Scaremongering about sea-level rise is an international pastime. Retired NASA scientist James Hanson appears to be leading the pack.

He has argued on "climate change" grounds that the global sea-level could plausibly rise by five metres this century. If so, a submerged Manhattan might become a modem-day Atlantis, a mecca for divers.

The UN International Protocol on climate change (IPCC) is widely depicted to be the authoritative voice of climate change scientists on sea-level rise and related matters.

IPCC's September 2013 report, Summary for Policymakers, assesses that the global average sea-level rose at an average annual rate of 1.7mm between 1901 and 2010. In 100 years that's a rise of only 0.17m.

Is Hanson a crank, a genius or both?

The IPCC does not predict future sea-level rise but it does make sea-level rise projections.

The September report projects global sea levels in 2081-2100 relative to 1986-2005 for four scenarios for future atmospheric greenhouse gas concentrations. The mid-points of the projected sea-level rises for these scenarios range from 0.41m to 0.64m.The extremes of the IPCC's "likely" range for these scenarios range from 0.26 metres to 0.98 metres.

What these projections are telling us is that the relevant lead authors think that if human emissions of these gases continue to increase sharply, the rate of sea-level rise will accelerate sharply, other things being equal.

Specifically, these scenarios all assume that the trend annual rate of sea-level rise has already accelerated to "about 3.7mm." For the scenario with the lowest projected sea-level rise to 2081-2100, this rises to a roughly steady rate of 4.5mm annually before 2050. However, the rate of increase keeps accelerating in the scenario with the highest projected rise reaching a stampeding rate of increase of 11mm a year by 2081-2100.

Yet, until there is unequivocal evidence of acceleration not due to natural variability, such projections remain speculative, if not heroic.

Perhaps unsurprisingly, the IPCC's politicised summary does not address the question of whether there is statistically significant evidence of a marked acceleration in the rate of sea-level rise in the observed historical record.

Instead, it seductively invites readers to assume a statistically significant acceleration by contrasting the long historical annual average of 1.7mm with higher annual averages for more recent periods, namely 2.0mm between 1971 and 2010 and 3.2mm between 1993 and 2010.

However, deep in the body of the main report the IPCC actually addresses the issue.

It reveals that in 2011, one study found zero acceleration in sea level rise between 1900 and 2010, and another found only a near-trivial rate of acceleration of 0.012mm a year. Both findings were statistically significant at the 90% confidence level.

I calculate that the latter rate of acceleration sustained for the next 100 years would produce a further sea-level rise of only 0.29 metres, starting from a trend rate of 2.3mm a year.

The IPCC explains that assumed thermal expansion of the ocean makes the largest contribution to the projected accelerations in sea-level rise. A supportive early 2013 posting on the IPCC-­related blog Real Climate shows fluctuations in the global average sea-level rise matched fluctuations in the global average temperature rise "rather well" between 1880 and 2000.

One difficulty is that this physical relationship implies the "hiatus" in global warming in the last 15 years should have been associated with a slow­down in trend sea-level rise from the historic average. This in itself suggests factors other than surface temperature rise might be behind the observed 3.2mm average annual rate of increase in the sea level in the 17 years from 1993-2010.

In short, ignore Hanson for now. Even the IPCC's acceleration in sea-level rise that remains speculative, statistically speaking.

To its credit, the IPCC does not pretend that its projections are predictions. It recognises that climate change scientists have no expertise per se in assessing how the distant future might unfold in respect of human emissions of greenhouse gases. Human inventiveness and behaviour in the form of mitigation are not readily explicable using the known laws of physics.

Yet, in New Zealand, the Ministry for the Environment is advising local governments to plan on the basis that the sea level in the 2090s will be 0.5m higher than in 1980-99, along with consideration of a 0.8m rise. It is even recommending assuming a rise of 10mm a year beyond 2100.

This comes as New Zealand's chief science adviser, Sir Peter Gluckman, has been calling for a greater weighting on objective evidence in policy formation.

Waiting for objective statistically-significant evidence of a marked acceleration in sea-level rise should not be too much to ask. It's not as if coast-loving property owners are facing an imminent, undetected risk.

Another policy consideration concerns the lack of a case for interfering in private risk-taking.

Some scientists and policy advisers seem to be fond of the vacuous "precautionary principle" mantra that uncertainty about science does not justify inaction. Of course not, but neither does it justify action.

Risk is a personal thing. The only rational approach to risk is to assess likelihoods, costs and benefits and one's own risk preferences.

Perhaps nervous investors should not buy beach houses - or invest in the share market. But risk was never a good policy reason for discouraging risk-takers from making risky investments using their own money.