ALBERT

Description: This study used the Community Atmospheric Model 3.5 (CAM3.5) to investigate the effects of carbonaceous aerosols on climate. The simulations include control runs with 3 times the mass of carbonaceous aerosols as compared to the model's default carbonaceous aerosol mass, as well as no-carbon runs in which carbonaceous aerosols were removed. The slab ocean model (SOM) and the fixed sea surface temperature (SST) were used to examine effects of ocean boundary conditions. Throughout this study, climate response induced by aerosol forcing was mainly analyzed in the following three terms: (1) aerosol radiative effects under fixed SST, (2) effects of aerosol-induced SST feedbacks, and (3) total effects including effects of aerosol forcing and SST feedbacks. The change of SST induced by aerosols has large impacts on distribution of climate response; the magnitudes in response patterns such as temperature, precipitation, zonal winds, mean meridional circulation, radiative fluxes, and cloud coverage are different between the SOM and fixed SST runs. Moreover, different spatial responses between the SOM and fixed SST runs can also be seen in some local areas. This implies the importance of SST feedbacks on simulated climate response. The aerosol dimming effects cause a cooling predicted at low layers near the surface in most carbonaceous aerosol source regions. The temperature response shows a warming (cooling) predicted in the north (south) high latitudes, suggesting that aerosol forcing can cause climate change in regions far away from its origins. Our simulation results show that direct and semidirect radiative forcing due to carbonaceous aerosols decreases rainfall in the tropics. This implies that carbonaceous aerosols have possibly strong influence on weakening of the tropical circulation. Most changes in precipitation are negatively correlated with changes of radiative fluxes at the top of model. The changes in radiative fluxes at top of model are physically consistent with the response patterns in cloud fields. On global average, low-level cloud coverage increases, and mid- and high-level cloud coverage decreases in response to changes in radiative energy induced by aerosol forcing. An approximated moisture budget equation was analyzed in order to understand physical mechanism of precipitation changes induced by carbonaceous aerosols. Our results show that changes in tropical precipitation are mainly dominated are mainly dominated by the dynamic effect (i.e., vertical moisture transport carried by the perturbed flow).

Description: Increasing concentrations of greenhouse gases are the primary contributor to the 0.8 °C increase in the global average temperature since the late 19th century, shortening cold seasons and lengthening warm seasons. The warming is amplified in polar regions, causing retreat of sea ice, snow cover, permafrost, mountain glaciers, and ice sheets, while also modifying mid-latitude weather, amplifying global sea level rise, and initiating high-latitude carbon feedbacks. Model simulations in which we reduced solar insolation over high latitudes not only cooled those regions, but also drew energy from lower latitudes, exerting a cooling influence over much of the hemisphere in which the reduction was imposed. Our simulations, which used the National Center for Atmospheric Research's CAM3.1 atmospheric model coupled to a slab ocean, indicated that, on a normalized basis, high-latitude reductions in absorbed solar radiation have a significantly larger cooling influence than equivalent solar reductions spread evenly over the Earth. This amplified influence occurred because high-latitude surface cooling preferentially increased sea ice fraction and, therefore, surface albedo, leading to a larger deficit in the radiation budget at the top of the atmosphere than from an equivalent global reduction in solar radiation. Reductions in incoming solar radiation in one polar region (either north or south) resulted in increased poleward energy transport during that hemisphere's cold season and shifted the Inter-Tropical Convergence Zone (ITCZ) away from that pole, whereas equivalent reductions in both polar regions tended to leave the ITCZ approximately in place. Together, these results suggest that, until emissions reductions are sufficient to limit the warming influence of greenhouse gas concentrations, polar reductions in solar radiation, if they can be efficiently and effectively implemented, might, because of fewer undesirable side effects than for global solar radiation reductions, be a preferred approach to limiting both high-latitude and global warming.

