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Importance of methane and nitrous oxide for Europe’s terrestrial greenhouse-gas balance
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Climate change negotiations aim to reduce net greenhouse-gas emissions by encouraging direct reductions of emissions and crediting countries for their terrestrial greenhouse-gas sinks. Ecosystem carbon dioxide uptake has offset nearly 10% of Europe’s fossil fuel emissions, but not all of this may be creditable under the rules of the Kyoto Protocol. Although this treaty recognizes the importance of methane and nitrous oxide emissions, scientific research has largely focused on carbon dioxide. Here we review recent estimates of European carbon dioxide, methane and nitrous oxide fluxes between 2000 and 2005, using both top-down estimates based on atmospheric observations and bottom-up estimates derived from ground-based measure- ments. Both methods yield similar fluxes of greenhouse gases, suggesting that methane emissions from feedstock and nitrous oxide emissions from arable agriculture are fully compensated for by the carbon dioxide sink provided by forests and grass- lands. As a result, the balance for all greenhouse gases across Europe’s terrestrial biosphere is near neutral, despite carbon sequestration in forests and grasslands. The trend towards more intensive agriculture and logging is likely to make Europe’s land surface a significant source of greenhouse gases. The development of land management policies which aim to reduce greenhouse-gas emissions should be a priority.
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commentary: the case for mandatory sequestration
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the fact that cumulative carbon dioxide emissions are more important than annual emission rates calls for a fresh approach to climate change mitigation. one option would be a mandatory link between carbon sequestration and fossil fuel extraction. FROM THE TEXT: With current emissions around 10 billion tonnes of carbon per year, and over three trillon tonnes still available in fossil fuel reserves (4,11), emissions need to fall,on average, by over 2% per year from now on to avoid releasing the trillionth tonne.The longer emissions are allowed to rise, the faster they will have to fall thereafter to stay within the same cumulative total.
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Permanent storage of carbon dioxide in geological reservoirs by mineral carbonation
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Anthropogenic greenhouse-gas emissions continue to increase rapidly despite efforts aimed at curbing the release of such gases. One potentially long-term solution for offsetting these emissions is the capture and storage of carbon dioxide. In principle, fluid or gaseous carbon dioxide can be injected into the Earth’s crust and locked up as carbonate minerals through chemical reactions with calcium and magnesium ions supplied by silicate minerals. This process can lead to near-permanent and secure sequestration, but its feasibility depends on the ease and vigour of the reactions. Laboratory studies as well as natu- ral analogues indicate that the rate of carbonate mineral formation is much higher in host rocks that are rich in magnesium- and calcium-bearing minerals. Such rocks include, for example, basalts and magnesium-rich mantle rocks that have been emplaced on the continents. Carbonate mineral precipitation could quickly clog up existing voids, presenting a challenge to this approach. However, field and laboratory observations suggest that the stress induced by rapid precipitation may lead to fracturing and subsequent increase in pore space. Future work should rigorously test the feasibility of this approach by addressing reaction kinetics, the evolution of permeability and field-scale injection methods.
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Southward movement of the Pacific intertropical convergence zone AD 1400–1850
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Tropical rainfall patterns control the subsistence lifestyle of more than one billion people. Seasonal changes in these rainfall patterns are associated with changes in the position of the intertropical convergence zone, which is characterized by deep convection causing heavy rainfall near 10◦ N in boreal summer and 3◦ N in boreal winter. Dynamic controls on the position of the intertropical convergence zone are debated, but palaeoclimatic evidence from continental Asia, Africa and the Americas suggests that it has shifted substantially during the past millennium, reaching its southernmost position some time during the Little Ice Age (AD 1400–1850). However, without records from the meteorological core of the intertropical convergence zone in the Pacific Ocean, quantitative constraints on its position are lacking. Here we report microbiological, molecular and hydrogen isotopic evidence from lake sediments in the Northern Line Islands, Galápagos and Palau indicating that the Pacific intertropical convergence zone was south of its modern position for most of the past millennium, by as much as 500 km during the Little Ice Age. A colder Northern Hemisphere at that time, possibly resulting from lower solar irradiance, may have driven the intertropical convergence zone south. We conclude that small changes in Earth’s radiation budget may profoundly affect tropical rainfall.
