The post Ocean heat storage: a particularly lousy policy target + Update at RealClimate finally presented a few realities of the global-warming metric known as ocean heat content—realities we have discussed numerous times. But they weren’t completely open about it and the other ocean temperature-related dataset, sea surface temperature.
That post by RealClimate founder Stefan Rahmstorf countered the 2014 comment Climate policy: Ditch the 2 °C warming goal by Victor and Fennel published in the journal Nature. Faced with the realities of the slowdown in surface temperature warming, Victor and Fennel proposed using a number of other metrics as indicators of global warming, including ocean heat content.
I’m not sure if Rahmstorf realizes what he has done. His post at RealClimate will be used enthusiastically by skeptics for years to come. Rahmstorf’s post will raise it’s lovely head every time alarmists, like those at SkepticalScience, attempt to use a continued rise in global ocean heat content to counter the continued divergence between climate models and surface temperatures. Example: The SkepticalScience post What has global warming done since 1998? can now be discarded.
Because Stefan Rahmstorf’s post is important I’ve archived it here.
A COUPLE OF INITIAL COMMENTS
1,) If you were to scroll through the comments on the RealClimate thread, you’ll find a few by Roger Pielke, Sr. Roger has been arguing for years that ocean heat content is the ideal metric for measuring global warming. I agree…ocean heat content would be the metric of choice IF (big if) the intents are to quantify how much heat is actually being stored in the oceans and then to compare those observations to theory and the outputs of climate models. But this is not a discussion of diagnosing global warming so that scientists can try to fix the flaws in climate models; this is a discussion of metrics that have value for policy decisions, and that, of course, makes the grand assumption that policies are required to combat surface warming, which many dispute.
We live on the surface; therefore, surface warming, or lack thereof, is the metric of choice. Land surface air temperatures mimic and exaggerate the surface temperatures of the oceans, not the oceans to depth. Evaporation from the oceans supplies the vast majority of the moisture in the atmosphere, and that evaporation takes place at the surface, not from the ocean depths.
2,) Yes, the heat content of the oceans does impact the rate of sea level rise, but even that impact is
miniscule minuscule. According to Levitus et al (2012), the trend of the thermosteric component of sea level rise is only 0.54 mm/year for the depths of 0-2000 meters for the period of 1955 to 2010. Then again, if the surfaces of the oceans and seas are going to rise to the levels reached during the last interglacial (period between ice ages), sea levels will be 6 to 9 meters (about 20 to 30 feet) than they are today. See:
- Kopp et al. (2013) Probabilistic assessment of sea level variations during the last interglacial stage, and
- Dutton & Lambeck (2012) Ice volume and sea level during the last interglacial.
No carbon dioxide policy is going to stop in the inland march of the oceans.
AN OCEAN WARMING OF 250000000000000000000000 JOULES “WOULD HAVE ESSENTIALLY ZERO IMPACT” ON THE SURFACE WHERE WE LIVE
We have presented and discussed the NODC’s vertically averaged temperature data for the depths of 0-2000 meters in numerous posts. We discussed how ocean heat content data is expressed in a very impressive term (10^22 Joules), but in terms we are more familiar, temperature, that warming is only measured in hundredths of a deg C. In the post Rough Estimate of the Annual Changes in Ocean Temperatures from 700 to 2000 Meters Based on NODC Data, we even presented the warming at the surface, at the depths of 0-700 meters, along with a rough estimate for the depths 700-2000 meters. See Figure 1, which was Figure 3 from that post.
Under the heading of “2. Ocean heat content has no direct relation to any impacts”, Stefan Rahmstorf writes (my boldface):
Ocean heat content has increased by about 2.5 X 1023 Joules since 1970 (IPCC AR5). What would be the impact of that? The answer is: it depends. If this heat were evenly distributed over the entire global ocean, water temperatures would have warmed on average by less than 0.05 °C (global ocean mass 1.4 × 1021 kg, heat capacity 4 J/gK). This tiny warming would have essentially zero impact.
That is, once the hypothetical heat from manmade global warming is absorbed by the oceans, it is no longer a concern to surface dwellers. Skeptics have been noting this for years. The addition “tiny warming” to ocean depths simply creates a tiny background warming that has “essentially zero impact” on the surface.
