I don't find that sentence confusing, but I definitely found the article poorly written. Here's the sixth nearly identical map and now let's repeat the exact same thing already stated five times or more while including one new sentence of detail. The density of actual information in the writing is so low that it's hard to follow.

The article is so repetitive and padded out with empty prose that I found myself ignoring whole paragraphs. It's like the author decided to describe what an el niño is, then describe what an el niño is, then describe a super el niño, then describe what an el niño is, then describe what an el niño is, then describe what an el niño is, then describe the possible consequences of a super el niño, then describe what an el niño is, then describe what an el niño is, and finally describe what an el niño is.

The author is saying the surface of the ocean (where it's easiest to measure temps, so we do that the most) is the 'footprint' or the full colum of warm water below it. Additionally, like a footprint the surface area it represents might be smaller too as warm water continues to rise.

I stumbled across this too but going back to the picture made the sentence easy to understand. Maybe the sentence would have worked better if it was located before the picture. (The picture shows a ramp going up to the right deep under the ocean surface - the subsurface warm core)

Area vs volume.

If these are the temperatures on the surface, think of the depths. There is a whole additional dimension to multiply (integrate) across.

The added energy is far greater than you might think simply by looking at the surface.

To use an extremely inappropriate metaphor, they’re saying the warm temperatures at the surface are just the tip of the iceberg.

Yea, I will admit my imagination started to go off as I read that