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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