DW: What do you think the future holds for small island nations now that US President Donald Trump has decided to pull the United States out of the Paris Agreement?

Professor Dr. Hans-Otto Pörtner: It will be an exacerbation of trends that are already in place, like the increase in sea level rise, the increase in temperature, and the impact on the sustainability of life on these island states. With the projections ahead of us, I'm afraid that some of them will not be sustainable, so people may have to abandon their territories and move to other places.

When the Paris Agreement was being negotiated in 2015 to prevent 2 degrees Celsius (3.6 degrees Fahrenheit) of warming since the industrial era, small island nations successfully campaigned for a stricter but secondary target to try to limit warming to 1.5 degrees Celsius. Since Trump's decision, is this target now completely unachievable?

Climate researcher Hans-Otto Pörtner

It's becoming more difficult to reach that target. We need a large scale transformation in society, industry and everyone using fossil fuels needs to find other ways of doing what they would like to do.

It's an ambitious target but it was within reach before. Now there's skepticism, but there is inertia to take action in many countries. The US certainly has been at the forefront with this surprising move to pull out. It was an expected move, but at the same time it's surprising that we still have individuals, and a fraction of the human population, who seem to think that ignoring climate change is an option. We really need to put measures into place to ensure we have a more sustainable future.

With Trump pulling out of the Paris Agreement, is there a chance that we can keep to the 2 degrees Celsius target if other countries still manage to drastically reduce their emissions?

It may well be that cities and states within the US will now be motivated by Trump's statements to take action. A number of places have already protested and said, "We want to continue our efforts to combat climate change. "At this level they may still reach the climate targets because all action is not necessarily at the level of federal government.

Local actors - the cities - are much more important because that's where people use the fossil fuels. They don't need to ask for permission from the federal government. So if states like California and cities like maybe New York continue to reduce emissions and put measures in place, it may well be that the US can bridge this period and still be on track.

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If the world remains on its current projected trends of temperature and sea level rises, how soon may small island states have to evacuate?

Think about the Maldives. Any of these low-lying islands that don't have natural elevations and are really close to the water line - clearly these would be under threat. A sea level rise by 1 meter (3.2 feet), which currently seems to be a conservative estimate by the end of the century, is already surpassing the limits of capacity for adaptation in many of these places.

It's not only that they are being flooded; it's also that salt water is entering the fresh water supply and agriculture will no longer be possible. There'll be no drinking water there. You can combat some of this with technology, but some of these countries are very poor, so it's a real challenge for them.

Some scientists talk about a 2 to 2.7-meter sea level rise by the end of the century. If we charge ahead with using fossil fuels and burning all of it, we even have the potential for a 65-meter sea level rise and this is something we do not want.

The precautionary measure is to take action and mitigate because it will otherwise get very difficult - we will get more heat waves and more damage to infrastructure. Those countries that have technologies will be able to adapt to some extent, but even in those cases there will be limits to adaptation.

Inhabitants of the Pacific island nation of Kiribati are on the frontline of sea level rise

If sea levels rise by currently estimated levels, could we then expect one of the largest migration crises of recent times?

Yes, I think the number of refugees is already higher than it has ever been before, and what is ahead of us under unabated climate change will exceed that by several dimensions. Some places on this planet will become uninhabitable, and this may concern the Arabic peninsula, areas in India, it may even concern some places around the Mediterranean.

Climate change will be very costly for those countries that can still pay for the damage. It will be a huge challenge to human civilization. It seems relatively clear that it doesn't cost the world to save the planet from the effects of climate change, but it will cost a lot to pay for all this damage ahead of us.

Professor Dr. Hans-Otto Pörtner, of German polar and marine research center the Alfred Wegener Institute, is co-chair of a working group for the United Nations' Intergovernmental Panel on Climate Change (IPCC) focusing on impacts, vulnerabilities and adaptation possibilities for humans under climate change.

This interview was conducted by Melanie Hall.