Summary for policymakers Technical summary 1. The climate system - an overview 2. Observed climate variability and change 3. The carbon cycle and atmospheric CO2 4. Atmospheric chemistry and greenhouse gases 5. Aerosols, their direct and indirect effects 6. Radiative forcing of climate change 7. Physical climate processes and feedbacks 8. Model evaluation 9. Projections of future climate change 10. Regional climate simulation - evaluation and projections 11. Changes in sea level 12. Detection of climate change and attribution of causes 13. Climate scenario development 14. Advancing our understanding Glossary Index Appendix.

Climate change 2001: the scientific basis

The planetary boundaries framework defines a safe operating space for humanity based on the intrinsic biophysical processes that regulate the stability of the Earth system. Here, we revise and update the planetary boundary framework, with a focus on the underpinning biophysical science, based on targeted input from expert research communities and on more general scientific advances over the past 5 years. Several of the boundaries now have a two-tier approach, reflecting the importance of cross-scale interactions and the regional-level heterogeneity of the processes that underpin the boundaries. Two core boundaries—climate change and biosphere integrity—have been identified, each of which has the potential on its own to drive the Earth system into a new state should they be substantially and persistently transgressed.

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Planetary boundaries: Guiding human development on a changing planet

Changes in Atmospheric Constituents and in Radiative Forcing

Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of black-carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottom-up inventory methods are 7500 Gg yr−1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m−2 with 90% uncertainty bounds of (+0.08, +1.27) W m−2. Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m−2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. When the principal effects of short-lived co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil fuel and biofuel) have an industrial-era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short-lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all short-lived species from black-carbon-rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2). The uncertainties in net climate forcing from black-carbon-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co-emitted organic carbon. In prioritizing potential black-carbon mitigation actions, non-science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near-term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black-carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.

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Bounding the role of black carbon in the climate system: A scientific assessment

Human activities are releasing tiny particles (aerosols) into the atmosphere. These human-made aerosols enhance scattering and absorption of solar radiation. They also produce brighter clouds that are less efficient at releasing precipitation. These in turn lead to large reductions in the amount of solar irradiance reaching Earth's surface, a corresponding increase in solar heating of the atmosphere, changes in the atmospheric temperature structure, suppression of rainfall, and less efficient removal of pollutants. These aerosol effects can lead to a weaker hydrological cycle, which connects directly to availability and quality of fresh water, a major environmental issue of the 21st century.

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Aerosols, climate, and the hydrological cycle

There have been several investigations to understand the impact of Saharan dust layer on radiative heat balance. However, there are few studies on the impact of dust over Asian regions which is unique in aerosol perspective because of co-existence of natural and anthropogenic aerosols. Here, we examine the surface cooling and lower atmospheric warming (and hence heating rate) due to dust over Afro-Asian regions using collocated data from METEOSAT (of ESA) and MODIS (of NASA). Large reduction of surface reaching solar radiation as much as 10 to 15 W $m^{-2}$ due to dust was observed simultaneous with a lower atmospheric warming of 0.3 to 0.5 K/day. During local noon warming was as large as 3K over desert regions. The large dust heating at source regions and its impact over ocean due to transported dust raises several issues which need to be understood.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006GL028623

