While much of the discussion of climate change revolves around carbon dioxide because of its primary role in increasing average global temperatures, many other chemicals other than CO2 can have an effect on the climate. Two such important contributors to global climate are the greenhouse gas methane (i.e. the primary component of natural gas) and black carbon (soot from incomplete burning). Methane is emitted by a variety of sources both natural and human such as wetlands and rice paddies, as I had discussed in a previous article about the methane cycle. Black Carbon is nothing more than the smoke common in wood fires or any kind of incomplete burning from diesel or coal burning. It also can absorb heat in the atmosphere and cause warming, and when deposited on snow or ice, can decrease its cooling ability by reducing its albedo or how much light it reflects. Both methane and soot have much shorter lifetimes than carbon dioxide in the atmosphere, at 11 years and weeks to months respectively compared to 170 years for carbon dioxide, making them attractive targets for short to medium term reductions in emissions related to climate change.
In addition to their effects on climate, methane and black carbon are also problematic emissions because they both act as precursors to photochemical smog and because black carbon is a big component of particulate emissions that are really bad for your lungs. Because reducing both these emissions would have a public health benefit beyond any changes to the climate, emission reductions of methane and black carbon could potentially be an important first stem to slowing climate change. In a recent Science article (gated), Drew Shindell, a NASA scientist, and a team of scientists and economists from around the world identified various technologies available now and possible regulations that could reduce methane and black carbon emissions immediately. They then calculating the costs of of implementing these new technologies and compared them to the potential benefits on public health, crop yields, and climate. The benefits of reducing both far outweighed the costs, in reduced impacts from changes in climate from differences in precipitation patterns, crop yields, and premature deaths. In particular, the reductions in black carbon resulted in an enormous benefit in terms of reduced premature deaths since particulate emissions such as black carbon are pretty bad for human health. Most of the benefits were realized in India and China (where black carbon emissions are much, much worse than in the United Stats or Europe) in the form of avoided changes in the precipitation patterns crucial for agriculture. However, the United States also benefited from potential increases in crop yields due to less photochemical smog from methane emissions. The ozone formed in photochemical smog, in addition to being a lung irritant, also reduces crop yields. Plants reduce atmospheric uptake in high concentrations of ozone because of its highly reactive nature, which can damage the inside of the plant as easily as it can damage your lung tissue.
For climate change, the full implementation of their recommended technologies and regulations would actually in the short to medium term help slow the pace of climate change relative to a "business as usual" scenario. This interactive graphic from NASA that accompanied this publication shows how under different "climate sensitivity" (how much the temperature changes in response to increased CO2) scenarios the implementation of CH4 (methane) and BC (black carbon) measures can reduce the expected warming. Depending on how sensitive the climate is to changes in CO2 these measures delay the time it takes to reach 2oC degree warming threshold that many scientists consider to be a dangerous threshold for the climate. This gives all of us some "breathing room" (so to speak) to develop alternatives to fossil fuel burning as a source of energy because even with these measures CO2 is still the primary mover of climate change. Many times it's extremely under-appreciated just how challenging it is to develop alternatives to fossil fuels as it may be decades before a true alternative can be developed. This study gives us hope that we can eventually tackle climate change without dramatic changes to living standards or a halt to the increasing prosperity that free trade and liberalizing reforms are bringing to the developing world.
Related Links:
An interview with Drew Shindell, the primary author of this study
Friday, January 13, 2012
Friday, January 6, 2012
Nuclear Bomb Testing and Boreal Forest Fires
One important piece of evidence that shows that climate change has been caused by humans is the decrease in the amount of radiocarbon (carbon-14) in CO2. You may be familiar with carbon-14 because of its use in radiocarbon dating in archaeology for objects up to 60,000 years old. For those unfamiliar, though, carbon-14 is produced in the upper atmosphere by reaction of neutrons from cosmic rays with nitrogen. The carbon-14 produced is then eventually transported to the lower atmosphere, where plants uptake the 14CO2 via photosynthesis. Once a plant dies, it stops absorbing carbon dioxide, which then causes any of the carbon-14 in the leftover plant material to radioactively decay. As such, fossil fuels, having been in the ground for millions of years, contain almost no carbon-14, so that when they are burned, the carbon dioxide produced has no carbon-14 content.
