Picture yourself on a beautiful beach, anywhere in the world. Your favourite beach, maybe. The waves are lapping on the shore, the Sun is sparkling over the water and there is a refreshing ocean breeze.

Now imagine this beach has gone forever. Sea level has risen and the shoreline has moved inland by hundreds of metres, drowning stretch after stretch of former coastline in the process.

It might be a struggle to envision dramatic transformations to such familiar places, but climate change scientists say there is overwhelming evidence that sea levels are indeed rising, and at a rapid rate. So how high will sea levels go? And at what cost to coastal communities?

It was in the early 20th Century that scientists first realised that sea levels were on the move.

In 1941 Beno Gutenberg, a geophysicist, analysed the data from tide gauges – instruments along coastlines that measure sea level – and noticed something odd. Over the period that reliable tide gauge data existed at the time – about 100 years – sea level was rising.

The melting of the world’s glaciers and ice sheets could release vast quantities of water into the oceans

Tide gauge data is now considered quite unreliable, but in 1993 NASA and the French space agency sent satellite-borne radar altimeters into space. Consequently we now have a much more accurate picture of sea level over the entire globe. These instruments confirmed that sea level is rising.

We now know that our warmer climate is driving the change. For instance, simple physics tells us that as water warms up, it expands.

“Thermal expansion through warmer ocean waters was the largest contributor to global sea level rise over the past century,” says John Krasting, a physical scientist at the US National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory.

This thermal expansion will continue, but there is another, more notorious problem that could lead to really dramatic changes in sea level in the future: the melting of the world’s glaciers and ice sheets could release vast quantities of water into the oceans. How bad could it get?

One way to answer these questions is to study past sea level changes.

The best estimates for mid-Pliocene sea levels range from 10 to 40m above present

Geologists can look at sedimentary rocks in order to locate past shorelines, effectively searching for fossilised beaches that will tell them what the height of the ocean was. Some scientists study clues to past sea level found in the shells of ancient organisms buried in ocean sediments and salt marshes.

One ancient time period of particular interest is the Pliocene – about 3 million years ago.

According to Maureen Raymo, a world-renowned climate scientist at the Lamont-Doherty Earth Observatory at Columbia University; “Temperatures in the Pliocene are estimated to have been 2 to 3C warmer than pre-industrial values, which means that they were 1 to 2C warmer than today.”

The temperatures in the Pliocene are similar to the 2C warming limit set by governments in Paris last year, making this time period very useful for understanding future sea level rise.

The last two decades have seen global sea level rise more than twice as fast as it did in the 20th Century

What is scary is that the best estimates for mid-Pliocene sea levels range from 10 to 40m above present. In other words, “the geologic record would say that this amount of warming would guarantee significant sea level rise”, says Raymo.

Back to the present. We have recently learned that it is not just the magnitude of sea level change we should worry about – it is the pace. A study published in March 2016 found that sea level rise in the 20th Century was faster than in any of the previous 27 centuries.

What is unique about this study is that the scientists brought together rigorous statistical techniques and much higher-resolution sea level records developed over the last decade to produce the first high-resolution global sea level database for the last 3,000 years.

Robert Kopp, a climate scientist at Rutgers University and the lead author of the study says; “This record shows us with 95% probability that it's been at least since 800 years BCE [or 2,800 years ago] that sea level has risen as fast as it did in the 20th Century.” And, he adds, “the last two decades have seen global sea level rise more than twice as fast as it did in the 20th Century”.

The study highlights the extreme sensitivity of sea level to even minor fluctuations in temperature.

“Fundamentally, this extraordinary rise in sea level goes hand in hand with an extraordinary rise in temperature,” says Kopp. “Physics tells us that global temperature change and global sea level change should go hand-in-hand. Our new record shows that they did so over the Common Era.”

We might see a total of 2m of sea level rise by 2100

Knowing that we are currently experiencing unprecedented sea level rise is helpful. But it does not tell us how high we might expect the oceans to rise in the future, which is vital information if we want to put in place appropriate coastal planning measures.

Another paper published at the same time looked at this second question. The authors used improved calculations to predict sea level rise and found that worldwide we can expect the oceans to be between 50cm and 130cm higher by the end of this century, if greenhouse gas emissions are not reduced rapidly.

This data also comes in line with the predictions made by the UN’s Intergovernmental Panel on Climate Change that sea level will rise by 50cm to 100cm by 2100.

There is a range in these predictions because a number of different emissions scenarios are used to make the calculations, and also because there are still uncertainties about when and how quickly ice sheets will melt.

