• Energy Research

AbstractThis paper summarizes the results of an intercomparison project with Earth System Models of Intermediate Complexity (EMICs) undertaken in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). The focus is on long-term climate projections designed to 1) quantify the climate change commitment of different radiative forcing trajectories and 2) explore the extent to which climate change is reversible on human time scales. All commitment simulations follow the four representative concentration pathways (RCPs) and their extensions to year 2300. Most EMICs simulate substantial surface air temperature and thermosteric sea level rise commitment following stabilization of the atmospheric composition at year-2300 levels. The meridional overturning circulation (MOC) is weakened temporarily and recovers to near-preindustrial values in most models for RCPs 2.6–6.0. The MOC weakening is more persistent for RCP8.5. Elimination of anthropogenic CO2 emissions after 2300 results in slowly decreasing atmospheric CO2 concentrations. At year 3000 atmospheric CO2 is still at more than half its year-2300 level in all EMICs for RCPs 4.5–8.5. Surface air temperature remains constant or decreases slightly and thermosteric sea level rise continues for centuries after elimination of CO2 emissions in all EMICs. Restoration of atmospheric CO2 from RCP to preindustrial levels over 100–1000 years requires large artificial removal of CO2 from the atmosphere and does not result in the simultaneous return to preindustrial climate conditions, as surface air temperature and sea level response exhibit a substantial time lag relative to atmospheric CO2.

Sunshade geoengineering ‐ the installation of reflective mirrors between the Earth and the Sun to reduce incoming solar radiation, has been proposed as a mitigative measure to counteract anthropogenic global warming. Although the popular conception is that geoengineering can re‐establish a ‘natural’ pre‐industrial climate, such a scheme would itself inevitably lead to climate change, due to the different temporal and spatial forcing of increased CO2 compared to reduced solar radiation. We investigate the magnitude and nature of this climate change for the first time within a fully coupled General Circulation Model. We find significant cooling of the tropics, warming of high latitudes and related sea ice reduction, a reduction in intensity of the hydrological cycle, reduced ENSO variability, and an increase in Atlantic overturning. However, the changes are small relative to those associated with an unmitigated rise in CO2 emissions. Other problems such as ocean acidification remain unsolved by sunshade geoengineering.

Predicting the impact of ongoing anthropogenic CO 2 emissions on calcifying marine organisms is complex, owing to the synergy between direct changes (acidification) and indirect changes through climate change (e.g., warming, changes in ocean circulation, and deoxygenation). Laboratory experiments, particularly on longer-lived organisms, tend to be too short to reveal the potential of organisms to acclimatize, adapt, or evolve and usually do not incorporate multiple stressors. We studied two examples of rapid carbon release in the geological record, Eocene Thermal Maximum 2 (∼53.2 Ma) and the Paleocene Eocene Thermal Maximum (PETM, ∼55.5 Ma), the best analogs over the last 65 Ma for future ocean acidification related to high atmospheric CO 2 levels. We use benthic foraminifers, which suffered severe extinction during the PETM, as a model group. Using synchrotron radiation X-ray tomographic microscopy, we reconstruct the calcification response of survivor species and find, contrary to expectations, that calcification significantly increased during the PETM. In contrast, there was no significant response to the smaller Eocene Thermal Maximum 2, which was associated with a minor change in diversity only. These observations suggest that there is a response threshold for extinction and calcification response, while highlighting the utility of the geological record in helping constrain the sensitivity of biotic response to environmental change.

AbstractIn an assessment of how Arctic sea ice cover could be remediated in a warming world, we simulated the injection of SO2 into the Arctic stratosphere making annual adjustments to injection rates. We treated one climate model realization as a surrogate “real world” with imperfect “observations” and no rerunning or reference to control simulations. SO2 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.

AbstractThe ultimate fate of (fossil fuel) CO2 emitted to the atmosphere is governed by a range of sedimentological and geological processes operating on timescales of up to the ca. hundred thousand year response of the silicate weathering feedback. However, how the various geological CO2 sinks might saturate and feedbacks weaken in response to increasing total emissions is poorly known. Here we explore the relative importance and timescales of these processes using a 3‐D ocean‐based Earth system model. We first generate an ensemble of 1 Myr duration CO2 decay curves spanning cumulative emissions of up to 20,000 Pg C. To aid characterization and understanding of the model response to increasing emission size, we then generate an impulse response function description for the long‐term fate of CO2 in the model. In terms of the process of carbonate weathering and burial, our analysis is consistent with a progressively increasing fraction of total emissions that are removed from the atmosphere as emissions increase, due to the ocean carbon sink becoming saturated, together with a lengthening of the timescale of removal from the atmosphere. However, we find that in our model the ultimate CO2 sink—silicate weathering feedback—is approximately invariant with respect to cumulative emissions, both in terms of its importance (it removes the remaining excess ~7% of total emissions from the atmosphere) and timescale (~270 kyr). Because a simple pulse‐response description leads to initially large predictive errors for a realistic time‐varying carbon release, we also develop a convolution‐based description of atmospheric CO2 decay which can be used as a simple and efficient means of making long‐term carbon cycle perturbation projections.

Climate change is predicted to alter temperature, carbonate chemistry, and oxygen availability in the oceans, which will affect individuals, populations and ecosystems. We use the fossil record of benthic foraminifers to assess developmental impacts in response to environmental changes during the Paleocene Eocene Thermal Maximum (PETM). Using an unprecedented number of µ-Computer Tomography scans, we determine size of the proloculus (first chamber), number of chambers, and final size of two benthic foraminiferal species which survived the extinction at Sites 690 (Atlantic sector, Southern Ocean, paleodepth 1900m), 1210 (central equatorial Pacific, paleodepth 2100m), and 1135 (Indian Ocean sector, Southern Ocean, 600-1000m). The population at shallowest Site 1135 does not show a clear response to the PETM, whereas those at the other sites record reductions in diameter or proloculus size. Temperature was similar at all sites, thus not likely the reason for differences between sites. At Site 1210, small size coincided with higher chamber numbers during the peak event, and may have been caused by a combination of low carbonate ion concentrations and low food supply. Dwarfing at Site 690 occurred at lower chamber numbers, and may have been caused by decreasing carbonate saturation at sufficient food levels to reproduce. Proloculus size varied strongly between sites and through time, suggesting a large influence of environment on both microspheric and megalospheric forms without clear bimodality. The effect of the environmental changes during the PETM was more pronounced at deeper sites, possibly implicating carbonate saturation. The data file contains information on morphometric data on benthic foraminifers of the PETM gathered by micro-CT. The data consists of proloculus volume (first chamber), number of chambers and diameter of Nuttallides truempyi and Oridorsalis umbonsaturs before, in the peak, recover and after the Carbon Isotope excursion associated with the PETM. Supplement to: Schmidt, Daniela N; Thomas, Ellen; Authier, Elisabeth; Saunders, David; Ridgwell, Andy (2018): Strategies in times of crisis—insights into the benthic foraminiferal record of the Palaeocene–Eocene Thermal Maximum. Philosophical Transactions of the Royal Society A-Mathematical Physical and Engineering Sciences, 376(2130), 20170328