[mhh__]

It made sense to me?

Is it an obscure idiom (Native English speaker, so hard to tell)?

[zeeZ]

I initially parsed it as five summers, each lasting 500 years, in a time span of 15 years. Logic then kicked in and "corrected" it to five summers, each as summer-y as 500 years worth of summers combined, in a time span of 15 years.

I had to read the article to finally made sense of it.

[baal80spam]

I'm confused, how do you know the summers in question have been the hottest, not the coldest for example?

[happytoexplain]

Assumption - corroborated by reading the article. That's often unavoidable by the nature of headlines. The fact that the headline writer successfully communicated the correct assumption means they did their job.