http://www.reportingclimatescience.com/news-stories/article/converting-wetlands-creates-radiative-forcing.html
26.03.2015 14:08 Age: 65 days >From the Helmholtz Centre Potsdam Natural wetlands usually emit methane and sequester carbon dioxide. Anthropogenic impacts however, in particular the conversion of wetlands into cropland, result in a significant increase in CO2 emissions, which overcompensate potential decreases in methane emission, caused by the reduction of wetlands. A large international research team now calculated that the conversion of arctic and boreal wetlands into agricultural land would result in an additional cumulative radiative forcing of about 0,1 mJ per square meter for the next 100 years. The conversion of temperate wetlands into agricultural land would even result in a cumulative radiative forcing of 0,15 mJ per square meter. Converting forested wetlands into managed forests also contributes to increased warming, albeit much less than the conversion of non-forested wetlands. Wetlands are unique ecosystems, which - under natural conditions - are the single largest natural source of the greenhouse gas methane (CH4) but at the same time an important sink for the greenhouse gas carbon dioxide (CO2). The climate footprint of these ecosystems depends on the balance of these two important greenhouse gases. Despite methane being 28 times more potent as a greenhouse gas than carbon dioxide (in a 100 year time span), the conversion of natural wetlands into agricultural or forested ecosystems and its associated decrease in methane emissions still leads to an overall warming effect. "The human impact on wetlands, such as drainage, results in a shift of the climate footprint of that wetland” says Torsten Sachs at the GFZ German Research Centre for Geosciences, co-author of the study. "The overall balance of these two differently active greenhouse gases and thus the climate footprint of a wetland over different time spans depend on the relative sign and magnitude of these ecosystem-atmosphere fluxes". The global impact is still rather uncertain due to large temporal and spatial variability and a lack of data on the complex interactions between environmental drivers such as temperatures of land, water, and sediment, water levels, vegetation, nutrient availability, among others, and the additional anthropogenic impacts such as land use change. To calculate the net ecosystem carbon balance of wetland ecosystems, the more than 40 member research team synthesized data from almost 30 differently affected arctic, boreal, and temperate study sites across the globe. Simultaneous measurements of the ecosystem-atmosphere CO2 and CH4 fluxes in continental North America, Greenland, Europe, and Russia were used for analyses and modeling. For sites with a full annual dataset of CO2 and CH4 fluxes, natural and converted sites were paired in all possible combinations within similar ecosystem types. "To determine the climate impact of the conversion, we used the difference of the net ecosystem carbon balance between the site pairs as series of consecutive annual mass pulses and integrated their effect on tropospheric greenhouse gas concentrations” explains GFZ researcher Sachs. The different radiative efficiencies and atmospheric residence times of the greenhouse gases were accounted for when the radiative forcing was calculated for the period from 2000 – 2100. This research was published in the the Proceedings of the National Academy of Science (PNAS). PNAS reports the significance of this research as: Wetlands are unique ecosystems because they are in general sinks for carbon dioxide and sources of methane. Their climate footprint therefore depends on the relative sign and magnitude of the land–atmosphere exchange of these two major greenhouse gases. This work presents a synthesis of simultaneous measurements of carbon dioxide and methane fluxes to assess the radiative forcing of natural wetlands converted to agricultural or forested land. The net climate impact of wetlands is strongly dependent on whether they are natural or managed. Here we show that the conversion of natural wetlands produces a significant increase of the atmospheric radiative forcing. The findings suggest that management plans for these complex ecosystems should carefully account for the potential biogeochemical effects on climate. Significant climate risks are associated with a positive carbon–temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the “cost” of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse–response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH4 emissions and cumulative CO2 exchange. Uncertain climate footprint of wetlands under human pressure by Ana Maria Roxana Petrescu et. al., published in the Proceedings of the National Academy of Science, PNAS Early Edition, doi: 10.1073/pnas.1416267112 Read the abstract and get the paper here. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout.
