Steve Connor: A worrying set of results for marine life – and for humans

The oceans are critical to our understanding of climate change because of the way they interact with the atmosphere. They absorb both heat and carbon dioxide and their moderating influence on global warming is undisputed – about half of the extra CO2 put into the atmosphere since industrial times has been absorbed by the oceans. It may come as a surprise to find that scientists are still trying to work out the scale of the temperature rise of the oceans in recent decades. But this is understandable given the size of the surface area covered by the oceans and the depths to which they extend.

Sea surface temperatures have been gathered over the decades by either throwing buckets over the side of ships, or by directly measuring the temperature of seawater drawn in through the engine intakes of ships. The discrepancy between these two methods has generated its own source of confusion over the accuracy of sea-surface temperatures.

The latest study concerns the temperature of the upper water level, down to about 700m. This temperature record is largely based on small devices originally developed by the military to assess sonar speeds. More recently, this method has been supplanted by more accurate devices that automatically sink and resurface.

It was crucial to know the depths of the two types of device when they recorded a temperature. Scientists also have to make sure all the devices are measuring temperatures and depths to the same scale, otherwise the recordings will be biased. The difficulty in making sure such recordings were accurate led to wider doubts about ocean temperatures.

Scientific observations on such a global scale in such an inhospitable terrain are not easy. There were bound to be doubts and discrepancies, but the latest study clears many up and has concluded that the oceans have warmed significantly over the past 20 years.

This has repercussions for rising sea levels, flow of ocean currents, and the continued existence of many forms of marine life. Coral reefs, for instance, are known to be sensitive to rising temperatures, as well as rising seawater acidity caused by the increasing amount of CO2 being absorbed by the oceans from the atmosphere.