Climate change: Threshold for dangerous warming will likely be crossed between 2027-2042.Science Daily headline, 21 December 2020.
We commonly see this kind of misusage. It has become so standard that the correct form is seldom seen these days. However, this usage is incorrect.
Likely has the same -ly suffix as many adverbs, so it looks like an adverb. Although many words can exist as several parts of speech depending on context, likely is not one of them. Likely is strictly an adjective. However, in the above example it’s being used as an adverb. That’s why it’s incorrect.
Likely doesn’t actually have an adverbial form. If we want that, it has to be probably.
[The] Threshold for dangerous warming will probably be crossed between 2027-2042.
Likely is a perfect synonym for probable. It’s correct when used that way. For example, the following events are likely, or the most likely outcome will be … .
Using the wrong form of a word as above is a mistake, and makes the writer seem illiterate. Readers who know better may stop reading.