ALTHOUGH carbon dioxide is the principal greenhouse gas in terms of long-term global warming potential, the atmospheric factors that determine warming and cooling are complex and scientists continue to discover new and surprising insights, writes WILLIAM REVILLE
For example, emissions of sulphur dioxide (SO2) have a cooling effect, so, as China and India reduce SO2 emissions, the rate of global warming will speed up. On the other hand, emissions of black carbon, largely in Asia, warm the planet, so, reducing these emissions could counter the declining SO2 emissions. And, finally, methane, the second most significant greenhouse gas after carbon dioxide, has recently been found to be more potent than previously thought. These findings were reviewed by A Ananthaswamy in the New Scientist (February 19th).
SO2 emissions from burning coal and oil react with water and oxygen in the air to form fine particles called sulphate aerosols. These aerosols are acidic and fall back to earth as acid rain that damages forests and wildlife. But these aerosols also reflect incoming sunlight back into space, cooling the Earth and mitigating the warming effect of increasing carbon dioxide levels.
The cooling that occurred in the northern hemisphere from the 1940s to the 1970s is largely explained by rising SO2 emissions that eventually plateaued and then reduced – European industrial sulphur dioxide emissions have been steadily decreasing over the past 30 years. Temperatures rose gradually and evenly in the southern hemisphere where there was relatively little SO2 emission.
When China and India seriously reduce SO2 emissions it is estimated that the diminution of the sulphate aerosol cooling effect could allow global warming to increase from the current 0.2 degrees to 0.3-0.4 degrees per decade. Recent work suggests that 75 per cent of all ecosystems on Earth would find it difficult to cope with rates of temperature increase greater than 0.3 degrees per decade.
In view of this it might be thought to be unwise to further reduce SO2 emissions. However, SO2 is a primary cause of acid rain and also causes lung and bronchial problems such as bronchitis and asthma, so there is no credible overall argument for continuing to emit high levels of SO2. Schemes have been proposed to artificially cool the Earth by pumping SO2 high into the atmosphere. This might well have the desired cooling effect but I don’t see how we could live with the side effects – we know about the acid rain and there might well be other side effects that we are unaware of at present. Black carbon is emitted during relatively low temperature burning of almost any material ranging from dung to diesel. Black carbon absorbs the warmth of sunlight and contributes to local warming. In the short term it may come second only to CO2 as a cause of global warming.
A brown haze overhangs much of Asia, mainly composed of black carbon emitted from low temperature burning of firewood, coal and cow dung. Black carbon reduces evaporation from oceans, thereby reducing rainfall, and it heats underlying snow and ice when it settles there. This slows down monsoons and speeds up the melting of Asian glaciers. As Ananthaswamy points out, there is a strong local contribution to climate change in Asia – it cannot all be attributed to emissions in the industrialised West. The recent retreat of Himalayan glaciers seems to be mainly caused by deposits of black carbon of local origin.
The Arctic seems to be the fastest warming part of the world. Falling levels of cooling aerosols and rising levels of carbon black seem to be at least as responsible as rising levels of greenhouse gases for warming in the Arctic.
Methane, emitted from landfills, livestock production and agriculture (particularly rice paddy fields) is a powerful greenhouse gas. Recent studies have shown that its warming greenhouse effect is 33 per cent greater than was previously thought. This is because of its negative effect on the formation of cooling aerosols.
The surprising consequences of the complex atmospheric interactions that contribute to global warming and cooling is illustrated by global dimming. Human emissions into the atmosphere, resulting in the large-scale formation of aerosols, has shaded Earth from sunshine by up to 10 per cent in some places – global dimming. You would think that this diminution in light would naturally reduce photosynthesis in plants and therefore reduce plant growth. The amazing thing is that plant growth is boosted by global dimming.
This is because the fine particles in the air scatter the light, so leaves receive light from all directions and fewer leaves are left in the shade. When we reduce pollution and the air gets cleaner, plant growth will fall.
Clearly, much remains to be learned about current climate change. As the old catchphrase puts it: “Where would you be going without a bell on your bike?”
William Reville is associate professor of biochemistry at University College Cork