Methane puzzle: acquittal for our ancestors

Not human, but astronomical factors caused the methane increase after the Ice Age

The German-French climate satellite will measure methane concentrations. The gas, like carbon dioxide, is responsible for global warming. Its effect is 25 times higher than that of CO2. Even when it comes to man-made increases in the amount of air in the atmosphere, methane has already clearly outperformed carbon dioxide: since pre-industrial times, atmospheric methane has more than doubled - the increase in carbon dioxide during this period was "merely " 30 percent. Like carbon dioxide, methane is one of the gases whose emission is to be reduced according to the Kyoto Protocol. © NASA
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The slow increase in atmospheric methane levels in the last 5, 000 years is exceptionally not a human plant, with the exception of the last two centuries. This was shown by a model analysis of several possible triggering factors now published in "Nature". Not the deforestation and agriculture of our ancestors are to blame, as some researchers suggest, but a change in solar radiation triggered by changes in the Earth's orbit.

Methane is one of the most potent greenhouse gases and its warming effect on the atmosphere is around 30 times higher than that of carbon dioxide. That man and his agriculture are to blame for the rapid increase of methane concentrations in the atmosphere in the last two hundred years, there is hardly any doubt. The cause of another, less pronounced increase in methane in the last 5, 000 years is extremely contentious. During this period, the greenhouse gas content increased from 550 parts per billion (ppb) to 770 ppb.

Man or nature - who was to blame?

According to some geoscientists, a human influence could also be responsible for this increase. In one study, they hypothesized that early agriculture, clearing of forests and other vegetation changes may have been sufficient to affect the methane content of the atmosphere. But this hypothesis is highly controversial, as many researchers believe that the interference with nature by the then much smaller human population were too low to have such a significant effect.

But if man was not, then what? And why did this increase occur only after the last ice age and not in the other interglacials? A possible new answer has now been found by a team of British scientists from the Universities of Bristol, Exeter and Sheffield. By combining several models and climate simulations, they have been able to compare the evolution of methane concentrations over the past 130, 000 years with those of several possible causes.

Modeling different "candidate" combinations

In a first step, the researchers used the Hadco Center's global coupled ocean atmosphere model HadCM3 to simulate how the climate system changes if only one or a combination of the potential outliers correspond to the real one is changed. The factors tested include the smallest changes in the Earth's orbit, an increase in greenhouse gases and the change in ice cover and sea level. In a second step, additional models were fed with these results to determine how the vegetation distribution, methane emissions and ultimately the methane concentration in the atmosphere changed. display

Methane emissions of the last 130, 000 years under different conditions Singarayer et al. / Nature

Changes in Earth's orbit and solar radiation as the main cause

The result: at least part of the methane increase is due to orbital factors due to minute changes in the Earth's orbit. In the model run, in which all other factors were frozen and only this was simulated according to the real conditions, already slightly increased values ​​for the later Holoz n were shown. All runs showed variations that correlated with variations in solar irradiance, especially in the tropics. The vegetation models showed that, especially in the southern hemisphere, large amounts of methane were produced by the plant world.

We conclude that the increase in methane in the late Holocene can be attributed primarily to increasing emissions of tropics to the southern hemisphere, so the researchers in their study. In contrast to the last interglacial period, these emissions are not offset by a corresponding decrease in emissions from the northern hemisphere.

Man - exceptionally innocent

Although the model calculations also showed a slight increase in atmospheric carbon dioxide and thus, in some cases, the signal of a human influence, this was not decisive for the increase in methane, the researchers say: Egal whether the slight increase in CO2 from the early to the late Holoz n is now natural or anthropogenic, it is definitely not the driving mechanism. The results of the sensitivity experiments strongly suggest that climate changes due to changes in solar radiation played a significant role in the increase of methane in the Sp t holocene. Accordingly, humans do not seem to blame for this development for once. (Nature, 2011; doi: 10.1038 / nature09739)

(Nature, 03.02.2011 - NPO)