Leonard Ornstein has agreed to write a guest weblog on ocean heat content as a diagnostic to assess global warming. The focus of our discussions by e-mail has been on the meaning of the term “heat in the pipeline”. Len has provided a guest weblog previously; see “How To Quickly Lower Climate Risks, At ‘Tolerable’ Costs?”. [my reply will appear tomorrow].
GUEST WEBLOG BY LEONARD ORNSTEIN
Roger has invited me to post some thoughts I have on two points concerning ocean heat, about which we appear to have some small differences.
A. Roger often suggests that the trend in global ocean heat content (GOHC) is a ‘better’ diagnostic than the trend in global mean surface temperature (GMST) for assessing what’s happening to the earth as a result of human-induced changes in the planetary environment. It’s my judgement that both diagnostics are useful – but that at present – the potential uncertainties in GOHC exceed even the large uncertainties in GMST:
Local heat content of the ocean closely tracks (temperature (°K) x heat capacity x volume). The heat capacity of liquid ocean water varies only slightly with temperature and density/salinity. Deep ocean water, below the thermocline (DOW) makes up about 90% of the ocean volume, has a temperature of about 3°C (~ 276° K) and a salinity of about 3.5%. Therefore, although the temperature of the DOW is colder than most of the sea surface (e.g., sea surface temperatures, SSTs, range from about 18°C (~ 291°K) to 29°C (~ 302°K) from about ± 50° Latitude to the equator), it stores much more heat than the upper 200 meters or so of the thermocline. The low temperature of the DOW is generated and maintained by a quasi-steady-state process. Deep water formation originates near the two poles, by the downwelling of cold dense surface water to create and maintain the famous thermohaline circulation (THC). Most of the THC ultimately upwells in the southern oceans as the completion of the meridional overturning circulation (MOC), with a delay of more than a millennium The DOW is the most poorly explored and measured volume of the ocean. A small brief slowdown of MOC would not necessarily change the GOHC, but would tend to produce a decrease in global mean SSTs that could decrease the GMST (and visa versa). The causal connections might not be readily observed. Such phenomena as ENSO, AMO and PDO represent quasi-cyclic, moderately well-observed exchanges of near-surface waters with deeper waters. When a volume of water is moved downward, some water must move up ‘to maintain the level of the oceans’. It may move up over a very large area below the thermocline, and so be almost imperceptible, as a thermal signature, or it may appear as Ekman-pumped, wind-driven upwelling, in local coastal areas, like with La Nina. But at present, a significant portion of the heat in near-surface water could be advected into the DOW without being observed because of the sparse sampling at depths below 2 km (Argo buoys go no deeper than 2km).
The sampling situation is the reverse for the atmosphere. The heat capacity varies with density, and therefore barometric pressure (altitude). So near-surface temperatures track a major portion of the heat content of the atmosphere. Although we would appreciate even better sampling, the density of sampling of global surface temperatures is quite high and we’re observing a substantial portion of the atmosphere.
For this reason, I’m somewhat more wary of the amount of uncertainty that is associated with GOHC than is Roger.
B. James Hansen has pointed out that “even if atmospheric composition and other climate forcings remain fixed at today’s values….additional global warming of …. ~ 0.6° C is “in the pipeline” and will occur in the future”.
Hansen et al. (2005) Science 308:1431 – 1434.
Roger interprets Hansen’s use of “in the pipeline” as an error in physics, because once heat has been radiatively deposited in the ocean it is “there”.
For example, see Roger’s:
I believe Jim and his colleagues have made their meaning quite clear, and their concept in no way represents ‘wrong’ physics.
In the opening sentences of the 2005 paper they state:
“Earth’s climate system has considerable thermal inertia. This point is of critical importance to policy- and decision-makers who seek to mitigate the effects of undesirable anthropogenic climate change. The effect of the inertia is to delay Earth’s response to climate forcings, i.e., changes of the planet’s energy balance that tend to alter global temperature.”
I believe this clearly describes what he means later, by “heat in the pipeline”. On page 1432, under “Earth’s energy imbalance” he states:
“We infer from the consistency of observed and modeled planetary energy gains that the forcing still driving climate change, i.e., the forcing not yet responded to, averaged ~0.75 W/m2 in the past decade and was ~0.85 T 0.15 W/m2 in 2003 (Fig. 1C). This imbalance is consistent with the total forcing of ~1.8 W/m2 relative to that in 1880 and climate sensitivity of ~2/3ºC per W/m2. The observed 1880 to 2003 global warming is 0.6º to
0.7ºC (11, 22), which is the full response to nearly 1 W/m2 of forcing. Of the 1.8 W/m2 forcing, 0.85 W/m2 remains, i.e., additional global warming of 0.85 x 0.67 ~ 0.6ºC is “in the pipeline” and will occur in the future even if atmospheric composition and other climate forcings remain fixed at today’s values (3, 4, 23).”
By “the forcing not yet responded to” Hansen means not yet responded to by the atmosphere. Roger can fault him here for his semantics; had he added “by the atmosphere” I believe his meaning would be perfectly clear, given the opening paragraph. And it makes perfect sense with his closing “will occur in the future”, meaning after the ocean has equilibrated with the atmosphere.
Roger emailed me:
“Here is what Jim Hansen wrote in 2000 [http://www.pnas.org/cgi/content/abstract/97/18/9875
In the 2000 quote above, I believe that it is fairly clear that “observed ocean heat storage” refers to that ocean heat that has not yet equilibrated with the atmosphere. And “a planetary imbalance of the same magnitude” means the stored heat has yet to equilibrate with the atmosphere and land surface, thus the planet is in imbalance with respect to the distribution of heat – and therefore, temperature.
Jim has been consistent. He could have been a bit more precise with the inclusion of explicit referents, e.g., “by the atmosphere”.