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OTEC Can Be a Big Global Climate Influence

Professor James Moum, physical oceanography, Oregon State University, commenting in LiveScience on the recently published study in the journal Nature Recent global-warming hiatus tied to equatorial Pacific surface cooling by Yu Kosaka & Shang-Ping Xie said, “Scientists have known that the eastern equatorial Pacific Ocean takes in a significant amount of heat from the atmosphere, but this new study suggests this small portion of the world’s oceans could have a big influence on global climate.”

As shown in the following diagram, this is the same area, which covers only about 8 percent of the globe’s surface, with the greatest difference between surface water temperatures and those at a depth of 1000 meters and accordingly it is the best area for producing power by the process of ocean thermal energy conversion or (OTEC), which could replicate the surface cooling effect identified in the study that has caused the so called global warming hiatus of the past 15 years.

 

According to the forthcoming U.N. Intergovernmental Panel on Climate Change report, the measured rate of warming during the past 15 years was about 0.09°F per decade, which is a decline of over 40 percent from the 1901-2012 average which saw the planet warm by 1.6°F or .145°F per decade.

Since carbon dioxide concentrations in the atmosphere have increased from 370 ppm to 400 ppm during the same period, the so called global warming hiatus has been seized on by climate change skeptics as evidence the climate system is less sensitive to increasing amounts of greenhouse gases than previously was thought.

Xie said in the LiveScience piece, “In our model, we were able to show two forces: anthropogenic forces to raise global average temperature, and equatorial Pacific cooling, which tries to pull the temperature curve down, almost like in equilibrium,”

The effect is similar to the El Niño and La Niña cycles, which are parts of a natural oscillation in the ocean-atmosphere system that occur every three to four years, and can impact global weather and climate conditions, Xie explained.

El Niño is characterized by warmer-than-average temperatures in the waters of the equatorial Pacific Ocean, while La Niña typically features colder-than-average waters.

While global surface temperatures have not warmed significantly since 1998, other studies have shown that Earth’s climate system continues to warm, with emerging evidence indicating that the deep oceans may be taking up much of the extra heat.

The following diagrams is from a paper World ocean heat content and thermosteric sea level change (0 – 2000 m), 1955 – 2010 by S. Levitus et al.

The study estimates the 0 – 2000 meter layer of the World Oceans have warmed 0.09 C and if all of that heat was instantly transferred to the lower 10 km of the global atmosphere it would result in a volume mean warming of 36 C.

Conversely a significant amount of surface heat can be moved to the deeper ocean with OTEC without causing an undue increase in the temperature of the deep water.

Kevin Trenberth and colleagues at the National Center for Atmospheric Research reanalyzed ocean temperature records between 1958 and 2009 and found that about 30 percent of the extra heat has been absorbed by the oceans and mixed by winds and currents to a depth below about 2,300 feet.

Oceans are well-known to absorb more than 90 percent of the excess heat attributed to climate change, but its presence in the deep ocean “is fairly new, it is not there throughout the record,” Trenberth said during a teleconference with NBC reporters in April.

To find out why, Trenberth’s team used a model that accounts for variables including ocean temperature, surface evaporation, salinity, winds and currents, and tweaked the variables to determine what causes the warming at depth.

“It turns out there is a spectacular change in the surface winds which then get reflected in changing ocean currents that help to carry some of the warmer water down to this greater depth,” Trenberth said. “This is especially true in the tropical Pacific Ocean and subtropics.”

The change in winds and currents, he added, appears related to a pattern of climate variability called the Pacific Decadal Oscillation which in turn is related to the frequency and intensity of the El Niño/La Niña phenomenon, which impacts weather patterns around the world.

The oscillation shifted from a positive stage to a negative stage at the end of the extraordinarily large El Niño in 1997 and 1998. The negative stage of the oscillation is associated more with La Niñas, which is when the tropical Pacific Ocean is cooler and absorbs heat more readily, Trenberth explained.

“So, some of this heat may come back in the next El Niño event … but some of it is probably contributing to the warming of the overall planet, the warming of the oceans. … It means that the planet is really warming up faster than we might have otherwise expected,” he said.

Even with this slowed rate of warming, the first decade of the 21st century was still the warmest decade since instrumental records began in 1850.

Susan Solomon, a climate scientist at MIT, commenting on the Kosaka/Xie study said with respect to the prospect of less future warming due to lower climate sensitivity to greenhouse gases, “this is the least consistent prospect with observations, not just of the past decade, but the previous 40 years.”

OTEC uses the temperature difference between cooler deep and warmer surface ocean waters to run a heat engine and produce useful work, usually in the form of electricity.

It too can have a big influence on global climate because it converts part of the accumulating ocean heat to work and about twenty times more heat is moved to the depths in a similar fashion to how Trenberth suggests the global-warming hiatus has come about.

The more energy produced by OTEC – done properly the potential is 30 terawatts – the more the entire ocean will be cooled and that heat converted to work will not return as will be the case when the oceans stop soaking up global-warming’s excess.

Kevin Trenberth estimates the oceans will eat global warming for the next 20 years.

Asked if the oceans will come to our climate rescue he said, “That’s a good question, and the answer is maybe partly yes, but maybe partly no.” The oceans can at times soak up a lot of heat. Some goes into the deep oceans where it can stay for centuries. But heat absorbed closer to the surface can easily flow back into the air. That happened in 1998, which made it one of the hottest years on record. Since then, the ocean has mostly been back in one of its soaking-up modes.

“They probably can’t go for much longer than maybe 20 years, and what happens at the end of these hiatus periods, is suddenly there’s a big jump [in temperature] up to a whole new level and you never go back to that previous level again,” Trenberth says.