Description: With human-induced climate change leading to amplified warming in high latitudes, mitigation alone is unlikely to be rapid enough to prevent significant, even irreversible, impacts. Model simulations in which solar insolation was arbitrarily reduced poleward of 51, 61, or 71° latitude in one or both hemispheres not only cooled those regions, but also drew energy from lower latitudes, exerting a cooling influence over much of the particular hemisphere in which the reduction was imposed. The simulations, conducted using the National Center for Atmospheric Research's CAM3.1 atmospheric model coupled to a slab ocean, indicated that high-latitude reductions in absorbed solar radiation have a significantly larger cooling influence than solar reductions of equivalent magnitude spread evenly over the Earth. This amplified influence occurred primarily because concentrated high-latitude reductions in solar radiation led to increased sea ice fraction and surface albedo, thereby amplifying the energy deficit at the top of the atmosphere as compared to the response for an equivalent reduction in solar radiation spread evenly over the globe. Reductions in incoming solar radiation in one polar region (either north or south) resulted in increased poleward energy transport during that hemisphere's cold season and shifted the Inter-Tropical Convergence Zone (ITCZ) away from that pole, whereas comparable solar reductions in both polar regions resulted in increased poleward energy transport, but tended to leave the ITCZ approximately in place. Together, these results suggest that, until emissions reductions are sufficient to limit the warming influence of increasing greenhouse gas concentrations, polar reductions in solar radiation, if they could be efficiently and effectively implemented, warrant further research as an approach to moderating the early stages of both high-latitude and global warming.

Description: A terrestrial biosphere model with dynamic vegetation capability, Integrated Biosphere Simulator (IBIS), coupled to the NCAR Community Atmosphere Model (CAM2) is used to investigate the multiple climate-forest equilibrium states of the climate system. A 1000-yr control simulation and another 1000-yr land cover change simulation that consisted of global deforestation for 100 yr followed by re-growth of forests for the subsequent 900 yr were performed. After several centuries of interactive climate-vegetation dynamics, the land cover change simulation converged to essentially the same climate state as the control simulation. However, the climate system takes about a millennium to reach the control forest state. In the absence of deep ocean feedbacks in our model, the millennial time scale for converging to the original climate state is dictated by long time scales of the terrestrial carbon stocks, biomass and soil carbon. Our idealized modeling study suggests that the equilibrium state reached after complete global deforestation followed by re-growth of forests is unlikely to be distinguishable from the control climate. The real world, however, could have multiple climate-forest states since our modeling study is unlikely to have represented all the essential ecological processes (e.g. altered fire regimes, seed sources and seedling establishment dynamics) for the re-establishment of major biomes.

Description: Global carbon budget studies indicate that the terrestrial ecosystems have remained a~large sink for carbon despite widespread deforestation activities. CO2-fertilization, N deposition and re-growth of mid-latitude forests are believed to be key drivers for land carbon uptake. In this study, we assess the importance of N deposition by performing idealized near-equilibrium simulations using the Community Land Model 4.0 (CLM4). In our equilibrium simulations, only 12–17% of the deposited Nitrogen is assimilated into the ecosystem and the corresponding carbon uptake can be inferred from a C : N ratio of 20:1. We calculate the sensitivity of the terrestrial biosphere for CO2-fertilization, climate warming and N deposition as changes in total ecosystem carbon for unit changes in global mean atmospheric CO2 concentration, global mean temperature and Tera grams of Nitrogen deposition per year, respectively. Based on these sensitivities, it is estimated that about 242 PgC could have been taken up by land due to the CO2 fertilization effect and an additional 175 PgC taken up as a result of the increased N deposition since the pre-industrial period. Because of climate warming, terrestrial ecosystem could have lost about 152 PgC during the same period. Therefore, since preindustrial times terrestrial carbon losses due to warming may have been approximately compensated by effects of increased N deposition, whereas the effect of CO2-fertilization is approximately indicative of the current increase in terrestrial carbon stock. Our simulations also suggest that the sensitivity of carbon storage to increased N deposition decreases beyond current levels, indicating climate warming effects on carbon storage may overwhelm N deposition effects in the future.