Of course, measuring carbon-14 in CO2 in the atmosphere has several complications associated with it. In an article in the Journal of Geophysical Research, a group of scientists from NASA, NOAA, and various universities around the world measured CO2 and its carbon-14 content from air samples collected on aircraft campaigns over the Arctic in Canada. As you might expect from my above explanation, CO2 from right over the Alberta tar sands has both higher concentrations and reduced carbon-14 content compared to typical levels for background CO2 from emissions from burning the oil extracted. Interestingly, some of the air samples that were far from any influence from civilization also had higher concentrations of CO2 but increased carbon-14 content. High levels of carbon monoxide in these samples along with some atmospheric modeling (and a little knowledge of local conditions) led the scientists to conclude that these measurements were sampling air from boreal forest fires in the Arctic. Why would this lead to higher carbon-14, though?
The above plot with the amount of carbon-14 in the atmosphere over the last few decades helps show what likely caused this increase in the carbon-14 content. During the 1950's and into the 1960's, the world's governments conducted many nuclear bomb tests in the atmosphere, which released a lot of neutrons into the atmosphere. Since neutrons cause the formation of carbon-14 from nitrogen as explained above, this nuclear testing caused a so-called "bomb spike" in carbon-14 until atmospheric nuclear testing was banned internationally in the 60's from health concerns about the release of other much more radioactive isotopes of elements such as strontium-90 into the atmosphere. Because plants continuously absorb CO2 throughout their lifetimes, decades-old trees in boreal forests in the Arctic could retain high levels of carbon-14 until burned by a sudden forest fire, releasing highly enriched carbon-14 back into the atmosphere.
The results in this paper highlight some of the challenges involved in the use of carbon-14 as a tracer of fossil fuel burning. However, it also shows the incredible utility of trace isotope studies in providing information about the chemical processes in the atmosphere. Using isotopes, much more information can be gained about a chemical than from simply looking at the amount present alone.
Of course, measuring carbon-14 in CO2 in the atmosphere has several complications associated with it. In an article in the Journal of Geophysical Research, a group of scientists from NASA, NOAA, and various universities around the world measured CO2 and its carbon-14 content from air samples collected on aircraft campaigns over the Arctic in Canada. As you might expect from my above explanation, CO2 from right over the Alberta tar sands has both higher concentrations and reduced carbon-14 content compared to typical levels for background CO2 from emissions from burning the oil extracted. Interestingly, some of the air samples that were far from any influence from civilization also had higher concentrations of CO2 but increased carbon-14 content. High levels of carbon monoxide in these samples along with some atmospheric modeling (and a little knowledge of local conditions) led the scientists to conclude that these measurements were sampling air from boreal forest fires in the Arctic. Why would this lead to higher carbon-14, though?
The above plot with the amount of carbon-14 in the atmosphere over the last few decades helps show what likely caused this increase in the carbon-14 content. During the 1950's and into the 1960's, the world's governments conducted many nuclear bomb tests in the atmosphere, which released a lot of neutrons into the atmosphere. Since neutrons cause the formation of carbon-14 from nitrogen as explained above, this nuclear testing caused a so-called "bomb spike" in carbon-14 until atmospheric nuclear testing was banned internationally in the 60's from health concerns about the release of other much more radioactive isotopes of elements such as strontium-90 into the atmosphere. Because plants continuously absorb CO2 throughout their lifetimes, decades-old trees in boreal forests in the Arctic could retain high levels of carbon-14 until burned by a sudden forest fire, releasing highly enriched carbon-14 back into the atmosphere.
The results in this paper highlight some of the challenges involved in the use of carbon-14 as a tracer of fossil fuel burning. However, it also shows the incredible utility of trace isotope studies in providing information about the chemical processes in the atmosphere. Using isotopes, much more information can be gained about a chemical than from simply looking at the amount present alone.
Wednesday, October 19, 2011
Climate Change, Photosynthesis, and El Niño
The way carbon is moved between the atmosphere, the biosphere, oceans, and other parts of the Earth system plays an important role in the current scientific consensus on climate change. While the various processes involved are known well enough to predict that the planet will continue warming if CO2 concentrations keep increasing from fossil fuel emissions, better understanding of any part of the so-called carbon cycle can improve predictions of how exactly how much warming will occur. The image below shows a model of the carbon cycle.
