Environment Counts | New evidence about anthropogenic aerosols and climate change – IPCC 5th ASSESSMENT Report
Author: Geoff Zeiss – Published At: 2014-09-15 17:28 – (2104 Reads)
The atmosphere is composed mostly of gases, but it also contains liquid and solid matter in the form of particles. Water aerosols are called clouds. The main constituents of non-water atmospheric aerosols are inorganic particles such as sulphate, nitrate, ammonium, and sea salt, organic aerosols (OA), black carbon (BC) from fossil fuel combustion, mineral dust (mostly desert dust) and primary biological aerosol particles (PBAPs)such as bacteria, fungal spores, and pollen. The majority of black carbon, sulphate, nitrate and ammonium come from anthropogenic sources, whereas sea salt, most mineral dust and PBAPs are predominantly of natural origin. Natural sulfur aerosols (sulfates) are formed from the sulfur dioxide(SO2) ejected by volcanoes.
The primary net effect of aerosol particles interacting with solar radiation (short wavelengths) is reflectance which cools the planet. But aerosol particles also interact with heat (long wavelengths) emanating from the Earth with a net warming effect. Aerosols can also serve as cloud condensation nuclei (CCN) and ice nuclei (IN) which contribute to cloud formation, but the mechanisms by which aerosols affect clouds and precipitation is very poorly understood.
Anthropogenic aerosols have been the strongest contributor to climate cooling except for brief periods with large volcanic eruptions. The cooling effect from anthropogenic aerosols has also grown stronger with time as industrial production has increased.
The major conclusion of AR5 is that aerosols are the dominant contributor to uncertainty in the net effect of anthropogenic sources during the Industrial Era. However, despite the large uncertainty range in the various contributions, there is a high confidence that aerosols have offset a substantial portion of greenhouse gas climate warming. IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis Chapters 7 and 8
Clouds andÂ aerosols
The atmosphere is composed mostly of gases, but it also contains liquid and solid matter in the form of particles. It is usual to distinguish these particles into atmospheric aerosol particles, cloud particles and precipitation. Particles in the atmosphere strongly influence the transfer of radiant energy thereby influencing the weather and climate. Atmospheric aerosols originate from two different pathways: emissions of primary particles such as smoke and formation of particles from gases by chemical reaction, for example. sulphates from sulphuric acid.
The main constituents of atmospheric aerosols are inorganic particles (such as sulphate, nitrate, ammonium, sea salt), organic aerosols (OA), black carbon (BC) from fossil fuel combustion, mineral dust (mostly desert dust) and primary biological aerosol particles (PBAPs) such as bacteria, fungal spores, and pollen. The majority of BC, sulphate, nitrate and ammonium come from anthropogenic sources, whereas sea salt, most mineral dust and PBAPs are predominantly of natural origin. Natural sulfur aerosols (sulfates) are formed from the sulfur dioxide(SO2) ejected by volcanoes. Scientists reserve the term aerosols for particulate matter in the atmosphere. Halocarbons, which consumer products often refer to as aerosols, are actually greenhouse gases.
Artificially injecting aerosols into the atmosphere has been proposed as a Solar Radiation Management (SRM) method, which could substantially offset a global temperature rise and partially offset some other impacts of global warming.
Clouds cover roughly two thirds of the globe. Clouds strongly affect the flows of both sunlight (warming the planet) and infrared heat (cooling the planet as it is radiated to space) through the atmosphere. The effect of clouds on the Earthâ€™s present-day top of the atmosphere (TOA) can be inferred from satellite data by comparing upwelling radiation in cloudy and non-cloudy conditions.
- By increasing reflectance of incoming solar radiation, clouds exert a global annual (shortwave) cooling effect of approximately â€“50 watts per square meter (W/m**2).
- Clouds also contribute to the greenhouse warming effect by reflecting heat (longwave radiation) back to the Earth’s surface of approximately +30 W/m**2, with a range of 10% or less between published satellite estimates.
