This brightening of the Sun has now been observed twice by satellite, and all the evidence suggests that the rhythm is ancient and variable. It also led scientists to study how the variable brightness might affect the Earth. She and her colleagues seek out historical temperature records to probe the likelihood that fluctuations in solar brightness heat and cool the Earth directly. Others say this mechanism is stronger. When other factors were added to their research model, that figure rose to 94 percent.
Many experts say that is too high.
So investigators are looking elsewhere. One suspect mechanism focuses on sunlight in the ultraviolet range. Short, potent and invisible, these rays account for most of the brightness change and have significant effects on Earth in addition to direct heating. Dr. Joanna Haigh of Imperial College in London has found that increases in heat and ozone at solar maximums might account not only for temperature shifts but for changes in the paths of winter storms. Her study predicted that storms sweeping across the Mediterranean would move northward, on average, about 400 miles during a solar maximum, a shift that would be consistent with observed changes in real weather patterns.