But isn't Northern Europe too cold for bees?

Part of Northern Europe is too cold for bees, but not all of it. This map of the distribution of honeybees in Europe:

• https://cdn.shopify.com/s/files/1/1879/2549/files/EU_Apis_Mellifera_L_Map.svg_501e682f-379a-410d-9cc9-14e7159eb80d_grande.png

shows the regions in southern Norway and Sweden where beekeeping is fairly straightforward. Denmark, also part of the Viking world, is also bee-friendly. Today, beekeeping extends north of this range, but generally requires the hives to be moved. The limit is due to the temperatures, but more the impact of the temperature on the duration of the flowering season than on the bees directly - if the bees can't collect enough honey to survive the winter in a too-short flowering season, they won't survive the winter. (Hives can be moved from place to place, to take advantage of regional availability of flowers, allowing bees to be kept, e.g., in Finland.)

Between the Medieval Warm Period and today, we have the Little Ice Age, from about 1300 to 1850 (not an actual Ice Age, but it was cooler than before). During this time, bees were successfully kept in Sweden, and it would have been easier before then (especially during the Medieval Warm Period).

For a study of the impact of the Little Ice Age on Scandinavian beekeeping, see:

• Erik Husberg, "Beekeeping in Scandinavia during 17th and 18th Century", https://www.evacranetrust.org/page/beekeeping-in-scandinavia-during-17th-and-18th-century

In the mid-18th century, about 30,000 hives were counted in Sweden.

Where did they get honey? Was trade even possible when they raided such a big part of Europe?

Apart from producing their own honey, it was possible to import it. The southern coast of the Baltic was a major honey (and wax) producing region, and the Baltic readily allowed importation.

Even though Vikings raided, Vikings also traded - these are not mutually exclusive activities. Indeed, they can enhance each other. Trading provides intelligence for raiding, and successful raiding can provide a lot of wealth for trading. Traders often like successful pirates - they can be good rich customers, willing to pay well.

The Vikings also imported wine, and often appear to have considered it superior. Compared to wine, mead was a cheaper local product (cheaper, but far from low-class and unfit for the high!).

For a graph showing the change in temperatures from the Medieval Warm Period to the Little Ice Age, see the bottom graph, (c), in https://commons.wikimedia.org/wiki/File:IPCC_FAR_Figure_7.1_Global_Temperatures.png

This previous answer might answer some of your questions:

• "Mead was really a common drink on the Viking Age? What is the story behind mead?" Answered by /u/y_sengaku

Isn't Northern Europe too cold for bees? Where did they get honey?

I'm in Northern Europe and I'm currenly watching 5 or 6 bees on my flowerbed. In truth, the range of apis melifera, the Western Honey Bee, covers Denmark as well as southern regions of Sweden and Norway. I wrote an answer here about the availability of honey and the keeping of beehives in contemporary 11th Century England that you might find useful.

While it appears that beekeeping was relatively commonplace, mead itself is likely to have remained a fairly high-status drink; the common alcoholic beverage of choice would have been ale.