Atmospheric warming due to dust absorption over Afro-Asian regions

Long-range transport of continental dust makes these particles a significant constituent even at locations far from their sources. It is important to study the temporal variations in dust loading over desert regions and the role of meteorology, in order to assess its radiative impact. In this paper, infrared radiance (10.5-12.5 mu m), acquired by the METEOSAT-5 satellite (similar to 5-km resolution) during 1999 and 2003 was used to quantify wind dependence of dust aerosols and to estimate the radiative forcing. Our analysis shows that the frequency of occurrence of dust events was higher during 2003 compared to 1999. Since the dust production function depends mainly on the surface wind speed over regions which are dry and without vegetation, the role of surface wind on IDDI was examined in detail. It was found that an increase of IDDI with wind speed was nearly linear and the rate of increase in IDDI with surface wind was higher during 2003 compared to 1999. It was also observed that over the Indian desert, when wind speed was the highest during monsoon months (June to August), the dust production rate was lower because of higher soil moisture (due to monsoon rainfall). Over the Arabian deserts, when the wind speed is the highest during June to August, the dust production rate is also highest, as soil moisture is lowest during this season. Even though nothing can be said precisely on the reason why 2003 had a greater number of dust events, examination of monthly mean soil moisture at source regions indicates that the occurrence of high winds simultaneous with high soil moisture could be the reason for the decreased dust production efficiency in 1999. It appears that the deserts of Northwest India are more efficient dust sources compared to the deserts of Saudi Arabia and Northeast Africa (excluding Sahara). The radiative impact of dust over various source regions is estimated, and the regionally and annually averaged top of the atmosphere dust radiative forcing (short wave, clear-sky and over land) over the entire study region (0-35 degrees N; 30 degrees-100 degrees E) was in the range of -0.9 to +4.5 W m(-2). The corresponding values at the surface were in the range of -10 to -25 W m(-2). Our studies demonstrate that neglecting the diurnal variation of dust can cause errors in the estimation of long wave dust forcing by as much as 50 to 100%, and nighttime retrieval of dust can significantly reduce the uncertainties. A method to retrieve dust aerosols during nighttime is proposed. The regionally and annually averaged long wave dust radiative forcing was +3.4 +/- 1.6 W m(-2).

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Dust aerosols over India and adjacent continents retrieved using METEOSAT infrared radiance Part II: quantification of wind dependence and estimation of radiative forcing

Surface physical properties, hydrodynamics, biochemical cues, orientation and temporal scales play an important role in invertebrate larval recruitment on artificial substrates. In the present study, invertebrate recruitment on four different substrates (acrylic, stainless steel, ceramic and concrete panels) was investigated in two different orientations (vertical and horizontal) in the central Red Sea. Results showed significant variations in the abundance of benthic invertebrates between the different substrates. While barnacles and bivalves preferred panels placed in vertical positions, the abundance of bryozoans was high on horizontal panels. Artificial panel submersion season plays a significant role in the recruitment of benthic invertebrates on surfaces in the Red Sea. In conclusion, this study supports the overall notion that marine invertebrate recruitment on hard substrates is regulated by a combination of factors which include substrate type, orientation and submersion season.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0025315418000887

Invertebrate recruitment on artificial substrates in the Red Sea: role of substrate type and orientation

-2 to around -18 Wm -2 brings out the importance of natur al aer osols in this region. The study also demonstrates the important role of wind speed on aerosol characteristics and hence its impact on direct and indirect radiative effects. The magnitude of indirect radiative effect (and uncertainty) is se veral- fold more than the direct radiative effect of sea -salt aerosols. DETERMINATION of the radiative effects of aerosols is currently one of the challenging problems in climate r e- search. Aerosols influence the earth's radiative balance directly by scattering inco ming short-wave radiation back to space and indirectly through their influence on cloud properties 1-3 . The indirect effect is considered to be one of the largest uncertainties in current global climate mo dels (GCMs) 3 . Accurately estimating the indirect rad iative effect of aerosols requires an understanding of the role of aerosols in influencing cloud properties. Aerosols act as cloud condensation nuclei (CCN) during the formation of clouds. An increase in aerosol concen - tration leads to an increase in c loud droplet number and hence increases the cloud albedo (also called as 'Twomey effect') 1 . Also, for a given water-vapour content, an i n- crease in aerosol concentration decreases the effective cloud-droplet size and hence possibly decreases the lif e- time of the clouds 1 . These changes in cloud albedo and cloud lifetime in turn influence the radiation budget of the earth. The influence of aerosols on cloud microphy - sics depends on several factors that are poorly unde rstood. There have been several studies to d etermine the aerosol indirect effect 2-10 . However, most of these studies were focused on determining the indirect effect of anthrop o- genic sulphate aerosols. Studies to determine the ind irect effect of natural aerosols are rather few. In this article, we have made preliminary estimates of the indirect effect of sea-salt aerosols, which is one of the major natural aer osol species, using the available database. The study was conducted using data obtained over Arabian Sea (AS) and North Indian Ocean (NIO) region adjoining the Indian subcontinent. This region is charac - terized by strong monsoon phenomena, where the wind reverses its direction with season. Hence th is region is influenced by winds from the continent during winter monsoon season (November-March) and from the ocean during summer monsoon season (June-September) 11 .

