Okay, first none of the projections, even business as usual, have all the ice in Antarctica melt even in the next few thousand years if we can stop increasing CO2 levels by 2100. So I’m going to assume in this answer that you want to know the worst possible scenarios for climate change, rather than the end result of all the ice in Antarctica melting, perhaps hundreds of thousands of years from now.

After all over those timescales it wouldn’t be a major issue to gradually move the population and who knows also what our technology will be capable of by then - we may be able to take the excess carbon dioxide out of the atmosphere and do many other things by then.

Also the melting ice sheet in Greenland is as much an issue as the Antarctica ice sheet over the next few centuries, so I’ll talk about them both.

SHORT SUMMARY

The ice in Greenland could melt completely over a thousand years with a 7 meters sea level rise. Both Greenland and the Western Antarctic ice sheet are losing ice at present and the rate of loss is accelerating. With business as usual, enough of the ice in Western Antarctica may melt by 2100 to cause a 10 foot rise in sea level. The rise would be uneven. Also the figures for 2100 could be underestimates. It’s hard to know because ice sheets melt in an uneven way, with sudden collapses leading to a large amount melting in one go rather than slowly and steadily, and the rate at which the ice is melting is increasing, which makes it hard to estimate exactly what will happen. The IPCC recommends that countries using their data recognize that the figures are conservative and allow for higher rises in sea level than their predictions.

The uneven sea level rises mean that some parts of the world will get much lower rises in sea level, or perhaps none at all, and others will get much more than the average. For instance Greenland ice attracts the sea towards it just by the gravitational effect of all that mass. That contributes a significant amount to the sea level as far south as the north of Scotland. So if the Greenland ice melts, then the overall sea level rises, but the sea level around Scotland doesn’t rise, because of the reduced gravity from the Greenland ice sheet. Instead some of the sea currently bunched up around Scotland and the northern Atlantic close to Greenland spreads out further south. But places like Florida and the Eastern coast of the USA then get a double whammy of a raised sea level due to more water in the seas from the melted ice, plus some of that extra sea level due to the reduced gravitational attraction of the Iceland ice sheet which causes sea currently humped up around Iceland to spread further away.

The local geology is also relevant. Netherlands can just increase the height of its dams. Other countries could protect themselves using dams and some regions though they would end up below sea level would not be flooded because there is higher land between them and the sea. But that only works if the geology is impervious to sea water.

So, Florida is one of the most impacted of all by a sea level increase, because it’s made up of a porous limestone rock. This makes it impossible to use dams like the ones in the Netherlands to keep the water out of Florida, because the sea can just seep through the rocks below the dams, making them useless. There isn’t any easy way to fix this. So even though it is a wealthy state that can easily afford technological solutions, there is nothing they can do with present day technology to keep the sea out. All they could do is to evacuate the flooded regions or raise the ground locally higher above sea level - or build Venice like cities on piles.

Some of the worst affected countries include Florida, the Bahamas and Bangladesh. 5% of the population of the Bahamas will be displaced by just a one meter increase in sea level, corresponding perhaps to a 1.5 or 2 C rise in temperature.

Coastal cities in the US like Miami and New York are also particularly vulnerable.

Then with “business as usual” low coral islands in the Western Pacific would disappear under the sea completely and their populations would need to be evacuated, so they are worst affected of all. The west coast of African and Eastern Australia would also be affected and some other places.

EVENTUAL RISE - SIX METERS?

The ice in Antarctica and Greenland is currently melting, so there’s no denying that the sea level will rise, the only question really is how high the sea level rise will be.

One paper suggested a 6 meter rise, though this is not the rise by 2100, but the eventual rise which might take anything between centuries and thousands of years. It did it by comparing the present with past warming episodes in previous interglacials.

With this study, cities in Japan would also be severely affected. According to one study, 18 million people in Tokyo and Osakai would find their homes are underwater with a rise in temperature of 2 C and 34 million people with a 4 C rise.

Miami, New Orleans and New York would also be affected with potentially 20 million people finding their homes below sea level eventually with a 4 C rise.

China is amongst the most affected of all with 134 million people who potentially may find their homes under water with a 4 C rise. That includes 44 million people in Shangai, Tianjin, Hong Kong and Taizhou.

India, Vietnam, Egypt, Brazil and the Philippines would also be affected with millions of people finding their homes are below sea level. For instance a significant part of the populations of Mumbai and Calcutta. You can find city by city projections here.

