The Earth has been releasing vast amounts of heat from deep Earth's interior to the surface since its formation, which primarily drives mantle convection and a number of tectonic activities. In this heat transport process the core-mantle boundary where hot molten core is in direct contact with solid-state mantle minerals has played an essential role to transfer thermal energies of the core to the overlying mantle. Although the dominant heat transfer mechanisms at the lowermost mantle is believed to be both conduction and radiation of the primary lowermost mantle mineral, bridgmanite, the radiative thermal conductivity of bridgmanite has so far been poorly constrained. Here we revealed the radiative thermal conductivity of bridgmanite at core-mantle boundary is substantially high approaching to similar to 5.3 +/- 1.2 W/mK based on newly established optical absorption measurement of single-crystal bridgmanite performed in situ under corresponding deep lower mantle conditions. We found the bulk thermal conductivity at core-mantle boundary becomes similar to 1.5 times higher than the conventionally assumed value, which supports higher heat flow from core, hence more vigorous mantle convection than expected. Results suggest the mantle is much more efficiently cooled, which would ultimately weaken many tectonic activities driven by the mantle convection more rapidly than expected from conventionally believed thermal conduction behavior. (C) 2021 The Author(s). Published by Elsevier B.V.

Climate change is expected to decrease heating demand and increase cooling demand for buildings and affect outdoor thermal comfort. Here, we project changes in residential heating degree-days (HDD) and cooling degree-days (CDD) for the historical (1981-2010) and future (2080-2099) periods in the United States using median results from the Climate Model Intercomparison Project phase 5 (CMIP5) simulations under the Representation Concentration Pathway 8.5 (RCP8.5) scenario. We project future HDD and CDD values by adding CMIP5 projected changes to values based on historical observations of US climate. The sum HDD + CDD is an indicator of locations that are thermally comfortable, with low heating and cooling demand. By the end of the century, station median HDD + CDD will be reduced in the contiguous US, decreasing in the North and increasing in the South. Under the unmitigated RCP8.5 scenario, by the end of this century, in terms of HDD and CDD values considered separately, future New York, NY, is anticipated to become more like present Oklahoma City, OK; Denver, CO, becomes more like Raleigh, NC, and Seattle, WA, becomes more like San Jose, CA. These results serve as an indicator of projected climate change and can help inform decision-making.

The claims of periodicity in impact cratering and biological extinction events are controversial. A newly revised record of dated impact craters has been analyzed for periodicity, and compared with the record of extinctions over the past 260 Myr. A digital circular spectral analysis of 37 crater ages (ranging in age from 15 to 254 Myr ago) yielded evidence for a significant 25.8 +/- 0.6 Myr cycle. Using the same method, we found a significant 27.0 +/- 0.7 Myr cycle in the dates of the eight recognized marine extinction events over the same period. The cycles detected in impacts and extinctions have a similar phase. The impact crater dataset shows 11 apparent peaks in the last 260 Myr, at least 5 of which correlate closely with significant extinction peaks. These results suggest that the hypothesis of periodic impacts and extinction events is still viable.

Carbon dioxide removal (CDR) from the atmosphere has been proposed as a measure for mitigating global warming and ocean acidification. To assess the extent to which CDR might eliminate the long-term consequences of anthropogenic CO2 emissions in the marine environment, we simulate the effect of two massive CDR interventions with CO2 extraction rates of 5 GtC yr(-1) and 25 GtC yr(-1), respectively, while CO2 emissions follow the extended RCP8.5 pathway. We falsify two hypotheses: the first being that CDR can restore pre-industrial conditions in the ocean by reducing the atmospheric CO2 concentration back to its pre-industrial level, and the second being that high CO2 emissions rates (RCP8.5) followed by CDR have long-term oceanic consequences that are similar to those of low emissions rates (RCP2.6). Focusing on pH, temperature and dissolved oxygen, we find that even after several centuries of CDR deployment, past CO2 emissions would leave a substantial legacy in the marine environment.