Anders Levermann, Research Domain Co-Chair for Adaptation at the Potsdam Institute for Climate Impact Research and one of the authors of the paper, explains that computer models for the large ice sheets on Greenland and Antarctica have significantly improved, but large uncertainties remain: especially with respect to the iceberg discharge off Antarctica.

Global temperature change and global sea level change should go hand-in-hand

A study published in March 2016 emphasises how large those uncertainties are.

Two researchers considered how warming could potentially undermine the ice cliffs around Antarctica and speed up the rate of ice loss there. This process, they say, could see Antarctica alone contribute as much as 1m to sea level by the end of the century – suggesting we might see a total of 2m of sea level rise by 2100.

A sea level rise of 2m is alarming – but compared to the Pliocene, when sea levels are thought to have been up to 40m higher than now, it is perhaps a surprisingly small figure. The reason for the difference comes down to uncertainty when looking at the past and predicting the future.

Raymo explains that, up until now, thermal expansion and the melting of mountain glaciers have dominated the sea level rise. In future, the melting of Greenland and Antarctic ice will play an increasingly larger role, but these changes will unfold potentially with timescales of centuries.

So how high could sea level really get?

Sea level in the Atlantic is rising faster than the Pacific

“Theoretically, if all the ice on the planet melted, sea level would rise by about 55m,” Raymo explains – although she adds that this is unlikely to occur anytime soon.

“The last time this happened on Earth, about 40 million years ago, atmospheric [carbon dioxide] levels may have been as high as 1000 parts per million, according to the latest research.” Carbon dioxide levels are currently just over 400 ppm.

But even if the maximum rise in sea level this century is unlikely to be more than 2m from the global mean, that will still be enough to inundate many low-lying coastal areas, increase the dangers of flooding and displace millions of people, unless costly sea defences are built.

And there is something else that sea defence planners might want to keep in mind. Regional sea level changes can deviate from the global mean, so some places get it worse than others.

To slow down the rise in sea level, we need to stop the rise in temperature

This regional variability is illustrated by a 2016 paper that found variations in sea level rise between the Atlantic and the Pacific. John Krasting, the lead author of the study explains; “our modelling study finds that sea level in the Atlantic is rising faster than the Pacific under present day carbon emission rates.” As a result, Atlantic coastal towns may be more affected by sea level rise than Pacific ones.

Could we slow down sea level rise?

Kopp says that we could in principle, but only if governments and individuals begin to take action. “To slow down the rise in sea level, we need to stop the rise in temperature, which means we need to phase out the use of carbon-emitting energy technologies.”

Other scientists agree that this plan is our only viable option.

There have been some other ideas, though. One of these was to pump water from the ocean onto Antarctica so that it would freeze again.

Scientists at the Potsdam Institute for Climate Impact have investigated this eccentric geo-engineering idea in a study published in March 2016. They found that the pumped water would freeze to solid ice as hoped, but the weight it would add to the Antarctic ice sheet would probably speed up ice-flows into the ocean.

Whatever happens, greenhouse gas emission reductions are vital

To store the water as ice for a millennium would require more than one-tenth of the present annual global energy supply to balance the current rate of sea level rise. So perhaps it is not the best solution.

“If we continue to do business as usual and churn out emissions,” says Levermann, “not even such an immense macro-adaptation project as storing water on Antarctica would suffice to limit long-term sea level rise.”

So whatever happens, greenhouse gas emission reductions are vital if sea level rise is to be kept manageable. Additionally, substantial investment into local coastal protection will be required.

Without that sort of investment, we might see a gradual abandonment of coastal areas. This would lead to a huge degree of turmoil when you consider that 44% of the world's population lives within 150km of the coast.

For a sense of how traumatic such coastal abandonment might be, consider this. Humans faced a similar scenario at the very end of the last ice age, and it left such an impression that stories from those difficult times have echoed down the years to the present.

Britain... was once connected to mainland Europe until rising seas drowned those settlements

Sea level was about 120m lower at the peak of the last ice age, 20,000 years ago, when huge volumes of seawater were locked away in glaciers and ice sheets.

As the ice melted the seas rose rapidly again until about 8,000 years ago when sea level began to stabilise. By then shorelines had retreated over 100km in some places.

In Australian aboriginal culture there are flooding legends from those times that have been passed down from generation to generation. They tell of drowned shorelines and lost hunting grounds.

There is also a lost world just off the British coast that was home to thousands of Stone Age settlers. Doggerland was an area of low-lying land that now lies under the North Sea and English Channel. Britain has not always been an island; it was once connected to mainland Europe until rising seas drowned those settlements.

The unpopular truth is that human-induced climate change, and the rise in sea level it is causing, is now changing our coastlines again. The impacts will be felt for generations to come.