A new analysis of top-to-bottom (0-5000 m) ocean heat content changes since the mid-1990s reveals that (a) large regions of the global ocean have undergone cooling, and (b) the overall net temperature change for 1994-2013 was a modest 0.02°C. In contrast, during the Holocene the oceans naturally warmed at a rate and magnitude several times greater than the last few decades, undermining claims that the modern era change is unusual or unprecedented.
Image Source: Wunsch, 2018
Global Warming = Ocean Warming
According to the United Nation’s Intergovernmental Panel on Climate Change (IPCC), the atmosphere accounts for only 1% of the heat energy change in the Earth system, whereas the 0-2000 meter layer of the ocean is where 93% of the globe’s overall heat/temperature change has occurred in recent decades.
“Ocean warming dominates the total energy change inventory, accounting for roughly 93% on average from 1971 to 2010 (high confidence). The upper ocean (0-700 m) accounts for about 64% of the total energy change inventory. Melting ice (including Arctic sea ice, ice sheets and glaciers) accounts for 3% of the total, and warming of the continents 3%. Warming of the atmosphere makes up the remaining 1%.” (IPCC, 2013)
Deep Ocean Temperatures Cooling Since The 1990s
The IPCC chooses to exclude the layer of the ocean below 2000 meters in their energy change inventories, even though about “52% of the ocean lies below 2000 m and about 18% below 3600 m” (Wunsch and Heimbach, 2014). Perhaps the reason the deep ocean is disregarded is because recent analysis has revealed that the deep oceans below 2000 meters have been cooling since 1993.
“A very weak long-term [1993-2011] cooling is seen over the bulk of the rest of the ocean below that depth [2000 m], including the entirety of the Pacific and Indian Oceans, along with the eastern Atlantic basin.” (Wunsch and Heimbach, 2014)
Image Source: (Wunsch and Heimbach, 2014)
Because of the cooling (thermal “retraction”) occurring below 2000 meters, the deep ocean removes a very uncertain -0.13 mm per year from global sea level change estimates (Llovel et al., 2014).
“Over the entire water column, independent estimates of ocean warming yield a contribution of 0.77 ± 0.28 mm yr−1 in sea-level rise … the deep ocean (below 2,000 m) contributes −0.13 ± 0.72 mm yr−1 to global sea-level rise.” (Llovel et al., 2014)
Top-To-Bottom Global Ocean Temps Have Changed By 0.02°C Since 1994…Large Regions Have Undergone Cooling
In a new paper, Harvard oceanographer Carl Wunsch provides an analysis of the overall temperature, salinity, and surface elevation changes in the global ocean for the 20 years between 1994-2013. He focuses on the profound uncertainties in the data analysis, and cautions that we still have much to learn about the ocean system.
Wunsch provides a novel assessment of the temperature data for the whole ocean (0-5000 m), as the record doesn’t end at the 2000 meter depth.
The results reveals that the global ocean has been cooling in some regions and warming in others for the last few decades, with an overall net change of 0.02°C (~0.001° per year) during 1994-2013.
Towards determining uncertainties in global oceanic
mean values of heat, salt, and surface elevation
“Lower-bounds on uncertainties in oceanic data and a model are calculated for the 20-year time means and their temporal evolution for oceanic temperature, salinity, and sea surface height, during the data-dense interval 1994–2013. … Trends [in temperature] are estimated as […] 0.0011 ± 0.0001 °C/y, with formal 2-standard deviation uncertainties. The temperature change corresponds to a 20-year average ocean heating rate of 0.48 ±0.1 W/m2 of which 0.1 W/m2 arises from the geothermal forcing. … The mean slope implies a change over 20 years [1994-2013] of 0.0213 ± 0.0014 °C.”
Image Source: Wunsch, 2018
2016 Paper Revealed Oceans Naturally Warm At Several Times The Magnitude/Rate Of Recent Decades
According to the IPCC, the global ocean temperature changed by just 0.015°C per decade at depths of 700 m in the 40 years between 1971-2010.
“In the upper 75 m of the ocean, the global average warming trend has been 0.11 [0.09 to 0.13]°C per decade over this time [1971-2010]. That trend generally lessens from the surface to mid-depth, reducing to about 0.04°C per decade by 200 m, and to less than 0.02°C per decade by 500 m. … decreasing to about 0.015°C per decade by 700 m.” (IPCC, 2013)
In contrast to these modern ocean temperature changes, a Geophysical Research Letters paper from 2016 revealed that during the Holocene the oceans naturally warmed by more than 2°C in a span of just 200 years — 0.1°C per decade — at the deeper 0-1000 m layer (Bova et al., 2016). This is several times the rate and magnitude the ocean is reported to have warmed during 1971-2010 (0.015°C per decade) at the shallower depth of 700 m.
In fact, (Bova et al., 2016) concluded that deep ocean temperature changes for the last 200 years are apparently so insignificant they are “below the detection limit”.
“The observational record of deep-ocean variability is short, which makes it difficult to attribute the recent rise in deep ocean temperatures to anthropogenic forcing. Here, we test a new proxy – the oxygen isotopic signature of individual benthic foraminifera – to detect rapid (i.e. monthly to decadal) variations in deep ocean temperature and salinity in the sedimentary record. We apply this technique at 1000 m water depth in the Eastern Equatorial Pacific during seven 200-year Holocene intervals. Variability in foraminifer δ18O [temperature proxy] over the past 200 years is below the detection limit, but δ18O signatures from two mid-Holocene intervals indicate temperature swings >2 °C within 200 years.” (Bova et al., 2016)
Comparing past “natural” changes to the recent changes in ocean temperature therefore would undermine claims that the modern version of “global warming” is unusual, unprecedented, or even remarkable.
Instead, perhaps we are merely witnessing “natural variability”.