Which world issue is going to define the first half of the 21st century?

The 21st century began on January 1, 2000. Since the first half of the 21st century won’t end for another 30 years, it is quite difficult to determine which world issue will define the first half of the 21st century. The only way to accurately determine and assess the issue that would define this time frame would be to wait until after the first half of the 21st century.

At this point, it seems like it is too early since we are not even halfway through the first half of the 21st century. However, if I had to determine which world issue would define the first eighteen years of the 21st century, it would have to be terrorist violence because it seems to continue to occur and many countries have been fighting this for a long time.