Recent increase in warming found in deep oceans
Why the Globe Hasn’t Warmed Much for the Past Decade
Climatecentral.org, March 27, 2013
Even the quickest glance at a graph of global temperatures makes it clear that the planet was warming sharply during the 1980s and 1990s. But while the 2000s were the hottest decade on record, the rate of warming slowed considerably after the turn of the current century — even while human emissions of heat-trapping greenhouse gas emissions have continued to grow. The question that has lingered is where’s all the extra heat going?
The answer, according to a new paper in Geophysical Research Letters, is that a lot of it is being stored in the deep ocean, more than a half-mile down. “We normally think about global warming as what we experience on the Earth's surface,” said co-author Kevin Trenberth, of the National Center for Atmospheric Research, in an interview. If extra heat is temporarily stored elsewhere thanks to natural climate variations, we won't necessarily notice it.
But sooner or later it will inevitably emerge, which means that the current slowdown in warming may well be balanced by a period of rapid warming in a few years — nobody knows how many — from now. Scientists have always said that global warming would proceed in fits and starts, not in a smooth upward trend in temperatures. This study offers one specific explanation of why that happens.
The natural variation in this case appears to be changes in wind patterns associated with the Pacific Decadal Oscillation or PDO, a gradual see-sawing of ocean surface temperatures and wind patterns that goes through warm and cold phases lasting several decades. (The more familiar El Nino/La Nina oscillation, by contrast, see-saws every few years).
According to Trenberth and his colleagues, deep ocean temperatures began to rise significantly starting in about 2000, at about the same time as trade winds in the Pacific were changing in strength, in turn affecting ocean currents, all very plausibly as a result of a shift in the PDO.
Ordinarily, heat trapped by greenhouse gases would warm the ocean’s surface water, but since warm water floats on top of colder water, the heat would have a hard time percolating to the depths. “You need something to push it down,” Trenberth said. That something could easily be strong prevailing winds, which can literally stir things up — or in this case, down.
Nobody can actually see this process in action; instead, Trenberth and his colleagues used sophisticated ocean-circulation models and fed in observed data about sea-surface temperatures, winds, currents and even changes in sea level, all of which affect how heat moves around. In the end, changes in the wind turned out to have the most profound effect. It’s still a circumstantial case, but, said Trenberth, “we find it very plausible that this is a real effect.”
Adding to their confidence is the fact that a similar mechanism, only in reverse, explains why 1998 remains one of the hottest years on record. “You can point to the PDO, which took extra heat out of the ocean,” Trenberth said. That pushed global warming along faster than it would naturally have happened.
Indeed, Trenberth, speculates that the PDO could also explain why temperatures rose so quickly during the 1980s and 1990s. “You can argue that the PDO was pulling heat from the ocean during that time, which is just when global warming took off. So it may well be that this natural variability has been modulating the way we see global warming for decades.”
In other words, the PDO is affecting how the ocean takes in the extra heat from manmade global warming, and is helping to influence the rate at which the extra heat gets released back into the atmosphere as well.
If that’s the case, then global temperatures are poised for another rapid rise when the PDO see-saws out of its current phase and begins pulling heat back out of the ocean — something that’s inevitable sooner or later, although nobody knows precisely when it might happen. When it does, the question will no longer be where all the extra heat has gone, but where’s all the extra heat coming from.
But the answer is likely to be exactly the same.
Study: Global Warming Accelerated in Past 15 Years
By Dana Nuccitelli, skepticalscience.com, March 27, 2013
A new study of ocean warming has just been published in Geophysical Research Letters by Balmaseda, Trenberth and Kallen (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.
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
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 transferred 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 certaion 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 700m, contributing significantly to an acceleration of the warming trend.
The warming of the oceans below 700 meters has also been identified by Letivus et al. (2012) and Von Schuckmann & Le Traon (2011), for example.
Some ‘Missing Heat’ Found
Kevin Trenberth 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 Nina 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 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.