In other words, a concentration of 1 ppb for a given gas means there is one molecule of that gas in every 1 billion molecules of air. Concentrations of these greenhouse gases are measured in parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt) by volume. As a result of human activities, however, these gases are entering the atmosphere more quickly than they are being removed, and thus their concentrations are increasing.Ĭarbon dioxide, methane, nitrous oxide, and certain manufactured gases called halogenated gases (gases that contain chlorine, fluorine, or bromine) become well mixed throughout the global atmosphere because of their relatively long lifetimes and because of transport by winds. Over time, these gases are removed from the atmosphere by chemical reactions or by emissions sinks, such as the oceans and vegetation, which absorb greenhouse gases from the atmosphere. When greenhouse gases are emitted into the atmosphere, many remain there for long time periods ranging from a decade to many millennia. and Global Greenhouse Gas Emissions indicators). Since the Industrial Revolution began in the 1700s, people have added a substantial amount of greenhouse gases into the atmosphere by burning fossil fuels, cutting down forests, and conducting other activities (see the U.S. Globally, the amount of ozone in the troposphere increased by about 12 percent between 19 (see Figure 5). .These chemicals have been released into the air for many years, but recently, international efforts have reduced emissions and phased out their use. Changes in stratospheric ozone reflect the effect of ozone-depleting substances. All of the decrease happened in the stratosphere, with most of the decrease occurring between 19. Overall, the total amount of ozone in the atmosphere decreased by more than 4 percent between 19 (see Figure 5).Concentrations of other halogenated gases have continued to rise, however, especially where the gases have emerged as substitutes for ozone-depleting chemicals (see Figure 4, right panel). As a result, concentrations of many major ozone-depleting gases have begun to stabilize or decline (see Figure 4, left panel). Some of these chemicals have been or are currently being phased out of use because they are ozone-depleting substances, meaning they also cause harm to the Earth’s protective ozone layer. Concentrations of many of the halogenated gases shown in Figure 4 were essentially zero a few decades ago but have increased rapidly as they have been incorporated into industrial products and processes.This increase is primarily due to agriculture. Levels have risen since the 1920s, however, reaching a new high of 334 ppb in 2021 (average of four sites in Figure 3). Over the past 800,000 years, concentrations of nitrous oxide in the atmosphere rarely exceeded 280 ppb.This increase is predominantly due to agriculture and fossil fuel use. The concentration of methane in the atmosphere has more than doubled since preindustrial times, reaching over 1,800 ppb in recent years (see the range of measurements for 20 in Figure 2).Almost all of this increase is due to human activities. Carbon dioxide concentrations have increased substantially since the beginning of the industrial era, rising from an annual average of 280 ppm in the late 1700s to 414 ppm in 2021 (average of five sites in Figure 1)-a 48 percent increase.Historical measurements show that the current global atmospheric concentrations of carbon dioxide, methane, and nitrous oxide are unprecedented compared with the past 800,000 years (see Figures 1, 2, and 3). Global atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and certain manufactured greenhouse gases have all risen significantly over the last few hundred years (see Figures 1, 2, 3, and 4).
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