In the timeline on the page, it uses both uses - "Summer temperatures drop by 1.5°C to 2.5°C" and "Summer temperatures
drop again by 1.4°C–2.7°C in Europe".
It took a few reads and an internal debate over whether European summer temperatures could possibly have been 4°C in the 6th century to understand what the author meant.
For your interpretation: "drop by X, to Y"
For author's intention: "drop by X-Y"