Abraham et al. (2013) explore the known unknowns in the oceans and global warming

Understanding how humans are changing the climate requires experts from many different areas. Physicists, chemists, engineers, mathematicians, biologists, atmospheric scientists, oceanographers, social scientists, the list goes on. Scientists studying the Earth's climate work out descriptions of how humans are interacting with the environment, how those interactions cause changes, and how measurements can be made.

The methods that have been developed to measure the Earth's climate include true engineering marvels. There are instruments on satellites that measure the rising sea levels and surface temperatures of oceans, land surfaces, and atmosphere. But satellite instruments can't see below the surface.

Perhaps the most important component of the Earth's climate, and perhaps the hardest to measure, is the oceans that cover over 70 percent of the Earth's surface. Over the past decades and even centuries, humans have used various techniques to measure oceans, from buckets that were dragged through the ocean waters to collect samples, to modern autonomous devices that measure the oceans day and night throughout the year and report data by satellite. A major new development since about 2005 is use of floats that pop up and down to sample the top 2000 meters of the ocean for temperature and salinity. These enable us to calculate the increase in heat and the changes to the acidity of the ocean waters.

It seems logical that throughout the decades, as our measurements have become more sophisticated, our understanding of the oceans has improved. That much is true. But, from a climate perspective, we must address how today's oceans differ from the oceans 10, 20, or 100 years ago. Sure, the oceans are warmer now because humans have loaded the atmosphere with heat-trapping gases, but how much warmer? How do we compare today's sophisticated measurements with yesteryears crude ones? For instance, if measurements in past decades were biased or their assessed depth was off, it could appear that the oceans have not warmed much in certain periods. Such errors would also have tremendous consequences for our predictions of what the climate will be like in the future.

This complicated topic is the subject of a recent paper my colleagues and I published in the journal Reviews of Geophysics. Nearly 30 of the world's top oceanographers collaborated on a massive study that not only went back through the history books to describe the evolution of ocean temperature measuring methods, but also looked forward to future measuring techniques.

The paper found that while all the evidence shows the Earth is warming, without pause, there are still unanswered questions and unmeasured parts of the oceans. Underneath ice sheets and deep in ocean basins are just two regions that need more attention. One of the world's pre-eminent oceanographers for, among other things, his important work measuring heat transferred to very deep ocean waters, is Dr. Gregory C. Johnson. Dr. Johnson works as an oceanographer at NOAA's Pacific Marine Environmental Laboratory in Seattle, Washington; he is also a co-author on the paper. He notes,

"This review points to the need to expand the innovative, year-round, broad-scale measurements of the upper half of the open ocean volume so successfully pioneered by the international Argo Program all the way down to the ocean floor and into the ice-covered polar regions, so we can make well-resolved, timely, and truly global assessments of the amount of heat being absorbed by the ocean."

In short, we are doing well, but we could do better with more deep-ocean measuring equipment.

Gregory Johnson, Photograph courtesy of NOAA
Gregory Johnson, Photograph courtesy of NOAA

A similar reaction comes from Dr. Kevin Trenberth, who not only is one of the world's top climate scientists, but is also recognized as a top communicator, winning the 2013 American Geophysical Union Climate Communication Award. Dr. Trenberth has been quite active in ocean heating studies, most recently publishing an important paper which calculated significant rates of heating in the ocean. He described this new study as,

"an excellent review of the history of ocean observations and very revealing about the problems, the issues, and the advances. Most people don't realize the state of the science of ocean observations and this paper is in that sense an expose."

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Posted by John Abraham on Thursday, 29 August, 2013

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