Black carbon (BC) is functionally defined as the absorbing component of atmospheric total carbonaceous aerosols (TC) and is typically dominated by soot-like elemental carbon (EC). However, organic carbon (OC) has also been shown to absorb strongly at visible to UV wavelengths and the absorbing organics are referred to as brown carbon (BrC), which is typically not represented in climate models. We propose an observationally based analytical method for rigorously partitioning measured absorption aerosol optical depths (AAOD) and single scattering albedo (SSA) among EC and BrC, using multiwavelength measurements of total (EC, OC, and dust) absorption. EC is found to be strongly absorbing (SSA of 0.38) whereas the BrC SSA varies globally between 0.77 and 0.85. The method is applied to the California region. We find TC (EC + BrC) contributes 81% of the total absorption at 675 nm and 84% at 440 nm. The BrC absorption at 440 nm is about 40% of the EC, whereas at 675 nm it is less than 10% of EC. We find an enhanced absorption due to OC in the summer months and in southern California (related to forest fires and secondary OC). The fractions and trends are broadly consistent with aerosol chemical-transport models as well as with regional emission inventories, implying that we have obtained a representative estimate for BrC absorption. The results demonstrate that current climate models that treat OC as nonabsorbing are underestimating the total warming effect of carbonaceous aerosols by neglecting part of the atmospheric heating, particularly over biomass-burning regions that emit BrC.short lived climate pollutants | aerosol forcing B lack carbon (BC) emitted from combustion sources such as automobile exhaust and biomass burning (1-3) absorbs solar radiation in both the visible and the near-infrared spectra and is estimated to be a principal contributor to global atmospheric warming (4). The short atmospheric lifetime of BC aerosol particles, typically of the order of 1 wk (5, 6), compared with greenhouse gases (which have atmospheric lifetimes of several years or decades) results in BC being not well mixed in the atmosphere but instead geographically and temporally correlated to emission sources. For this reason, reducing BC emissions is an attractive control strategy for climate change that is expected to have a more immediate and regional impact (4,7,8). The state of California appears to be a successful example where aggressive control policies for vehicular diesel emissions and domestic wood burning have produced a near 50% decrease in BC concentrations (9). This decline in conjunction with the near-static concentrations of primarily scattering aerosol particles (such as sulfates) may have led to a large negative change in the direct radiative forcing (9).A simplification in such model estimates of aerosol forcing is that BC is considered to be equivalent to elemental carbon (EC), and the organic fraction of carbonaceous aerosols [organic carbon (OC)] is treated as scattering and is therefore found to have a cooling...

Abstract. The Paris Agreement of December 2015 stated a goal to pursue efforts to keep global temperatures below 1.5 • C above preindustrial levels and well below 2 • C. The IPCC was charged with assessing climate impacts at these temperature levels, but fully coupled equilibrium climate simulations do not currently exist to inform such assessments. In this study, we produce a set of scenarios using a simple model designed to achieve long-term 1.5 and 2 • C temperatures in a stable climate. These scenarios are then used to produce century-scale ensemble simulations using the Community Earth System Model, providing impact-relevant long-term climate data for stabilization pathways at 1.5 and 2 • C levels and an overshoot 1.5 • C case, which are realized (for the 21st century) in the coupled model and are freely available to the community. Here we describe the design of the simulations and a brief overview of their impact-relevant climate response. Exceedance of historical record temperature occurs with 60 % greater frequency in the 2 • C climate than in a 1.5 • C climate aggregated globally, and with twice the frequency in equatorial and arid regions. Extreme precipitation intensity is statistically significantly higher in a 2.0 • C climate than a 1.5 • C climate in some specific regions (but not all). The model exhibits large differences in the Arctic, which is ice-free with a frequency of 1 in 3 years in the 2.0 • C scenario, and 1 in 40 years in the 1.5 • C scenario. Significance of impact differences with respect to multi-model variability is not assessed.

The rate of increase of global‐mean precipitation per degree global‐mean surface temperature increase differs for greenhouse gas and aerosol forcings and across emissions scenarios with differing composition of change in forcing. We investigate whether or not the rate of change of extreme precipitation also varies across the four emissions scenarios that force the Coupled Model Intercomparison Project, version 5 multimodel ensemble. In most models, the rate of increase of maximum annual daily precipitation per degree global warming in the multimodel ensemble is statistically indistinguishable across the four scenarios, whether this extreme precipitation is calculated globally, over all land, or over extratropical land. These results indicate that in contrast to mean precipitation, extreme precipitation depends on the total amount of warming and does not depend on emissions scenario in most models.

Though highly motivated to slow the climate crisis, governments may struggle to impose costly polices on entrenched interest groups, resulting in a greater need for negative emissions. Here, we model wartime-like crash deployment of direct air capture (DAC) as a policy response to the climate crisis, calculating funding, net CO2 removal, and climate impacts. An emergency DAC program, with investment of 1.2–1.9% of global GDP annually, removes 2.2–2.3 GtCO2 yr–1 in 2050, 13–20 GtCO2 yr–1 in 2075, and 570–840 GtCO2 cumulatively over 2025–2100. Compared to a future in which policy efforts to control emissions follow current trends (SSP2-4.5), DAC substantially hastens the onset of net-zero CO2 emissions (to 2085–2095) and peak warming (to 2090–2095); yet warming still reaches 2.4–2.5 °C in 2100. Such massive CO2 removals hinge on near-term investment to boost the future capacity for upscaling. DAC is most cost-effective when using electricity sources already available today: hydropower and natural gas with renewables; fully renewable systems are more expensive because their low load factors do not allow efficient amortization of capital-intensive DAC plants.

