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Laurentide ice-sheet retreat continued into the mid-Holocene. Speleothem-based precipitation records suggest the cessation of melt led to the establishment of the present precipitation patterns associated with the North Atlantic Oscillation. The North Atlantic Oscillation is the dominant atmospheric pressure mode in the North Atlantic region and affects winter temperature and precipitation in the Mediterranean, northwest Europe, Greenland, and Asia 1 . The index 1 that describes the sea-level pressure difference between Iceland and the Azores is correlated with a dipole precipitation pattern over northwest Europe and northwest Africa. How the North Atlantic Oscillation will develop as the Greenland ice sheet melts is unclear 2 . A potential past analogue is the early Holocene, during which melting ice sheets around the North Atlantic 3 , 4 freshened surface waters, affecting the strength of the meridional overturning circulation 5 . Here we present a Holocene rainfall record from northwest Africa based on speleothem δ 18 O and compare it against a speleothem-based rainfall record from Europe 6 . The two records are positively correlated during the early Holocene, followed by a shift to an anti-correlation, similar to the modern record, during the mid-Holocene. On the basis of our simulations with an Earth system model, we suggest the shift to the anti-correlation reflects a large-scale atmospheric and oceanic reorganization in response to the demise of the Laurentide ice sheet and a strong reduction of meltwater flux to the North Atlantic, pointing to a potential sensitivity of the North Atlantic Oscillation to the melting of ice sheets.

Uncertainties about the properties and amounts of atmospheric black carbon complicate efforts to understand its regional and global effects on climate. The black soot coming out of the tailpipes of diesel trucks is a nuisance familiar to every highway traveler. Soot also endangers the health of untold numbers of women and their families exposed to smoke from traditional cookstoves burning biofuels and coal. But in addition to irritating our noses and lungs, this pollutant, also known as black carbon (BC), is the strongest absorber of solar radiation in the atmosphere. The magnitude of global warming from BC, as well as its regional effects, has been the subject of intense debate. In a recent comprehensive assessment, Bond et al. ( 1 ) have synthesized available model results and observations, and propose a \"best estimate\" for BC's global climate forcing. Their estimate is almost twice as high as values commonly discussed ( 2 ). What causes such large discrepancies between estimates, and what are the implications for the global and regional climate effects of BC?

Low clouds play a key role in the Earth-atmosphere energy balance and influence agricultural production and solar-power generation. Smoke aloft has been found to enhance marine stratocumulus through aerosol-cloud interactions, but its role in regions with strong human activities and complex monsoon circulation remains unclear. Here we show that biomass burning aerosols aloft strongly increase the low cloud coverage over both land and ocean in subtropical southeastern Asia. The degree of this enhancement and its spatial extent are comparable to that in the Southeast Atlantic, even though the total biomass burning emissions in Southeast Asia are only one-fifth of those in Southern Africa. We find that a synergetic effect of aerosol-cloud-boundary layer interaction with the monsoon is the main reason for the strong semi-direct effect and enhanced low cloud formation in southeastern Asia. Biomass burning emissions have been shown to influence clouds in the Atlantic, but its influence in other regions is not well known. Here, the authors show that biomass burning aerosols increase the low-cloud cover over subtropical southeastern Asia by a similar magnitude than over the Atlantic.

Marine low clouds play an important role in the climate system, and their properties are sensitive to cloud condensation nuclei concentrations. While new particle formation represents a major source of cloud condensation nuclei globally, the prevailing view is that new particle formation rarely occurs in remote marine boundary layer over open oceans. Here we present evidence of the regular and frequent occurrence of new particle formation in the upper part of remote marine boundary layer following cold front passages. The new particle formation is facilitated by a combination of efficient removal of existing particles by precipitation, cold air temperatures, vertical transport of reactive gases from the ocean surface, and high actinic fluxes in a broken cloud field. The newly formed particles subsequently grow and contribute substantially to cloud condensation nuclei in the remote marine boundary layer and thereby impact marine low clouds. Globally, new particle formation represents a major source of cloud condensation nuclei. Here, the authors present evidence of frequent occurrence of new particle formation in the upper part of remote marine boundary layer following cold front passages.

