Higher atmospheric CO2 triggers release of potent greenhouse gases
A new research study led by Trinity College Dublin in Ireland suggests that soil releases the potent greenhouse gases methane and nitrous oxide when a bigger concentration of carbon dioxide (CO2) is found in the atmosphere. The findings, published in the journal Nature, are funded in part by a Marie Curie Actions grant under the EU's Seventh Framework Programme (FP7). The researchers believe that the capacity of land ecosystems to mitigate global warming has been overestimated. Earth continues to be adversely impacted by human intervention, primarily through land use changes, deforestation and the continued burning of fossil fuels. This activity leads to the increase of CO2 in the atmosphere and, in turn, global warming. To date, researchers believed that because plant growth accelerates following a surge of CO2 levels - since stimulated assimilation of carbon by plants can fuel soil carbon input and soil carbon storage - the ecosystems on land could also contribute to de-escalating atmospheric CO2 levels and thus slow climate change. This study shows that this may not be the case. The radiative forcing of terrestrial ecosystems is not determined by their uptake and release of CO2 alone. The soil emissions of methane and nitrous oxide may occur in lower atmospheric concentrations than does CO2, but the ramifications on a global level are significantly greater: 298 times higher for nitrous oxide and 25 times higher for methane. 'This feedback to our changing atmosphere means that nature is not as efficient in slowing global warming as we previously thought,' explains lead author Dr Kees Jan van Groenigen, research fellow at the Department of Botany at the School of Natural Sciences, Trinity College Dublin. Dr van Groenigen and colleagues from the United States collated all published research to date from 49 experiments performed in agricultural fields, wetlands, forests and grasslands primarily in Europe, Asia and North America. All experiments focused on measuring how CO2 in the atmosphere impacts the capacity of soil to take up or release the nitrous oxide and methane gases. Using meta-analysis, the researchers show how increasing CO2 stimulates both nitrous oxide and methane emissions; the former affects upland soils and the latter impacts rice paddies and natural wetlands. 'Until now, there was no consensus on this topic, because results varied from one study to the next,' says Professor Craig Osenberg of the University of Florida in the United States, co-author of the study. 'However, two strong patterns emerged when we analysed all the data: firstly more CO2 boosted soil emissions of nitrous oxide in all the ecosystems, and secondly, in rice paddies and wetlands, extra CO2 caused soils to release more methane.' Wetlands and rice fields are two major sources of methane emissions into the atmosphere. According to the researchers, specialised microscopic organisms in soil are responsible. Just like humans respire oxygen, these microorganisms respire both nitrate and CO2 chemicals, and generate methane as well. Because they do not need oxygen to subsist, they thrive when atmospheric CO2 concentrations rise. 'The higher CO2 concentrations reduce plant water use, making soils wetter, in turn reducing the availability of oxygen in soil, favouring these microorganisms,' Dr van Groenigen says. They also flourish thanks in part to the rising CO2 levels that accelerate plant growth. The extra plant growth provides the soil microorganisms with more energy, effectively creating a stronger metabolism. The team says this extra plant growth could help the ecosystems slow climate change. Increased CO2 leads to more plant growth, which results in more CO2 uptake through photosynthesis. But the researchers postulate that some of the extra CO2 also helps microorganisms release their nitrous oxide and methane into the atmosphere. This offsets any action performed by plants to cool the planet. 'It's an ecological point and counterpoint: the more the plants soak up CO2, the more microbes release these more potent greenhouse gases,' explains Professor Bruce Hungate of Northern Arizona University in the United States, co-author on the study. 'The microbial counterpoint is only partial, reducing the cooling effect of plants by about 20 %.' They point out, however, that it is an ecological surprise; climate models will have to factor this in for the future. 'By overlooking the key role of these two greenhouse gases, previous studies may have overestimated the potential of ecosystems to mitigate the greenhouse effect,' Dr van Groenigen concludes.For more information, please visit: Trinity College Dublin:http://www.tcd.ie/Nature:http://www.nature.com/
Countries
Ireland, United States