Ocean heat flux and open ocean wind energy

I posted this on our professional web site (http://carnegieenergyinnovation.org), but thought it useful to repost here.

Anna Possner  and I recently published a study in the Proceedings of the National Academy of Sciences, titled “Geophysical potential for wind energy over the open oceans“.

It is well known that there is enough wind energy to power civilization,  and that wind speeds tend to be higher over the open oceans.  What wasn’t known was whether winds over the oceans tend to be stronger because the ocean presents a smooth surface without any mountains, trees, or houses to slow the flow, or whether there really is something special about the oceans that promotes stronger winds.

Anna performed a set of numerical climate model simulations to evaluate the maximum rate at which the atmosphere could transport kinetic energy (wind energy) downward to the surface, and how that varied from place to place and from season to season.

We noticed that there was a very strong correlation between the rate at which energy could be extracted near the surface (which becomes limited by the rate at which the atmosphere can transport energy downward) and the amount of heat  streaming out of the ocean into the atmosphere. The following figure embellishes Figure S10 from the Supporting Material for the paper mentioned above.

The land surface can store heat in the summer and release it in the winter but cannot transport heat from one place to another. In contrast, ocean currents can transport heat from low latitude to high latitude regions. Ocean heat transport can generate surface temperature contrasts (between land and sea and between different ocean areas). These temperature contrasts can contribute to unsettled and stormy activity in the atmosphere that can bring wind energy downward from the middle of the atmosphere towards the surface. Buoyancy forces may also play a role, with heat of the lower atmosphere inducing some rising motion that is compensated for by downward moving air with higher amounts of kinetic energy.

The specific mechanisms are yet to be worked out in detail, but the figure above shows a relatively tight and unexpected correlation between ocean-to-atmosphere heat fluxes and the maximum sustainable wind-energy extraction rates in those locations.

Anders Levermann  asked me about global warming. Global warming induces a heat flux from the atmosphere to the ocean and so tends to reduce net ocean-to-atmosphere heat fluxes. Furthermore, heating of the ocean surface and increased high-latitude precipitation tend to increase the vertical stability of the upper ocean, inhibiting vertical mixing, and thus reducing atmosphere-ocean heat exchange. These changes are likely to be small relative to the magnitudes present in the background state, but would indicate that while today there is a huge wind energy resource in some open ocean environments, it could be a little smaller in a global warming scenario.

The scientific novelty of our study is in showing that the ocean really is different from the land when it comes to wind power potential, and that difference is largely due to the fact that the ocean can transport heat but the land cannot.  There were several good press accounts of our work, and these press accounts emphasized the resource size. We appreciate the coverage we got and understand that science journalists need to focus on what will be most interesting to a broad audience and not necessarily on the contribution that will be most interesting to our scientific colleagues.

Some of the best journalistic accounts of our work can be found in these links (sorry if you are a journalist who did a great job but I didn’t see it or forgot to link to you here):

Chris Mooney, Washington Post
Bob Berwyn, InsideClimateNews
Eli Kintisch, Science

Saying “there is enough wind energy over the open oceans to power civilization” is a little like saying “there is enough solar energy over the Sahara desert to power civilization.” It is true but of little practical value if there are other ways to provide that energy that are much cheaper and easier, and perhaps with less adverse environmental consequence. Nevertheless, I think our study gives a green light to those developing floating wind farm technologies and suggests that they can focus on low-cost resolution of engineering challenges — and that they don’t need to worry about running out of resource.


Imagine it is year 2100 and the world is a good place to be.

To create something, we must visualize it first.

Every human artifact exists first in our minds and then only later in reality.

If we want a world where nearly everyone can experience a deep sense of well-being, and where nature can flourish, we need to visualize that world and attempt to create it.

Many people have been thinking about transition paths to better futures, and often these discussions get hung up in the difficulty of getting anything done in a political environment that is oriented to short-sighted self-service of the privileged few.

Much of the climate conversation seems to hover around doom and gloom, generating feelings of frustration and despair. This negative messaging is not getting us to where we want to be.

Another approach is to attempt to visualize a range of futures that we would like to live in (or like to see the world living in) and then try to understand which of these possible futures has a feasible path leading to it from where we are now.

What would have to become true in order to make each of those futures possible? How likely is it that each of those things would become true? If we construct many such lists for many such futures, we can ask what are the items that show up again-and-again on nearly every list, and therefore is a likely requirement for making good futures be feasible futures.

This idea of trying to visualize outcomes that we want and then figure out transition paths to those outcomes has become a major organizing theme for much of the work in my group. (See http://CarnegieEnergyInnovation.orghttp://CarnegieEnergyInnovation.org)

I recently tweeted “Imagine it is year 2100 and the poorest parts of the world are prosperous and carbon free. What had to be true to make that happen?” I was surprised by the amount of conversation that this tweet generated. It suggests people are ready for some positive messaging around the climate issue.

But positive messaging must be realistic. While it is important to visualize positive futures, we should not delude ourselves into thinking that those positive futures will be easy to create.