Changing Global Cloudiness

Clouds and Climate Change
Until recently, scientists did not know whether clouds had a net cooling or heating effect on global climate. Clouds reflect solar radiation, which tends to cool the climate, but they also help contain the energy that the Earth would otherwise emit to space, which tends to warm the climate. Measurements made in the 1980s by NASA's Earth Radiation Budget Experiment (ERBE) satellite demonstrated that clouds have a small net cooling effect on the current global climate (Figure 2). As human-induced processes (e.g., deforestation, release of greenhouse gases) increasingly affect our climate, it becomes more critical to gain a precise understanding of how climate variations can alter the physical and chemical processes that govern cloud formation and dissipation.

Cloud Forcing
Figure 2: Cloud Radiative Forcing determined from 1985 to 1986. Net cloud forcing is the result of two opposing effects: (1) greenhouse heating by clouds (or positive forcing) — clouds trap heat coming from Earth's surface that would otherwise be lost to space, and (2) cooling by clouds (or negative forcing) — clouds reflect incoming solar radiation back to space. The relatively large areas where cooling is the greatest are represented by colors that range from yellow to green to blue. In some areas, the effect of the clouds is to produce some warming as shown by colors that range from orange to red to pink. Overall, clouds have the effect of lessening the amount of heating that would otherwise be experienced at Earth's surface-a cooling effect. (Earth Radiation Budget Experiment data on the Earth Radiation Budget Satellite and the NOAA-9 satellite. Data processed at NASA Langley Research Center; image produced at the University of Washington).

We must also determine to what extent resulting changes in cloud patterns will affect climate as a result of their effects on Earth's "energy balance." If we measure the total amount of solar energy Earth receives from the sun, then we deduct from that number the total amount of sunlight reflected and the heat emitted from Earth, then we arrive at a number referred to as Earth's "energy budget." Because global measurements to date suggest that the amount of incoming radiant energy is roughly equal to the amount that is outgoing, we say there is an "energy balance." But a small percentage change in cloud cover could have a significant effect on Earth's energy balance.

A key to predicting climate change is to observe and understand the global distribution of clouds, their physical properties (such as thickness and droplet size), and their relationship to regional and global climate. Because clouds change rapidly over short time and space intervals, they are difficult to simulate in computer models and, therefore, their contribution to climate change is difficult to quantify. Improving our understanding of clouds and refining our predictive models of their behavior will also aid weather forecasting.

Current global change models are hampered by insufficient understanding of certain fundamental physical processes. NASA's Terra spacecraft will obtain a variety of precise measurements of cloud properties to feed improved climate models. Terra will yield improvements in our knowledge of cloud water droplet and ice crystal content, as well as how the feedback loops between Earth's air, land, and ocean work from day to day.

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Changing Global Cloudiness
How do Clouds Form?
Clouds and Climate Change
Surface-Based and Satellite Cloud Observations
Terra and Cloud Observations