Scientists’ best guess on sea level rise this century has increased considerably on its last projections in 2007. The Intergovernmental Panel on Climate Change (IPCC) now estimates seas will rise between 26 and 82 centimetres.

So what’s changed?

Best estimates

Between 1901 and 2010, global sea levels rose by 19 centimetres – an average of about 1.7 millimetres per year. But looking at the last few decades, it’s clear sea level rise is speeding up. Between 1993 and 2010, sea levels rose by 3.2 mm per year – almost twice the long term average.

Since the 1970s (at least), it’s very likely that there has been a human contribution to the rise in sea levels, the report states. ‘Very likely’ here means the IPCC considers there is a 90 per cent chance.

With greenhouse gas emissions set to continue rising, the IPCC projects more and faster sea level rise by the end of the 21st century. Exactly how much will depend on how we choose to address climate change, as the graph below shows.

The IPCC’s four new scenarios of future climate change, called the Representative Concentration Pathways (RCP), all assume some carbon cuts take place – with the exception of the highest emissions scenario RCP8.5.

Here’s the sea level rise projected by the end of the century (the period 2081-2100) for each scenario:

Adapted from Figure 13.10 – Chapter 13 of the IPCC’s Fifth Assessment Report.

If governments achieve drastic emissions cuts from 2020 onward (RCP2.6), sea levels are projected to rise by between 26 and 54 cm on 1986-2005 levels by the end of the century. The average within that range – shown as a line through the middle of the left-hand grey box – is 40cm.

Under scenarios where emissions stabilise by the end of the century (RCP4.5) or soon after (RCP6.0), sea levels are projected to rise by between 32 and 62 cm (47cm on average).

Under a scenario where emissions continue to rise rapidly (RCP8.5), sea levels are projected to rise by between 45 and 82 cm, or 62cm on average.

Sea level rise doesn’t happen uniformly around the world. But 70 per cent of coastlines worldwide will experience close to this global average, the report says. Some regions will experience considerably more or less, but overall, the report says, 95 per cent of the ocean’s area will experience sea level rise.

Where’s the rise coming from?

Working with those averages, the IPCC breaks down future sea level rise into its contributing factors. Here again are the four scenarios plotted by us, based on IPCC figures:

Image by Carbon Brief, created using data in Table 13.5 – Chapter 13 of the IPCC’s Fifth Assessment Report.

Under all scenarios, the expansion of water as it warms is the biggest cause of sea level rise – accounting for between 30 and 55 per cent of the increase. The oceans have absorbed more than 90 per cent of the heat trapped by greenhouse gas emissions since the 1970s. As temperatures rise, more heat will enter the sea, which means water will expand further.

Glacier melt is predicted to be the next biggest contributor to future sea level rise. Glacier melting is already speeding up and almost all the world’s glaciers are shrinking, says the IPCC. Under the highest emissions scenario, RCP8.5, ice loss from glaciers could drive sea levels up by as much as 16cm by the end of the century.

Melting of the Greenland ice sheet, including surface melt, is expected to be the next biggest driver of sea level rise. Like many other ice-covered regions, Greenland’s ice loss is speeding up. Between 1992 and 2001 the ice sheet was losing 34 billion tonnes of ice per year, but between 2002-2011 that increased six-fold to 215 billion tonnes per year. Under the highest emissions scenario RCP8.5, Greenland’s shrinking ice is expected to raise sea levels 12cm by the end of the century.

Projecting future ice loss from Antarctica is harder, as different regions of the ice sheet are changing in different ways. While solid ice is being lost from parts of the Antarctic Peninsula and in West Antarctica, the surface of the ice sheet is gaining mass as more snow falls on it. Overall, though, models predict it will cause sea levels to rise by about 5cm in most scenarios.

Higher sea levels are possible

Sea level rise of between 26 and 82 centimetres is the range the IPCC considers likely. The report doesn’t rule out the possibility of higher sea levels, but says there isn’t enough evidence to say what the chances are.

The likely estimates come from models that simulate all the known processes that contribute to sea level rise – and how those processes are likely to be affected if temperatures continue to rise.

Another set of models take a different approach – they look at how sea levels have changed In response to warming in the past and extend the relationship forward in time. These models give much higher projections for how much sea level rise we can expect by the end of the century – up to twice as large as the IPCC report states.

At the moment, however, there is very little agreement among this type of model. So while the new report discusses their estimates, scientists don’t consider them reliable enough to include in the overall assessment.

Why are the projections higher than last time?

The overall figures for sea level rise are considerably higher than in the last IPCC report, back in 2007. At that time, a different set of emissions scenarios projected sea levels would rise by between 18 and 59cm.

Back then, that amount of sea level rise was considered enough to put between 300 and 560 million people worldwide at risk of flooding.

Over the past six years, scientists’ ability to measure changes in ice loss has increased, particularly for glaciers and in Greenland. Data collected using satellites and field experiments has allowed scientists to identify that a much larger area is losing ice than was realised in 2007, says the new report.

There’s also been more research studying how ice sheets respond to a changing climate. That knowledge allowed scientists to improve climate models, to provide more realistic projections of future sea levels. Back in 2007, the IPCC was only able to include the loss of solid ice from Greenland and Antarctica in a very simplistic way. Now it uses models to simulate changes in the ice sheets.

In short, the science is more advanced this time around. However, caveats are important. Some of its ice sheet models are still pretty new, which introduces a degree of uncertainty into some parts of the projection.

But overall, the new projections should be considered pretty robust. Depending on what we do, between 26 and 82 centimetres of sea level rise is likely by the end of the century – a change to the planet that governments around the world will need to prepare for.