BUT THE OCEAN SURFACES ARE WARMING AT A MUCH HIGHER RATE
Stefan Rahmstorf continues under that heading (my boldface):
The only reason why ocean heat uptake does have an impact is the fact that it is highly concentrated at the surface, where the warming is therefore noticeable (see Fig. 1). Thus in terms of impacts the problem is surface warming – which is described much better by actually measuring surface temperatures rather than total ocean heat content. Surface warming has no simple relation to total heat uptake because that link is affected by ocean circulation and mixing changes. (By the way, neither has sea-level rise due to thermal expansion, because the thermal expansion coefficient is several times larger for warm surface waters than for the cold deep waters – again it is warming in the surface layers that counts, while the total ocean heat content tells us little about the amount of sea-level rise.)
(Note to Stefan Rahmstorf: Your Figure 1 is not about surface warming.)
But, what Stephan Rahmstorf has failed to present, the ocean surfaces are not cooperating with the climate models used by the IPCC for their 5th Assessment Report. Over the past 3+ decades, the surfaces of the global oceans have warmed at a rate that’s about half the rate simulated by climate models. See Figure 2.
That illustration was included as Figure 3 in the recent post On The Recent Record-High Global Sea Surface Temperatures – The Wheres and Whys.
Not too coincidentally…if memory serves, the observed ocean warming to depth is about half that predicted by climate models. That is, there is still a lot of missing theoretical heat. Maybe the feedbacks that drive the extra hypothetical warming of the oceans in climate models aren’t as positive as climate scientists believe. It will likely be decades, though, before the climate science community admits that.
PROBLEMS WITH MEASURING OCEAN HEAT CONTENT…AND THE SUBSURFACE TEMPERATURES OF THE OCEANS
We have presented the problems with ocean heat content data and with the reference subsurface temperature measurements in numerous posts in recent years. For examples, see:
- Is Ocean Heat Content Data All It’s Stacked Up to Be?
- AMAZING: The IPCC May Have Provided Realistic Presentations of Ocean Heat Content Source Data
And there are additional posts under the category Ocean Heat Content Problems.
Stephan Rahmstorf continued in his blog post under the heading of “3. Ocean heat content is difficult to measure.”
The reason is that you have to measure tiny temperature changes over a huge volume, rather than much larger changes just over a surface. Ocean heat content estimates have gone through a number of revisions, instrument calibration issues etc. If we were systematically off by just 0.05 °C throughout the oceans due to some instrument drift, the error would larger than the entire ocean heat uptake since 1970. If the surface measurements were off by 0.05 °C, this would be a negligible correction compared to the 0.7 °C surface warming observed since 1950.
As we’ve been saying for years, ocean heat content data (and the temperature readings to ocean depths upon which they are based) is a problematic dataset. Even during the ARGO era. See Paul Voosen’s October 2011 article Provoked scientists try to explain lag in global warming. It includes quotes from a handful of well-known climate scientists—including Kevin Trenberth. Voosen had this to say about Trenberth’s opinion of ARGO:
Trenberth questions whether the Argo measurements are mature enough to tell as definite a story as Hansen lays out. He has seen many discrepancies among analyses of the data, and there are still “issues of missing and erroneous data and calibration,” he said. The Argo floats are valuable, he added, but “they’re not there yet.”
And they still aren’t supporting hypothetical human-induced global warming. In an Open Letter to Kevin Trenberth – NCAR, I presented an updated graph of the NODC’s vertical mean temperature anomaly data for the Indian, Pacific, North Atlantic and South Atlantic Oceans, for the depths of 0-2000 meters, during the ARGO era (starting in 2003). See Figure 3 below (which was Figure 5 in the linked post). The flatness of the Pacific trend indicates there has not been a substantial increase in the subsurface temperatures of the Pacific Ocean as a whole to depths of 2000 meters over the past 11 years…same with the North Atlantic. Manmade greenhouse gases cannot explain the warming in the South Atlantic and Indian Oceans, when they obviously have had no impact on the warming of the North Atlantic and Pacific Oceans to 2000 meters over the past 11 years.
ARE YOU NEW TO DISCUSSIONS OF OCEAN WARMING?
For approaching 6 years, I have been presenting how ocean heat content data and satellite-era data sea surface temperature data indicate that naturally occurring, coupled ocean-atmosphere processes were responsible for their warming. See the illustrated essay The Manmade Global Warming Challenge (42mb). It’s free. And don’t let the “naturally occurring, coupled ocean-atmosphere processes” scare you. They’re not difficult to comprehend.