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At last, all the major emitters of greenhouse gases (GHGs) have agreed under the Copenhagen Accord that global average temperature increase should be kept below 2°C. This study develops the criteria for limiting the warming below 2°C, identifies the constraints imposed on policy makers, and explores available mitigation avenues. One important criterion is that the radiant energy added by human activities should not exceed 2.5 (range: 1.7-4) watts per square meter (Wm −2 ) of the Earth's surface. The blanket of man-made GHGs has already added 3 (range: 2.6-3.5) Wm −2 . Even if GHG emissions peak in 2015, the radiant energy barrier will be exceeded by 100%, requiring simultaneous pursuit of three avenues: (i) reduce the rate of thickening of the blanket by stabilizing CO 2 concentration below 441 ppm during this century (a massive decarbonization of the energy sector is necessary to accomplish this Herculean task), (ii) ensure that air pollution laws that reduce the masking effect of cooling aerosols be made radiant energy-neutral by reductions in black carbon and ozone, and (iii) thin the blanket by reducing emissions of short-lived GHGs. Methane and hydrofluorocarbons emerge as the prime targets. These actions, even if we are restricted to available technologies for avenues ii and iii, can reduce the probability of exceeding the 2°C barrier before 2050 to less than 10%, and before 2100 to less than 50%. With such actions, the four decades we have until 2050 should be exploited to develop and scale-up revolutionary technologies to restrict the warming to less than 1.5°C. December 7-19, 2009, in Copenhagen to arrive at an international agreement for mitigating climate change. An agreement could not be reached; instead, the COP-15 arrived at the so-called "Copenhagen Accord" (CHA). Of the 193 nations that attended, including the leaders of major developed and developing nations, all but a few nations (Bolivia, Cuba, Nicaragua, Sudan, and Venezuela) supported the accord. The most significant part of the succinct three-page 12-paragraph CHA (1) is the following: "We underline that climate change is one of the greatest challenges of our time" in its opening paragraph, followed by the second paragraph, which begins with "We agree that deep cuts in global emissions are required according to science, and as documented by the IPCC Fourth Assessment Report with a view to reduce global emissions so as to hold the increase in global temperature below 2 degrees Celsius, and take action to meet this objective consistent with science and on the basis of equity." Targets for greenhouse gas (GHG) emission reductions as required by Appendix 1 of the CHA have already been provided by over 100 countries, including most if not all of the major emitters. The initial response to the CHA was one of disappointment (2), particularly because it did not include binding targets for reductions in CO 2 emissions. As such, the CHA is considered to be just a political document (2).The present article, on the other hand, argues that an agreement...

Methane mitigation is essential for addressing climate change, but the value of rapidly implementing available mitigation measures is not well understood. In this paper, we analyze the climate benefits of fast action to reduce methane emissions as compared to slower and delayed mitigation timelines. We find that the scale up and deployment of greatly underutilized but available mitigation measures will have significant near-term temperature benefits beyond that from slow or delayed action. Overall, strategies exist to cut global methane emissions from human activities in half within the next ten years and half of these strategies currently incur no net cost. Pursuing all mitigation measures now could slow the global-mean rate of near-term decadal warming by around 30%, avoid a quarter of a degree centigrade of additional global-mean warming by midcentury, and set ourselves on a path to avoid more than half a degree centigrade by end of century. On the other hand, slow implementation of these measures may result in an additional tenth of a degree of global-mean warming by midcentury and 5% faster warming rate (relative to fast action), and waiting to pursue these measures until midcentury may result in an additional two tenths of a degree centigrade by midcentury and 15% faster warming rate (relative to fast action). Slow or delayed methane action is viewed by many as reasonable given that current and on-the-horizon climate policies heavily emphasize actions that benefit the climate in the long-term, such as decarbonization and reaching net-zero emissions, whereas methane emitted over the next couple of decades will play a limited role in long-term warming. However, given that fast methane action can considerably limit climate damages in the near-term, it is urgent to scale up efforts and take advantage of this achievable and affordable opportunity as we simultaneously reduce carbon dioxide emissions.

The origin of sedimentary dolomite is a subject of long-standing enigma that still awaits resolution. Previous studies have shown that room temperature synthesis of abiotic dolomite is rarely achieved and primary (proto-)dolomite precipitation is closely associated with microbial activities. In this study, we demonstrate through laboratory carbonation experiments that highly negative-charged clay minerals (as indicated by the values of zetal potential) such as illite and montmorillonitecan aid the precipitation of abiotic proto-dolomite under ambient conditions, whereas nearly-neutral charged kaolinite exerts negligible influence on such process. In comparison to montmorillonite, illite has higher surface-charge density, thus is more effective in catalyzing protodolomite precipitation. Furthermore, the signal of proto-dolomite in carbonate neoformations is enhanced with increasing concentrations of illite or montmorillonite. On the basis of these results, we suggest that clay minerals catalyze dolomite formation perhaps via electrostatic binding of Mg 2+ and Ca 2+ ions and simultaneously desolvating these strongly hydrated cations, a crucial step for dolomite crystallization. The resulting proto-dolomites display the morphologies, textures, and structures similar to those of biogenic dolomite reported before, which are considered precursors of ordered sedimentary dolomite. Therefore, our results offer a possible route to authigenic dolomite found in sedimentary environments.