Measuring the way temperature changes is relatively straightforward, using thermometers, and nowadays satellites, but it’s always been more difficult to work out what’s happening to sea level.

Apart from the problem of tides, which vary according to the moon’s position, there are storms and waves that must make it impossible to use a standard ruler-type measurement. Again it would help if there wasn’t high and low pressure in the atmosphere changing the water level on a daily basis.

Also, the land doesn’t stay still as it can be moving up and down – suddenly with earthquakes – but generally more slowly as it adjusts to the last Ice Age. Scotland is still rising now that the weight of the ice is not depressing the land, and the London area is sinking because of the sediment from the rivers deposited in the southern North Sea.

Despite these difficulties, tide gauges – wide tubes with a small hole below water level – work quite well, to an accuracy of 3mm, and this is now improved with modern satellite information. Regular readings allow us to work out the average sea level, and the worrying evidence is that it is rising.

The key is the increase in temperature caused by the extra CO2 in the atmosphere because of human activity, particularly in the last century, as this causes warmer water to expand, and land-based glaciers and ice caps to melt.

Most of the rise that has happened recently is due to expansion as the water warms up, deeper and deeper down. The rate might seem small – a thousand litres expands by only two litres when the temperature rises from 4degsC, when it is most dense, to 25C – but there is a prodigious volume of water that has warmed by more than a degree in the last half-century.

At the end of the Ice Age, some 15,000 years ago, the sea level was more than 100m lower than now, so it’s clear that if the Greenland Ice Cap, parts of Antarctica and the temperate glaciers melt, the rise could be substantial.

The expectation was that the rise would be 50cm by 2100, but recent evidence is that the melt is now faster and it could well be above a metre, and even as high as two. Indeed, the Dutch are planning for an above-a-metre rise by 2100 and 4m a century later.

The increase in closures of the Thames Barrage, from four times in the Eighties to more than 75 in the last decade, suggests that sea level is rising more rapidly, and this is serious news for heavily-populated deltas such as the Nile, the Mississippi and Bangladesh.