There's no shortage of researchers lining up to tell us that climate change poses a big threat to coastal cities and their populations - from Dhaka to New Orleans to Mombasa. At the recent launch of the U.N.'s State of the World's Cities 2008/2009 report, lead author Eduardo Lopez Moreno noted that 3,351 of the world's cities are located in what's known as the "low elevation coastal zone" - less than 10 metres above sea level.

"In case there is an increase in sea level, there will be a displaced population of more or less 400 million people," he told reporters. It's a jaw-droppingly big figure. But how accurate are such warnings, and how useful are they?

The UN-HABITAT tome also says sea levels rose by an estimated 17 centimetres last century, and "conservative global mean projections for sea level rise between 1990 and 2080 range from 22 centimetres to 34 centimetres".

That doesn't sound like very much, so does it mean we don't really need to do anything, or at least not for a couple more decades? And if the water goes up less than half a metre, will 400 million people actually end up being displaced?

Worryingly, no one seems to have a definitive answer to these questions. According to a report from the U.N. news service IRIN, nowhere is the debate more heated than in Bangladesh - often cited as a country under threat from rising seas.

Scientists and researchers have been thrashing out arguments in the nation's newspapers, with arguments not only about how fast ice caps and glaciers are melting - and what this would do to sea levels - but also whether new land created by silt deposits in the Bay of Bengal could offset the impact of a rising ocean.

IRIN quotes leading climate scientist James Hansen, whose paper on what target should be set for CO2 levels in the atmosphere sparked the Bangladesh controversy: "... it is quite possible that rising sea level will be matched or exceeded by rising silt levels, but when we hit the point of ice sheet disintegration there is no way to keep up with rising sea level. Of course, my hope is that we will not follow business-as-usual, in which case Bangladesh could indeed be in good shape."

Confused? Me too. Faced with the uncertainties of the predictions above, it's tempting to wait and see, to put off action until we know more, to hope for the best.

But for aid agencies trying to help people who can no longer grow crops because of persistent drought or whose homes and possessions have been swept away by floods, those simply aren't viable options.

'ANALYTICAL STONE WALL'

As highlighted in the latest issue of Forced Migration Review (FMR) - which focuses on climate change and displacement - humanitarian and development workers are under pressure to respond to the consequences of global warming without really knowing what they're up against.

The U.N. deputy high commissioner for refugees, Craig L Johnstone, describes the status quo in stark terms - arguing that we've hit an "analytical stone wall" and are in "desperate need of a better understanding of the size and the characteristics of this issue".

Oli Brown, a programme manager with the International Institute for Sustainable Development, notes in an article entitled "The numbers game" that even the generally accepted estimate of 200 million climate migrants by 2050 from Oxford University professor Norman Myers was calculated - by the academic's own admission - using some "heroic extrapolations".

"Intuitively we know that climate change migration is likely to be a serious issue in future. We just don't know how serious," Brown writes. "And it is hard to persuade policymakers of its importance without concrete (or at least more sophisticated) figures."

The article argues that getting a better handle on climate-related migration will require an effort to develop "objective and empirically-based detailed numerical scenarios". And to produce these, we need more advanced computer models, better base-line data and and increased capacity of institutions and governments to track the movement of forced migrants within and across national boundaries, Brown says.

But even if we did have a more accurate grasp of how many people were going to be displaced by climate change and when, it's pretty unclear how the international community would respond.

HOW TO RESPOND?

UNHCR's Johnstone says that while his agency has to be ready to assume its share of the responsibility, there's still a need to consider whether existing legal mechanisms coupled with careful coordination will be enough, or whether additional frameworks should be created.

Maria Stavropoulou, a consultant who has worked with the U.N. refugee agency, argues that most cases of enviromental migration could be dealt with under the current legal regimes for refugees and internally displaced people. Additional measures might be needed for those whose states could disappear as a result of climate change (for example, small island nations like Tuvalu or Kiribati) and to prevent the deportation of people from countries hit by a natural disaster who don't have refugee status, she says.

One of the reasons why many experts do not endorse a new climate-change refugee regime is the difficulty of identifying exactly why people move. "It will be generally be impossible to say whether a degradation in ecosystems leading to displacement has climate change as a major causative factor," Stavropoulou writes.

Arguably, this may be just the excuse governments are looking for to avoid stumping up cash to assist climate change migrants. UNHCR's Johnstone plainly states that "funding availability will be problematic".

Worse still he warns: "On the basis of our experiences to date, I predict that dealing with the resettlement of those who have been forcibly displaced by climate change will be a formidable, and possibly insurmountable, task."

Given this bleak picture, should humanitarians simply throw in the towel?

The answer from practitioners is that, despite the inadequacy of data and forecasts, they should get on with the job of helping vulnerable communities prepare better for rising weather disasters and addressing the different factors that might push them out of their homes, including climate change.

"While some improvements may be possible, the complexity of both climate modelling and social systems means that clear reliable projections of future trends in key areas such as migration, conflict, urbanisation and financial cost are impossible," write Jenty Kirsch-Wood and Anne-Marie Linde, officers at the U.N. Office for the Coordination of Humanitarian Affairs (OCHA), and Jacob Korreborg of the Danish Ministry of Climate and Energy. "We must act on a sound analysis of past trends - and on best guesses."

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