Do you use the subjective verb in the conditional clause and the non-subjective verb in the main clause?

Question

The verb of the second sentence, have, is difficult for me to understand. Why isn't it the subjective form, would have? The preceding conditional clause If glaciers started reforming contains the past tense, started, which is the subjective form. Why not in the main clause?

Here’s an interesting thought. If glaciers started reforming, they have a great deal more water now to draw on — Hudson Bay, the Great Lakes, the hundreds of thousands of lakes of Canada, none of which existed to fuel the last ice sheet — so they would grow very much quicker. And if they did start to advance again, what exactly would we do? Blast them with TNT or maybe nuclear missiles? Well, doubtless we would, but consider this. In 1964, the largest earthquake ever recorded in North America rocked Alaska with 200,000 megatons of concentrated might, the equivalent of 2,000 nuclear bombs. Almost 3,000 miles away in Texas, water sloshed out of swimming pools. A street in Anchorage fell twenty feet. The quake devastated 24,000 square miles of wilderness, much of it glaciated. And what effect did all this might have on Alaska’s glaciers? None.

A Walk in the Woods: Rediscovering America on the Appalachian Trail By Bill Bryson

Answer 1

That glaciers have a great deal of water to draw on is not conditioned on them starting to reform. That's just a fact. The would you're expecting appears later: "... they would grow".

The clarity could be improved by removing so and inserting since:

Here’s an interesting thought. If glaciers started reforming, since they have a great deal more water now to draw on — Hudson Bay, the Great Lakes, the hundreds of thousands of lakes of Canada, none of which existed to fuel the last ice sheet— so they would grow very much quicker.

You could even reorder the clauses:

Here’s an interesting thought: since they have a great deal more water now to draw on — Hudson Bay, the Great Lakes, the hundreds of thousands of lakes of Canada, none of which existed to fuel the last ice sheet— if glaciers started reforming they would grow very much quicker.

or

Here’s an interesting thought: if glaciers started reforming they would grow very much quicker since they have a great deal more water now to draw on — Hudson Bay, the Great Lakes, the hundreds of thousands of lakes of Canada, none of which existed to fuel the last ice sheet.