True Christians must affirm the full, without-error-ness of God’s Word, the Bible. If the words God gave to man have error, have falsehood, then God has falsehood. And this cannot be.
Because Christians must affirm that God’s Word is inerrant, without error in anything to which it speaks in the autographs, all Christians must affirm a global, not a regional, flood.
Here are 9 simple reasons why.
1. Genesis is a history book, not poetic or apocryphal. It recounts what happened in history. Quotations from individuals in history may contain other genres of communication. But the prose of Genesis is history, and to be read as such.
2. Genesis 6:13 “ And God said to Noah, “I have determined to make an end of all flesh, for the earth is filled with violence through them. Behold, I will destroy them with the earth.” (ESV)
God’s destructive target is not simply all men, but the earth itself. He intends to destroy the globe with his flood, a violent prefiguration of his final judgment.
3. Genesis 6:17 “ For behold, I will bring a flood of waters upon the earth to destroy all flesh in which is the breath of life under heaven. Everything that is on the earth shall die.” (ESV)
God will bring a flood of waters upon the earth to destroy all flesh. Moses specifies the “all flesh” with “in which is the breath of life”. And Moses specifies where this flesh dwells with “under heaven”. So if it is flesh with the breath of life under heaven, it will be destroyed by the flood from God.
4. Genesis 7:4 “ For in seven days I will send rain on the earth forty days and forty nights, and every living thing that I have made I will blot out from the face of the ground.” (ESV)
What will God do? He will blot out every living thing that he has made from the face of the ground. So if you are a living thing, made by God, and on the face of the ground, and you are not are on the ark, you will be blotted out by God by his flood. A regional flood cannot accomplish calamity and judgment of this scale. Only a global flood can do this.
5. Genesis 7:19-20 “ And the waters prevailed so mightily on the earth that all the high mountains under the whole heaven were covered.  The waters prevailed above the mountains, covering them fifteen cubits deep.” (ESV)
All the high mountains under the whole heaven were covered. And by how much water were these high mountains on earth covered? Twenty-two and a half feet! More than two basketball hoops high! Over Everest! Good luck containing that water to a region.
6. Genesis 7:21-23 “ And all flesh died that moved on the earth, birds, livestock, beasts, all swarming creatures that swarm on the earth, and all mankind.  Everything on the dry land in whose nostrils was the breath of life died.  He blotted out every living thing that was on the face of the ground, man and animals and creeping things and birds of the heavens. They were blotted out from the earth. Only Noah was left, and those who were with him in the ark.” (ESV)
Moses writes that all flesh died. And then he explicitly states what he means by all flesh. All flesh that moved on the earth, all birds, all livestock, all beasts, all swarming creatures that swarm on the earth, and all mankind. And then Moses repeats himself with added strength. Everything on the dry land in whose nostrils was the breath of life died. And then he repeats himself again, just in case you were wondering if he accidentally misspoke or overspoke. Was anyone left? Could Moses just have meant that a multitude of great magnitude perished? No, he meant everyone, everyone except Noah and those who were with him on the ark. The only place on the planet with land dwelling and air soaring survivors was the one boat.
7. Genesis 8:17; 9:1 “ Bring out with you every living thing that is with you of all flesh—birds and animals and every creeping thing that creeps on the earth—that they may swarm on the earth, and be fruitful and multiply on the earth… And God blessed Noah and his sons and said to them, “Be fruitful and multiply and fill the earth.” (ESV)
In these verses we are reminded of Genesis 1:28, where man is commanded by God to be fruitful and multiply and fill the earth and subdue it. This is a command with the entire globe in view. It is part of the covenant with all creation. And it is reiterated by God to Noah, the flood an act of God destroying the earth he had made and starting anew with humanity, Noah and his family the restarting family. The entire globe having been destroyed, they now have marching orders to fill and subdue and teem upon the surface of the entire globe. This is the post-fall reiteration of the covenant of creation, and it is creationwide.
8. Genesis 9:11 “ I establish my covenant with you, that never again shall all flesh be cut off by the waters of the flood, and never again shall there be a flood to destroy the earth.” (ESV)
Part of the Noahic covenant is God’s commitment to never again do what he did in the flood, namely, to cut off all flesh by the waters of the flood, to destroy the earth by a flood. This is what he had done; and this is what he committed to not repeat. Regional floods have occurred throughout history. Animals and men have been swallowed by flood and tsunami. But never again has the entire globe been covered by the waters of the deep and of the heavens. And it never shall. Fire, not water, is the future of the globe.
9. Genesis 9:13-17 “ I have set my bow in the cloud, and it shall be a sign of the covenant between me and the earth.  When I bring clouds over the earth and the bow is seen in the clouds,  I will remember my covenant that is between me and you and every living creature of all flesh. And the waters shall never again become a flood to destroy all flesh.  When the bow is in the clouds, I will see it and remember the everlasting covenant between God and every living creature of all flesh that is on the earth.”  God said to Noah, “This is the sign of the covenant that I have established between me and all flesh that is on the earth.” (ESV)
The sign of God’s covenant promise to Noah to never flood the whole earth again is the rainbow. God’s having acted against the entire planet, he sets his promisory sign around the whole planet. Rainbows are not like the northern lights, visible to only a region of the earth. They can be found wherever rain and clouds are found.