ALBERT

Description: Earth radiation management has been suggested as a way to rapidly counteract global warming in the face of a lack of mitigation efforts, buying time and avoiding potentially catastrophic warming. We compare six different radiation management schemes that use surface, troposphere and stratosphere interventions in a single climate model in which we projected future climate from 2020 to 2099 based on RCP4.5. We analyze the surface air temperature responses to determine how effective the schemes are at returning temperature to its 1986-2005 climatology and analyze precipitation responses to compare side effects. We find crop albedo enhancement is largely ineffective at returning temperature to its 1986-2005 climatology. Desert albedo enhancement causes excessive cooling in the deserts and severe shifts in tropical precipitation. Ocean albedo enhancement, sea-spray geoengineering, cirrus cloud thinning and stratospheric SO 2 injection have the potential to cool more uniformly, but cirrus cloud thinning may not be able to cool by much more than 1 K globally. We find that of the schemes potentially able to return surface air temperature to 1986-2005 climatology under future greenhouse gas warming, none has significantly less severe precipitation side effects than other schemes. Despite different forcing patterns, ocean albedo enhancement, sea-spray geoengineering, cirrus cloud thinning and stratospheric SO 2 injection all result in large scale tropical precipitation responses caused by Hadley cell changes and land precipitation changes largely driven by thermodynamic changes. Widespread regional scale changes in precipitation over land are significantly different from the 1986-2005 climatology and would likely necessitate significant adaptation despite geoengineering.

Description: Proposals to geoengineer Earth's climate by cirrus cloud thinning (CCT) potentially offer advantages over solar radiation management schemes: amplified cooling of the Arctic and smaller perturbations to global mean precipitation in particular. Using an idealized climate model implementation of CCT in which ice particle fall speeds were increased 2×, 4×, and 8× we examine the relationships between effective radiative forcing (ERF) at the top of atmosphere (TOA), near surface temperature and the response of the hydrological cycle. ERF was non-linear with fall speed change and driven by the trade-off between opposing positive shortwave and negative longwave radiative forcings. ERF was –2.0 Wm -2 for both 4× and 8× fall speeds. Global mean temperature decreased linearly with ERF while Arctic temperature reductions were amplified compared with the global mean change. The change in global mean precipitation involved a rapid adjustment, which was linear with the change in the net atmospheric energy balance (~ -1%/Wm -2 ), and a feedback response (~2%/ o C). Global mean precipitation and evaporation increased strongly in the first year of CCT. Intensification of the hydrological cycle was promoted by: intensification of the vertical overturning circulation of the atmosphere, changes in boundary layer climate favorable for evaporation, and increased energy available at the surface for evaporation (from increased net shortwave radiation and reduced sub-surface storage of heat). Such intensification of the hydrological cycle is a significant side effect to the cooling of climate by CCT. Any accompanying negative cirrus cloud feedback response would implicitly increase the costs and complexity of CCT deployment.

Description: In an assessment of how Arctic sea ice cover could be remediated in a warming world, we simulated the injection of SO 2 into the Arctic stratosphere making annual adjustments to injection rates. We treated one climate model realisation as a surrogate ‘real world’ with imperfect ‘observations’ and no re-running or reference to control simulations. SO 2 injection rates were proposed using a novel model predictive control regime which incorporated a second simpler climate model to forecast ‘optimal’ decision pathways. Commencing the simulation in 2018, Arctic sea ice cover was remediated by 2043 and maintained until solar geoengineering was terminated. We found quantifying climate side effects problematic because internal climate variability hampered detection of regional climate changes beyond the Arctic. Nevertheless, through decision maker learning and the accumulation of at least 10 years’ time series data exploited through an annual review cycle, uncertainties in observations and forcings were successfully managed.

Description: ABSTRACT A validation experiment, carried out in a scaled field setting, was attempted for the long electrode electrical resistivity tomography method in order to demonstrate the performance of the technique in imaging a simple buried target. The experiment was an approximately 1/17 scale mock-up of a region encompassing a buried nuclear waste tank on the Hanford site. The target of focus was constructed by manually forming a simulated plume within the vadose zone using a tank waste simulant. The long electrode results were compared to results from conventional point electrodes on the surface and buried within the survey domain. Using a pole-pole array, both point and long electrode imaging techniques identified the lateral extents of the pre-formed plume with reasonable fidelity but the long electrode method was handicapped in reconstructing vertical boundaries. The pole-dipole and dipole-dipole arrays were also tested with the long electrode method and were shown to have the least favourable target properties, including the position of the reconstructed plume relative to the known plume and the intensity of false positive targets. The poor performance of the pole-dipole and dipole-dipole arrays was attributed to an inexhaustive and non-optimal coverage of data at key electrodes, as well as an increased noise for electrode combinations with high geometric factors. However, when comparing the model resolution matrix among the different acquisition strategies, the pole-dipole and dipole-dipole arrays using long electrodes were shown to have significantly higher average and maximum values within the matrix than any pole-pole array. The model resolution describes how well the inversion model resolves the subsurface. Given the model resolution performance of the pole-dipole and dipole-dipole arrays, it may be worth investing in tools to understand the optimum subset of randomly distributed electrode pairs to produce maximum performance from the inversion model.

