the fourth angel poured out his bowl on the sun
Again, we see similarities between the bowl and trumpet judgments. In the fourth trumpet judgment a third of the sun was struck (Rev. Rev. 8:12+). But the fourth trumpet also targeted the moon and stars, whereas the fourth bowl only affects the sun. This alternation in the suns normal operation will serve as one of many signs in the sun, in the moon, and in the stars during the time of the end (Luke Luke 21:25). See commentary on Revelation 6:12.
to scorch men with fire
To scorch is καυματίσαι [kaumatisai] , to Mat. 13:6).1 The intense heat they endure is an indication of judgment (Rev. Rev. 14:18+). Those John saw coming out of the Great Tribulation shall neither hunger anymore nor thirst anymore; the sun shall not strike them, nor any heat (Rev. Rev. 7:16+). Although this may refer to tribulation in general, the possibility exists that it may also speak about the relief provided to those saints who lived on the earth at the time of the suns increased intensity prior to their martyrdom.2 See commentary on Revelation 7:16.
One purpose in God striking the sun is found in the ages-long idolatry of men where they have worshiped the sun, moon, and stars rather than acknowledging their Creator. Thus, that which men have worshiped now becomes the source of their curse.3 Men are to be taught that the very things in which they have trusted or to which they have given their worship are to be the sources of their most terrible punishments.4
Another serious consequence of the suns intense heat will be the melting of the polar ice caps. The resulting rise in the oceans water level will inundate coastal regions, flooding areas miles inland with the noxious waters of the dead oceans. Widespread damage and loss of life will accompany that flooding, adding further to the unspeakable misery of the devastated planet. Transportation by sea will become impossible.5
1 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: University of Chicago Press, 2000), 425.
2 This plague will apparently affect the whole of mankind, including the saints alive at the time (Rev. Rev. 7:16+).Monty S. Mills, Revelations: An Exegetical Study of the Revelation to John (Dallas, TX: 3E Ministries, 1987), Rev. 16:8.
3 Concerning worship of heavenly bodies: Gen. Gen. 11:4; Deu. Deu. 4:19; Deu. 17:3; 2K. 2K. 17:16; 2K. 23:5, 2K. 23:11; 2Chr. 2Chr. 33:3; Job Job 31:26-28; Isa. Isa. 47:13; Jer. Jer. 8:2; Jer. 10:2; Jer. 19:13; Acts Acts 7:42; Rom. Rom. 1:25; Rev. Rev. 8:12+.
5 John MacArthur, Revelation 12-22 : The MacArthur New Testament Commentary (Chicago, IL: Moody Press, 2000), Rev. 16:8.