In particular, the approximation of the conversion of CO2 to sugars by plants during photosynthesis (called primary production) could have its range of possible quantities narrowed. In a recent study in the journal Nature, the authors use measurements the rare, stable oxygen isotope, oxygen-18, in CO2 to estimate the value of primary production. Since CO2 can exchange oxygen atoms with water in leaves without undergoing photosynthesis, the ratio of oxygen-18 to oxygen-16 in CO2 is related to how much CO2 is converted to sugars in plants. Using these measurements, they actually found that primary production might be greater than previously thought! Their estimate has the advantage of not relying on assumptions about biology and provides further constraints on primary production. This increase in the estimate of primary production certainly may turn out to be good news because it could mean that CO2 concentrations will rise (slightly) more slowly. However, especially because primary production does not count how much of the sugars produced are consumed by the plant itself (and thus changed back into CO2), it does unfortunately not mean that the biosphere can completely offset all changes in CO2 from fossil fuel emissions.
By observing the oxygen-18 to oxygen-16 ratio over 30 years at various locations around the world, the scientists found occasional small increases in the ratio from year to year. Oddly enough, these increases occurred at the same time as El Niño! They explained this increase in the oxygen-18 to oxygen-16 ratio by decreases in rainfall over rain forests (where lots of photosynthesis occurs) in Southeast Asia and northern South America during El Niño. Because the water with oxygen-18 is "heavier" than water with oxygen-16, the water in clouds tends to have more oxygen-16 since "lighter" oxygen-16 containing water evaporates first. During periods with less rainfall, the water still evaporates over rainforests, reducing the amount of oxygen-16 in water in the soil. Without enough rainfall to return oxygen-16 back to the ground, the relative amount of oxygen-18 increases, thus the water in plant leaves also has more oxygen-18. Although this effect is very small (a 0.05% change), it can be measured in CO2. As the southern ocean returns to a La Niña pattern and rainfall in these regions increases, the oxygen-18 to oxygen-16 ratio in CO2 eventually returns to its "normal" level. The scientists used the rate of change in oxygen-18 to calculate an estimate of primary productivity. Additional measurements of oxygen-17 to oxygen-16 ratios may provide additional constraints to help improve this estimate further. (For a more detailed explanation of isotopes in geology and chemistry, see this previous post)
(h/t Jeremiah J.)
In particular, the approximation of the conversion of CO2 to sugars by plants during photosynthesis (called primary production) could have its range of possible quantities narrowed. In a recent study in the journal Nature, the authors use measurements the rare, stable oxygen isotope, oxygen-18, in CO2 to estimate the value of primary production. Since CO2 can exchange oxygen atoms with water in leaves without undergoing photosynthesis, the ratio of oxygen-18 to oxygen-16 in CO2 is related to how much CO2 is converted to sugars in plants. Using these measurements, they actually found that primary production might be greater than previously thought! Their estimate has the advantage of not relying on assumptions about biology and provides further constraints on primary production. This increase in the estimate of primary production certainly may turn out to be good news because it could mean that CO2 concentrations will rise (slightly) more slowly. However, especially because primary production does not count how much of the sugars produced are consumed by the plant itself (and thus changed back into CO2), it does unfortunately not mean that the biosphere can completely offset all changes in CO2 from fossil fuel emissions.
By observing the oxygen-18 to oxygen-16 ratio over 30 years at various locations around the world, the scientists found occasional small increases in the ratio from year to year. Oddly enough, these increases occurred at the same time as El Niño! They explained this increase in the oxygen-18 to oxygen-16 ratio by decreases in rainfall over rain forests (where lots of photosynthesis occurs) in Southeast Asia and northern South America during El Niño. Because the water with oxygen-18 is "heavier" than water with oxygen-16, the water in clouds tends to have more oxygen-16 since "lighter" oxygen-16 containing water evaporates first. During periods with less rainfall, the water still evaporates over rainforests, reducing the amount of oxygen-16 in water in the soil. Without enough rainfall to return oxygen-16 back to the ground, the relative amount of oxygen-18 increases, thus the water in plant leaves also has more oxygen-18. Although this effect is very small (a 0.05% change), it can be measured in CO2. As the southern ocean returns to a La Niña pattern and rainfall in these regions increases, the oxygen-18 to oxygen-16 ratio in CO2 eventually returns to its "normal" level. The scientists used the rate of change in oxygen-18 to calculate an estimate of primary productivity. Additional measurements of oxygen-17 to oxygen-16 ratios may provide additional constraints to help improve this estimate further. (For a more detailed explanation of isotopes in geology and chemistry, see this previous post)
(h/t Jeremiah J.)
