ozone molecules with which it comes into contact. Crutzen’s work was not widely accepted at the time, but it helped pave the way for the efforts of two American chemists, F. Sherwood Rowland and Mario J. Molina of the University of California at Irvine, two American chemists who, in 1973, began studying the impacts of CFCs on the Earth’s atmosphere.
The pair presented a theory – later known as the Rowland-Molina Hypothesis – that CFC molecules were stable enough to remain in the atmosphere until they reached the stratosphere, where they would ultimately (after 50–100 years) be broken down by ultraviolet radiation, releasing a chlorine atom which might catalyze the destruction of ozone. (In fairness, several other scientists, including Ralph Cicerone, Michael McElroy, Richard Stolarski, and Steven Wofsy had independently theorized that chlorine could catalyze ozone loss, but none had realized that CFCs were a potentially large source of chlorine.)
The Rowland-Molina Hypothesis was swiftly confirmed by three separate research teams. They discovered that one liberated chlorine atom could destroy 100,000 ozone molecules in the atmosphere 5, with potentially grave consequences for the ozone layer.
After publishing their pivotal paper in June 1974, Rowland and Molina appeared before the U.S. House of Representatives in December 1974, where they testified about the harm caused by CFCs to the protective ozone layer. In the wake of their testimony, significant funding was made available to research the problem and verify the initial findings. In 1976, the U.S. National Academy of Sciences (NAS) confirmed the accuracy of the ozone depletion hypothesis. 6
Twenty years later, in recognition of the importance of their work, Rowland and Molina (together with the Dutch chemist Paul Crutzen) were awarded the 1995 Nobel Prize for Chemistry.
What Was The Reaction Of The Aerosol Propellant Industry?
The Rowland-Molina hypothesis was vigorously disputed by the aerosol and halocarbon industries. The chair of the board of DuPont was quoted as saying that ozone depletion theory was “utter nonsense”. Robert Abplanalp, President of Precision Valve Corporation and designer of the first aerosol spray can valve, complained to the Chancellor of the University of California about Rowland’s public statements 7
Even after the evidence was overwhelming, the Alliance for Responsible CFC Policy – a trade association representing the CFC industry – was still insisting that the scientific evidence was ambivalent. Even as late as March 1988, DuPont Chairman Richard E. Heckert wrote in a letter to the United States Senate, “At the moment, scientific evidence does not point to the need for dramatic CFC emission reductions. There is no available measure of the contribution of CFCs to any observed ozone change…” 8 It’s attitudes like this that fuel calls to rename our era the Anthropocene epoch, in order to reflect our blindness to the results of our actions.
When Did Ozone Depletion Start?
By 1980, scientists began to notice significant decreases in atmospheric ozone levels. Satellite measurements confirmed that ozone concentrations occurring in sampled columns of air were declining by roughly 5 percent a year, mostly towards the polar regions, with the smallest decreases occurring in the tropics. A corresponding increase of ground level UV radiation was also reported.
However, it was only in 1985, when a team of English scientists found a hole in the ozone layer over Antarctica (later confirmed to be the result of CFCs), that Rowland and Molina’s earlier work was fully validated. (For more, see below.)
Ozone Depletion In 4 Simple Steps
Step 1. Human activities release gases containing chlorine and bromine. They include CFCs (the most famous), as well as hydrochlorofluorocarbons (HCFCs), hydrobromoflurocarbons (HBFCs), halons, carbon tetrachloride, methyl chloroform, and methyl bromide.
Step 2. These gases congregate in the lower atmosphere because they do not dissolve easily in rain or snow.
Step 3. Natural air currents transport these gases up into the stratosphere. Here, they break down (dissociate) after exposure to sunlight, which converts them to more reactive forms of gas. To be specific, when they dissociate, the gases release chlorine atoms (or bromine or other halogen atoms), which then go on to eradicate ozone.
