Earth’s atmosphere is crucial for living beings on the surface. It protects from the harmful radiation beaming from the Sun and other sources in space. It also directly regulates the planets climate. So, if you find any significant changes in the climate, one of the first suspect would be a changing atmosphere. Indeed, scientists have noted a correlation between increased CO2 (carbon-dioxide) in the atmosphere and global temperatures, over broad ranges of time (1,2).
Like CO2, other gases like methane (CH4) and water vapour (H2O) also cause an increase in temperature of the planet. They do so by blocking heat to escape from the surface of the planet. This effect is called the greenhouse effect and causes global warming leading to climate change.
Methane is the second largest contributor to global warming attributed to human activity, after carbon dioxide. Fossil fuel extraction and its use is known to be a major human activity leading to more methane in the atmosphere. But, compared to carbon dioxide and other greenhouse gases, methane has a relatively short shelf-life. It lasts, on average, only around nine years in the atmosphere. Compare this to carbon dioxide which can persist for even up to a 100 years. So, curbing methane emissions can give us relatively quick insights in to how modifying our fuel sources can have an impact on the climate.
Methane (CH4) released into the atmosphere can be distinguished into two categories,
- Methane containing a heavier isotope of carbon called C-14 – This kind of methane is found in biological sources such as plants and animals. For instance, it is released when cows burp (belch) or from landfills and rice fields.
- Methane which contains the normal carbon, C-12 – This kind of methane does not contain the heavier carbon isotope and is primarily present in fossil fuels. When we burn fossil fuels, we release this methane into the atmosphere. This kind of methane is also released from natural geological sources like seeps and mud volcanoes.
So we can measure C-12 abundance in atmospheric methane and this gives us an estimate of how much methane comes from natural geological sources and human activity combined. But how can we pin point the amount of methane contributed by human activity?
Snapshots from Ice
In the vast ice caps of the poles on our planet, lies historical data. As ice forms, through accumulation of snow, it traps air molecules in small gaps. So, the deeper we dig into ice, the trapped air is more and more from the past. Using this principle, scientists can study the components of the air (atmosphere) from the past.
Scientist recently published results from this study in a Nature paper (3). By measuring the ratio of carbon-12 and carbon-14 isotopes in air from more than 200 years ago, the researchers found that almost all of the methane emitted to the atmosphere was biological in nature until about 1870. Then onward, methane released from fossils began to rise rapidly. The timing coincides with a sharp increase in the use of fossil fuels by humans.
The measurements revealed a striking deviation from the previous estimations. The levels of naturally released fossil methane are about 10 times lower than previous research reported (3). Given the total fossil emissions measured in the atmosphere today, the researchers deduced that the man-made fossil component is 25-40 percent higher than what was previously reported.
Results from this study give more weightage to human contribution towards methane emissions. If most of the methane emission is coming from human activity, it means we can control it. Given that methane has a shorter shelf-life in the atmosphere, it is tempting to speculate that regulations in fossil fuel usage would bring faster results than previously expected. But first, we must convince the “deniers”.
- 1.Lüthi, D. et al. High-resolution carbon dioxide concentration record 650,000–800,000 years before present. Nature 379–382 (2008). doi:10.1038/nature06949
- 2.Shakun, J. D. et al. Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation. Nature 49–54 (2012). doi:10.1038/nature10915
- 3.Hmiel, B. et al. Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions. Nature 409–412 (2020). doi:10.1038/s41586-020-1991-8