Thursday, October 6, 2011
Super soggy air on Mars
Using observations from the SPICAM instrument on the European Space Agency's Mars probe Mars Express, French scientists discovered that the amount of water vapor in the upper Martian atmosphere sometimes far exceeds expectations. While some water vapor is formed from sublimation (evaporation) off ice on the Marian surface, the amount found in the upper atmosphere is greater than the temperature would predict. In terms of relative humidity, the humidity of the air was observed to approach almost as much as 1000%! Scientists call such air "supersaturated" in water, since the air is holding far more water than it would otherwise.
How could so much extra water end up in the atmosphere of Mars? The authors suggest that the low pressure and lack of dust particles in the upper atmosphere make condensation into ice very difficult, so that the water simply stays in vapor form. Simultaneous measurements of the amount of dust show that this could indeed be the case. These results fundamentally change scientists understanding of the water cycle on Mars as water vapor exists in much higher concentrations at higher altitudes than previous thought. Greater amounts of water vapor in the upper Martian atmosphere imply that a larger amount of water is able to escape Mars's gravity than previously thought. Models of the chemistry of the Martian atmosphere are also affected by water - despite its relatively low amounts (even with this result) water acts as a catalyst in many chemical cycles in the Martian atmosphere.
How could so much extra water end up in the atmosphere of Mars? The authors suggest that the low pressure and lack of dust particles in the upper atmosphere make condensation into ice very difficult, so that the water simply stays in vapor form. Simultaneous measurements of the amount of dust show that this could indeed be the case. These results fundamentally change scientists understanding of the water cycle on Mars as water vapor exists in much higher concentrations at higher altitudes than previous thought. Greater amounts of water vapor in the upper Martian atmosphere imply that a larger amount of water is able to escape Mars's gravity than previously thought. Models of the chemistry of the Martian atmosphere are also affected by water - despite its relatively low amounts (even with this result) water acts as a catalyst in many chemical cycles in the Martian atmosphere.
Thursday, September 22, 2011
The Scientific Process and Ship Wakes
This article from the open-access journal Atmospheric Chemistry and Physics was actually rejected to be published by the journal in 2010. Because of the open-access model of the journal, the originally submitted article along with reviewer comments can be viewed despite the article's rejection. In the article, the authors measured the amount of light reflected by the wakes from shipping barges in the northern Pacific Ocean from airplanes. They then used their results to estimate the increase in reflected sunlight from increases in shipping across the Pacific, since this would have a (very) small cooling effect on the climate. Unfortunately, while this was a somewhat clever idea, the reviewers thought that there were far too many uncertainties in both their measurements and calculations so that the reported number was an overestimate of an already small number.
While this article was rejected, because of the open-access nature of the journal, it shows how the scientific process of peer review works. Typically, when science and the scientific method is taught in schools, the curriculum focuses on the experimental process of science. While certainly this is very important, science also has important social ways of processing new information. If a group of scientists do a series of experiments that reach new and interesting conclusions, they will try to get their results published to a scientific journal. (And present this information at conferences, universities, research centers, etc.) Upon submission to a journal, two (or more) other scientists from the same research field will read the initial paper, either recommending that the results be published along with potential changes or that the paper be rejected outright. If the paper is rejected, the authors can appeal to have it accepted, but this is not usually granted. The authors then edit the paper based on the suggestions of the reviewers, and it is resubmitted and published in the next issue of the journal. Usually a reader only sees the final product of this process in a scientific journal, but in the case of an open-access journal such as Atmospheric Chemistry and Physics the entire process is visible to the reader (and the journal is free online). Even articles that eventually are rejected such as this one are immediately put on the journal's website after a quick review process to ensure the article is relevant.
Peer review is an important process to ensure quality as scientists being human after all are prone to misjudgment, bias, or error. While science is constantly changing over time as new ideas or methods are realized, it's important to make sure these ideas are plausible or methods actually work! While there are many cases of new ideas overturning the scientific consensus (I'm looking at you, quantum mechanics) it's often after these new ideas have been thoroughly scrutinized and verified through the process of peer review, discussions at scientific conferences, and independent testing of observations or experiments. The process is not without its faults, of course! Occasionally a scientist will unethically use peer review to block the publication of results that contradict their own. (Yes, it's kind of silly. There is the joke: "The debates in science are so fierce because the stakes are so low." I've seen people spend 15 minutes at a conference arguing about what the proper term for something is.) In those cases, an open access journal such as Atmospheric Chemistry and Physics provides the kind of transparency that makes such cases apparent to improve the scientific process. Hopefully with the growing use of information technology (I know I rarely ever read the actual printed journal) more publishers decide to switch to an open-access model not only for transparency but to improve access to scientific journals to the general public.