Organic nucleation is an important source of atmospheric aerosol number concentration, especially in pristine continental regions and during the preindustrial period. Here, we improve on previous simulations that overestimate boundary layer nucleation in the tropics and add changes to climate and land use to evaluate climate forcing. Our model includes both pure organic nucleation and heteromolecular nucleation of sulfuric acid and organics and reproduces the profile of aerosol number concentration measured in the Amazon. Organic nucleation decreases the sum of the total aerosol direct and indirect radiative forcing by 12.5%. The addition of climate and land use change decreases the direct radiative forcing (−0.38 W m −2 ) by 6.3% and the indirect radiative forcing (−1.68 W m −2 ) by 3.5% due to the size distribution and number concentration change of secondary organic aerosol and sulfate. Overall, the total radiative forcing associated with anthropogenic aerosols is decreased by 16%. Organic nucleation is an important source of atmospheric aerosol number concentration, especially in pristine continental regions and during the preindustrial period. Here the authors find a 16% reduced radiative forcing associated with anthropogenic aerosols when including organic nucleation together with climate and land use change.

Since the publication of the compilation of biomass burning emission factors by Andreae and Merlet (2001), a large number of studies have greatly expanded the amount of available data on emissions from various types of biomass burning. Using essentially the same methodology as Andreae and Merlet (2001), this paper presents an updated compilation of emission factors. The data from over 370 published studies were critically evaluated and integrated into a consistent format. Several new categories of biomass burning were added, and the number of species for which emission data are presented was increased from 93 to 121. Where field data are still insufficient, estimates based on appropriate extrapolation techniques are proposed. For key species, the updated emission factors are compared with previously published values. Based on these emission factors and published global activity estimates, I have derived estimates of pyrogenic emissions for important species released by the various types of biomass burning.

The optical properties of airborne mineral dust depend on its mineralogy, size distribution, and shape, and they might vary between different source regions. To date, large differences in refractive index values found in the literature have not been fully explained. In this paper we present a new dataset of complex refractive indices (m=n-ik) and single-scattering albedos (SSAs) for 19 mineral dust aerosols over the 370–950 nm range in dry conditions. Dust aerosols were generated from natural parent soils from eight source regions (northern Africa, Sahel, Middle East, eastern Asia, North and South America, southern Africa, and Australia). They were selected to represent the global-scale variability of the dust mineralogy. Dust was resuspended into a 4.2 m3 smog chamber where its spectral shortwave scattering (βsca) and absorption (βabs) coefficients, number size distribution, and bulk composition were measured. The complex refractive index was estimated by Mie calculations combining optical and size data, while the spectral SSA was directly retrieved from βsca and βabs measurements. Dust is assumed to be spherical in the whole data treatment, which introduces a potential source of uncertainty. Our results show that the imaginary part of the refractive index (k) and the SSA vary widely from sample to sample, with values for k in the range 0.0011 to 0.0088 at 370 nm, 0.0006 to 0.0048 at 520 nm, and 0.0003 to 0.0021 at 950 nm, as well as values for SSA in the range 0.70 to 0.96 at 370 nm, 0.85 to 0.98 at 520 nm, and 0.95 to 0.99 at 950 nm. In contrast, the real part of the refractive index (n) is mostly source (and wavelength) independent, with an average value between 1.48 and 1.55. The sample-to-sample variability in our dataset of k and SSA is mostly related to differences in the dust iron content. In particular, a wavelength-dependent linear relationship is found between the magnitude of k and SSA and the mass concentrations of both iron oxide and total elemental iron, with iron oxide better correlated than total elemental iron with both k and SSA. The value of k was found to be independent of size. When the iron oxide content exceeds 3 %, the SSA linearly decreases with an increasing fraction of coarse particles at short wavelengths (< 600 nm). Compared to the literature, our values for the real part of the refractive index and SSA are in line with past results, while we found lower values of k compared to most of the literature values currently used in climate models. We recommend that source-dependent values of the SW spectral refractive index and SSA be used in models and remote sensing retrievals instead of generic values. In particular, the close relationships found between k or SSA and the iron content in dust enable the establishment of predictive rules for spectrally resolved SW absorption based on particle composition.

