The human impact on methane emissions
Last week we saw that the start date of the human influence on methane emissions is still not agreed on, but can be as early as 5000 years ago. It is clear, however, that the most significant rise in atmospheric methane concentrations is synchronous with the start of the industrial revolution.
The industrial revolution marked a major turning point in the way people lived and their relationship with the environment. Machinery replaced manual labour, fossil fuels were introduced in the textile and metal industry and agricultural changes increased the food supply. All these changes resulted in a better economy, longer life span of people, increased energy use - and a surge in methane emissions.
The exploitation of coal, oil and gas was one reason for this steady rise. Intensive livestock farming and, to a lesser extent, the spread of rice paddies, was the other. Additionally, farm land sizes increased heavily due to the replacement of manual labour by machines. This expansion of agriculture went together with deforestation - and burning of forests also produces methane (fig. 1)
So since 1750, methane levels have more than doubled from about 700 ppb in 1850 to 1800 ppb now (fig. 1, blog 1) with about two thirds of it being anthropogenic.
But this steady rise slowed down in the 1980s and remained constant from 1990. Scientist were stoked and hope arose that the methane problem was solved. False hope - in 2007 methane emissions abruptly started to rise again (fig. 2)
It is important to understand the dynamics behind these fluctuations - if we know what the drivers are behind the changes, we might get insight in how to constrain methane growth.
The stabilization of methane emissions is, according to most studies, linked to a stabilization in fossil fuel emissions (e.g. Aydin et al., 2011; Simpson et al., 2012; Kirschke et al., 2013; Bousquet et al., 2006). According to Schaefer et al. (2016), the stabilisation of fossil fuel emissions could be related to the collapse of the Soviet Union in 1991. Because of poor technology and pipelines, soviet gas production was suspected to be leaky. The fall of the Soviet union reduced the gas production and replaced their technology by Western technology.
Scientist were more puzzled about the increase in methane emissions after 2007. Fracking, especially in the US, has been blamed for it by some. Especially 'fugitive' fossil fuel emissions, leaking methane that accidentally comes free during oil and gas operations, have received much attention. Because these emissions are accidental, they are hard to quantify. Other scientists thought an increase in natural sources was to blame, not fossil fuels. Schwietzke et al. (2016) found that methane emissions from fossil fuels are 20-60% higher than estimated, but not increasing. Schaefer et al. (2016) blamed the farmers, especially in China and India. These growing countries with large populations are increasing in their rice fields and the number of livestock. Nisbet et al. (2016) argued that rising temperatures and changes in the precipitation pattern have expanded tropical wetlands, which caused the methane increase. In this scenario, global warming would go together with rising methane. Another recent study did not look at methane sources, but focussed on the OH-sink of methane and found that a decline in this sink could have contributed to the methane rise.
It's probably a combination of things. These uncertainties show once more how little we know about methane sources and the dynamics of the cycle. Nevertheless, methane emissions must slow down. Next week we will look at the future of methane concentrations and what we can do to constrain them.
The industrial revolution marked a major turning point in the way people lived and their relationship with the environment. Machinery replaced manual labour, fossil fuels were introduced in the textile and metal industry and agricultural changes increased the food supply. All these changes resulted in a better economy, longer life span of people, increased energy use - and a surge in methane emissions.
The exploitation of coal, oil and gas was one reason for this steady rise. Intensive livestock farming and, to a lesser extent, the spread of rice paddies, was the other. Additionally, farm land sizes increased heavily due to the replacement of manual labour by machines. This expansion of agriculture went together with deforestation - and burning of forests also produces methane (fig. 1)
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| Figure 1 | The sources of anthropogenic methane emissions in millions short tons per year. NOAA |
So since 1750, methane levels have more than doubled from about 700 ppb in 1850 to 1800 ppb now (fig. 1, blog 1) with about two thirds of it being anthropogenic.
 |
| Figure 2 | Changes in atmospheric methane levels from 1983 to 2015. Top: Global methane levels. Bottom: methane growth rate. |
It is important to understand the dynamics behind these fluctuations - if we know what the drivers are behind the changes, we might get insight in how to constrain methane growth.
The stabilization of methane emissions is, according to most studies, linked to a stabilization in fossil fuel emissions (e.g. Aydin et al., 2011; Simpson et al., 2012; Kirschke et al., 2013; Bousquet et al., 2006). According to Schaefer et al. (2016), the stabilisation of fossil fuel emissions could be related to the collapse of the Soviet Union in 1991. Because of poor technology and pipelines, soviet gas production was suspected to be leaky. The fall of the Soviet union reduced the gas production and replaced their technology by Western technology.
Scientist were more puzzled about the increase in methane emissions after 2007. Fracking, especially in the US, has been blamed for it by some. Especially 'fugitive' fossil fuel emissions, leaking methane that accidentally comes free during oil and gas operations, have received much attention. Because these emissions are accidental, they are hard to quantify. Other scientists thought an increase in natural sources was to blame, not fossil fuels. Schwietzke et al. (2016) found that methane emissions from fossil fuels are 20-60% higher than estimated, but not increasing. Schaefer et al. (2016) blamed the farmers, especially in China and India. These growing countries with large populations are increasing in their rice fields and the number of livestock. Nisbet et al. (2016) argued that rising temperatures and changes in the precipitation pattern have expanded tropical wetlands, which caused the methane increase. In this scenario, global warming would go together with rising methane. Another recent study did not look at methane sources, but focussed on the OH-sink of methane and found that a decline in this sink could have contributed to the methane rise.
It's probably a combination of things. These uncertainties show once more how little we know about methane sources and the dynamics of the cycle. Nevertheless, methane emissions must slow down. Next week we will look at the future of methane concentrations and what we can do to constrain them.
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