Description: Solar radiation management schemes could potentially alleviate the impacts of global warming. One such scheme could be to brighten the surface of the ocean by increasing the albedo and areal extent of bubbles in the wakes of existing shipping. Here we show that ship wake bubble lifetimes would need to be extended from minutes to days, requiring the addition of surfactant, for ship wake area to be increased enough to have a significant forcing. We use a global climate model to simulate brightening the wakes of existing shipping by increasing wake albedo by 0.2 and increasing wake lifetime by ×1440. This yields a global mean radiative forcing of -0.9 ± 0.6 Wm -2 (-1.8 ± 0.9 Wm -2 in the Northern Hemisphere) and a 0.5 °C reduction of global mean surface temperature with greater cooling over land and in the Northern Hemisphere, partially offsetting greenhouse gas warming. Tropical precipitation shifts southwards but remains within current variability. The hemispheric forcing asymmetry of this scheme is due to the asymmetry in the distribution of existing shipping. If wake lifetime could reach ~3 months, the global mean radiative forcing could potentially reach -3 Wm -2 . Increasing wake area through increasing bubble lifetime could result in a greater temperature reduction but regional precipitation would likely deviate further from current climatology as suggested by results from our uniform ocean albedo simulation. Alternatively, additional ships specifically for the purpose of geoengineering could be used to produce a larger and more hemispherically symmetrical forcing.

Description: In this paper, we breakdown the temperature response of coupled ocean-atmosphere climate models into components due to radiative forcing, climate feedback, and heat storage and transport to understand how well climate models reproduce the observed 20th century temperature record. Despite large differences between models' feedback strength, they generally reproduce the temperature response well but for different reasons in each model. We show that the differences in forcing and heat storage and transport give rise to a considerable part of the intermodel variability in global, Arctic, and tropical mean temperature responses over the 20th century. Projected future warming trends are much more dependent on a model's feedback strength, suggesting that constraining future climate change by weighting these models on the basis of their 20th century reproductive skill is not possible. We find that tropical 20th century warming is too large and Arctic amplification is unrealistically low in the Geophysical Fluid Dynamics Laboratory CM2.1, Meteorological Research Institute CGCM232a, and MIROC3.2(hires) models because of unrealistic forcing distributions. The Arctic amplification in both National Center for Atmospheric Research models is unrealistically high because of high feedback contributions in the Arctic compared to the tropics. Few models reproduce the strong observed warming trend from 1918 to 1940. The simulated trend is too low, particularly in the tropics, even allowing for internal variability, suggesting there is too little positive forcing or too much negative forcing in the models at this time. Over the whole of the 20th century, the feedback strength is likely to be underestimated by the multimodel mean.

Description: Abstract The lakes that form via ice‐rich permafrost thaw emit CH4 and CO2 to the atmosphere from previously frozen ancient permafrost sources. Despite this potential to positively feedback to climate change, lake carbon emission sources are not well understood on whole‐lake scales, complicating upscaling. In this study, we used observations of radiocarbon (14C) and stable carbon (13C) isotopes in the summer and winter dissolved CH4 and CO2 pools, ebullition‐CH4, and multiple independent mass balance approaches to characterize whole‐lake emission sources and apportion annual emission pathways. Observations focused on five lakes with variable thermokarst in interior Alaska. The 14C age of discrete ebullition‐CH4 seeps ranged from 395 ± 15 to 28,240 ± 150 YBP across all study lakes; however, dissolved 14CH4 was younger than 4,730 YBP. In the primary study lake, Goldstream L., the integrated whole‐lake 14C age of ebullition‐CH4, as determined by three different approaches, ranged from 3,290 to 6,740 YBP. A new dissolved‐14C‐CH4‐based approach to estimating ebullition 14C age and flux showed close agreement to previous ice‐bubble surveys and bubble‐trap flux estimates. Differences in open water versus ice‐covered dissolved gas concentrations and their 14C and 13C isotopes revealed the influence of winter ice trapping and forcing ebullition‐CH4 into the underlying water column, where it comprised 50% of the total dissolved CH4 pool by the end of winter. Across the study lakes, we found a relationship between the whole‐lake 14C age of dissolved CH4 and CO2 and the extent of active thermokarst, representing a positive feedback system that is sensitive to climate warming.