Sea Level Change and Drowned Forests
Why Does the Sea Rise and Fall?
Sea level is not fixed. Over geological time, it has risen and fallen by hundreds of metres — and those changes have left striking evidence in the geological record. The primary control on global sea level is how much water is stored as ice. When continental ice sheets grow, they lock up vast amounts of water as solid ice, and sea levels around the world fall. When those ice sheets melt, the water returns to the oceans and sea levels rise. This is eustatic sea level change — a global shift driven by changes in ocean water volume.
Understanding the link between ice and sea level is central to this unit. It connects the icehouse and greenhouse climate states you studied in the previous lesson directly to the landscape evidence we can see on coastlines and on the seabed today. Exam questions often ask you to explain the mechanism behind sea level change, so mastering the cause-and-effect chain is essential.
During an icehouse period, continental ice sheets grow by accumulating snow and ice. This locks water away from the ocean, reducing its volume. Global sea levels therefore fall — by as much as 120 metres during the peak of the last glaciation about 20,000 years ago. During warming, ice sheets melt and return water to the ocean, causing sea levels to rise.
Drowned Forests: Evidence for Rising Seas
Drowned forests are one of the most dramatic pieces of evidence for post-glacial sea level rise. Around the coasts of Britain — at places such as Borth in Wales, Redcar in Yorkshire and various beaches in Cornwall — ancient tree stumps are exposed at low tide. These are the remains of forests that grew on dry land thousands of years ago, during and after the last glaciation when sea levels were much lower than today.
As the ice sheets of the last glaciation melted (beginning around 18,000 years ago), meltwater flooded back into the ocean and sea levels rose rapidly. The forests that had colonised low-lying coastal land were progressively submerged. Peat layers associated with the tree roots are often preserved beneath beach sediment, and radiocarbon dating of this peat confirms ages of roughly 4,000–8,000 years for many British drowned forests — showing that significant sea level rise occurred in the early Holocene after the main ice retreat.
During the peak of the last glaciation, sea levels were roughly 120 metres lower than today and a large area of dry land connected Britain to mainland Europe across what is now the southern North Sea. Geologists call this area Doggerland. It was a landscape of rivers, lakes and forests, inhabited by early humans. As the Pleistocene ice sheets melted during the Holocene, sea levels rose and Doggerland was progressively flooded. By about 8,500 years ago, Britain had become an island. Trawler nets in the North Sea occasionally dredge up worked flint tools and animal bones, confirming the former existence of this inhabited landscape beneath the sea.
The Scale of Ice: How Much Sea Level Change?
The size of an ice sheet determines how much sea level falls when it forms. The larger the ice extent, the more water is removed from the oceans. During the Last Glacial Maximum around 20,000 years ago, ice sheets covered much of North America, Scandinavia, and Britain as far south as Norfolk, the Midlands and south Wales. Global sea levels were approximately 120 metres below their current level. Conversely, if the current Antarctic ice sheet were to melt completely, sea levels would rise by approximately 58 metres — enough to flood all coastal cities on Earth.
At the peak of the last glaciation (approximately 20,000 years ago), global sea levels were about 120 metres lower than present. This was enough to expose a land bridge between Britain and Europe (Doggerland) and to leave drowned forests along Britain’s current coastline as evidence of the lower sea level.
Global Warming, Global Cooling, and Ice Extent
The chain of causation runs in both directions. Global warming reduces ice extent by increasing melt rates. Reduced ice extent means less water stored as ice, so sea levels rise. Global cooling has the opposite effect: more snow falls than melts, ice sheets grow, water is removed from the oceans, and sea levels fall.
This process operates on a range of timescales. Orbital cycles (changes in the shape of Earth’s orbit and the tilt of its axis) drive glacial–interglacial cycles over periods of 20,000–100,000 years. Over millions of years, atmospheric carbon dioxide levels — influenced by volcanic activity and the weathering of rocks — are the dominant control on global temperature and therefore on ice volume and sea level. Both mechanisms are explored in the following lessons.
Eustatic sea level change is a global process, but local sea level change is also influenced by isostasy. When a thick ice sheet sits on a continent, its weight depresses the crust downward. When the ice melts, the crust slowly rebounds upward — a process called post-glacial isostatic rebound. In Scotland, the crust is still rising by about 1–2 mm per year because of the unloading of Pleistocene ice. This means local relative sea level in Scotland is falling even as global eustatic sea level rises. Southern England, further from the centre of ice loading, is slowly tilting downward into the North Sea. Isostasy and eustasy act simultaneously and must be distinguished in exam answers about sea level change.
Reading Sea Level Change from the Rock Record
Past sea level changes are also recorded in sedimentary sequences. When sea level rises, marine sediments (muds, sands, limestones) are deposited over formerly terrestrial or coastal deposits. When sea level falls, erosion may occur and a gap in the record — an unconformity — can result. Sequences of alternating marine and terrestrial sediments indicate repeated sea level rises and falls, and are common throughout the British Palaeozoic and Mesozoic rock record.