Description: We assess uncertainties in projected oceanic uptake of anthropogenic CO2 associated with uncertainties in model ocean transport using a suite of climate/carbon-cycle models. In response to a CO2 pulse emission of 590 Pg C (corresponding to an instantaneous doubling of atmospheric CO2 from 278 to 556 ppm), the fraction of CO2 emitted absorbed by the ocean (model mean ±2σ) is 37±8%, 56±10%, and 81±4% in year 30, 100, and 1000 after the emission pulse, respectively. Modeled oceanic uptake of excess CO2 on timescales from decades to about a century is strongly correlated with simulated present-day uptake of chlorofluorocarbons (CFCs) and anthropogenic CO2, while the amount of excess CO2 absorbed by the ocean from a century to a millennium is strongly correlated with modeled radiocarbon in the deep Southern and Pacific Ocean. The rates of surface-to-deep ocean transport are determined for individual models from the instantaneous doubling CO2 experiments, and they are used to calculate oceanic uptake of CO2 in response to emission pulses of 1000 and 5000 Pg C. These results are compared with simulated oceanic uptake of CO2 from a number of model simulations with the coupling of climate-ocean carbon cycle and without it. This comparison demonstrates that the impact of different ocean transport rate across models on the oceanic uptake of anthropogenic CO2 is of similar magnitude as that of climate-carbon cycle feedbacks in a single model associated with changes in temperature, circulation, and marine biology, emphasizing the importance of ocean transport in the fate of anthropogenic CO2.

Description: This study used Community Atmospheric Model 3.5 (CAM3.5) to investigate the effects of carbonaceous aerosols on climate. The simulations include control runs with carbonaceous aerosols and no carbon runs in which carbonaceous aerosols were removed. The Slab Ocean Model (SOM) and the fixed Sea Surface Temperature (SST) were used to examine effects of ocean boundary conditions. Throughout this study, climate response induced by aerosol forcing was mainly analyzed in the following three terms: (1) aerosol radiative effects under fixed SST, (2) effects of aerosol-induced SST feedbacks , and (3) total effects including effects of aerosol forcing and SST feedbacks. The change of SST induced by aerosols has large impacts on distribution of climate response, the magnitudes in response patterns such as temperature, precipitation, zonal winds, mean meridional circulation, radiative fluxes and cloud coverage are different between the SOM and fixed SST runs. Moreover, different spatial responses between the SOM and fixed SST runs can also be seen in some local areas. This implies the importance of SST feedbacks on simulated climate response. The aerosol dimming effects cause a cooling predicted at low layers near the surface in most of carbonaceous aerosol source regions. The temperature response shows a warming (cooling) predicted in the north (south) high latitudes, suggesting that aerosol forcing can cause climate change in regions far away from its origins. Our simulation results show that warming of the troposphere due to black carbon decreases rainfall in the tropics. This implies that black carbon has possibly strong influence on weakening of the tropical circulation. Most of these changes in precipitation are negatively correlated with changes of radiative fluxes at the top of model. The changes in radiative fluxes at top of model are physically consistent with the response patterns in cloud fields. On global average, low-level cloud coverage increases, mid- and high-level cloud coverage decreases in response to changes in radiative energy induced by aerosol forcing. An approximated moisture budget equation was analyzed in order to understand physical mechanism of precipitation changes induced by carbonaceous aerosols. Our results show that changes in tropical precipitation are mainly dominated by dynamic effect, i.e. vertical moisture transport carried by the perturbed flow.

