Differences in climate, such as the difference between now and the Mesozoic era, occur with the sun as an almost unchanging constant, and take hundreds of thousands if not many millions of years to take affect. They aren't the result of the sun changing in any way; they happen due to changes in the earth's tilt, orbital path, and atmospheric content. No natural phenomenon can account for the speed at which the climate is changing, and even if it were a natural phenomenon the sun would have little to do with it.