While this article was rejected, because of the open-access nature of the journal, it shows how the scientific process of peer review works. Typically, when science and the scientific method is taught in schools, the curriculum focuses on the experimental process of science. While certainly this is very important, science also has important social ways of processing new information. If a group of scientists do a series of experiments that reach new and interesting conclusions, they will try to get their results published to a scientific journal. (And present this information at conferences, universities, research centers, etc.) Upon submission to a journal, two (or more) other scientists from the same research field will read the initial paper, either recommending that the results be published along with potential changes or that the paper be rejected outright. If the paper is rejected, the authors can appeal to have it accepted, but this is not usually granted. The authors then edit the paper based on the suggestions of the reviewers, and it is resubmitted and published in the next issue of the journal. Usually a reader only sees the final product of this process in a scientific journal, but in the case of an open-access journal such as Atmospheric Chemistry and Physics the entire process is visible to the reader (and the journal is free online). Even articles that eventually are rejected such as this one are immediately put on the journal's website after a quick review process to ensure the article is relevant.
Peer review is an important process to ensure quality as scientists being human after all are prone to misjudgment, bias, or error. While science is constantly changing over time as new ideas or methods are realized, it's important to make sure these ideas are plausible or methods actually work! While there are many cases of new ideas overturning the scientific consensus (I'm looking at you, quantum mechanics) it's often after these new ideas have been thoroughly scrutinized and verified through the process of peer review, discussions at scientific conferences, and independent testing of observations or experiments. The process is not without its faults, of course! Occasionally a scientist will unethically use peer review to block the publication of results that contradict their own. (Yes, it's kind of silly. There is the joke: "The debates in science are so fierce because the stakes are so low." I've seen people spend 15 minutes at a conference arguing about what the proper term for something is.) In those cases, an open access journal such as Atmospheric Chemistry and Physics provides the kind of transparency that makes such cases apparent to improve the scientific process. Hopefully with the growing use of information technology (I know I rarely ever read the actual printed journal) more publishers decide to switch to an open-access model not only for transparency but to improve access to scientific journals to the general public.
Wednesday, September 14, 2011
Variations in Volcanic Dust High Up
Last month, in Science magazine, Susan Solomon,* an atmospheric chemist at the National Oceanic and Atmospheric Administration (NOAA) and her colleagues presented satellite measurements of sulfate aerosols in volcanic dust in the stratosphere, the upper region of the atmosphere that contains the ozone layer. These measurements showed that the levels of volcanic dust in the stratosphere actually vary significantly even in the absence of major volcanic eruptions such as the Mount Pinatubo eruption in 1991.
Aerosols such as this volcanic dust scatter and reflect light from the sun, thus causing a net cooling of the climate. The volcanic dust in the stratosphere actually increased (mostly from natural volcanic events) enough from 2000 to 2010 to decrease the heat trapped by the atmosphere by 0.1 W/m2. For comparison, the increase in CO2 during the same period increased the heat trapped by the atmosphere by 0.28 W/m2, thus this volcanic dust cancelled out some of the warming that would have occurred from CO2 alone. While this is certainly a good thing as it has slowed the pace of global warming, it is unclear whether this increase in volcanic dust will continue due to the unpredictable nature of volcanic activity. For example, if by 2020 volcanic dust were to return to levels seen in the 1960, any cooling effect would disappear and cause average global temperature to increase by 0.06oC in addition to any changes from increased greenhouse gases. Despite the inherit unpredictability of volcanic activity, understanding that volcanic dust does have variable effects on the climate over time can help better constrain the possible range of any future changes in the climate.
