One of the benefits of urban living is that you rarely have to worry about where your waste goes. Between widespread recycling schemes, reliable sewage systems and regular bin collections, cities make it very easy to throw away anything you might want to get rid of. But, as I have been known to say repeatedly in this column, there is no “away.” Lots of what you dispose of spends hundreds of years in huge landfills on the edges of your nearest city.
And the Lawrence Berkeley National Laboratory has shown that these piles of waste are changing much more than the urban skyline. In a new paper (£) published in the journal, Geophysical Research Letters, they’ve proved that landfills responsible for the vast majority of all the methane emitted in the San Francisco Bay area.
Generally, when people talk about methane, the word "cows" is rarely far behind. Thanks to their diet, cows burp, fart and poo a lot, which are all methane-producing processes. With somewhere in the region of 1.5 billion heads of cattle on Earth (as of August 2015), this "activity" adds up, so they are an important source of the gas. But in urban areas, other sources dominate, and as this study shows, its landfills that take the top spot.
There, methane is produced by the decomposition of waste. Initially, when waste is added to a landfill, it emits very little methane because it first decomposes aerobically (in the presence of oxygen from the air). But once it’s been covered over by other waste, the oxygen can’t get to it, so the decomposition becomes anaerobic. In this environment, oxygen-hating bacteria flourish, and as they break down the waste, they produce methane as a by-product.
By taking measurements from six sites across the Bay, and feeding it into the latest atmospheric models, the Berkeley Lab team showed that methane emissions from landfills have historically been underestimated by 1.3 – 2.3 times. So there might be twice as much of the gas as we thought. They also analyzed other gases, including toluene and ethane, which are collectively called volatile organic compounds (VOCs). Biological sources of methane (like wetlands, cows and landfills) tend not to emit VOCs, while sources of fossil methane (like natural gas) do. So, by measuring quantities of these compounds, it was possible to identify the source of the methane emissions. Somewhere in the region of 82% of total emissions were found to be biological, with the remaining 17% linked to natural gas leaks.
The reason that these numbers are important is that, like carbon dioxide and nitrous oxide, methane is a greenhouse gas. Greenhouse gases (GHGs) act a bit like fluffy duvets for the Earth. Their molecules allow the sun’s light to enter the atmosphere, but they stop its heat from escaping, which warms up the atmosphere (and the planet). Having some GHGs is a good thing – without them, our world would be very, very cold – but the levels we’ve been pumping into the atmosphere since the early days of industrialization have tipped this delicate balance, heating the Earth and changing our climate.
Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities, mainly the burning of fossil fuels. But you might not know that, if you’re in the U.S., methane takes second place. It spends a lot less time in the atmosphere than CO2 does, but it is much more efficient at trapping radiation, so has a demonstrably larger impact on the climate.**
California is regularly lauded for its environmental efforts, and in AB32 (the California Global Warming Solutions Act) it set out some of its goals around greenhouse gases. The state aims to reduce GHG emissions by 30% in 2020, taking them back to what they were in 1990. By 2050, they’re targeting an 80% reduction. Lead scientist of the study, Dr. Marc Fischer said, "We believe methane is 10-15% of California's total greenhouse gas emissions on a 100-year timescale. If we are to reduce total emissions by 80% in 2050, we would have a very much harder time doing that if we don't also reduce methane."
This study (along with several others, including this one from November 2016) are amongst the first to quantify greenhouse gas emissions from multiple sources. And with a challenge as big as mitigating the impact of climate change, we need all the data we can get.
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** This is measured by looking at a compound's Global Warming Potential (GWP) – it was developed to give us a way to directly compare the impact of greenhouse gases on climate change. Carbon dioxide is used as the reference gas, so has a GWP = 1. According to the EPA, methane’s GWP = 28-36 over 100 years.
