The seventh seal that Jesus opens leads to the blowing of trumpets by seven angels. Blowing of trumpets was often a call to battle in the ancient world, and we can see from the details of what was signified by the trumpets that a battle, or series of battles, is about to take place between heaven and earth.
The seven trumpets divide into two sections. Trumpets one to four are described in 8:6-12. Then 8:13 contains an introduction to the remaining trumpets, and a lot more happens when they are blown than took place under trumpets one to four. Trumpets five, six and seven are each said to bring woe, and since trumpet seven is concerned with the Day of Judgement (11:15) we should not be surprised that it is connected to a woe.
The first four trumpets are concerned with judgements that affect everyday life. In the first trumpet, loss of trees affected produce and shelter in ancient times, and loss of grass affected pastures. The consequences of the second trumpet were destruction of ships, which would affect trade, and fish, which would affect food supplies. The third trumpet was connected to a basic need of humans, which is water supply, and when that is affected, the outcome is serious. Often confusion in political leaders was described by using cosmic disturbances, so the fourth trumpet points to confusion and upheaval in the ruling classes. We see here the consequences of rebelling against God and failing to recognise the authority of Jesus.
The first four trumpets describe the outpouring of various divine judgements on the earth, sea, creatures and cosmic bodies, with each of the judgements affecting one third. Although these judgements are serious and affect a lot of people, as well as other daily circumstances, they are not yet the full judgement that will occur when the seventh trumpet is blown. So these four trumpets are reminders that greater judgement is coming.
There seems to be a similarity between judgements connected to the four trumpets with the plagues that God sent on Egypt at the Exodus. Since the plagues were indicators that redemption for Israel was near, so John is being told that signs of divine judgement on the earth are indicators that eternal redemption is about to happen for those who follow Jesus.