Contrary to some views, there are very few matters within the science of global warming and climate change which are simple whatever side of the debate is considered. This is especially true of certain headline numbers that colour some commentaries.
Take the oceans as an example. The last ice age has been followed by several thousand years of very slow variation in sea levels, less than 2mm rises per decade. But over the last hundred years, changes in average sea levels have increased ten-fold to a steady annual rate of 2mm per year and a total of about 20cm since 1910.
More recent data since 1991 suggest this annual change may have lifted to 3mm per year, and it is argued that the steepening gradient of increase is evidence of the effects of global warming of the oceans arising from use of fossil fuels and the greenhouse effect caused by emissions.
The IPCC has modelled future scenarios which point to a further rise of approximately 40cm by the end of the 21st century albeit with wide margins of uncertainty. Some communities and seaside councils are planning for even more. The historical numbers are not too controversial but their interpretation and the driving mechanisms which inform the model forecasts are.
Firstly, let’s consider the measurement task. On average, the depth of the oceans is nearly 4000 metres, or 4 million mm. Measuring annual changes of 2 to 3mm in the position of the surface of the sea is quite a challenge especially as most water surfaces, affected by tides and waves, will not stay obligingly still during the process. The same can be said of newly recognised shifts in the sea floor.
The measurement accuracy needs to be within about 1mm – i.e. less than a part per million. Today’s techniques using satellite-borne laser altimeters and tidal gauges achieve such precision, and historical tidal and other data enable a reconstruction of trends over centuries. So, we know that sea levels are, on average, rising and that this could be a problem. Coastal communities may become at risk, damage from storm surges is amplified, coastal flooding and contamination of freshwater supplies may follow.
But some areas are rising faster than others, and some are even falling. Where continental data is available, such as for Australia and the US, there is considerable coastal variation caused by the movement and plasticity of the land masses themselves. And computer models struggle to explain such variations or to forecast future patterns confidently.
The interpretation of our oceans is no less complex than, indeed is coupled to, the understanding of our climate. No matter the simple headlines – the science is seriously complicated.
As our environment warms, the oceans heat up and thermal expansion follows producing rising sea levels. As mountain snow and ice, polar ice caps, and glaciers melt, the volume of our oceans increases as do sea levels.
But Antarctic ice, accounting for more than 80 per cent of the world’s land-based ice that affects sea levels, appears to be accumulating not melting. Also, continental plates flex – northwest Australia is bending downwards relative to southeast Australia. Plus, earthquakes produce unpredictable local variations in sea levels unrelated to global warming.