Understanding cloud processes has been recognized for decades as a dominant source of uncertainty in our understanding of changes in the climate system, but has never been systematically assessed by the IPCC before AR5.
There are two important sources of energy, the sun and the Earth itself. The sun radiates energy at short wavelengths from the ultraviolet to the visible that impinges on the Earth’s atmosphere. The Earth itself also radiates energy, but in the form of heat at long (infrared) wavelengths.
The energy budget of the Earth is broken down into contributions from different energy fluxes. Energy fluxes due to different processes are often referred to as radiative forcings (RF) and expressed as energy per square meter per second or watts per square meter (W/m**2). They include
- Incoming solar TOA (at the top of the atmosphere) = average solar radiation (short wavelengths) impinging on top of Earth’s atmosphere
- Solar reflected TOA = solar radiation reflected by Earth’s atmosphere including clouds and aerosols
- Solar down surface = solar radiation hitting Earth’s surface
- Solar absorption surface = solar radiation absorbed by Earth’s surface
- Solar reflected surface = solar radiation reflected by Earth’s surface (surface albedo)
- Thermal up surface = heat (long wavelengths) radiated by Earth’s surface to atmosphere
- Sensible heat = heat exchanged between Earth’s surface and atmosphere due to convection
- Thermal outgoing TOA = heat radiated from Earth’s atmosphere to space
- Greenhouse gas effect = back radiation to the surface from heat retained on Earth’s surface by greenhouse gases (CO2, CH4, N2O)
- Evaporation = heat conveyed from Earth’s surface to atmosphere by evaporation of water
The largest anthropogenic contributions to warming the climate are greenhouse gases, specifically CO2 and CH4. But these are partially offset by the generally cooling effect of aerosols.
Aerosols and theÂ climate
Aerosol particles interact with solar radiation (short wavelengths) primarily by reflectance with a net cooling effect. To a lesser extent they interact with terrestrial heat radiation (long wavelengths) primarily by reflecting heat back to the Earth thereby contributing to the greenhouse effect. This aerosol-radiation affect is well understood. But aerosols also interact with clouds. For example, aerosols can serve as cloud condensation nuclei (CCN) and ice nuclei (IN) upon which cloud droplets and ice crystals form. The aerosol-cloud effect is complex and only beginning to be understood. Since AR4, research continues to identify new pathways through which aerosols may affect the radiative properties of clouds, as well as the intensity and geographical distribution of precipitation. Progress in understanding aerosol-cloud interactions and their effects on climate is also limited by inadequate observational tools, but new and improved observational aerosol data sets have emerged since AR4. Long-term aerosol mass concentrations are being measured more systematically by global and regional networks. Over most regions the majority of the optically active aerosol resides in the lowest 1 to 2 km so for example, aerosols are being measured by the Aerosol Robotic Network (AERONET) and other ground-based networks. This is being complemented by satellite-based sensors such as MODIS, MISR, POLDER, and PARASOL. Recently, the CALIPSO spaceborne lidar is being used to measure aerosols.
|Type of aerosol process||Warming/Cooling||Radiative forcing in watts/square meter (W/m**2)|
|aerosolâ€“radiation interactions||Cooling||â€“0.45 (â€“0.95 to +0.05)|
|aerosolâ€“cloud interaction||Cooling||â€“0.45 (â€“1.2 to 0.0)|
|black carbon (BC) on snow and ice||Warming||0.04 (0.02 to 0.09)|
|Total aerosol effect (excluding BC on snow and ice)||Cooling||â€“0.9 (â€“1.9 to â€“0.1)|
Anthropogenic impacts onÂ climate
have been the strongest contributor to climate cooling except for brief periods with large volcanic eruptions. The cooling effect from anthropogenic aerosols has also grown stronger with time as industrial production has increased.