Step 4. These reactive atoms can destroy tens of thousands of ozone molecules before being removed from the stratosphere. It is calculated that one CFC molecule takes an average of about five to seven years to move from the ground level up to the upper atmosphere, and it can stay there for about a century, destroying up to one hundred thousand ozone molecules during that time. 9
The loss of ozone is now universally recognized as being caused by rising levels of chlorine and bromine in the stratosphere stemming from the manufacture of CFCs and other halocarbons. This has been confirmed both by computer models and by satellite data showing that chlorine and bromine gases in the stratosphere react with and destroy ozone.
What Is The Ozone Hole?
The “ozone hole” is not actually a hole in the ozone layer, it is merely an area of the stratosphere located above Antarctica that has extremely low concentrations of ozone. It was first documented in 1985 by a group of three British Antarctic Survey (BAS) scientists – Joseph C. Farman, Brian G. Gardiner, and Jonathan D. Shanklin – who measured the phenomenon from their base at Halley Bay, Antarctica.
From the late 1970s, a rapid decrease in ozone (up to 60 percent below the global average) had been observed in the spring (September to November) over Antarctica. Farman’s paper analyzing the ozone depletion led to the discovery that meteorological conditions over the South Pole – specifically the wind system known as the circumpolar vortex, or polar winter vortex – were causing the formation of polar stratospheric clouds (PSCs).
Studies showed that CFCs and other ODSs adhered to the ice crystals in the polar clouds, allowing chemical reactions to occur in which less-reactive chlorine molecules were converted into more-reactive variants, such as molecular chlorine (C12). Then, in the sunny spring, sunlight completed the process by breaking down the molecular chlorine into new atoms that destroyed the ozone. Ozone depletion continued until the end of the polar vortex in November.
Why Isn’t There An Ozone Hole Above The Arctic?
Because although polar stratospheric clouds do form in the Arctic, they don’t last long enough to allow the same amount of damage to the ozone layer. Nonetheless, decreases of up to 40 percent in Arctic ozone have been recorded, usually coinciding with unseasonably cold temperatures in the lower stratosphere, and always coinciding with large increases in reactive chlorine.
How Is Ozone Measured?
Although ground monitoring and research stations continue to be important, the most precise ozone data comes from satellites like Aura (EOS CH-1), a multi-national NASA scientific satellite in orbit around the Earth, studying the Earth’s ozone layer, air quality and climate. Its payload includes the Ozone Monitoring Instrument (OMI) which studies tropospheric ozone, cloud pressure and coverage, as well as different types of aerosol (smoke, dust, and sulfates). Scientists also receive data from the Suomi NPP and NOAA-20 satellites, which carry the Ozone Mapping and Profiler Suite (OMPS), a complex set of instruments that analyze the worldwide distribution of ozone and how it is distributed vertically within the stratosphere.
How Bad Is Ozone Depletion In The Antarctic?
Global ozone depletion occurs mainly in polar regions (but see HIPERION report, below). The Antarctic ozone layer was severely affected from the mid-1980s onwards. The damage is seasonal, occurring mostly in late winter and early spring (August–November). Peak damage is usually seen in early October when ozone is often wholly eradicated over a range of altitudes, thus reducing total ozone by as much as two-thirds in some locations. This severe depletion constitutes the “ozone hole” which is visible in satellite imagery of the Antarctic continent.
How Bad Is Ozone Depletion In The Arctic?
Significant depletion of the ozone layer above the North Pole is apparent in most years during the late winter/early spring (January-March). However, the maximum damage is less pronounced than that seen in the Antarctic and also varies significantly from year to year. Thus, the large, reoccurring “ozone hole,” seen above the Antarctic does not happen in the Northern Hemisphere.
How Bad Is Ozone Depletion Globally?
Damage to the global ozone layer began gradually in the 1980s and reached its apogee of about 5 percent during the early 1990s. The depletion has reduced since then and now averages roughly 3.5 percent over the globe. The average ozone loss is very small at the equator and increases toward the poles.