The bottom line is global-warming needs to be put on a permanent hiatus and the world needs more zero emissions energy.

OTEC provides both.

Content Discussion

Clifford Goudey's picture
Clifford Goudey on September 5, 2013

Jim, it’s an interesting idea and deserving of further examination.  However, the positives aren’t quite so clear to me.

You wrote, “Conversely a significant amount of surface heat can be moved to the deeper ocean with OTEC without causing an undue increase in the temperature of the deep water..”

That increase in deep-water temperature is predictable and therefore “due.” 

There are three unfortunate consequences of AGW: generally warmer climates, more weather variability and storm severity, and sea-level rise.  Of the three, the latter is going to cost us the most due to coastal inundation.  It is the increase in average temperature of the oceans that is causing the expansion. The redistribution of the local temperatures using OTEC will not ameliorate sea-level rise.

You also wrote, “The more energy produced by OTEC … the more the entire ocean will be cooled and that heat converted to work will not return as will be the case when the oceans stop soaking up global-warming’s excess.” 

I question that.  The extracted work will be dissipated in the form of heat into the atmosphere and can then return in the manner you’ve described.  However, aren’t those areas of warm surface water important in the re-radiating of heat back into space?  I’m not convinced that the shifting of this heat to greater depths is a winning strategy.  Logically, wouldn’t cooler surface waters in temperate regions be the place where atmospheric heat is absorbed?

I think we need to look elsewhere for the permanent hiatus we seek.

Clifford Goudey's picture
Clifford Goudey on September 5, 2013

I question that graph sourced from Berényi.  The thermal coefficient of expansion for sea water increases monotonically with increasing pressure.  See: http://publishing.cdlib.org/ucpressebooks/view?docId=kt167nb66r&chunk.id=d3_4_ch03&toc.id=ch03&toc.depth=1&brand=eschol&anchor.id=tab009#X  Therefore driving heat to depth is exactly the wrong thing to do.

Thermal expansion accounts for more than half of sea-level rise occurring in the 20th century, so I disagree with your dismissal of it as “very small” factor.  With an average ocean depth of 4,267 meters, it does not take much of a temperature rise to have a significant effect along the coast. That said, I am aware that land ice melting has recently become an increased factor.

The image you present on the delta-T of surface and 1,000-m depth is just that and we can not assume that it also represents where heat is migrating from the atmosphere to the ocean.  I believe that to be a more complicate matter relating to temperature differences, wind and currents.

I agree on the point of the Kosaka & Xie study, but for the reasons I’ve already pointed out, I see potential downsides.

Clifford Goudey's picture
Clifford Goudey on September 5, 2013

I’m not sure is it fair to look at the warm surface water that is cooled and moved to depth and not the cold deep water that is warmed and moved toward the surface.  I remain unconvinced.

Clifford Goudey's picture
Clifford Goudey on September 5, 2013

The point is, ultimately there are two discharge streams: the slightly cooled surface water and the slightly warmed surface water.  Both those streams are enormous relative to the power generated due to the low efficiency of the Rankine cycle operating at a low delta T. 

The volume change of each stream must be accounted for and they must both be discharged.  The net effect on the ocean’s volume is small but there is some heat removed in the form of work.  Ultimately this work is dumped into the atmosphere in the form of heat as the power is dissipated by the user.

Clifford Goudey's picture
Clifford Goudey on September 5, 2013

Clifford Goudey's picture
Clifford Goudey on September 5, 2013

Jim, you wrote, “Work is dumped into the atmosphere in the form of heat with the production and use of any energy.”  Almost.  The exceptions are wind, hydro, wave, and tidal or ocean current power.  In all four cases those resources are doomed to be dissipated in the form of heat.  If the wind, for example, is intercepted by a wind turbine, there is less wind remaining to be dissipated.  So the power gets dissipated as heat by the user somewhere else.  

Robert Bernal's picture
Robert Bernal on September 5, 2013

All generated energy turns back to heat, however, there can be differences due to escape back into space, in which case, (I believe) only CSP does. Imagine the few hundred thousand square miles or so of mirrors needed to power a future world economy. Some small percent should be reflected back into space, but this OTEC  idea got me wondering… if predomintly cold water is discharged at the surface, less clouds may form, thus less heat reflected to space. Which brings me to another conclusion. Warming air will become a detriment for solar because of the increased clouds. Wind will most probaly benefit (just store it in molten salts, too, at the efficiency loss of the steam or Brayton cycle).

Hopefully, this should be very trivial compared to the warming not displaced, otherwise., no matter the drawbacks. This is why we need to come up with the least espensive, most abundant, relatively safe (and robust) form of clean energy.

Clifford Goudey's picture
Clifford Goudey on September 6, 2013

Jim, expanding on what I explained above – due to friction, the wind and the ocean currents eventually get dissipated – ending up as heat.  A cascade of water converts potential energy to kinetic energy and then when it lands it gets converted to heat.  Waves encounter a shore and what isn’t reflected is converted to heat.  If any of these resources are intercepted by an energy conversion device, that flux otherwise destined to become heat is extracted as useful work.  All but wind power results in reducing the heat going into the ocean and therefore reduces sea-level rise associated with thermal expansion.  Similarly, wind turbines remove energy from the air that would ultimetely be dissipated as heat due to friction.

It is important to keep in mind that this removal is either temporary or, more likely, imediately converted to heat in the atmosphere as the energy is used.

Maybe we should be having this conversation over a beer.

Robert Bernal's picture
Robert Bernal on September 6, 2013

Yikes! Guess it would work for a while, being that cooler ocean surfaces would delay the effects of excess CO2, but eventually, that would come at the price of warming the interior, slightly. Methane hydrates at the sea floor needs to be kept cool, lest another ocean anoxic event is triggered.