Despite claims and reports blaming global warming on human activity, it has reached a point where Earth is itself contributing to climate change.

Until now, the Earth's landmass had been considered a 'sink' for carbon dioxide, soaking up some of the emissions of the greenhouse gas from human activity.

But after being overwhelmed by other greenhouse gas emissions, scientists now believe the land has reached saturation point and the emission of gas from plants, for example, is contributing to global warming.

A new study reveals that human activity, including intensive farming (stock image), has boosted emissions of nitrous oxide and methane from the land. Researchers say that these additional greenhouse emissions are outweighing the cooling effect of the land soaking up carbon dioxide

In a new study, an international team of researchers has demonstrated that emissions of methane and nitrous oxide have 'overwhelmingly' surpassed the land's ability to soak up carbon dioxide.

They suggest that this saturation means the land may now actually be contributing to climate change, instead of slowing it down.

The scientists looked at the 'biogenic fluxes' of the three main greenhouse gases over the last three decades, and subtracted out emissions that existed in pre-industrial times.

HUMANS SHIFT THE BALANCE The planet's landmass acts as a 'sink' to soak up greenhouse gases such as methane, nitrous oxide and carbon dioxide, from the atmosphere. While levels of these gases naturally fluctuate - as they are released into the atmosphere from 'biogenic' sources such as plants, animals and bacteria - their levels have been shifted by human activity. Adding fertilisers to the soil to grow crops has increased the amount of nitrous oxide released, while rearing livestock has boosted levels of methane. In total, these additional emissions of the gases are outweighing the cooling effective of the land soaking up carbon dioxide, leading to a net warming effect. The study did not look at carbon dioxide produced by burning fossil fuels. Advertisement

These biogenic sources include gas emitted by plants, animals, and microbes, such as methane produced by wetlands, and nitrous oxide released by soil.

But the amounts of these gases have been changed by human activity, plus new sources created by sewage, fertilisers, and cattle.

The scientists added up all the biogenic emissions of carbon dioxide, methane, and nitrous oxide, then subtracted those that occurred naturally.

The study did not include gas emissions from fossil fuel burning or natural gas production.

The surprise finding in the report is that human impact on biogenic methane and nitrous oxide emissions far outweighs the impact on the uptake of carbon dioxide.

In other words, they say this so-called terrestrial biosphere is now contributing to climate change rather than mitigating climate change - and it's all because of human action.

Scientists at the Carnegie Institute's department of global ecology in Washington DC acknowledge that this runs counter to conventional thinking.

They said that previous studies had focused only on carbon dioxide rather than considering methane and nitrous oxide, and had emphasised the mitigating effect of carbon uptake.

Researchers say levels of greenhouse gases naturally fluctuate as they are released into the atmosphere from natural sources. But their levels have been shifted by human activity (pictured in the figure). This includes adding fertilisers to soil, and rearing livestock, which has boosted levels of nitrous oxide and methane

Human-induced emissions of the gases in southern Asia - which contain thje majority of the world's paddy fields (stock image) and account for 60 per cent of fertiliser use - were found to have a larger net warming effect than other areas

Lead author Hanqin Tain, director of the International Center for Climate and Global Change Research at Auburn University, said: 'This reveals for the first time that human activities have transformed the land biosphere to become a contributor to climate change.'

They added that reducing the emissions of methane and nitrous oxide from agriculture, particularly in southern Asia, could help to mitigate climate change.

The findings have been published in the journal Nature.

If the land is losing its ability to soak up carbon dioxide as the scientists claim, then it marks a worrying trend.

Researchers have previously suggested that the world's oceans arelosing their ability to absorb greenhouse gases.

This points to a shift in the fine global balance, where carbon dioxide is absorbed from the atmosphere by the land and oceans.

Alongside increased atmospheric concentration of the gas, the warming oceans are not able to store as much carbon, meaning they are able to soak up less carbon dioxide.