| Atmospheric aerosols, these tiny particles suspended in the air, play a very important role for
the Earth-atmosphere climate system on both global and regional scales through various
mechanisms and physical processes. The climatic effects of aerosols are determined by
modifications they induce on the various components of the Earth’s radiation budget. Despite
the progress that has been made lately, there is still much to learn about the climatic role of
aerosols in various aspects. One of the most important issues that has to be addressed is the
spatial and temporal variability, especially the temporal variability of aerosol properties and
their consequent radiative effects. For example, there is uncertainty with regard
to aerosol radiative properties and whether or not aerosol loads are increasing or
decreasing with time, and what the consequences are. Moreover, the extent to which
aerosols cool or warm the planet is not clear, as well as the contribution to this
cooling/warming by aerosols of natural and anthropogenic origin. Given that the
aerosol radiative effects, especially on radiation reaching the Earth’s surface and in
the atmosphere, cannot be directly measured/observed, models are necessary to
overcome this problem. Specifically, radiative transfer models (RTMs) are able
to calculate the radiation fluxes within the entire Earth-atmosphere system from
regional to planetary scale, and the flux changes caused by aerosols. Yet, what is more
interesting for models is that they allow us to study in detail the space and time resolved
aerosol radiative effects and their sensitivity to various physical parameters. Using
RTMs the aerosol direct effect on solar radiation can be determined at the top of the
atmosphere (DRETOA) in the atmosphere (DREatm) and at the Earth’s surface
(DREsurf).
Using a detailed radiative transfer model together with climatological input
data for surface and atmospheric variables, the direct radiative effects of aerosols
(DREs) were determined over the globe for a period of more than two decades,
covering the eighties (1980s), the nineties (1990s) and the early 20th century (2000s).
The climatological input data are representative for each region of the globe (2.5Ë
x2.5Ë latitude-longitude resolution) and for each month and year, and were taken
from various satellite databases (e.g. International Satellite Cloud Climatology
Project, ISCCP, Total Ozone Mapping Spectrometer, TOMS, Moderate Resolution
Imaging Spectroradiometer, MODIS) and reanalysis datasets (e.g. National Centers for
Environmental Prediction, NCEP, National Center for Atmospheric Research, NCAR).
The inter-annual variability of aerosol DREs was then estimated and subsequently
tendencies and trends of DREs were determined. These trends were considered
with regard to current discussions on global dimming and brightening (GDB) to
assess the effects of aerosols on these climatic processes. An attempt was made to
quantify the overall changes in surface solar radiation (i.e. GDB) to compare them
against quality surface measurements (Baseline Surface Radiation Network, BSRN,
Global Energy Balance Archive, GEBA) and to identify their causes. In addition, the
effect of aerosols on the thermal dynamics of the Earth-atmosphere system was
investigated and possible effects on clouds and precipitation were also examined. |