Answer to Question 1
Answer: Any two of the following:
- The number of days with frost has decreased over many parts of the midlatitude regions.
- There has been a decrease in the number of extreme cold events across much of the world, and extreme warm events have become more frequent.
- Snow cover has decreased in most areas (especially in the Northern Hemisphere), and those decreases have mostly been driven by increasing temperature. In places where snow cover increased, increasing precipitation (rather than cooling) was the cause.
- The breakup date for river and lake ice occurred earlier by an average of 6.5 days per century, and the freeze-up date occurred later by an average of 5.8 days per century.
- From 1901 to 2002 the maximum extent of seasonally frozen ground declined by about 7 percent in the Northern Hemisphere.
Answer to Question 2
Answer: Data for the last 12,000 years show a definite 1,500-year cycle in solar radiation that matches well with debris deposited on the floor of the North Atlantic by icebergs. This has led some researchers to conclude that solar forcing underlies at least part of the temperature changes observed in the Holocene.
The first dark spot observed on Uranus. Image obtained by the HST ACS in 2006.
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