The big question is how long this would take. If it’s a thousand years, then that’s time to move the population of the cities or respond in other ways. If it’s a century or two, it’s a major upheaval. It is difficult though to model this exactly- as it depends on the behaviour of both the Greenland and Antarctic ice sheets, and the local effects, e.g. how much a particular country is affected, will depend on the gravitational effects of the remaining ice, which depends on which of those melts the most, as well as other effects such as the distribution of the sea over the globe changing as its temperature changes.

I’ve used material from different sources here. They don’t always say the same thing and as you’ll see there is a fair bit of uncertainty in the projections. That’s normal for cutting edge science which is what this is. The general trend is the same for all the sources, but they differ in detail - how much each ice sheet would be affected, and now much etc.

IN DETAIL: UNEVEN EFFECTS OF SEA LEVEL RISE

The IPCC has already said that the sea levels will rise by up to one meter by 2100 if we don't use stronger methods to limit greenhouse gases. These graphs are from their summary for policy makers from 2014.

There the RCPs are the Representative Concentration Pathways considered by the IPCC. RCP 8.5 is business as usual. RCP 2.6 has emissions peak before 2020 with an immediate and rapid decrease in emissions, roughly what the Paris agreement is attempting to achieve. RCP 5.6 has a peak around 2040 and RCP 6.0 has a peak at 2080.

This shows the temperature changes for the business as usual and the rapid reductions scenarios.

• RCP 8.5 is "business as usual" with emissions continuing to rise. Temperature rise by 2100: 2.6°C to 4.8°C, sea rise 0.45 to 0.82 m
• RCP 6.0 has emissions peak at 2080, temperature rise 1.4°C to 3.1°C
• RCP 4.5 has a peak around 2040, temperature rise 1.1°C to 2.6°C
• RCP 2.6 has them peak before 2020, temperatures: 0.3°C to 1.7°C. Sea level rise 0.2 to 0.6m

The temperature increases there are measured relative to the average for 1986–2005

There are many uncertainties there as you can see by the range of values with 2.2 °C difference between the projections for 2100. It's also possible that the temperatures are above or below those numbers. The same applies even more for the rising oceans. You might think that a sea level rise has to happen uniformly over the entire world, as after all the oceans are interconnected. But no, they don't rise uniformly. As an example, New York experiences a much higher sea level rise than normal while London will have a much lower sea level rise than normal for the same average sea level rise.

There are several effects here

• Parts of the Earth are still rising as a result of the ice sheets withdrawal at the end of the last ice age
• With a warmer ocean, the distribution of water across the globe changes
• The mass of ice in Greenland and Antarctica exerts a gravitational pull on the water closest to their coasts, raising the sea level there .As the ice melts then this gravitational pull is reduced, so the water previously piled up around Greenland and Antarctica gets redistributed over the globe.

You might wonder how something so small as an ice sheet can have enough gravitational pull to raise sea levels. But a mountain sized mass just one kilometer away from you has four times the pull as the same amount of matter two kilometers away and a ten thousandth of the pull as the same amount a hundred kilometers away.

So, although the way the maths works is that it adds up so that the mass of the entire Earth has the same effect on you as a point with the same mass as the Earth at its center, it is not all concentrated there at all of course. The matter that is close to you has most effect, counterbalanced by a tiny pull from large amounts of matter far away from you. So a small change in the mass distribution close to where you live can have a significant effect on the sea levels around the coast.

Of course any mountains close to the shore will exert a gravitational pull on the sea, but those are constant unchanging effects (except as a result of major volcanic eruptions). So,in normal situations, only the ice sheets, by melting, can change that pull.

They used two scenarios in their models. Their high end scenario contributes 30 cm of sea level rise by 2100 and their mid range scenario contributes 7 cm by then. The increase in the rate of ice loss per year (every year) for the two scenarios were 2.5 and 19.1 gigatons per year respectively. The rate of ice loss increased by on average 14.5 Gt per year each year over the period from 1992 to 2012 so at present we are running closer to the high end scenario than the low end one.

In this figure, from the paper describing this research in 2013, the green line outlines a region with a 0.5 meter sea level rise for the scenario of a 7 cm (MR) rise in average sea levels.

In this figure, the green line outlines a region with a 0.5 meter sea level rise, and the blue line outlines a region with a 1 meter sea level rise for the scenario of a 30 cm (HE) rise in average sea levels.