Description: A terrestrial biosphere model with dynamic vegetation capability, Integrated Biosphere Simulator (IBIS2), coupled to the NCAR Community Atmosphere Model (CAM2) is used to investigate the multiple climate-forest equilibrium states of the climate system. A 1000-year control simulation and another 1000-year land cover change simulation that consisted of global deforestation for 100 years followed by re-growth of forests for the subsequent 900 years were performed. After several centuries of interactive climate-vegetation dynamics, the land cover change simulation converged to essentially the same climate state as the control simulation. However, the climate system takes about a millennium to reach the control forest state. In the absence of deep ocean feedbacks in our model, the millennial time scale for converging to the original climate state is dictated by long time scales of the vegetation dynamics in the northern high latitudes. Our idealized modeling study suggests that the equilibrium state reached after complete global deforestation followed by re-growth of forests is unlikely to be distinguishable from the control climate. The real world, however, could have multiple climate-forest states since our modeling study is unlikely to have represented all the essential ecological processes (e.g. altered fire regimes, seed sources and seedling establishment dynamics) for the re-establishment of major biomes.

Description: Global carbon budget studies indicate that the terrestrial ecosystems have remained a large sink for carbon despite widespread deforestation activities. CO2 fertilization, N deposition and re-growth of mid-latitude forests are believed to be key drivers for land carbon uptake. In this study, we assess the importance of N deposition by performing idealized near-equilibrium simulations using the Community Land Model 4.0 (CLM4). In our equilibrium simulations, only 12–17% of the deposited nitrogen is assimilated into the ecosystem and the corresponding carbon uptake can be inferred from a C : N ratio of 20 : 1. We calculate the sensitivity of the terrestrial biosphere for CO2 fertilization, climate warming and N deposition as changes in total ecosystem carbon for unit changes in global mean atmospheric CO2 concentration, global mean temperature and Tera grams of nitrogen deposition per year, respectively. Based on these sensitivities, it is estimated that about 242 PgC could have been taken up by land due to the CO2 fertilization effect and an additional 175 PgC taken up as a result of the increased N deposition since the pre-industrial period. Because of climate warming, the terrestrial ecosystem could have lost about 152 PgC during the same period. Therefore, since pre-industrial times terrestrial carbon losses due to warming may have been more or less compensated by effects of increased N deposition, whereas the effect of CO2 fertilization is approximately indicative of the current increase in terrestrial carbon stock. Our simulations also suggest that the sensitivity of carbon storage to increased N deposition decreases beyond current levels, indicating that climate warming effects on carbon storage may overwhelm N deposition effects in the future.

Description: We compare modeled oceanic carbon uptake in response to pulse CO2 emissions using a suite of global ocean models and Earth system models. In response to a CO2 pulse emission of 590 Pg C (corresponding to an instantaneous doubling of atmospheric CO2 from 278 to 556 ppm), the fraction of CO2 emitted that is absorbed by the ocean is: 37±8%, 56±10%, and 81±4% (model mean ±2σ ) in year 30, 100, and 1000 after the emission pulse, respectively. Modeled oceanic uptake of pulse CO2 on timescales from decades to about a century is strongly correlated with simulated present-day uptake of chlorofluorocarbons (CFCs) and CO2 across all models, while the amount of pulse CO2 absorbed by the ocean from a century to a millennium is strongly correlated with modeled radiocarbon in the deep Southern and Pacific Ocean. However, restricting the analysis to models that are capable of reproducing observations within uncertainty, the correlation is generally much weaker. The rates of surface-to-deep ocean transport are determined for individual models from the instantaneous doubling CO2 simulations, and they are used to calculate oceanic CO2 uptake in response to pulse CO2 emissions of different sizes pulses of 1000 and 5000 Pg C. These results are compared with simulated oceanic uptake of CO2 by a number of models simulations with the coupling of climate-ocean carbon cycle and without it. This comparison demonstrates that the impact of different ocean transport rates across models on oceanic uptake of anthropogenic CO2 is of similar magnitude as that of climate-carbon cycle feedbacks in a single model, emphasizing the important role of ocean transport in the uptake of anthropogenic CO2.