4.3 Carbon dioxide (CO₂), methane (CH₄) and human activity

In recent decades, the understanding of the reality of climate change has moved from one of slow and gradual change over deep time to clear evidence that there have been naturally-occurring climate changes of several degrees Celsius and sea-level rises of half a metre within timescales of a decade or so (IPCC, 2013). In fact, research by Steffensen et al. (2008) on the Younger Dryas using ice core data revealed that central Greenland cooled by a staggering 2–4 °C in just 1–3 years, around ten times faster than the highest reconstructed rates of warming over the past two centuries (Box et al., 2009).

While the Younger Dryas and the 8.2 ka event were entirely natural, there is an additional human contribution to consider. But when exactly did the human contribution begin? Often the phrase ‘pre-industrial levels’ is used to mean ‘before significant anthropogenic changes started’, but it is not specific. Could humans have influenced the climate before the Industrial Revolution of the 18th century?

Another very significant greenhouse gas is methane (CH₄), and 1 cubic metre of methane in the atmosphere can be over 25 times more effective at trapping heat than the same amount of CO₂. Past atmospheric methane concentrations can also be directly measured from ice cores.

Over the last quarter of a million years, CH₄ concentration and the variation of solar radiation reaching the Earth attributed to the Milankovitch cycle are positively correlated (Figure 24(a)). But this correlation dramatically breaks down in the most recent data of the Holocene.

The latest 5000 years of methane data show that the atmospheric concentration has risen dramatically out of synchrony with the solar radiation (Figure 24(b)). Three metres down in the EPICA ice core, dating to the 1820s, methane concentrations are as high as any period in the entire ice core record – approaching 800 ppb (parts per billion). Carbon dioxide has a similar break from the expected downward trend, although starting earlier, at about 8000 years ago.

If the ‘normal’ trend of methane and carbon dioxide was downwards, along with the Milankovitch cycle, then where have the extra gases come from?

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Figure 24 (a) The atmospheric concentration of methane, and changes in solar radiation reaching the Earth’s surface due to the Milankovitch cycles; (b) observed and expected atmospheric methane levels over the last 11 000 years up to 1937

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This figure consists of two panels. The first panel shows the atmospheric concentration of methane and solar radiation from the Milankovitch cycle. The horizontal axis is time measured in years before present (BP) and the minimum is 250 000 years BP with ticks every 50 000 years. The maximum on this axis is 0 which is the present day. The left-hand vertical axis is methane in the EPICA ice core in parts per billion (ppb): the minimum is 300 ppb and the maximum is 900 ppb with ticks every 100 ppb. The right-hand vertical axis is solar radiation in watts per square metre: the minimum is 440 watts per square metre and the maximum is 540 watts per square metre with ticks every 20 watts per square metre. The panel has a blue-dashed line which shows the solar radiation and it is a smoothly varying curve cycling from about 440 watts per square metre to 510 watts per square metre on a period of approximately 25 000 years. An orange line shows the methane concentration in the EPICA ice core. Although it is not smoothly varying, methane concentration does cycle in a similar way to the solar radiation between the values of about 400 ppb and 600-700 ppb, such that high solar radiation corresponds to high methane, and low solar radiation corresponds to low methane. This is so throughout virtually the entire record, although this pattern is broken in the last 5 000 years, where methane levels do not reduce and instead rapidly increase to around 900 ppb. The second panel shows just methane concentration over the last 11 500 years. The horizontal axis is time measured in years before present (BP). The minimum is 11 500 years BP with ticks at 10 000 and 5 000 years BP. The maximum on this axis is 0 which is the present day. The vertical axis is methane concentration in parts per billion (ppb): the minimum is around 410 ppb and the maximum is 800 ppb with ticks every 50 ppb from 450 to 750 ppb. From 11 500 to 5 000 years BP the methane concentration in the core is on a downwards trend from approximately 700 ppb to 550 ppb. The trend is what would be expected from the previous panel and after 5000 years BP the trend is marked with an arrow continuing downwards reaching approximately 450 ppb at 0 on the horizontal axis. However, the methane concentration departs from this trend at 5 000 years BP and reverses direction to become a virtual mirror image of the previous 5 000 years, reaching greater than 700 ppb at 0 on the horizontal axis. So there is a departure from the expected trend of methane of the last 250 000 years.

Figure 24 (a) The atmospheric concentration of methane, and changes in solar radiation reaching the Earth’s surface due to the ...

Carbon dioxide and methane are by-products of civilisation. You often think of these by-products as beginning with the Industrial Revolution, but their story begins far earlier. In 2003, climate scientist William Ruddiman proposed that society had been altering the levels of these gases – and therefore influencing global climate – for many thousands of years. In his own words:

Ruddiman suggests that these activities could have increased atmospheric concentrations by up to around 40 ppm for CO₂ and 300 ppb for methane, increasing the expected natural levels by around 15% and 70% respectively (Ruddiman et al., 2016).

The amounts of CO₂ and methane added to the atmosphere – and therefore the degree to which human activities changed the climate – during the agricultural era are still being debated, but now, of course, the human effect on these gases is clear.

As with CO₂, since the Industrial Revolution the atmospheric concentration of methane has rapidly increased and currently is over 1800 ppb. Virtually all of that rise has been from anthropogenic sources, including major food production activities such as growing rice and cattle. Not only were humans possibly affected by climate change during the Holocene, but the impact by humans on the planet had already been started thousands of years before the Egyptian Pyramids were built. How then are these changes being seen today?