Future atmospheric methane concentrations in the context of the stabilization of greenhouse gas concentrations

Abstract. Tropospheric CH 4 concentration depends, according to modeled tropospheric processes, on many factors, including emissions of CH 4 as well as NO x and CO. Illustrative analyses of the relation between emissions and CH 4 concentration give some guidance on the role of CH 4 in the stabilization of greenhouse gas concentrations. The contribution of CH 4 to radiative forcing at the time of stabilization is expected to be modest, provided CH 4 and CO emissions do not go far beyond current rates. However, in cases leading to stabilization the potential mitigation of increases in radiative forcing by methane control could be comparable to that of CO 2 control over the next century. Whether or not this potential is realized will depend partially on the cost of deep reductions of CH 4 , NO x , CO, or CO 2 emissions over the next century, which is not known

Climate and carbon budget implications of linked future changes in CO2 and non-CO2 forcing

The approximate proportional relationship between cumulative carbon emissions and instantaneous global temperature rise (the carbon budget approximation) has proven to be a useful concept to translate policy-relevant temperature objectives into CO _2 emissions pathways. However, when non-CO _2 forcing is changing along with CO _2 forcing, errors in the approximation increases. Using the GCAM model to produce an ensemble of ∼3000 scenarios, we show that linked changes in CO _2 forcing, aerosol forcing, and non-CO _2 greenhouse gas (GHG) forcing lead to an increase in total non-CO _2 forcing over the 21st century across mitigation scenarios. This increase causes the relationship between instantaneous temperature and cumulative CO _2 emissions to become more complex than the proportional approximation often assumed, particularly for low temperature objectives such as 1.5 °C. The same linked changes in emissions also contribute to a near-term increase in aerosol forcing that effectively places a limit on how low peak temperature could be constrained through GHG mitigation alone. In particular, we find that 23% of scenarios that include CCS (but only 1% of scenarios that do not include CCS) achieve a temperature objective of 1.5 °C without temperature overshoot

The Interplay Between Bioenergy Grass Production and Water Resources in the United States of America

We apply a land surface model to evaluate the interplay between potential bioenergy grass (Miscanthus, Cave-in-Rock, and Alamo) production, water quantity, and nitrogen leaching (NL) in the Central and Eastern U.S. Water use intensity tends to be lower where grass yields are modeled to be high, for example in the Midwest for Miscanthus and Cave-in-Rock and the upper southeastern U.S. for Alamo. However, most of these regions are already occupied by crops and forests and substitution of these biome types for ethanol production implies trade-offs. In general, growing Miscanthus consumes more water, Alamo consumes less water, and Cave-in-Rock consumes approximately the same amount of water as existing vegetation. Bioenergy grasses can maintain high productivity over time, even in water limited regions, because their roots can grow deeper and extract the water from the deep, moist soil layers. However, this may not hold where there are frequent and intense drought events, particularly in regions with shallow soil depths. One advantage of bioenergy grasses is that they mitigate nitrogen leaching relative to row crops and herbaceous plants when grown without applying N fertilizer; and bioenergy grasses, especially Miscanthus, generally require less N fertilizer application than row crops and herbaceous plants

Role of the Freight Sector in Future Climate Change Mitigation Scenarios

The freight sector’s role is examined using the Global Change Assessment Model (GCAM) for a range of climate change mitigation scenarios and future freight demand assumptions. Energy usage and CO₂ emissions from freight have historically grown with a correlation to GDP, and there is limited evidence of near-term global decoupling of freight demand from GDP. Over the 21^st century, greenhouse gas (GHG) emissions from freight are projected to grow faster than passenger transportation or other major end-use sectors, with the magnitude of growth dependent on the assumed extent of long-term decoupling. In climate change mitigation scenarios that apply a price to GHG emissions, mitigation of freight emissions (including the effects of demand elasticity, mode and technology shifting, and fuel substitution) is more limited than for other demand sectors. In such scenarios, shifting to less-emitting transportation modes and technologies is projected to play a relatively small role in reducing freight emissions in GCAM. By contrast, changes in the supply chain of liquid fuels that reduce the fuel carbon intensity, especially deriving from large-scale use of biofuels coupled to carbon capture and storage technologies, are responsible for the majority of freight emissions mitigation, followed by price-induced reduction in freight demand services

Emissions and Atmospheric CO 2 Stabilization: Long-Term Limits and Paths

carbon cycle, CO 2 , emissions, scenario, stabilization,