Then the fourth poured out his ceremonial bowl upon the sun: and it was given to it to scorch the people with fire. And the people were scorched with immense heat, and they blasphemed the name of God, the one having authority over these plagues, and they did not repent to give to him glory.
Revelation 16:8-9

The very first thing this passage made me think of was climate change and global warming. I am not saying this is a prophecy beginning to find its fulfillment in the effects of our own colossal mismanagement of our planet. But, you have to admit, it is a compelling corollary.
Note the response of the people. God has sent warning after warning. Now earth itself is dying, for as resilient as our world is, this depiction is of a dying planet. So far, all the water of earth has become either flowing or coagulated blood. There is not one, even one, life left in the seas, lakes, rivers, and streams. Whatever water the remainder of living things is getting is coming from some manufactured means.
And now the sun is flaring, with UV rays and scorching incalescence that sweeps the planet in a massive heatwave. Yet, those people who remain do not finally cry out to God for rescue.
Far from it.
They blaspheme God, instead. They go on a suicidal mission of refusing to acknowledge God’s sovereignty and glory, refusing to turn to the one being in all the universe who could help them.

Historicist
Scholars see this fourth bowl being poured out congruously with the second and third bowls, as all three represent a part of what happened to the Roman Church during the French Revolution.
Because the sun is often used as a metaphor for a great ruler, this must have to do with leaders of that time. Some understand John’s vision as a darkening of the sun, meaning a decline in power for kings and potentates in those nations which supported the Roman Church. Others focus on the German emperor as a prominent leader of that time whose influence and power were greatly reduced.
Still others look to Napoleon Bonaparte, whose conquering and colonizing of Europe was like a sun scorching all in its path. In a matter of eight years, Napoleon subdued and absorbed what was left of the Holy Roman Empire, so that it no longer existed as such.
Interestingly, Napoleon’s golden emblem visually evokes the brightness of the sun, with lightning bolts emanating from the eagle.

Preterist
Scholars note there is no evidence of a an unusual heatwave, or scorching sun, or any other natural phenomena that would match this fourth bowl. However, if the sun is taken as a metaphor for a mighty leader – political or religious – then a number of candidates come to mind.
Zealots
If the seven bowls describe the demise of Judea and any Jewish hegemony in Palestine, then perhaps the Zealots are in view. Once Jerusalem was under siege, several different militant sects sought to gain control of the city and therefore the resistance to Rome. In their efforts, they destroyed the caches of food of those they were endeavoring to dominate, to starve them into submission, and practiced other forms of tyranny on their fellow citizens.
In Book IV, Chapters 3 through 6, of his The Wars of the Jews: or History of the Destruction of Jerusalem, Josephus described the revolt of the Zealot factions against the process of electing Ananus ben Ananus to a provisional government soon after Vespasian laid siege to Jerusalem. The Zealots stormed the temple in protest and took possession of it, so Ananus sent his own soldiers to block all exits. Somehow, the Zealots managed to get word to the neighboring Idumeans pleading with them to prevent Ananus from handing Jerusalem over to Rome. The Idumeans came, freed the Zealots, and together, they massacred many Jerusalem citizens.
Perhaps this story could be applied to the fourth bowl, full of scorching.
Reversal of Blessing
Some see this bowl as a direct reversal of God’s protective presence during the Hebrews’ forty-year wilderness wandering, when God’s pillar of cloud and fire covered them from the sun’s searing heat and the night’s bone-chilling cold.
The Lord went in front of them in a pillar of cloud by day, to lead them along the way, and in a pillar of fire by night, to give them light, so that they might travel by day and by night. Neither the pillar of cloud by day nor the pillar of fire by night left its place in front of the people.
“You who live in the shelter of the Most High,
who abide in the shadow of the Almighty,
will say to the Lord, ‘My refuge and my fortress;
my God, in whom I trust.’”“You will not fear the terror of the night
Exodus 21-22; Psalm 91:1, 5-6 (NRSVUE)
or the arrow that flies by day
or the pestilence that stalks in darkness
or the destruction that wastes at noonday.”
When Moses gave his final will and testament to the people, he told them to permanently mark the land with God’s Law.
“Then Moses and the elders of Israel charged all the people as follows:
“Keep the entire commandment that I am commanding you today.
‘On the day that you cross over the Jordan into the land that the Lord your God is giving you, you shall set up large stones and cover them with plaster
‘You shall write on them all the words of this law when you have crossed over, to enter the land that the Lord your God is giving you …’”
Deuteronomy 21:1-3 (NRSVUE, modifications added)
One of the warnings embedded in God’s Law spoke of the very bowl that was now being poured out.
“‘The Lord will afflict you with consumption, fever, inflammation, with fiery heat and drought, and with blight and mildew; they shall pursue you until you perish.'”
Deuteronomy 28:22 (NRSVUE, emphases added)

More details Fourth Vial poured on the sun | By Anonymous – Public Domain
Futurist
Expositors point out a symmetry between the fourth trumpet and the fourth bowl. In the first, the sun, moon, and stars dim for a time. Now, the sun chars the earth. Symbolically, this represents the restored Roman Empire, now the sole world-wide government, ruling as Satan’s authority on earth. What was beneficial for humanity – governments and ruling bodies – now scalds and shrivels all it touches.
But a literal meaning could also be portrayed. There might be a supernatural event affecting the sun’s reach through space, or even a natural but prophetically uncanny solar flare which burns through the earth’s protective ozone layer with UV radiation.
A solar storm of significant magnitude actually occurred on September 2, 1859 which sent charged particles slicing into earth’s atmosphere, shorting out all the telegraph wires in the U.S. and Europe, as well as igniting numerous fires. A smaller storm in 1989 cut the power to one of Canada’s provinces.
One commentator warned that an unleashing of nuclear warheads would create the same disaster, releasing the equivalent of a sun’s burning rays at close range.


Spiritual
In a more general sense, perhaps this bowl is akin to a natural disaster, yet focused as judgment against the unrepentant in a similar way that the stars poetically fought against Sisera in Deborah’s song.
The martyrs, who had been given white robes and an invitation to rest, had also been given promises from God.
“They will no longer hunger, nor will they thirst any longer, nor may the sun fall upon them, nor any burning heat.
“For the Lamb up on the center of the throne will shepherd them and will guide them towards living fountains of water, and God will wipe away every tear from their eyes.”
Revelation 7:16-17
Those who receive this scorching heat were the ones who had put the martyrs to death. It should come as no surprise that they blaspheme God. The heat reveals the deepest recesses of their hearts and souls: they are sworn enemies of God and all that is holy.

The four perspectives taken from Revelation: Four Views A Parallel Commentary, edited by Steve Gregg