Do Volcanoes Affect The Ozone Layer?
Yes, any mass eruption of stratospheric particles into the atmosphere as a result of volcanic activity, will affect the ozone layer. However, recorded volcanic activity cannot account for the observed decreases in worldwide ozone since 1980. Nonetheless, if large volcanic eruptions do occur in the coming decades, ozone depletion will rise for several years afterwards.
How do volcanic eruptions affect the ozone layer? Because satellite observation has shown that chemical reactions on the surfaces of volcanically produced particles will boost ozone depletion by increasing the quantity of the reactive chlorine gas chlorine monoxide (C1O).
What Are The Effects Of Ozone Depletion?
The thinning of the ozone layer increases UVB levels on the surface of the Earth which could lead to serious cell damage in humans, including cancer. 10 Human health has been the focus of public concern over the ozone issue and was also the basic reason for establishing the Montreal Protocol. As stated above, the range of possible health issues included sunburn, skin cancer, DNA problems and cataracts. Indeed, scientists have calculated that for every one percent decrease in ozone the incidence of skin cancer would rise by two percent. 11
However, while decreases in ozone levels are theoretically linked to increases in surface UVB, there is a lack of hard evidence (across all latitudes) linking loss of ozone to higher incidence of health problems in human beings. This is partly a causal issue – UVA is not absorbed by ozone, making it impossible to (e.g.) separate cancers caused by UVB from those caused by UVA. Even so, evidence of sorts continues to pile up.
For example, one medical study showed that a 10 percent rise in UVB radiation was associated with a 19 percent rise in melanomas for men and 16 percent for women. 12 A study of people in Punta Arenas, in southern Chile (within the ozone hole over Antarctica), revealed a 56 percent rise in melanoma and a 46 percent rise in non-melanoma skin cancer over a period of 7 years, in parallel with lowered ozone and raised UVB levels. 13
An even more clear-cut study of eye conditions was prompted by epidemiological studies indicating a link between ocular cortical cataracts and UVB exposure. It involved a detailed investigation of ocular exposure to UVB among Chesapeake Bay watermen. 14 In this highly exposed group of predominantly white males, the evidence linking cortical opacities to UV exposure was the strongest to date. Based on these results, ozone depletion is predicted to cause hundreds of thousands of additional cataracts by 2050. 15
The other main effect of ozone depletion is climate change. Ozone is a greenhouse gas, so as it reduces it cools the Antarctic. This has influenced the orbit of the westerly winds of the Southern Annular Mode weather cycle, pulling them closer to land, which has increased the speed of the wind, making Antarctica even cooler and drier.
Hiperion Report Highlights Global Health Risks Of Ozone Depletion
According to most studies, the issue of ozone depletion impacts mostly on polar regions. However, in Autumn 2008, Ecuador’s National Space Agency produced a study called HIPERION. Using ground instruments in Ecuador as well as 28 years’ worth of data from 12 satellites of several countries, the study discovered that the level of UV radiation reaching equatorial latitudes was much higher than expected, with the UV Index reaching a record 24 in Quito. The World Health Organization (WHO) considers 11 as an extreme level involving a high degree of health risk. The report’s conclusion was that reduced ozone levels around the equatorial zone of the planet are endangering large populations in these areas. 16 A little later, the Peruvian Space Agency (CONIDA) produced its own study, which corroborated the findings of the Ecuadorian report.
Does Ozone Depletion Affect Animals?
Yes, it seems so. The best example features a study by researchers at the Institute of Zoology in London, which found that whales off the coast of California have suffered a sharp increase in sun damage. Photographs and skin biopsies revealed clear evidence of acute and severe sunburn. 17
Are Crops Affected?
