The Seven Bowls Judgment: 16:1-21.
The bowl judgments are similar to trumpet judgment but greater in intensity and resemble the plagues on Egypt. Chapter 15 was a prelude to this judgment. These seven judgments occur in rapid succession. John heard a mighty voice from the Temple saying to the seven angels, "Go your ways and pour out the bowls containing the wrath of God upon the earth". This is the final judgment of God. The trumpet judgment will affect only one third of the world but this would affect the whole world. So the first angel leaves heaven and pour out the wrath of God out of his bowl on the earth.
 
The first bowl: 16:1-2. 
When the angel pours out the first bowl on the earth, a horrible malignant sore breaks out on everyone who has received the mark of the beast (16:2). It is an incurable rotten sore similar to the boils inflected on the Egyptian (Exodus 9:9-11). The extent of this affliction is limited to those who have worshiped the beast.  Some scholars suggest that it is worse than cancer caused by germ warfare.

The second bowl:  16:3.
When the second bowl is poured out upon the sea, it becomes like the blood of a dead man. Everything in the sea will die.  Some suggest that 80% of the living organism is in the seas.  It is the sea that provides our proteins and minerals today.  50% of oxygen comes from the boiling of the sea water. The stench and disease that will come out of this judgment will be unimaginable. This plague resembles the first of the ten plagues of Egypt(Exodus 7:14-18).
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The third bowl:  16:4-7.
When the third angel pours out his bowl on the rivers, spring will turn to blood. This will contaminate the fresh drinking water supply.  John heard the angel saying “Even so, Lord God Almighty, true and righteous are your judgments (16:4-7). This will be an answer to the cry of the martyrs under the altar (6:9, 10). God says, Vengeance is mine, I will repay, says the Lord (Romans 12:19). Satan was spilling the blood of the saints during these years. Satan is getting what he deserves. As the tribulation draws to a close, God will deal in harsh judgment. 

The fourth bowl: 16:8-9.
When the angel pours out the fourth bowl on the sun, it will scorch everyone with its fire. This will be the real global warming experience. People will be exposed to solar radiation. This can cause sun-burns and cancer in the body. If there is a slight tilt in the earth axis and if the o zone barrier is depleted, it can happen. Some suggest that due to rationing of energy, there will be no air conditioning to provide any relief from scorching heat.

The fifth bowl:  16:10-11.
When the angel pours out the fifth bowl on the throne of the beast, his kingdom will plunge into darkness. This judgment may be poured out on the ten nations of the revived Roman Empire. Unlike other times, there is total darkness all over the world (Exodus 10:21-23). People will gnash their tongue in pain and  blaspheme the God of heaven for their pain and sores. Yet they refused to repent of their sins and turn to God (16: 11).  

The sixth bowl:  16:12-16.
When the sixth angel poured out his bowl on the great Euphrates River, it would be dried up so that the kings of the east could march their armies without hindrance. This river is 1800 miles long and too deep to cross.  Now the kings from the east and their armies could march towards Armageddon for the battle. John sees three frog-like spirits energizing the dragon, the beast and the false prophet for battle against God. They are demonic spirits capable of performing miracles. The armies of the nations will gather at the plain of Megiddo for the battle against the Lord. Here the Lord promises a special blessing on the saints who are found pure and waiting for His return (Revelation 16:15).

The seventh bowl: 16:17-21.
When the seventh angel poured out his bowl in the air, a mighty shout came from the throne in the Temple saying “It is finished” (16:17). The wrath of God is finished by this final judgment. There will be violent thundering, lightning and earthquakes. The great city of Babylon will split into three sections. And cities of many nations, including Jerusalem, will become heaps of rubble. Every  island disappears and all the mountains will be leveled. This may happen because of nuclear explosions. And great hailstones weighing about 100 pounds will fall on people from heaven. But people will not repent of their sins and give glory to God. Instead they will blaspheme the name of God and will harden their hearts because of the severity of the judgment. They will continue to worship the beast.  Through the outpouring of the judgments, God is also bringing the final salvation to His people.  So believers can find confidence and hope in God despite their trials and tribulations.