The result of this is that places like Scotland and Scandinavia may see almost zero change in sea level from ice melting, because they are influenced by the mass loss from Greenland. Only the thermal expansion of the oceans will matter there. While with the high end scenario for global warming, sea level rises could reach a meter in the Western Pacific. There, many people live on low islands made up of coral and may need evacuating. It also has severe impacts on some of the coastal regions of North and South America, the Caribbean, the West coast of Africa, Eastern Australia and some other places. In the Americas, the Bahamas are particularly affected as is Florida and many coastal cities. Discussion of this here: Ice Melt Means Uneven Sea Level Rise Around the World

One of the big uncertainty there is about the rate of melting of the ice sheets in Greenland and Antarctica. The rate at which they are melting has continued to increase, year after year, and the estimates depend on how much this rate continues to increase. More on that in the next section.

See the article in the Guardian about the guide to these scenarios

This again shows the uneven sea level rises for two of the scenarios:

Sea level rises for 2030, 2060 and 2090 for the scenarios RCP4.5 (middle range) and RCP8.5 ("business as usual"). The black lines are contours for the middle of the range sea level rise so mark the boundaries of areas of the sea that rise less than normal and areas that rise more than normal.

This map is not entirely consistent with the previous one. This is the more recent paper from 2016, so probably the more accurate. However it is also very low resolution.

LONGER TERM EFFECTS - COLLAPSE OF THE WESTERN ANTARCTIC ICE SHELF

But what happens further into the future in the future? Well eventually, with "business as usual", the Greenland ice sheet is likely to melt completely leading to a sea level rise of 7 meters average, and much higher in some parts of the world. The threshold for this to happen is probably less than 4 C relative to pre-industrial and it may happen even with a 1 C rise. But this is likely to take a thousand years (it takes a long time to melt all that ice). What about the nearer future, in the 22nd century?

In these new studies, a team of glaciologists using satellite and air measurements say that the ice in Western Antarctica has already started a process that is probably impossible to stop. With ice penetrating satellite radar mapping of the terrain beneath the ice (using the EU Sentinel 1 satellites), they say that here are no mountains or hills significant enough to slow the collapse. The fastest melting glacier, Smith glacier, is losing 70 meters thickness of ice a year. It's grounding line - the point at which it starts to float on the sea - is retreating two kilometers a year and has been doing that since 2011, is continuing unabated.

There are six glaciers that will collapse, enough to raise the sea level another four feet. But these may collapse other glacier leading to a rise of sea levels triple that. A separate team studying just one of the glaciers, Thwaite glacier, came to the same conclusion that collapse is inevitable. That is, will happen anyway, based on the CO2 emissions so far.

If so then this would cause a 10 foot rise in sea level. This would cause issues for coastal cities like New York and low lying countries like the Netherlands and Bangladesh which is the area in the world likely to be most affected by sea level rise since much of the country is not far above sea level.

They spotted a new rift which may lead for a large ice sheet to break off again, like the giant 225 square mile "iceberg" of 2015.

Rift in Pine Island Glacier ice shelf, Credit NASA/Nathan Kurtz.

They say there that many think it is inevitable that the Western Antarctic ice sheet will disappear. The main Antarctic ice sheet is still growing. Antarctica has been growing steadily all through the glacial and interglacial periods which is why we have these long ice cores to look at the temperature changes in Antarctica in exquisite detail. If we continue "business as usual" it will stop growing and eventually thousands of years into the future, melt completely. But that is not a risk at present. The risk is just from the Western ice sheet. But because the ice rests on land, rather than on the sea, adding this to the oceans will raise the height of the oceans, and they estimate by 10 feet so about 3 meters.

So what effect will it have if they are right? Not end of civilization. But some major issues. Florida is amongst the most affected since the underlying geology is porous limestone. This means it will be impossible to build conventional flood barriers as the sea will just percolate through the rock beneath them. So it seems inevitable that Florida will be flooded if sea levels rise. Only mitigation, such as evacuating the affected regions, or preventing climate change is possible as a way ahead.

Here it is as it is now.

Florida at current sea level (click to go to interactive map) - The areas shaded are not mapped, so ignore them.

Here it is at 3 feet

Florida at 3 feet - a level likely to be reached by 2100 with "business as usual" - shaded areas are not mapped

Florida at six feet, a level likely to be reached in the 22nd century if the West Antarctic ice continues to melt. Some think we may reach that level sooner, even by 2100, with "business as usual" Again, shaded areas not mapped.

