Well, incoming solar radiation is reflected back out into space as well by the same processes.

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The Northern Lights happen when incoming solar radiation hits the earth's upper atmosphere and excites atoms to a new energy state, emitting colours which is energy in the form of light.

BBC: Aurora Borealis

Melting on the ice sheet's surface acts as a feedback mechanism, Dr van den Broeke explained, because the liquid water absorbs more and reflects less of the incoming solar radiation - resulting in a heating of the ice.

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Regardless of which theory is correct (and both might be wrong), the effect of fewer cosmic rays would be fewer clouds and since clouds tend to cool the earth by reflecting incoming solar radiation away from it a warmer planet.

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Other techniques aim to enhance marine cloud reflectivity by introducing sea salt aerosols in low clouds, mimic the effects of volcanic eruptions by injecting sulphate aerosols into the lower stratosphere, or place shields or deflectors in space to reduce the amount of incoming solar radiation.

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