Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth’s radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable and arguable lines of evidence, including modelling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of −1.60 to −0.65 W m−2, or −2.0 to −0.4 W m−2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted towards more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds.

https://hal.archives-ouvertes.fr/hal-02322106/document

Bounding global aerosol radiative forcing of climate change

The global response to warming of 1.5oC comprises transitions in land and ecosystem, energy, urban and infrastructure, and industrial systems. The feasibility of mitigation and adaptation options, and the enabling conditions for strengthening and implementing the systemic changes, are assessed in this chapter.

Strengthening and Implementing the Global Response

The Indian summer monsoon is influenced by numerous factors, including aerosol-induced changes to clouds, surface and atmospheric heating, and atmospheric circulation. An analysis of satellite data and global climate model simulations suggests that dust aerosol levels over the Arabian Sea, West Asia and the Arabian Peninsula are positively correlated with the intensity of the Indian summer monsoon.

Short-term modulation of Indian summer monsoon rainfall by West Asian dust

It has been predicted, by theory and models, that heavy precipitation will increase with climate change and this is now being seen in observations. Emergence of signals such as this will enable testing of predictions, which should increase confidence in them. Environmental phenomena are often observed first, and then explained quantitatively. The complexity of processes, the range of scales involved, and the lack of first principles make it challenging to predict conditions beyond the ones observed. Here we use the intensification of heavy precipitation as a counterexample, where seemingly complex and potentially computationally intractable processes manifest themselves to first order in simple ways: heavy precipitation intensification is now emerging in the observed record across many regions of the world, confirming both theory and model predictions made decades ago. As the anthropogenic climate signal strengthens, there will be more opportunities to test climate predictions for other variables against observations and across a hierarchy of different models and theoretical concepts.

Observed heavy precipitation increase confirms theory and early model

Assessment of Climate Change over the Indian Region : A Report of the Ministry of Earth Sciences (MoES), Government of India

The net warming effect of cirrus clouds has driven part of the geoengineering research toward the idea of decreasing their occurrence frequency by seeding them with efficient ice nucleating particles. We study responses of cirrus clouds to simplified global seeding strategies in terms of their radiative fluxes with the help of the ECHAM-HAM general circulation model. Our cirrus scheme takes into account the competition between homogeneous and heterogeneous freezing, preexisting ice crystals, and the full spectrum of updraft velocities. While we find that the cirrus cloud radiative effect evaluated from our model is positive and large enough (5.7 W/m2) to confirm their geoengineering potential, none of the seeding strategies achieves a significant cooling due to complex microphysical mechanisms limiting their climatic responses. After globally uniform seeding is applied, we observe an increase in cirrus cloud cover, a decrease in ice crystal number concentration, and a decrease in ice crystal radius. An analysis of their respective radiative contributions points to the ice crystal radius decrease as the main factor limiting seeding effectiveness.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015JD024666

Why cirrus cloud seeding cannot substantially cool the planet

Climate engineering is a potential means to offset the climate warming caused by anthropogenic greenhouse gases. Suggested methods broadly fall into two categories. Methods in the first category aim to remove carbon dioxide (CO2) from the atmosphere, whereas those in the second aim to alter Earth's radiation balance. The most prominent and best researched climate engineering approach in the second category is the injection of atmospheric aerosol particles or their precursor gases into the stratosphere ( 1 ), where these particles reflect solar radiation back to space. Climate engineering through cirrus cloud thinning, in contrast, mainly targets the long-wave radiation that is emitted from Earth.

https://science.sciencemag.org/content/sci/357/6348/248.full.pdf

A cirrus cloud climate dial

What Drives the Life Cycle of Tropical Anvil Clouds

Cirrus clouds impact the planetary energy balance and upper-tropospheric water vapor transport and are therefore relevant for climate. In this study cirrus clouds at temperatures colder than −40°C simulated by the ECHAM–Hamburg Aerosol Module (ECHAM-HAM) general circulation model are compared to Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite data. The model captures the general cloud cover pattern and reproduces the observed median ice water content within a factor of 2, while extinction is overestimated by about a factor of 3 as revealed by temperature-dependent frequency histograms. Two distinct types of cirrus clouds are found: in situ–formed cirrus dominating at temperatures colder than −55°C and liquid-origin cirrus dominating at temperatures warmer than −55°C. The latter cirrus form in anvils of deep convective clouds or by glaciation of mixed-phase clouds, leading to high ice crystal number concentrations. They are associated with extinction coefficie...

https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-16-0608.1

Cirrus Cloud Properties as Seen by the CALIPSO Satellite and ECHAM-HAM Global Climate Model

. Recent satellite observations show efficient vertical transport of Asian pollutants from the surface to the upper-level anticyclone by deep monsoon convection. In this paper, we examine the transport of carbonaceous aerosols, including black carbon (BC) and organic carbon (OC), into the monsoon anticyclone using of ECHAM6-HAM, a global aerosol climate model. Further, we investigate impacts of enhanced (doubled) carbonaceous aerosol emissions on the upper troposphere and lower stratosphere (UTLS), underneath monsoon circulation and precipitation from sensitivity simulations. The model simulation shows that boundary layer aerosols are transported into the monsoon anticyclone by the strong monsoon convection from the Bay of Bengal, southern slopes of the Himalayas and the South China Sea. Doubling of emissions of both BC and OC aerosols over Southeast Asia (10° S–50° N, 65–155° E) shows that lofted aerosols produce significant warming (0.6–1 K) over the Tibetan Plateau (TP) near 400–200 hPa and instability in the middle/upper troposphere. These aerosols enhance radiative heating rates (0.02–0.03 K day−1) near the tropopause. The enhanced carbonaceous aerosols alter aerosol radiative forcing (RF) at the surface by −4.74 ± 1.42 W m−2, at the top of the atmosphere (TOA) by +0.37 ± 0.26 W m−2 and in the atmosphere by +5.11 ± 0.83 W m−2 over the TP and Indo-Gangetic Plain region (15–35° N, 80–110° E). Atmospheric warming increases vertical velocities and thereby cloud ice in the upper troposphere. Aerosol induced anomalous warming over the TP facilitates the relative strengthening of the monsoon Hadley circulation and increases moisture inflow by strengthening the cross-equatorial monsoon jet. This increases precipitation amounts over India (1–4 mm day−1) and eastern China (0.2–2 mm day−1). These results are significant at the 99 % confidence level.

/pdf/potential-impact-of-carbonaceous-aerosol-on-the-upper-3e1yjmojln.pdf

Potential impact of carbonaceous aerosol on the upper troposphere and lower stratosphere (UTLS) and precipitation during Asian summer monsoon in a global model simulation

Blaž Gasparini

Papers

Why cirrus cloud seeding cannot substantially cool the planet

A cirrus cloud climate dial

What Drives the Life Cycle of Tropical Anvil Clouds

Cirrus Cloud Properties as Seen by the CALIPSO Satellite and ECHAM-HAM Global Climate Model

Potential impact of carbonaceous aerosol on the upper troposphere and lower stratosphere (UTLS) and precipitation during Asian summer monsoon in a global model simulation