The Role of Urbanization in the Global Carbon Cycle

Urban areas account for more than 70% of CO2 emissions from burning fossil fuels. Urban expansion in tropics is responsible for 5% of the annual emissions from land use change. Here, I show that the effect of urbanization on the global carbon cycle extends beyond these emissions. I quantify the contribution of urbanization to the major carbon fluxes and pools globally and identify gaps crucial for predicting the evolution of the carbon cycle in the future. Urban residents currently control ~22 (12–40)% of the land carbon uptake (112 PgC/yr) and ~24 (15–39)% of the carbon emissions (117 PgC/year) from land globally. Urbanization resulted in the creation of new carbon pools on land such as buildings (~6.7 PgC) and landfills (~30 PgC). Together these pools store 1.6 (±0.3)% of the total vegetation and soil carbon pools globally. The creation and maintenance of these new pools has been associated with high emissions of CO2, which are currently better understood than the processes associated with the dynamics of these pools and accompanying uptake of carbon. Predictions of the future trajectories of the global carbon cycle will require a much better understanding of how urban development affects the carbon cycle over the long term.Peer Reviewe

Bio-energy retains its mitigation potential under elevated CO2

Background If biofuels are to be a viable substitute for fossil fuels, it is essential that they retain their potential to mitigate climate change under future atmospheric conditions. Elevated atmospheric CO2 concentration [CO2] stimulates plant biomass production; however, the beneficial effects of increased production may be offset by higher energy costs in crop management. Methodology/Main findings We maintained full size poplar short rotation coppice (SRC) systems under both current ambient and future elevated [CO2] (550 ppm) and estimated their net energy and greenhouse gas balance. We show that a poplar SRC system is energy efficient and produces more energy than required for coppice management. Even more, elevated [CO2] will increase the net energy production and greenhouse gas balance of a SRC system with 18%. Managing the trees in shorter rotation cycles (i.e. 2 year cycles instead of 3 year cycles) will further enhance the benefits from elevated [CO2] on both the net energy and greenhouse gas balance. Conclusions/significance Adapting coppice management to the future atmospheric [CO2] is necessary to fully benefit from the climate mitigation potential of bio-energy systems. Further, a future increase in potential biomass production due to elevated [CO2] outweighs the increased production costs resulting in a northward extension of the area where SRC is greenhouse gas neutral. Currently, the main part of the European terrestrial carbon sink is found in forest biomass and attributed to harvesting less than the annual growth in wood. Because SRC is intensively managed, with a higher turnover in wood production than conventional forest, northward expansion of SRC is likely to erode the European terrestrial carbon sink

Investigating the Balance Between Timber Harvest and Productivity of Global Coniferous Forests Under Global Change

A widely used assumption in forestry is that the demand for timber will exceed the maximum level available from forests on a sustainable basis. In this study, measurements of extracted timber and modeled forest productivity were used to investigate the relationship between harvested timber and natural forest productivity for current conditions, and under global change scenario. The analysis was confined to coniferous forests and countries that have coniferous forests within their territories. Annual round wood production from the database of Food and Agriculture Organization was used as an approximation of annual timber harvest for each country. Annual stem primary productivity of coniferous forests was estimated using the BIOME-BGC model. Based on the current rates, annual timber extraction was extrapolated for each country for the next 80 years. Then, on a country basis, the timber harvest was related to the modeled forest stem productivity, assuming that the area of coniferous forest would stay unchanged for the next 80 years.The results of this study suggest that global coniferous forests currently produce more wood than people consume, but that this gap will narrow in the future. The results also suggest that wood extraction may reach forest regrowth by the middle of the next century, even though most coniferous forests are located in high latitudes and may have an accelerated stem growth associated with the joint effect of climate change and elevated carbon dioxide concentration in the atmosphere