Posted on 25 March 2013 by dana1981

A new study of ocean warming has just been published in Geophysical Research Letters by Balmaseda, Trenberth, and Källén (2013).  There are several important conclusions which can be drawn from this paper.

• Completely contrary to the popular contrarian myth, global warming has accelerated, with more overall global warming in the past 15 years than the prior 15 years.  This is because about 90% of overall global warming goes into heating the oceans, and the oceans have been warming dramatically.

• As suspected, much of the 'missing heat' Kevin Trenberth previously talked about has been found in the deep oceans.  Consistent with the results of Nuccitelli et al. (2012), this study finds that 30% of the ocean warming over the past decade has occurred in the deeper oceans below 700 meters, which they note is unprecedented over at least the past half century.

• Some recent studies have concluded based on the slowed global surface warming over the past decade that the sensitivity of the climate to the increased greenhouse effect is somewhat lower than the IPCC best estimate.  Those studies are fundamentally flawed because they do not account for the warming of the deep oceans.

• The slowed surface air warming over the past decade has lulled many people into a false and unwarranted sense of security.

The main results of the study are illustrated in its Figure 1.

Figure 1: Ocean Heat Content from 0 to 300 meters (grey), 700 m (blue), and total depth (violet) from ORAS4, as represented by its 5 ensemble members. The time series show monthly anomalies smoothed with a 12-month running mean, with respect to the 1958–1965 base period. Hatching extends over the range of the ensemble members and hence the spread gives a measure of the uncertainty as represented by ORAS4 (which does not cover all sources of uncertainty). The vertical colored bars indicate a two year interval following the volcanic eruptions with a 6 month lead (owing to the 12-month running mean), and the 1997–98 El Niño event again with 6 months on either side. On lower right, the linear slope for a set of global heating rates (W/m²) is given.

The Data

In this paper, the authors used ocean heat content data from the European Centre for Medium-Range Weather Forecasts' Ocean Reanalysis System 4 (ORAS4).  A ‘reanalysis’ is a climate or weather model simulation of the past that incorporates data from historical observations.  In the case of ORAS4, this includes ocean temperature measurements from bathythermographs and the Argo buoys, and other types of data like sea level and surface temperatures.  The ORAS4 data span from 1958 to the present, and have a high 1°x1° horizontal resolution, as well as 42 vertical layers.  As the authors describe the data set,

"ORAS4 has been produced by combining, every 10 days, the output of an ocean model forced by atmospheric reanalysis fluxes and quality controlled ocean observations."

Accelerated Global Warming

As illustrated in Figure 1 above, the study divides ocean warming into three layers for comparison – the uppermost 300 meters (grey), 700 meters (blue), and the full ocean depth (violet).  After each of the Mt. Agung, Chichón, and Pinatubo volcanic eruptions (which cause short-term cooling by blocking sunlight), a distinct ocean cooling event is observed in the data.  Additionally, after the very strong El Niño event of 1998, a cooling of the upper 300 and 700 meters of oceans is visible as a result of heat being transfered from the surface ocean to the atmosphere.

One of the clearest features in Figure 1 is the rapid warming of the oceans over the past decade.  As we have previously discussed, the warming of the shallower oceans has slowed since around 2003, which certain climate contrarians have cherrypicked to try and argue that global warming has slowed.  However, more heat accumulated in the deeper oceans below 700 meters during this period.  The authors describe the ocean warming since 1999 as,

"the most sustained warming trend in this record of OHC.  Indeed, recent warming rates of the waters below 700m appear to be unprecedented."

Their results in this respect are very similar the main conclusion of Nuccitelli et al. (2012), in which we noted that recently, warming of the oceans below 700 meters accounts for about 30% of overall ocean and global warming.  Likewise, this new study concludes,

"In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend."

The warming of the oceans below 700 meters has also been identified by Levitus et al. (2012) and Von Schuckmann & Le Traon (2011), for example.

Some 'Missing Heat' Found

Kevin Trenberth's past comments about 'missing heat' drew considerable attention.  The phrase refers to the fact that the heat accumulation on Earth since about 2004 (e.g. from warming oceans, air, and land, and melting ice) that instruments were able to measure could not account for the amount of global heat accumulation we expected to see, based on the energy imbalance caused by the increased greenhouse effect, as measured by satellites at the top of the Earth's atmosphere.

These new estimates of deeper ocean heat content go a long way towards resolving that 'missing heat' mystery.  There is still some discrepancy remaining, which could be due to errors in the satellite measurements, the ocean heat content measurements, or both.  But the discrepancy is now significantly smaller, and will be addressed in further detail in a follow-up paper by these scientists.

So what's causing this transfer of heat to the deeper ocean layers?  The authors suggest that it is a result of changes in winds related to the negative phase of the Pacific Decadal Oscillation and more frequent La Niña events.

Good News for Climate Sensitivity?  Probably Not

Recently there have been some studies and comments by a few climate scientists that based on the slowed global surface warming over the past decade, estimates of the Earth's overall equilibrium climate sensitivity (the total amount of global surface warming in response to the increased greenhouse effect from a doubling of atmospheric CO2, including amplifying and dampening feedbacks) may be a bit too high.  However, as we previously discussed, these studies and comments tend to neglect the warming of the deep oceans below 700 meters.

Does the warming of the deep ocean support these arguments for lower equilibrium climate sensitivity?  Probably not, as Trenberth explained (via personal communication),

"it contributes to the overall warming of the deep ocean that has to occur for the system to equilibrate.  It speeds that process up.  It means less short term warming at the surface but at the expense of a greater earlier long-term warming, and faster sea level rise."

So the slowed warming at the surface is only temporary, and consistent with the 'hiatus decades' described by Meehl et al. (2011).  The global warming end result will be the same, but the pattern of surface warming over time may be different than we expect.

The real problem is that in the meantime, we have allowed the temporarily slowed surface warming to lull us into a false sense of security, with many people wrongly believing global warming has paused when in reality it has accelerated.

Global Warming Wake Up Call

Perhaps the most important result of this paper is the confirmation that while many people wrongly believe global warming has stalled over the past 10–15 years, in reality that period is "the most sustained warming trend" in the past half century.  Global warming has not paused, it has accelerated.

The paper is also a significant step in resolving the 'missing heat' issue, and is a good illustration why arguments for somewhat lower climate sensitivity are fundamentally flawed if they fail to account for the warming of the oceans below 700 meters.

Most importantly, everybody (climate scientists and contrarians included) must learn to stop equating surface and shallow ocean warming with global warming.  In fact, as Roger Pielke Sr. has pointed out, "ocean heat content change [is] the most appropriate metric to diagnose global warming."  While he has focused on the shallow oceans, actually we need to measure global warming by accounting for all changes in global heat content, including the deeper oceans.  Otherwise we can easily fool ourselves into underestimating the danger of the climate problem we face.

Note: this post has been incorporated into the new rebuttal to the myth "No long tail means climate sensitivity is low", as well as "global warming stopped in [insert date]" and "Trenberth can't account for the lack of warming".

Also see this Carbon Brief post on the Balmaseda et al. paper.