Climate change in response to a change in external forcing can be understood in terms of fast response to the imposed forcing and slow feedback associated with surface temperature change. Previous studies have investigated the characteristics of fast response and slow feedback for different forcing agents. Here we examine to what extent that fast response and slow feedback derived from time-mean results of climate model simulations can be used to infer total climate change. To achieve this goal, we develop a multivariate regression model of climate change, in which the change in a climate variable is represented by a linear combination of its sensitivity to CO2 forcing, solar forcing, and change in global mean surface temperature. We derive the parameters of the regression model using time-mean results from a set of HadCM3L climate model step-forcing simulations, and then use the regression model to emulate HadCM3L-simulated transient climate change. Our results show that the regression model emulates well HadCM3L-simulated temporal evolution and spatial distribution of climate change, including surface temperature, precipitation, runoff, soil moisture, cloudiness, and radiative fluxes under transient CO2 and/or solar forcing scenarios. Our findings suggest that temporal and spatial patterns of total change for the climate variables considered here can be represented well by the sum of fast response and slow feedback. Furthermore, by using a simple 1-D heat-diffusion climate model, we show that the temporal and spatial characteristics of climate change under transient forcing scenarios can be emulated well using information from step-forcing simulations alone.

Anthropogenic emissions of carbon dioxide (CO2) are causing ocean acidification, lowering seawater aragonite (CaCO3) saturation state (Omega(arag)), with potentially substantial impacts on marine ecosystems over the 21st Century. Calcifying organisms have exhibited reduced calcification under lower saturation state conditions in aquaria. However, the in situ sensitivity of calcifying ecosystems to future ocean acidification remains unknown. Here we assess the community level sensitivity of calcification to local CO2-induced acidification caused by natural respiration in an unperturbed, biodiverse, temperate intertidal ecosystem. We find that on hourly timescales nighttime community calcification is strongly influenced by Omega(arag), with greater net calcium carbonate dissolution under more acidic conditions. Daytime calcification however, is not detectably affected by Omega(arag). If the short-term sensitivity of community calcification to Omega(arag) is representative of the long-term sensitivity to ocean acidification, nighttime dissolution in these intertidal ecosystems could more than double by 2050, with significant ecological and economic consequences.

Approximately one-quarter of the anthropogenic carbon dioxide released into the atmosphere each year is absorbed by the global oceans, causing measurable declines in surface ocean pH, carbonate ion concentration ([CO32-]), and saturation state of carbonate minerals (Omega)(1). This process, referred to as ocean acidification, represents a major threat to marine ecosystems, in particular marine calcifiers such as oysters, crabs, and corals. Laboratory and field studies(2,3) have shown that calcification rates of many organisms decrease with declining pH, [CO32-], and Omega. Coral reefs are widely regarded as one of the most vulnerable marine ecosystems to ocean acidification, in part because the very architecture of the ecosystem is reliant on carbonate-secreting organisms(4). Acidification-induced reductions in calcification are projected to shift coral reefs from a state of net accretion to one of net dissolution this century(5). While retrospective studies show large-scale declines in coral, and community, calcification over recent decades(6-12), determining the contribution of ocean acidification to these changes is difficult, if not impossible, owing to the confounding effects of other environmental factors such as temperature. Here we quantify the net calcification response of a coral reef flat to alkalinity enrichment, and show that, when ocean chemistry is restored closer to pre-industrial conditions, net community calcification increases. In providing results from the first seawater chemistry manipulation experiment of a natural coral reef community, we provide evidence that net community calcification is depressed compared with values expected for pre-industrial conditions, indicating that ocean acidification may already be impairing coral reef growth.

Many previous studies have shown that a solar forcing must be greater than a CO2 forcing to cause the same global mean surface temperature change but a process-based mechanistic explanation is lacking in the literature. In this study, we investigate the physical mechanisms responsible for the lower efficacy of solar forcing compared to an equivalent CO2 forcing. Radiative forcing is estimated using the Gregory method that regresses top-of-atmosphere (TOA) radiative flux against the change in global mean surface temperature. For a 2.25% increase in solar irradiance that produces the same long term global mean warming as a doubling of CO2 concentration, we estimate that the efficacy of solar forcing is similar to 80% relative to CO2 forcing in the NCAR CAM5 climate model. We find that the fast tropospheric cloud adjustments especially over land and stratospheric warming in the first four months cause the slope of the regression between the TOA net radiative fluxes and surface temperature to be steeper in the solar forcing case. This steeper slope indicates a stronger net negative feedback and hence correspondingly a larger solar forcing than CO2 forcing for the same equilibrium surface warming. Evidence is provided that rapid land surface warming in the first four months sets up a land sea contrast that markedly affects radiative forcing and the climate feedback parameter over this period. We also confirm the robustness of our results using simulations from the Hadley Centre climate model. Our study has important implications for estimating the magnitude of climate change caused by volcanic eruptions, solar geoengineering and past climate changes caused by change in solar irradiance such as Maunder minimum.