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Committed terrestrial ecosystem changes due to climate change
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Targets for stabilizing climate change are often based on considerations of the impacts of different levels of global warming, usually assessing the time of reaching a particular level of warming. However, some aspects of the Earth system, such as global mean temperatures1 and sea level rise due to thermal expansion2 or the melting of large ice sheets3 , continue to respond long after the stabilization of radiative forcing. Here we use a coupled climate–vegetation model to show that in turn the terrestrial biosphere shows significant inertia in its response to climate change. We demonstrate that the global terrestrial biosphere can continue to change for decades after climate stabilization. We suggest that ecosystems can be committed to long-term change long before any response is observable: for example, we find that the risk of significant loss of forest cover in Amazonia rises rapidly for a global mean temperature rise above 2 ◦ C. We conclude that such committed ecosystem changes must be considered in the definition of dangerous climate change, and subsequent policy development to avoid it.
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Riverine carbon dioxide release
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Inland waters are increasingly recognized as important to the global carbon cycle. Detailed measurements in the United States suggest that significant amounts of carbon dioxide are released from streams and rivers, particularly the smaller ones.
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Wood and river landscapes
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The influence of trees and dead wood on river dynamics has long been overlooked. Recent work suggests that large wood pieces can stabilize the land surface, contributing to a large-wood cycle that profoundly affects floodplain morphology and ecology.
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Response of the North Atlantic storm track to climate change shaped by ocean– atmosphere coupling
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A poleward shift of the mid-latitude storm tracks in response to anthropogenic greenhouse-gas forcing has been diagnosed in climate model simulations1,2. Explanations of this effect have focused on atmospheric dynamics3–7 . However, in contrast to storm tracks in other regions, the North Atlantic storm track responds by strengthening and extending farther east, in particular on its southern flank8. These adjustments are associated with an intensification and extension of the eddy- driven jet towards western Europe9 and are expected to have considerable societal impacts related to a rise in storminess in Europe10–12. Here, we apply a regression analysis to an ensemble of coupled climate model simulations to show that the coupling between ocean and atmosphere shapes the distinct storm-track response to greenhouse-gas forcing in the North Atlantic region. In the ensemble of simulations we analyse, at least half of the differences between the storm-track responses of different models are associated with uncertainties in ocean circulation changes. We compare the fully coupled simulations with both the associated slab model simulations and an ocean-forced experiment with one climate model to establish causality. We conclude that uncertainties in the response of the North Atlantic storm track to anthropogenic emissions could be reduced through tighter constraints on the future ocean circulation.
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Editorial : Beyond forest carbon
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The preservation of forests, both on land and in mangrove swamps, has received much attention in the move to protect biological carbon stores. Less conspicuous communities of organisms deserve some scrutiny, too.
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Reduction in carbon uptake during turn of the century drought in western North America
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Fossil fuel emissions aside, temperate North America is a net sink of carbon dioxide at present1–3. Year-to-year variations in this carbon sink are linked to variations in hydroclimate that affect net ecosystem productivity3,4. The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming5–8. Here, we examine the effect of the turn of the century drought in western North America on carbon uptake in the region, using reanalysis data, remote sensing observations and data from global monitoring networks. We show that the area-integrated strength of the western North American carbon sink declined by 30–298Tg C yr−1 during the 2000–2004 drought. We further document a pronounced drying of the terrestrial biosphere during this period, together with a reduction in river discharge and a loss of cropland productivity. We compare our findings with previous palaeoclimate reconstructions7 and show that the last drought of this magnitude occurred more than 800 years ago. Based on projected changes in precipitation and drought severity, we estimate that the present mid-latitude carbon sink of 177–623 Tg C yr−1 in western North America could disappear by the end of the century.
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