Tensions between Shia and Sunni Muslims are not the most serious threat to world security. The most serious threat to world security right now is the rise of communist nations, like North Korea and China, and the fact that they will not disclose how many dangerous things they have to other nations.
The centuries old sectarian conflict between the followers of Sunni Islam and Shia Islam is a cause for great concern, and a major factor in regional instability in the Muslim world, but the most serious threat to world security? Even more serious than climate change, nuclear war and nuclear meltdowns, economic crisis, and the growing gap between rich and poor? No.