Yes. An increase in UV levels would affect crops. Many species of plants, including a major crop like rice, rely on cyanobacteria living in their roots to retain nitrogen. Unfortunately, cyanobacteria would be seriously affected by any increase in UV radiation. 18 According to experts, plants have a limited ability to adapt to increased levels of UVB, therefore plant growth can be directly affected by UVB radiation. 19
What Was The Global Response To Ozone Depletion?
Recognition of the global risk to health resulting from the possible disintegration of the protective ozone layer triggered an international effort to restrict and ultimately ban the use of CFCs and other halocarbons. It led to the 1987 landmark “Montreal Protocol on Substances That Deplete the Ozone Layer” which began the gradual elimination of CFCs and other known ODSs (from 1993 onwards) and targeted a 50 percent reduction in global consumption by 1998. A series of amendments to the Montreal Protocol reinforced controls on CFCs and other halocarbons. CFCs were banned in the 1990s; HCFCs and HFCs are due to be phased out by 2030 and 2050 respectively.
By 2005 the use of ozone-depleting chemicals controlled by the agreement had fallen by 90–95 percent among signatory states. Without the Montreal Protocol, it is estimated that the U.S. alone would have suffered an additional 280 million cases of skin cancer, 1.5 million deaths from skin cancer, and 45 million cataracts. 20 21
Compared to the United Nations Framework Convention on Climate Change (1992), the Kyoto Protocol (1997), or the Paris Climate Agreement 2015, The Montreal Protocol has been a wonderful success. 22
Are Ozone Levels In The Atmosphere Improving?
It looks like it. In the 1990s, following the Montreal Protocol, ozone levels began to stabilize and have now started to recover. They are projected to reach pre-1980 levels before 2075. 23 However, certain anomalies continue to crop up – a recent study, for example, warned of possible violations of the Montreal Protocol. Fortunately, satellite surveillance should expose the culprits and maintain a robust standard of compliance. In the meantime, scientists are developing new, environmentally-friendly alternatives to CFCs and the like, to reduce our dependence on high risk chemicals.
How Long Before The Ozone Layer Recovers Completely?
In 2018, more than 30 years after the Montreal Protocol, NASA researchers documented the first hard evidence that the Antarctic ozone is recovering as a result of the ban on CFCs. Ozone depletion in the region was down 20 percent since 2005. Furthermore, at the end of 2018, the United Nations issued a scientific assessment stating that the ozone layer was indeed improving and predicting that it would recover completely in the Northern Hemisphere (excluding the polar region) by the 2030s, followed by the Southern Hemisphere in the 2050s and polar regions by 2060. 20
In 1994, the United Nations General Assembly voted to designate September 16 as the International Day for the Preservation of the Ozone Layer, or “World Ozone Day”, to commemorate the signing of the Montreal Protocol on that date in 1987.
Does Ozone Depletion Have Anything To Do With Global Warming?
Not directly, although the loss of ozone has led to less solar radiation being trapped in the stratosphere, thus counteracting some of the rise in atmospheric temperature caused by increasing carbon dioxide levels. Although as the ozone layer is slowly replenished over the coming decades, this cooling effect will recede.
That said, the very chemicals designed to replace CFCs – the HFCs, PFCs, sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), and other F-gases – all have a very high global warming potential (GWP) relative to other greenhouse gases like carbon dioxide or methane, so even small atmospheric levels have a disproportionately large impact on global temperatures.
So even though they are less destructive towards the ozone layer than the earlier CFCs, fluorinated gases are extremely harmful as far as climate change is concerned. Indeed, they are among the worst greenhouse gases on the planet.
To put them in context, one of the most widely used hydrofluorocarbons, known as HFC-23 traps 12,400 times more heat in the atmosphere than CO2, while Sulphur hexafluoride (SF6) traps 23,500 times more than CO2. 24
The ozone issue was the first global alert concerning the environment. In some ways, therefore, it helped to prepare the ground for the climate crisis that followed. Having heard about the destructive impact of nitrous oxide and fluorocarbon gases on the ozone layer, we can all better understand the greenhouse effect and the heat-trapping impact of carbon dioxide (CO2) and methane (CH4).