Implications
In a broader sense, aerosols (i.e. dust and liquid droplets floating in air) are an important part of the climate change picture that are often under-discussed compared to poor, infamous CO2. Changes in aerosols caused by humans from land use, transportation (autos, trains, etc.), and industry (smoke stacks) since the industrial revolution have increased the light reflected by the Earth. Much like the volcanic dust in the stratosphere, this has reduced the heat trapped by the atmosphere, partially offsetting the increased heat trapped by greenhouse gases. In the period from 1940 to 1980, the combined effect of these two might have canceled each other out for a time, leading some to speculate about the possibility of global cooling in the 1970's. Of course, this hypothesis has not been borne out by the data since then. Aerosols have negative effects on humans directly through inhalation or indirectly through smog and acid rain, so government regulation of these pollutants has reduced their concentration in the atmosphere. While this is an obviously good thing for human society, it had the unfortunate effect of reducing their cooling effect on the atmosphere!
Studies like this one are especially important for improving climate predictions, as aerosols are the least well-understood part of climate change, especially because of their indirect effects on cloud formation. (See this chart from the IPCC and notice the very large black bars on aerosols compared to other factors.) They're certainly understood enough to predict that the Earth is warming and will continue to warm without any changes in human activity, but predicting how much the climate might change in the future is constrained largely by the uncertainty in the effects of aerosols. Better understanding the impact of aerosols, both human-made and natural, can improve the uncertainty in future predictions of climate. This, in turn, can provide better estimates of the costs and benefits of any potential emissions reductions or even geoengineering.
*Susan Solomon has won the US National Medal of Science for her work on understanding the cause of ozone depletion, and was one of the co-chairs of the physical science report for the International Panel on Climate Change.
Aerosols such as this volcanic dust scatter and reflect light from the sun, thus causing a net cooling of the climate. The volcanic dust in the stratosphere actually increased (mostly from natural volcanic events) enough from 2000 to 2010 to decrease the heat trapped by the atmosphere by 0.1 W/m2. For comparison, the increase in CO2 during the same period increased the heat trapped by the atmosphere by 0.28 W/m2, thus this volcanic dust cancelled out some of the warming that would have occurred from CO2 alone. While this is certainly a good thing as it has slowed the pace of global warming, it is unclear whether this increase in volcanic dust will continue due to the unpredictable nature of volcanic activity. For example, if by 2020 volcanic dust were to return to levels seen in the 1960, any cooling effect would disappear and cause average global temperature to increase by 0.06oC in addition to any changes from increased greenhouse gases. Despite the inherit unpredictability of volcanic activity, understanding that volcanic dust does have variable effects on the climate over time can help better constrain the possible range of any future changes in the climate.
Implications
In a broader sense, aerosols (i.e. dust and liquid droplets floating in air) are an important part of the climate change picture that are often under-discussed compared to poor, infamous CO2. Changes in aerosols caused by humans from land use, transportation (autos, trains, etc.), and industry (smoke stacks) since the industrial revolution have increased the light reflected by the Earth. Much like the volcanic dust in the stratosphere, this has reduced the heat trapped by the atmosphere, partially offsetting the increased heat trapped by greenhouse gases. In the period from 1940 to 1980, the combined effect of these two might have canceled each other out for a time, leading some to speculate about the possibility of global cooling in the 1970's. Of course, this hypothesis has not been borne out by the data since then. Aerosols have negative effects on humans directly through inhalation or indirectly through smog and acid rain, so government regulation of these pollutants has reduced their concentration in the atmosphere. While this is an obviously good thing for human society, it had the unfortunate effect of reducing their cooling effect on the atmosphere!
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Studies like this one are especially important for improving climate predictions, as aerosols are the least well-understood part of climate change, especially because of their indirect effects on cloud formation. (See this chart from the IPCC and notice the very large black bars on aerosols compared to other factors.) They're certainly understood enough to predict that the Earth is warming and will continue to warm without any changes in human activity, but predicting how much the climate might change in the future is constrained largely by the uncertainty in the effects of aerosols. Better understanding the impact of aerosols, both human-made and natural, can improve the uncertainty in future predictions of climate. This, in turn, can provide better estimates of the costs and benefits of any potential emissions reductions or even geoengineering.
*Susan Solomon has won the US National Medal of Science for her work on understanding the cause of ozone depletion, and was one of the co-chairs of the physical science report for the International Panel on Climate Change.
Thursday, September 8, 2011
The Decade-long Mystery of Atmospheric Methane
In a recent paper in Atmospheric Chemistry and Physics, scientists from the Institute for Marine and Atmospheric Research Utrecht compared the results from an atmospheric model of methane to observations of methane and its carbon isotope ratios from various atmospheric monitoring stations around the world. (Full disclosure: I worked at IMAU for 3 months on a fellowship with the PI on an unrelated research project) From 1998 to 2006, the amount of methane (CH4) in the atmosphere stopped growing even though emissions from human civilization during this time increased. Based on a model that takes into account the flow of methane into the atmosphere from natural and human sources and the flow of methane out of the atmosphere from the natural sinks, methane should have increased during this time period.
