global dimming

A bright world,
aerosols and the
hydrological cycle
Joaquim Ballabrera
Unitat de Tecnologia Marina, CSIC
30.000 barceloneses mueren al año
por la contaminación de la ciudad
El codirector del Centro de Investigación en
Epidemología Ambiental (CREAL) Jordi Sunyer
explicó hoy que en el área metropolitana de
Barcelona se registran anualmente 30.000
muertes relacionadas con la polución
atmosférica, en el marco de la Jornada Europea
de Ciudades y Regiones para la Mejora de la
Calidad del Aire que se celebra en Barcelona.
Published by La Razón on June 17, 2010.
The Keeling curve shows a steady increase
in mean atmospheric CO2 concentration
from about 315 ppm in 1958 to 385 ppm in
2008. The increase in atmospheric CO2 is
considered to be largely due to the
combustion of fossil fuels.
The rate of annual warming over the 1901–2000 period is estimated to
be 0.078C/decade. Warming is not continuous but occurs principally
over two periods (about 1920–45 and since 1975).
A warm atmosphere can hold much more water vapor than a cold one. The
water carrying capacity of the air is given by the vapor pressure dependence
on temperature (Clausius-Clapeyron). Other things being equal (winds,
cloudiness etc) we may expect an intensification of the water cycle of 7%
per degree of global warming. This means more intense droughts in some
regions, stronger rainfall in others.
In many ways, society is
more vulnerable to
changes in the water
cycle than to temperature
changes alone.
There was a widespread reduction in solar radiation at the Earth’s surface,
often referred to as global dimming, lasted from the mid-1950s until the
mid-1980s when a recovery, referred to as global brightening, started.
Temporal dynamics of warm season (May–
September) pan evaporation (mm) for China as
a whole, 1955–2000. The thick line represents
the pan evaporation trend with the rate (mm
decade1) shown in the lower right corner of the
chart. (Liu et al. 2004, JGR).
By increasing aerosol and cloud optical depth,
anthropogenic emissions of aerosols and their precursors
contribute to a reduction of solar radiation at the surface.
Couplings Between Changes in the Climate System and
Biogeochemistry, 4th Assessment Report of the IPCC panel
The net aerosol forcing over the 20th century from
inverse estimates based on the observed warming likely
ranges between –1.7 and –0.1 W m–2.
Understanding and Attributing Climate Change, 4th
Assessment Report of the IPCC panel (2007).
Sometimes called “global dimming”, this phenomena
has reversed since about 1990. Over the period 1984 to
2001, surface solar radiation has increased by about
0.16 W m–2 yr–1 on average (Pinker et al., 2005).
Records from six widely separated measurement sites show the reversal
from global dimming to global brightening occurred almost
simultaneously some 20 years ago. Why?
Besides the magnitude and scale of the observed changes, there are
further globally observed indications:
1. Different trends between nighttime and daytime temperatures [Wild et
al., 2007];
2. Observations of pan-evaporation (reduction of evaporation while the
world is warming up). Radiance, winds, convective reduction ?
3. Distinct patterns in ocean heat uptake (recent decreases since 2003).
All these phenomena are consistent with multidecadal variations in
surface solar radiation. These changes observed at the surface can
differ from radiative flux changes at the tropopause where forcings
are typically calculated.
Aerosol Climate Impact
• Aerosols scatter and
absorb shortwave solar
• Aerosols absorb and emit
thermal (longwave) radiation.
Aerosols modify the
microphysical properties
of clouds by acting as
cloud condensation
nuclei and ice nuclei.
Clouds intercept both heat from the sun and heat radiated from the Earth.
Their effects are complex and vary in time, location, and altitude. Usually
during the daytime the interception of sunlight predominates, giving a
cooling effect; however, at night the re-radiation of heat to the Earth slows
the Earth's heat loss.
Aerosol type
Atmospheric loading (1012 g)
Global average column
loading (g)
Mineral dust
Sea salts
Natural and anthropogenic
Anthropogenic biomass burning
Oxidation of naturally emitted
organic compounds
Natural and anthropogenic nitrates
Approx. annual rates of
sulfur transfer in Tg S
Dry Wet
224 (25 Anth)
Dry Wet
The major removal process for
sulphate aerosol rain, which is
very effective. Water vapor
residence time is about eight
days. In the case of the
sulphate aerosols in the
atmosphere, the residence
time is 5 days.
L.D. Danny Harvey, Global
Warming. The hard science,
Prentice Hall, London, 2000.
The 2001 study by researchers at the Scripps Institution of
Oceanography concluded that the imbalance between global dimming
and global warming at the surface leads to weaker turbulent heat fluxes
to the atmosphere. This means globally reduced evaporation and hence
precipitation occur in a dimmer and warmer world, which could ultimately
lead to a more humid atmosphere in which it rains less.
Total annual rain
Light rain events
Light rain events
Over the last 50 years, the southern part of eastern China has seen
increased amounts of total rainfall per year. The northern half has seen
less rain and more droughts. But light rainfall that sustains crops
has decreased everywhere.
Air pollution could
hamper our ability
to grow food.