This is the 1494th most frequent Russian word.
обещают
"Обещают" means "they promise."
Here, 'обещают' is used to indicate a promise or commitment made by 'они' (they) regarding their arrival in the morning.
Они обещают приехать утром.
They promise to arrive in the morning.
In this sentence, 'обещают' represents the act of making a promise that is part of general human expectation.
Когда люди обещают, они должны держать слово.
When people promise, they should keep their word.
'Обещают' here showcases a declaration or assurance given by scientists about a potential development.