Why would anyone care about methane? Because methane absorbs infrared radiation (heat) strongly, it is the second most important greenhouse gas on the atmosphere of Earth despite its relatively short lifetime in the atmosphere (~9 years). Its concentration has actually increased in the atmosphere from 700 parts per billion in 1750 to nearly 1900 parts per billion today. This increase in methane has led to more heat being trapped by the atmosphere, partially contributing to climate change.
Because the sources and sinks of methane are not completely understood, models that try to take all of them into account cannot predict the "leveling off" of methane observed in the atmosphere from 1998 to 2006. The authors use a mathematical model similar to the one displayed below that includes estimated values for the natural sources of methane (mostly from bacteria in wetlands), the human sources of methane (fossil fuel mining/extraction, rice paddies, waste and water treatment, biomass (i.e. wood) burning, and livestock), and the natural sinks of methane (mostly hydroxyl free radicals (OH) produced from water exposed to sunlight). They then do a "sensitivity analysis" where each major source or sink is increased or decreased within the model to show how changes in these can potentially affect the methane calculated by the model. By increasing the sink through increased hydroxyl radical concentrations or by decreasing the source from wetlands, the model calculates a trend similar to atmospheric observations of methane. How can we tell which of these may have caused this change in the trend, though?
This diagram depicts the flow of methane from sources into the atmosphere as well as the sinks that consume methane.
A. Permafrost, Glaciers, and Ice Cores B. Wetlands C. Forest Fires D. Rice Paddies E. Animals F. Plants G. Landfills H. Waste Water Treatment Facilities I. Hydroxyl Radical J. Chlorine Radical
Image by Olivia Shoup and used under the Creation Commons Attribution Share-Alike 3.0 license
Observations and modeling of the carbon-13 to carbon-12 ratio in methane can help provide additional constraints on understanding the sources and sinks of methane. (For more about isotopes and chemistry and how it relates to the atmosphere, see this part of a previous post on a related topic.) Since each source and sink has a relatively different carbon-13 to carbon-12 ratio, any changes to these sources and sinks in the model will also affect the modeled carbon-13 to carbon-12 ratio. These results then can be compared to the atmospheric record of carbon-13 to carbon-12 in methane to see if any change in the estimated value for a source or sink is justified. Using the isotope ratio as a guide, a decrease in wetland methane emissions in the model, while it can bring the methane concentration in the model close to that observed in nature, results in a significant increase in the carbon-13 to carbon-12 ratio, which is not observed. In contrast, an increase in the hydroxyl radical sink will bring the methane in the model close to that observed in nature, but without changing the modeled isotope ratio. Such an increase in hydroxyl radical concentration has been independently proposed, but this hypothesis is hard to confirm since hydroxyl radicals are difficult to observe directly due to their very low lifetime (less than one second).
Using both the atmospheric concentration and carbon isotope ratios of methane, the scientists in this study were able to identify potential causes of the slow down in the growth of methane in the atmosphere from 1998 to 2006. The ways methane is produced or released in the atmosphere may have been reduced during this time period, but this is not necessarily consistent with either the carbon isotope ratios or other outside estimates of these sources. The primary way methane is destroyed in the atmosphere, through reaction with hydroxyl radicals (OH), may have also increased during this time period. This is consistent with the isotope evidence as well as other studies relating to hydroxyl radicals. A combination of these effects is also possible, but the combined effect would need to be consistent with the isotope evidence as well. This kind of modeling study demonstrates how measuring the isotope ratios in an atmospheric gas can be useful for understanding its chemical and biological activity in the atmosphere.
Implications
Using a model such as this along with atmospheric observations, scientists can develop a better understanding of how methane or other atmospheric gases move in and out of the atmosphere. This is crucial to understanding the potential range of impacts from a given public policy towards environmental pollutants, whether related to smog or climate change. Because of its short lifetime and significant impacts not only on the climate but as a precursor to photochemical smog (ozone pollution), it has been suggested that reductions in methane (and other pollutants) that are technologically feasible now could "buy time" on climate change while having significant public health benefits. While a certain degree of uncertainty does exist about any form of public policy (who knows if an asteroid might hit the Earth tomorrow or nuclear war could break out?), developing as complete an understanding of the physical basis behind any proposed policy as possible is key to helping best estimate the cost and benefits of any decision.
