1.
You use ~ to indicate that the statement you are about to make is a consequence of what you have just said. (FORMAL)
The trade imbalance is likely to rise again in 1990. Hence a new set of policy actions will be required soon...
= therefore, thus
ADV: ADV cl/group
2.
You use ~ in expressions such as ‘several years ~’ or ‘six months ~’ to refer to a time in the future, especially a long time in the future. (FORMAL)
The gases that may be warming the planet will have their main effect many years ~.
ADV: amount ADV