Florida before and after a 3 meter sea level rise due to melting ice from the Antarctic and Greenland ice sheets. Image made using the Eustatic Sea Level Change tool from Virtual Earth System Laboratory (VESL).

Florida is already getting affected more than usual by hurricanes because of the one foot the sea has risen by so far. See Goodbye Miami for an article in Rolling Stone magazine about these issues.

However, remember that Florida is in an area that is much more affected by the global sea level rise than the average, so it may be flooded much more than this. The Bahamas are also impacted. Just a 1.5 or 2 C rise in temperature will already have a significant impact there. 5% of the population of the Bahamas would be displaced if the sea level rose by one meter.

SURGING SEAS RESULTS

Shows the effects on Japan of a 4 C rise (left) and 2 C rise (right), image from Climate Central

Thirty-four million people in Japan, 25 million the United States, 20 million in the Philippines, 19 million Egypt and 16 million in Brazil.

The worst affected cities in the US include Miami, New Orleans and New York.

145 million people in China live in cities that would be underwater with a 4 C rise in temperature.

Tokyo, Osaka, other mega-cities will be swamped by surging sea levels, even at 2 degree rise: study | The Japan Times

NEW YORK

This is the projection for New York city which experiences higher sea level rises than the global average:

• Projections for sea level rise in New York City are 11 to 21 inches by the 2050s, 18 to 39 inches by the 2080s, and the highest estimates (shown as + in the diagram above) could reach as high as 6 feet by 2100.

New York City Panel on Climate Change 2015 Report Executive Summary

Here is their detailed map of possible sea flooding zones for New York city by various dates from the 2020s to 2100.

SEA LEVEL MAPS

You can have an explore of the world to get a rough first idea of the effects of local sea level rises with this interactive map

Global sea level rise map. This map was created by Alex Tingle using Google maps and NASA elevation data.

This shows Florida, the Bahamas and Cuba after a 9 meter rise, which isn’t going to happen any time soon, like in the next century or two, but may be the very long term effect of all that ice melting.

It uses NASA altitude data which is approximate. It also just maps a new sea level against the topography of google maps. Places that are inland below sea level of course will not experience sea level rises even if shown flooded in the diagram (you need to trace out to see if there is a connection with the sea). But in the case of Florida, since the underlying rock is porous, then the map probably pretty much shows what the effect would be no matter what flood defenses are used.

Also this map doesn't take account of the uneven nature of sea level rise around the world. So you can’t just dial in the projected global sea level rise. You need to dial in your local sea level rise if you know what it is projected to be.

SURGING SEAS MAP

For a more accurately done map - but one that is slower to navigate unless you have a fast internet connection, see the Surging Seas. It takes account of local differences when working out sea level effects and also takes account of areas contiguous to the sea. To find out when your area will be affected by the various sea level rises, then click on Projections on the map and then choose the relevant scenario and it will show the date by which that sea level rise is projected to happen, if it happens before 2200.

Use this to get a better idea of what the effects would be. But for sea level rise especially, its estimates for when you will reach a particular sea level for the various scenarios may be an underestimate, as the IPCC projections are regarded as quite conservative.

NOAA MAP

The maps I did for Florida used the NOAA map, which is detailed and accurate but is only available for the USA. You can find it here, Sea Level Rise Viewer.

CONVERSION OF GLOBAL TO LOCAL SEA LEVEL RISES

One big problem I had with the material here on flooding, is that I can't find a good high resolution map to convert global sea level rises into local sea level rises. E.g. how much of a rise do you get at Florida for a global sea level rise of 1 meter? The sea level maps suggest anything between over 3 meters for the first map (if it scales up similarly to the 30 cm rise), and a little over 1 meter for the second one. The second one is published three years later but is very low resolution. Both indeed are so low resolution it is hard to be sure what the situation is for a small region such as Florida

The IPCC summary for policy makers from 2014. reports says

" Sea level rise will not be uniform across regions. By the end of the 21st century, it is very likely that sea level will rise in more than about 95% of the ocean area. About 70% of the coastlines worldwide are projected to experience a sea level change within ±20% of the global mean"

But it doesn't say how to identify the 30% of coastlines that have sea level rises more than 20% above or below the global sea level rise, or how to find out how much of a rise they get.

The Storm Surge site lets you show when the sea level rise you dial in is reached for a few selected points on the map according to the two scenarios they give but they don't say how to convert global to local sea level rises either.

Does anyone reading this know of anything more detailed we can use for this?