Why would anyone care about methane? Because methane absorbs infrared radiation (heat) strongly, it is the second most important greenhouse gas on the atmosphere of Earth despite its relatively short lifetime in the atmosphere (~9 years). Its concentration has actually increased in the atmosphere from 700 parts per billion in 1750 to nearly 1900 parts per billion today. This increase in methane has led to more heat being trapped by the atmosphere, partially contributing to climate change.
Because the sources and sinks of methane are not completely understood, models that try to take all of them into account cannot predict the "leveling off" of methane observed in the atmosphere from 1998 to 2006. The authors use a mathematical model similar to the one displayed below that includes estimated values for the natural sources of methane (mostly from bacteria in wetlands), the human sources of methane (fossil fuel mining/extraction, rice paddies, waste and water treatment, biomass (i.e. wood) burning, and livestock), and the natural sinks of methane (mostly hydroxyl free radicals (OH) produced from water exposed to sunlight). They then do a "sensitivity analysis" where each major source or sink is increased or decreased within the model to show how changes in these can potentially affect the methane calculated by the model. By increasing the sink through increased hydroxyl radical concentrations or by decreasing the source from wetlands, the model calculates a trend similar to atmospheric observations of methane. How can we tell which of these may have caused this change in the trend, though?
A. Permafrost, Glaciers, and Ice Cores B. Wetlands C. Forest Fires D. Rice Paddies E. Animals F. Plants G. Landfills H. Waste Water Treatment Facilities I. Hydroxyl Radical J. Chlorine Radical
Image by Olivia Shoup and used under the Creation Commons Attribution Share-Alike 3.0 license
Observations and modeling of the carbon-13 to carbon-12 ratio in methane can help provide additional constraints on understanding the sources and sinks of methane. (For more about isotopes and chemistry and how it relates to the atmosphere, see this part of a previous post on a related topic.) Since each source and sink has a relatively different carbon-13 to carbon-12 ratio, any changes to these sources and sinks in the model will also affect the modeled carbon-13 to carbon-12 ratio. These results then can be compared to the atmospheric record of carbon-13 to carbon-12 in methane to see if any change in the estimated value for a source or sink is justified. Using the isotope ratio as a guide, a decrease in wetland methane emissions in the model, while it can bring the methane concentration in the model close to that observed in nature, results in a significant increase in the carbon-13 to carbon-12 ratio, which is not observed. In contrast, an increase in the hydroxyl radical sink will bring the methane in the model close to that observed in nature, but without changing the modeled isotope ratio. Such an increase in hydroxyl radical concentration has been independently proposed, but this hypothesis is hard to confirm since hydroxyl radicals are difficult to observe directly due to their very low lifetime (less than one second).
Using both the atmospheric concentration and carbon isotope ratios of methane, the scientists in this study were able to identify potential causes of the slow down in the growth of methane in the atmosphere from 1998 to 2006. The ways methane is produced or released in the atmosphere may have been reduced during this time period, but this is not necessarily consistent with either the carbon isotope ratios or other outside estimates of these sources. The primary way methane is destroyed in the atmosphere, through reaction with hydroxyl radicals (OH), may have also increased during this time period. This is consistent with the isotope evidence as well as other studies relating to hydroxyl radicals. A combination of these effects is also possible, but the combined effect would need to be consistent with the isotope evidence as well. This kind of modeling study demonstrates how measuring the isotope ratios in an atmospheric gas can be useful for understanding its chemical and biological activity in the atmosphere.
Implications
Using a model such as this along with atmospheric observations, scientists can develop a better understanding of how methane or other atmospheric gases move in and out of the atmosphere. This is crucial to understanding the potential range of impacts from a given public policy towards environmental pollutants, whether related to smog or climate change. Because of its short lifetime and significant impacts not only on the climate but as a precursor to photochemical smog (ozone pollution), it has been suggested that reductions in methane (and other pollutants) that are technologically feasible now could "buy time" on climate change while having significant public health benefits. While a certain degree of uncertainty does exist about any form of public policy (who knows if an asteroid might hit the Earth tomorrow or nuclear war could break out?), developing as complete an understanding of the physical basis behind any proposed policy as possible is key to helping best estimate the cost and benefits of any decision.
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