Direct and indirect radiative effects of sea-salt aerosols over Arabian Sea

In this paper, we discuss the measurements of spectral surface reflectance (rholambda s) in the wavelength range 350-2500 nm measured using a spectroradiometer onboard a low-flying aircraft over Bangalore (12.95degN, 77.65deg E), an urban site in southern India. The large discrepancies in the retrieval of aerosol properties over land by the Moderate-Resolution Imaging Spectroradiometer (MODIS), which could be attributed to the inaccurate estimation of surface reflectance at many sites in India and elsewhere, provided motivation for this paper. The aim of this paper was to verify the surface reflectance relationships assumed by the MODIS aerosol algorithm for the estimation of surface reflectance in the visible channels (470 and 660 nm) from the surface reflectance at 2100 nm for aerosol retrieval over land. The variety of surfaces observed in this paper includes green and dry vegetations, bare land, and urban surfaces. The measured reflectance data were first corrected for the radiative effects of atmosphere lying between the ground and aircraft using the Second Simulation of Satellite Signal in the Solar Spectrum (6S) radiative transfer code. The corrected surface reflectance in the MODIS's blue (rho470 s), red (rho660 s), and shortwave-infrared (SWIR) channel (rho2100 s) was linearly correlated. We found that the slope of reflectance relationship between 660 and 2100 nm derived from the forward scattering data was 0.53 with an intercept of 0.07, whereas the slope for the relationship between the reflectance at 470 and 660 nm was 0.85. These values are much higher than the slope ( ~0.49) for either wavelengths assumed by the MODIS aerosol algorithm over this region. The reflectance relationship for the backward scattering data has a slope of 0.39, with an intercept of 0.08 for 660 nm, and 0.65, with an intercept of 0.08 for 470 nm. The large values of the intercept (which is very small in the MODIS reflectance relationships) result in larger values of absolute surface reflectance in the visible channels. The discrepancy between the measured and assumed surface reflectances could lead to error in the aerosol retrieval. The reflectance ratio (rho660 s/rho2100 s) showed a clear dependence on the NDVI SWIR where the ratio increased from 0.5 to 1 with an increase in NDVI SWIR from 0 to 0.5. The high correlation between the reflectance at SWIR wavelengths (2100, 1640, and 1240 nm) indicated an opportunity to derive the surface reflectance and, possibly, aerosol properties at these wavelengths. We need more experiments to characterize the surface reflectance and associated inhomogeneity of land surfaces, which play a critical role in the remote sensing of aerosols over land.

How Good is the Assumption About Visible Surface Reflectance in MODIS Aerosol Retrieval Over Land? A Comparison With Aircraft Measurements Over an Urban Site in India

Sathianeson Satheesh

Papers

Atmospheric warming due to dust absorption over Afro-Asian regions

Dust aerosols over India and adjacent continents retrieved using METEOSAT infrared radiance Part II: quantification of wind dependence and estimation of radiative forcing

Invertebrate recruitment on artificial substrates in the Red Sea: role of substrate type and orientation

Direct and indirect radiative effects of sea-salt aerosols over Arabian Sea

How Good is the Assumption About Visible Surface Reflectance in MODIS Aerosol Retrieval Over Land? A Comparison With Aircraft Measurements Over an Urban Site in India