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UNSW : UNSW Atmosphere Study Guide
Atmosphere Study Guide 30 Explain (article three) As carbon dioxide levels creep ever higher, scientists are working to put greenhouse gas in its place. We investigate carbon sequestration. ONE MORNING EACH WEEK a scientist takes a stroll on the barren upper slopes of Hawaii’s Mauna Loa volcano, with a basketball- sized glass sphere in hand. The researcher faces the wind and strides forward while twisting open a stopcock in the sphere. With a whoosh lasting no more than a few seconds, 5 L of the most pristine air on the planet replaces the vacuum inside the thick-walled orb. Every couple of weeks, a more thoroughly wrapped-up researcher at the South Pole conducts the same ritual. At these remote sites and dozens of others around the world, instruments also sniff the air, adding measurements of atmospheric chemistry to a dataset that stretches back for more than 50 years. This nearly continuous record is from one of the longest-running experiments in history, says Ralph F. Keeling, a climate scientist at Scripps Institution of Oceanography in La Jolla, California. Several trends emerge from the data, says Keeling. First, in the Northern Hemisphere, the atmospheric concentration of carbon dioxide (CO2) rises and falls by about seven parts per million (ppm) over the course of a year. The concentration typically peaks each May, then drops as the hemisphere’s flush of new plant growth converts the gas into sprouts, vegetation and wood. In October, the decomposition of newly fallen leaves again boosts CO2 levels. Populations of algae at the base of the ocean’s food chain follow the same trend, waxing each spring and waning each autumn. A second trend is that each year’s 7 ppm variation in CO2 is superimposed on an average concentration that’s rising steadily. Today’s average is more than 380 ppm, compared with 315 ppm 50 years ago, and 280 ppm prior to the Industrial Revolution. And it’s still rising by about 2 ppm each year, mainly due to combustion of fossil fuels. Largely because CO2 traps heat, Earth’s average temperature has climbed by about 0.74° C over the past century. In the next 20 years, the average global temperature is projected to rise at least another 0.4° C. Stopping additional temperature increases depends on limiting - if not eliminating - the rise in CO2 levels. And as Keeling says, “It’s clear that if we want to stabilise CO2 concentrations in the atmosphere, we need to stop the rise in fossil fuel emissions.” But halting the increase in atmospheric CO2 doesn’t necessarily mean losing fossil fuels. Many experts think that capturing CO2 emissions, rather than simply reducing them, could ultimately provide climate relief. Possible solutions range from boosting natural forms of carbon capture and storage - fertilising the oceans to enhance algal blooms, say, or somehow augmenting the soil’s ability to hold organic matter - to schemes for snatching CO2 from smoke stacks and disposing of it deep underground or in seafloor sediments. Success in sequestering carbon depends on meeting two major challenges: how to remove CO2 from the air (or prevent it from getting there in the first place) and what to do with it once it has been collected. DOING IT IN THE WOODS Organisms at the base of the world’s food chains soak up quite a bit of CO2 - currently about 2% of the atmosphere’s stockpile each growing season. That gas, plus sunlight and other nutrients, is converted into carbon- rich sugars and biological tissues that nourish humans and all other animals. Unfortunately, most of that carbon makes its way back to the atmosphere rather quickly: animals metabolise their food and breathe out CO2 , and decomposition of dead plants and animals also releases large amounts of greenhouse gas. Over the long haul, though, ecosystems can sequester significant amounts of carbon. For instance, 20 to 30%of the carbon in the world’s soil is locked in the Northern Hemisphere’s peat lands, wetlands that accumulate plant matter and soil, with most of that accumulating since the end of the last Ice Age about 11,000 years ago. Recent data suggest that North American ecosystems sequester, on average, 505 million tonnes of carbon each year. Most of that sequestered carbon - about 301 million tonnes - is locked away in forests or in the wood products harvested from them, notes Anthony W. King, an ecosystem scientist at Oak Ridge National Laboratory in Tennessee, U.S . Some researchers, including Ning Zeng, an atmospheric scientist at the University of Maryland in College Park, seek to harness the prodigious carbon-storing power of forests. Right now, forest floors worldwide are lined with coarse wood - everything from twigs and limbs shed during growth to entire fallen trees - containing about 65 billion tonnes of carbon, says Zeng. Left undisturbed, that material would return its carbon to the atmosphere via decomposition or fire. But bury that wood in an oxygen-poor environment and the carbon could be locked away for centuries. Furthermore, Zeng notes, each year the world’s forests produce enough coarse wood to lock away about 10 billion tonnes of carbon. Burying just half of that would significantly counteract the 6.9 billion tonnes released into the atmosphere via fossil fuel emissions. Carbon busters Many think that capturing CO2 emissions could provide climate relief. iSTOCKPHOTO