3.1 Climate processes; external and internal controls  (Page 4/19)

Changes in albedo can create a positive feedback    that reinforces a change in the climate. A positive feedback is a process which amplifies the effect of an initial change. If the climate cools, (the initial change), snow covers more of the surface of the land, and sea-ice covers more of the oceans. Because snow has a higher albedo than bare ground, and ice has a higher albedo than water, this initial cooling increases the amount of sunlight that is reflected back into space, cooling the Earth further (the amplification, or positive feedback). Compare the brightness of Figure Surface of Earth with Cloud Cover Removed with a similar photo montage from February (Figure Surface of the Earth in February with Cloud Cover Removed ): the extra snow has increased the Earth's albedo. Imagine what would happen if the Earth produced even more snow and ice as a result of this further cooling. The Earth would then reflect more sunlight into space, cooling the planet further and producing yet more snow. If such a loop continued for long enough, this process could result in the entire Earth being covered in ice! Such a feedback loop is known as the Snowball Earth hypothesis, and scientists have found much supporting geological evidence. The most recent period in Earth's history when this could have occurred was around 650 Million years ago. Positive feedbacks are often described as "runaway" processes; once they are begun they continue without stopping.

Albedo does not explain everything, however. The Earth and the Moon both receive the same amount of insolation. Although the Moon is only slightly more reflective than the Earth, it is much colder. The average temperature on Earth is 15 ^o C, while the Moon's average temperature is -23 ^o C. Why the difference? A planet's energy balance is also regulated by its atmosphere. A thick atmosphere can act to trap the energy from sunlight, preventing it from escaping directly into space. Earth has an atmosphere while the Moon does not. If the Earth did not have an atmosphere, it would have an average temperature of -18 ^o C; slightly warmer than the Moon since it has a lower albedo.

How does the atmosphere trap the energy from the Sun? Shouldn't the Earth's atmosphere reflect as much incoming radiation as it traps? It is true the atmosphere reflects incoming solar radiation—in fact, only around half the insolation that strikes the top of the atmosphere reaches the Earth's surface. The reason an atmosphere generally acts to warm a planet is that the nature of light radiation changes as it reaches the planet's surface. Atmospheres trap more light than they reflect.