OTHER PLACES VULNERABLE TO SEA LEVEL RISES

World wide then naturally the Netherlands are amongst the most affected. As a rich country they would be able to increase the height of their flood defenses but it would be expensive. Other coastal areas in France, Belgium, Denmark and the UK (e.g. the Norfolk broads) would also be affected.

Here is the area as it is now:

And after a 3 meter rise. Remember only places that are shown as blue and connected to the sea would actually be flooded. Unless the geology is porous, inland areas below sea level would not be affected, and the Netherlands particularly can be expected to build better flood defenses, though a 3 meter increase in height of them would be an expensive undertaking. However I'm not sure what the local sea level rise would be there, for a 3 meter global rise.

Then finally, this shows the effect for Bangladesh of a 3 meter rise. This is likely to be an overwhelming humanitarian issue for a poor country with a huge population and they would surely need external help to deal with the issues. I'll do it as links to the images:

Bangladesh as it is now

And after a 3 meter sea level rise such as might happen by 2100 if the western Antarctic ice sheet melts

(I will see if I can get some better maps for Bangladesh and for the Netherlands area)

A ten foot rise in sea level will not mean the end of civilization at all, but it will lead to major problems for several particularly vulnerable spots world wide.

Even if we stop all CO2 releases, unless we actively remove it from the atmosphere, then it's going to continue to warm the oceans, and melt the ice long into the future. That's because of the long time that CO2 remains in the atmosphere, the yet slower response of the ocean levels to the warming Earth, and the even slower response of the melting ice sheets. The Greenland ice sheets are melting fast, losing 270 billion tons of ice a year. There is a lot of ice there. It should take thousands of years for it to all melt. But if it all melts then the sea levels will eventually rise by 7 meters.

That should be no problem, if it happens slowly, we have to abandon coastal cities such as New York, London, etc. Or we build massive flood barriers.

Or we actively remove CO2 from the atmosphere and so prevent the flooding over thousands of years timescales. Over such large timescales , we could do any of that.

NEED FOR POLITICAL DECISIVE ACTION

So, there is still a lot of uncertainty in the models, and they will continue to refine them surely for decades into the future. There are many details that they can model and look into. Also computers will continue to become more powerful, permitting more detailed models taking account of more and more effects. Although they think they have a reasonable understanding of what will happen, there is plenty of reason to continue to debate it and to work on the fine details of what is going to happen. How much will the temperature rise with business as usual? Or have we got it all wrong and it won't 'rise at all? Or what if it rises much more than expected?

At that point it becomes a political rather than a scientific decision. The scientists have done the best they can with their models, and continue to do so. They provide their best projections for the future, along with estimates of how certain they are. As is usual in science, then there are a few outliers in both directions, scientists that think the effects will be more than predicted and others who think they will be less than predicted. And sometimes the minority view in science turns out to be right. If we wait for certainty however, it will be too late to act, if the IPCC is right that we need to act swiftly to prevent the worst of the effects.

The politicians and the general public who vote for them then have to decide what to do with this information. In the past, this has been a matter for a great deal of political debate world wide. But now, in the rest of the world outside the US, that debate has already reached a conclusion, and we've moved on to action. There were many actions we could have done, but this was the decision embodied in the Paris agreement. While in the US politicians are still debating it, and indeed Trump has just left the Paris agreement, making the US the only country other than Syria and Nicaragua who are not party to the Paris agreement - and in the case of Nicaragua it has withdrawn as a symbolic protest because it thinks that the Paris agreement doesn’t go far enough. and is actually doing more than is needed for the Paris agreement in its actions against climate change. So the only country other than the US not in the agreement is Syria, and in the case of Syria it’s not that they object to the agreement particularly, it is rather that it’s understandable because of the chaos and war in their country that they haven’t managed to join it yet.

This image is being shared on social media as "Politicians Discussing Global Warming." - although that's not the original name of this work, originally called "Follow the Leaders" It was an installation by the sculptor Isaac Cordal in Berlin in 2011. The new title is more powerful though. At some point we have to stop debating and act. The rest of the world have decided they have had enough debate and it is time to act already. But US is still debating and has elected a climate skeptic president who has withdrawn from the Paris agreement.

There are many other effects of course other than flooding.

This answer uses material from my Order Patterned With Chaos - How Climate Is Predicted For Decades - With Exact Forecasts Only For Days - which I wrote in December 2016, so I’ve updated some of it for this answer.