Many terrestrial surfaces are covered by photoautotrophic communities, which are capable of synthesizing their own food from inorganic substances using sunlight. According to an analysis of previously published data, these communities account for nearly half of the biological nitrogen fixation on land. Many terrestrial surfaces, including soils, rocks and plants, are covered by photoautotrophic communities, capable of synthesizing their own food from inorganic substances using sunlight as an energy source 1 , 2 . These communities, known as cryptogamic covers, comprise variable proportions of cyanobacteria, algae, fungi, lichens and bryophytes, and are able to fix carbon dioxide and nitrogen from the atmosphere 3 . However, their influence on global and regional biogeochemical cycling of carbon and nitrogen has not yet been assessed. Here, we analyse previously published data on the spatial coverage of cryptogamic communities, and the associated fluxes of carbon and nitrogen, in different types of ecosystem across the globe. We estimate that globally, cryptogamic covers take up around 3.9 Pg carbon per year, corresponding to around 7% of net primary production by terrestrial vegetation. We derive a nitrogen uptake by cryptogamic covers of around 49 Tg per year, suggesting that cryptogamic covers account for nearly half of the biological nitrogen fixation on land. We suggest that nitrogen fixation by cryptogamic covers may be crucial for carbon sequestration by plants.

Rock art originated some 46,000 years ago and can provide unique insights into the minds of our human ancestors. However, dating of these ancient images, especially of petroglyphs, remains a challenge. In this study, we explore the potential of deriving age estimates from measurements of the areal densities of manganese (D Mn ) and iron (D Fe ) in the rock varnish on petroglyphs, based on the concept that the amount of varnish that has regrown on a petroglyph since its creation, relative to the surrounding intact varnish, is a measure of its age. We measured D Mn and D Fe by portable X-ray fluorescence (pXRF) on dated Late Pleistocene and Holocene rock surfaces, from which we derived accumulation rates of Mn and Fe in the rock varnish. The observed rates were comparable to our previous findings on basalt surfaces in North America. We derived age estimates for the rock art at four sites in the northern Great Basin region of North America based on D Mn measurements on the petroglyphs and intact varnish. They suggest that rock art creation in this region began around the Pleistocene/Holocene transition and continued into the Historic Period, encompassing a wide range of styles and motifs. Evidence of reworking of the rock art at various times by Indigenous people speaks of the continued agency of these images through the millennia. Our results are in good agreement with chronologies based on archeological and other archaeometric techniques. While our method remains subject to significant uncertainty with regard to the absolute ages of individual images, it provides the unique opportunity to obtain age estimates for large ensembles of images without the need for destructive sampling.

Biomass burning (BB) aerosols can influence regional and global climate through interactions with radiation, clouds, and precipitation. Here, we investigate the impact of BB aerosols on the energy balance and hydrological cycle over the Amazon Basin during the dry season. We performed simulations with a fully coupled meteorology–chemistry model, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), for a range of different BB emission scenarios to explore and characterize nonlinear effects and individual contributions from aerosol–radiation interactions (ARIs) and aerosol–cloud interactions (ACIs). The ARIs of BB aerosols tend to suppress low-level liquid clouds by local warming and increased evaporation and to facilitate the formation of high-level ice clouds by enhancing updrafts and condensation at high altitudes. In contrast, the ACIs of BB aerosol particles tend to enhance the formation and lifetime of low-level liquid clouds by providing more cloud condensation nuclei (CCN) and to suppress the formation of high-level ice clouds by reducing updrafts and condensable water vapor at high altitudes (>8 km). For scenarios representing the lower and upper limits of BB emission estimates for recent years (2002–2016), we obtained total regional BB aerosol radiative forcings of −0.2 and 1.5 W m−2, respectively, showing that the influence of BB aerosols on the regional energy balance can range from modest cooling to strong warming. We find that ACIs dominate at low BB emission rates and low aerosol optical depth (AOD), leading to an increased cloud liquid water path (LWP) and negative radiative forcing, whereas ARIs dominate at high BB emission rates and high AOD, leading to a reduction of LWP and positive radiative forcing. In all scenarios, BB aerosols led to a decrease in the frequency of occurrence and rate of precipitation, caused primarily by ACI effects at low aerosol loading and by ARI effects at high aerosol loading. The dependence of precipitation reduction on BB aerosol loading is greater in a strong convective regime than under weakly convective conditions. Overall, our results show that ACIs tend to saturate at high aerosol loading, whereas the strength of ARIs continues to increase and plays a more important role in highly polluted episodes and regions. This should hold not only for BB aerosols over the Amazon, but also for other light-absorbing aerosols such as fossil fuel combustion aerosols in industrialized and densely populated areas. The importance of ARIs at high aerosol loading highlights the need for accurately characterizing aerosol optical properties in the investigation of aerosol effects on clouds, precipitation, and climate.