- “Twenty Questions and Answers About the Ozone Layer” (PDF). Scientific Assessment of Ozone Depletion: 2010. World Meteorological Organization. 2011.
- “Ozone: Good Up High, Bad Nearby” (PDF). EPA.
- “Surface exposure to sunlight stimulates CO2 release from permafrost soil carbon in the Arctic.” Rose M. Cory, Byron C. Crump, Jason A. Dobkowski, and George W. Kling. PNAS February 26, 2013 110 (9) 3429-3434.
- “Ozone depletion and climate change: impacts on UV radiation.” Bais AF, McKenzie RL, Bernhard G, Aucamp PJ, Ilyas M, Madronich S, Tourpali K. Photochem Photobiol Sci. 2015 Jan;14(1):19-52.
- Basic Ozone Layer Science. EPA.gov
- “Halocarbons, effects on stratospheric ozone.” National Academy of Sciences (1976). Washington, DC.
- “Ozone crisis: The 15-year evolution of a sudden global emergency.” Sharon Roan. (1989) p.56, ISBN 0-471-52823-4.
- “Du Pont: A Case Study In the 3D Corporate Strategy”. Greenpeace. 1997.
- Rusch, George M. “Chlorofluorocarbons .” Encyclopedia of Public Health (2002) – Encyclopedia.com
- “Health and Environmental Effects of Ozone Layer Depletion”. EPA. February 15, 2013.
- “Impacts of a Projected Depletion of the Ozone Layer”. Consequences. 1 (2). De Gruijl, Frank R. (Summer 1995).
- “Average midrange ultraviolet radiation flux and time outdoors predict melanoma risk”. Cancer Res. 62 (14): 3992–6. PMID 12124332. Fears, T. R.; Bird, C. C.; Guerry d, 4th; Sagebiel, R. W.; Gail, M. H.; Elder, D. E.; Halpern, A.; Holly, E. A.; Hartge, P.; Tucker, M. A. (2002).
- “Skin cancer and ultraviolet-B radiation under the Antarctic ozone hole: southern Chile, 1987–2000”. Photodermatol Photoimmunol Photomed. 18 (6): 294–302. Abarca, J. F.; Casiccia, C. C. (December 2002).
- “Sunlight exposure and risk of lens opacities in a population-based study: the Salisbury Eye Evaluation project”. JAMA. 280 (8): 714–8. PMID 9728643. West, S. K.; Duncan, D. D.; Munoz, B.; Rubin, G. S.; Fried, L. P.; Bandeen-Roche, K.; Schein, O. D. (1998).
- “Ozone depletion will bring big rise in number of cataracts”. BMJ. 331 (7528): 1292–1295. Dobson, R. (2005).
- The HIPERION Report (PDF) (Report). Ecuadorian Civilian Space Agency. 2008.
- “Sunburned whales: Troubling environment news of the week”. The Washington Post. November 11, 2010.
- “Photoecophysiology of cyanobacteria”. Recent Research Developments in Photochemistry and Photobiology. 3: 91–101. Sinha, R. P.; Singh, S. C.; Hader, D. P. (1999).
- “Health and Environmental Effects of Ozone Layer in Plants”. U.S Environmental Protection Agency. February 15, 2013.
- “Ozone depletion, explained.” Christina Nunez. National Geographic. April 18, 2019.
- “Updating Ozone Calculations and Emissions Profiles for Use in the Atmospheric and Health Effects Framework Model.” U. S. Environmental Protection Agency. 2015.
- “Without the Ozone Treaty You’d Get Sunburned in 5 Minutes.” Stephen Leahy. National Geographic. Sept 25, 2017.
- “The Antarctic Ozone Hole Will Recover”. NASA. June 4, 2015.
- Source: IPCC. Fifth Assessment Report. 2014.