The types of climate impacts (also known as radiative forcing) that are considered in AR5 are
- Solar radiation
- Black carbon (BC) on snow and ice
- Statospheric water from CH4
- Tropospheric ozone
- Other well-mixed green house gases
- Aerosol-radiation interaction
- Aerosol-cloud interaction
- Surface albedo (land use change)
- Stratospheric ozone
- Volcanic aerosols (mostly sulphates)
Mineral dust (mostly desert dust), sulphates, nitrates, and organic carbon aerosols are cooling. Carbon monoxide, volatile organic compounds, and nitrogen oxides react to create a variety of aerosols that are cooling. The effect of aerosols on cloud formation is also cooling. But black carbon (soot) deposited on ice and snow decreases the reflectance of these surfaces and is warming. Combining all these effects and even with the uncertainty in the aerosol contributions, the estimated increase in radiative forcing since 1750 is between 1.13 and 3.33 watts per square meter. The relative magnitudes and error bars for anthropogenic sources are shown in the illustration.
Comparison of the estimates of the various types of radiative forcing in AR5 with previous reports is shown in Table 8.6. A major conclusion of AR5 is that aerosols are the greatest source of uncertainty in understanding anthropogenic climate warming. The IPCC estimates the uncertainty associated with several important aerosol processes as low.
|Type of radiative forcing||Confidence level|
|aerosol-radiation interactions (direct effect)||HIGH|
|Rapid adjustment aerosol-radiation interactions||LOW|
|Total aerosol effect||MEDIUM|
|Surface albedo (BC aerosol on snow and ice)||LOW|
The IPCC AR5 has estimated the total contributions to anthropogenic radiative forcing from the beginning of the industrial era (1750) through 2011.
Over the Industrial Era (1750 through the present) anthropogenic CO2 and other greenhouse gases have been the dominant contributor to climate change, except for shorter periods with strong volcanic eruptions. The time evolution shows an almost continuous increase in the magnitude of anthropogenic radiative forcing, both greenhouse gases and aerosols.
Cooling due to aerosolâ€“radiation interactions was weak until 1950 but strengthened in the latter half of the last century especially in the period 1950 to 1980. Although there is “high confidence” for a substantial increase in aerosol cooling in the period 1950â€“1980, there is much more uncertainty in the relative change in global mean aerosol cooling over the last two decades (1990â€“2010).
Conclusions from AR5 aboutÂ aerosols
Some of the important conclusions of the contributors to AR5 about aerosols are;
- The large uncertainty in the impact of aerosols on the climate is the dominant contributor to the uncertainty in overall net Industrial Era climate change. Despite the large uncertainty range, there is “high confidence” that aerosols have offset a substantial portion of global greenhouse gas warming. The effect of natural aerosols over the last 15 years has “likely” offset a substantial fraction (at least 30%) of the anthropogenic warming.
- The effect of volcanic aerosols on the climate is well understood and is greatest for a short period (~2 years) following volcanic eruptions. There have been no major volcanic eruptions since Mt Pinatubo in 1991, but several smaller eruptions have caused cooling for the years 2008â€“2011 of â€“0.11 (â€“0.15 to â€“0.08) W/m**2 as compared to 1750 and â€“0.06 (â€“0.08 to â€“0.04) W/m**2 as compared to 1999â€“2002.
- The total anthropogenic radiative forcing (warming) over the Industrial Era is 2.3 (1.1 to 3.3) W/m**2. It is “certain” that the total anthropogenic radiative forcing is positive, that is, warming. The total anthropogenic radiative forcing estimate for 2011 is 43% higher compared to the AR4 estimate for the year 2005.
- There is “very high confidence” that the effect of natural aerosols is a small fraction of the effect of anthropogenic sources during the Industrial Era except for brief periods following large volcanic eruptions. In particular, robust evidence from satellite observations of the solar irradiance and volcanic aerosols demonstrates a near-zero (â€“0.1 to +0.1 W/m**2 ) change in the effect of natural aerosols compared to the increase in anthropogenic radiative forcing (warming) of 1.0 (0.7 to 1.3) W/m**2 from 1980 to 2011.
Material in this article is from;