The world’s oceans have absorbed more than 90 percent of global warming since the mid-20th century, making accurate measurements of deep ocean temperatures vital to predicting how much global temperatures and sea levels are going to rise.
In 1999, a group of 30 countries launched the Argo program as the first global, subsurface ocean observing system.
It will improve on the earlier patchwork of observations dating as far back as Britain’s HMS Challenger in 1873, which dropped a weighted thermometer overboard on a hemp line 8mm thick.
The Argo floats are rather more sophisticated with an inflatable chamber and a pump that changes the buoyancy of the float by changing its volume. It can sink from the surface to a depth of 2,000m and then resurface, measuring temperatures and salinity as it goes.
The data so far confirm a warming trend in the oceans over the past century.
Significantly, two recent publications suggest that the deep oceans have warmed particularly quickly in the past decade.
That is important because it may help explain a recent lull in surface warming and so confirm that greenhouse gas emissions are still warming the Earth as much as previously predicted.
There is consistent evidence that the world is absorbing more heat than it radiates back into space, mostly because of man-made greenhouse gases, which absorb infra-red radiation and prevent it from leaking back into space.
Measuring changes in ocean temperatures over time will help quantify that energy imbalance.
Argo data can also add detail to the expected impact on world temperatures from greenhouse gas emissions by helping unpick contributions from other sources such as volcanoes, solar cycles and other man-made pollution.
The initial goal of the Argo program was to obtain more than 100,000 profile observations per year, each from zero to 2000m in depth, from 3,300 floats spaced evenly across the globe.
The 30 countries achieved the 3000-float array in late 2007 and have maintained that number since then, according to the recent “State of the climate in 2012,” published in the Bulletin of the American Meteorological Society.
Each profile of ocean temperature and salinity is recorded in a 10-day cycle and subsequently transmitted via satellite.
There are some caveats regarding the results derived so far.
Argo is a recent dataset and so may not be able to distinguish longer natural cycles of ocean warming. And there are difficulties comparing observations before and after the Argo data because of differences in sampling method, instrument and geographical distribution.
In addition, there were initial problems with reliability of the Argo data, which meant accurate conclusions could only be drawn from 2005, and some instrument errors may remain undetected.
The Argo data still can help resolve the issue of a lull in the past decade of warming at the Earth’s surface, where measurements are taken using land-based weather stations, satellites and ocean buoys.
According to those surface temperatures, 11 of the 12 hottest years in the past 150 have been since 2000, NASA data show, illustrating a clear warming trend.
However, the warming trend on land has slowed in the past decade, while warming of the sea surface has stalled.
Warming also seems to have slowed in the ocean to depths of about 700m.