[ Pobierz całość w formacie PDF ]
.Methane in the atmosphere chemically decomposes and loses its potency as a greenhouse gas in eight to 12 years, so it has a less persistent effect than carbon dioxide.If we significantly reduce our methane emissions, within a decade its effect as a heating agent and producer of lower-atmosphere ozone would be diminished, and a successful longer-term strategy to stop most human-caused methane emissions would take it off the agenda as a greenhouse gas of lasting concern.Levels of nitrous oxide (known popularly as ‘laughing gas’) have also increased — they are up by 16 per cent since 1750.While relatively small in concentration, the gas has an effect three hundred times more powerful than carbon dioxide, making its overall contribution to global warming about one-tenth that of carbon dioxide.The majority of nitrous oxide is emitted naturally from tropical soils and oceans.The human activity that produces most nitrous oxide is agriculture (through the use of fertilisers), but jet engines, some industrial processes, and cars with catalytic converters that burn fossil fuels also contribute to its production.The gas persists in the atmosphere for about 120 years before being broken down by the effect of sunlight; nonetheless, it is slowly accumulating in the air as a consequence of additional human-caused emissions.A number of other gases (known as ‘trace gases’) that are emitted in smaller quantities from industrial processes — including hydroflourocarbons, sulfur hexafluoride, and perfluorocarbon — contribute to global warming, but on a smaller scale.These gases, together with carbon dioxide, methane, and nitrous oxide, are known as the ‘Kyoto gases’, because they are defined under the agreement to control emissions that was established by the Kyoto Protocol in 1997.While human activity since 1750 has raised the carbon dioxide level by 38 per cent to 387 parts per million in 2008, the effect of all the Kyoto gases together is calculated to be equivalent to 455 parts per million of carbon dioxide.Human activities also contribute to the greenhouse effect by releasing non-gaseous substances such as aerosols, which are small particles that exist in the atmosphere.Aerosols include black-carbon soot, organic carbon, sulphates, nitrates, as well as dust from smoke, manufacturing, windstorms, and other sources.Aerosols have a net cooling effect because they reduce the amount of sunlight that reaches the ground, and they increase cloud cover.This effect is popularly referred to as ‘global dimming’, because the overall aerosol impact is to reduce, or dim, the sun’s radiation, thus masking some of the effect of the greenhouse gases.This is of little comfort, however, because aerosols, or airborne particle pollution, last only about ten days before being washed out of the atmosphere by rain; so we have to keep putting more into the air to maintain the temporary cooling effect.Unfortunately, the principal source of aerosols is the burning of fossil fuels, which causes a rise in carbon dioxide levels and global warming that lasts for many centuries.The dilemma is that if you cut the aerosols, the globe will experience a pulse of warming as their dimming effect is lost; but if you keep pouring aerosols together with carbon dioxide into the air, you cook the planet even more in the long run.There has been a necessary effort to reduce emissions from some aerosols because they cause acid rain and other forms of pollution.However, in the short term, this is warming the air as well as making it cleaner.The total effect of aerosol cooling is generally estimated to be less than 1 degree; however, work by Nicolas Bellouin and a team from the UK Met Office that was published in Nature, in December 2005, found that the cooling effect of aerosols at around 1.4 degrees is much greater than most current climate models estimate.The corollary is that, since aerosols emissions continue to decline, they will be less able to create a cooling effect, and therefore future global warming from greenhouse gases will be greater than presently indicated.This view is consistent with the idea that climate sensitivity is higher than is generally taken to be the case, as we discussed in Chapter 6.This has led Meinrat Andreae of the Max Planck Institute for Chemistry in Mainz, Germany, to conclude that a doubling of pre-industrial carbon dioxide levels by 2100 would produce a 6-degree increase, which would ‘be comparable to the temperature change from the previous ice age to the present [and] so far outside the range covered by our experience and scientific understanding that we cannot with any confidence predict the consequences for the Earth’.Andreae’s collaborator, Chris Jones, warns: ‘Now we are taking our foot off the brake, but we don’t know how fast we will go.Because we don’t know exactly how strong the aerosol cooling has been, we do not know how strong the greenhouse warming will be.’While most aerosols act to cool the planet, one component, black carbon, has the opposite effect [ Pobierz całość w formacie PDF ]