Clouds are regulators of the radiative heating of the planet. They
reflect incoming shortwave (SW) solar radiation, while absorbing the
longwave (LW) radiation emitted by the warmer earth, and emitting energy
to space at the colder temperatures of the cloud tops. Small changes in
the cloud-radiative forcing fields can play a significant role as a
climate feedback mechanism, yet the lack of quantitative estimates of
the global distributions of cloud-radiative forcing has limited our
understanding of the effect of clouds on the radiative balance of the
earth. The Earth Radiation Budget Experiment (ERBE) was launched in
1984 in an effort to gather quantitative data on the SW and LW
components of cloud forcing.
The ERBE includes three satellites in different orbits. Each satellite used a scanner to measure the reflected SW and the emitted LW fluxes at the top of the atmosphere. The difference between the incident flux and the reflected SW flux gives the solar energy absorbed by the surface-atmosphere column. The emitted LW flux is the energy radiated away by the column. The difference between the absorbed and emitted flux is the net radiative heating of the surface-atmosphere column. Data from the three satellites were combined to produce diurnal average fluxes on a daily basis, which were then averaged over the days in the month to produce monthly analysis data.
The combined effect of LW absorption and emission, the greenhouse effect, results in clouds overall reducing LW emissions to space. Results of the ERBE indicate that LW cloud forcing reaches peak values of 50 to 100 W/m2 over tropical regions, compared to 330 W/m2 for clear sky LW radiation emission. LW cloud forcing decreases towards the poles. Emission from optically thick high clouds reduce emission more than low clouds, causing cloud forcing to be highest in regions with extensive cirrus cloud decks.
ERBE data show SW cloud forcing peaking in the mid-latitudes. Cloud systems associated with the mid-latitude storm tracks and extensive stratus decks over the colder oceans are responsible for a reduction of absorbed solar radiation exceeding 100 W/m2 for these regions. In tropical regions, the LW and SW terms nearly cancel each other with net forcing averaging 10 W/m2. The worlds major deserts showed negligible SW forcing, while the snow-covered regions of the Arctic and the Antarctic had positive SW forcing terms, mostly because of the high albedos of these locations. The polar caps and the major deserts are the two brightest parts of the globe respectively, reflecting as much as 40 percent of the incident solar radiation.
When averaged over the globe, cloud forcing reduces the absorbed solar radiation by 44.5 W/m2, and the LW emissions to space by 31.3 W/m2, thus reducing the radiative heating of the planet by 13.2 W/m2. The size of the forcing suggests that the planet would be significantly warmer without the current cloud-radiative forcing. In fact, several global climate model studies have suggested that the negative cloud-radiative forcing is three to five times as large as the doubled carbon dioxide forcing.
In summary, the cloud forcing concept contributes to our understanding of climate and climate change. A change in climate can perturb the cloud forcing, which in turn can feed back into the initial climate change. Thus, observations of long-term changes in cloud forcing can provide insights into the nature of the cloud-climate feedback. We must be able to predict changes in cloud forcing if we are to predict the total response of climate to various perturbations.
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