Sunday, October 12, 2014

Beyond the Carbon Footprint: the Contribution of Methane and Nitrous Oxide to Global Warming

(contributed by Emma Seddon)

The latest IPCC report on climate change, published in 2014, continues to hammer home the message that evidence for global warming is unequivocal: ‘Human interference with the climate system is occurring, and climate change poses risks for human and natural systems.’ It stresses our need for managing the risks and adapting to them, and that it is possible to avert catastrophic change without sacrificing our living standards. In other words, as the leader of the IPCC team, Professor Ottmar Edenhofer, put it: ‘It doesn’t cost the world to save the planet.’

The report continues to warn us about soaring carbon dioxide emissions, which are now almost double the previous rate. Its analysis concludes that rapid action on implementing low-carbon energy policies would still keep the average global temperature rise to only 2 degrees Centigrade by 2050. However, the stress that has always been put on the reduction of carbon dioxide emissions means that the dangers of other pollutants and greenhouse gases, such as methane and nitrous oxide, if not underestimated, can be neglected by policymakers. The WMO Greenhouse Gas Bulletin compares the relative levels of all greenhouse gases that persist in the atmosphere, including carbon dioxide, methane, nitrous oxide, hydroflurocarbons and perflurocarbons, which it describes as ‘the main drivers of climate change’.

The need for action

The urgent need for action on climate change is a message that can sometimes be lost on people; it can seem like an old story, just part of the background noise of politics. As long as the politicians talk about it, we can be lulled into a false sense of security that everything that can be done is being done: it is everyone’s problem, but not everyone’s to deal with. If we do care, we are content to let others worry about a solution as there are so many other environmental problems to solve. These include the destruction of our forests, our disappearing wildlife and pollution of the land and air by a range of industrial waste products, including the growing problem of industrial waste from the pharmaceutical industry, which contaminates our water supply. In the case of pharmaceutical waste, this is a problem that directly affects the daily lives of our families but which can be dealt with on a local level; however, the problem of rising methane levels is not so easily managed and is more global in scale.

Rising levels of greenhouse gases

To achieve the required change in the Earth’s net radiative flux (RF) or the ability of the Earth to reflect or retain solar radiation—to halt or reverse global warming, which is a result of the greenhouse effect of the long-lived greenhouse gasses (LLGHG)—we would need to make significant reductions in our emission levels of these gases. Next to carbon dioxide, methane is the next most prevalent LLGHG, which since 1980 has increased its RF factor from around 0.8 W/m2 (watts per square meter) to 1.0 W/m2, compared to carbon dioxide, which has risen from 1.0 W/m2 to nearly 3.0 W/m2. The concentrations of carbon dioxide, nitrous oxide and methane in the atmosphere have increased substantially since 1750, coinciding with the expansion of industrialization. Figures for 2011 put this increase at 40%, 20% and 150% respectively, with methane clearly showing the highest percentage rise. These levels exceed by far the highest concentrations of any LLGHG seen in the ice core records for the past 800,000 years. The most recent analysis of globally averaged mole fractions (the average amount of a constituent in a mixture) of carbon dioxide, nitrous oxide and methane show that these increases have continued, with 2013 levels recorded at 142%, 121% and 253% of pre-1750 levels respectively.

Methane in the atmosphere accounts for ~17% net RF attributed to LLGHGs. 40% of methane comes from natural sources such as wetlands and the action of termites; the latter is estimated to be around 208,000,000 metric tons annually. The other 60% is the result of our actions: from the exploitation of fossil fuels, landfills, the burning of biomass, rice-growing and ruminant livestock. By 2013, this had increased by 253% of the pre-industrial level. However, there are considerable inter-annual differences in the percentage of methane in the atmosphere, which studies have suggested are the result of increased emissions from tropical wetlands and from industrial sources in the northern hemisphere, including leakage from gas and oil wells, from coal mining and waste treatment plants.

Nitrous oxide in the atmosphere accounts for ~6% net RF attributed to LLGHGs. Although this is the third most common greenhouse gas in the atmosphere, it has the greatest effect on the depletion of stratospheric ozone. Before industrialization, nitrous oxide in the atmosphere was in balance, with emissions from the oceans and soils equaling losses of the gas in the stratosphere. Industrialization has changed this picture. Emissions from the use of agricultural fertilizers, either directly from the fields or indirectly from the contamination of rivers, and from the burning of fossil fuels and biomass, have significantly increased emission levels, which now exceeds stratospheric losses to produce a net mole fraction rise of 121% over pre-industrialization levels.

Ocean acidification

The oceans absorb a quarter of our carbon dioxide emissions, which reduces the effect of increased levels; emissions from the burning of fossil fuels still contribute to a rise in atmospheric percentages of carbon dioxide, but the net global warming effect is reduced. However, the absorption of carbon dioxide alters the acidity of the oceans by altering the carbonate system, forming carbonic acid that breaks down into hydrogen and carbonate ions. An increase in hydrogen ions results in increased acidity. The carbonate ions have a limiting effect, which binds with the hydrogen ions to form bicarbonate, which reduces the capacity of the upper levels of the oceans to absorb carbon dioxide. This is now an unprecedented 70% of the oceans’ pre-industrialization absorption capacity, a figure that may be reduced drastically to only 20% by the end of the century.

These levels of degradation may be reduced by up to 10% by climate change, but the increase of fresh water in the oceans from the Arctic ice melt could significantly accelerate the rate of acidification. The ice melt also releases quantities of carbon dioxide and methane into the atmosphere, which was either trapped in the ice crystals or present through the decay of ancient organic matter, and this accelerates the rate of atmospheric warming.


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