Posted by Greg Laden
The sun warms the Earth’s surface. Additional greenhouse gases and associated positive feedbacks (like, additional additional greenhouse gasses) increase that effect. So, it gets warmer, and by “it” we mean the “surface” of the Earth. This is usually measured as the temperature near the surface across the land and the surface of the sea (Sea Surface Temperature or SST). But over 90% of the heat added by global warming goes into the ocean.
We know how much heat goes into the ocean (other than SST) two ways. One is direct measurements using equipment that samples water at depth, and the other is by super amazing precise measurements of how big the ocean is (reflected in the altitude of the surface), which increases as it heats up. Direct measurements are sparse and do not go back a long ways, and are very rare in the southern hemisphere compared to the northern hemisphere.
A paper just out in Nature Climate Change looks more closely at the Southern Hemisphere by combining direct measurements, estimates from ocean expansion, and some fancy modeling. The study suggests that the estimate of heat held in the upper 700 meters of the ocean in the Southern Hemisphere since 1970 was several percent too low owing to the lack of good data.
The study looks at the period from 1970 to 2004, prior to the deployment of some (but not yet sufficient) improved measurement technology. Study author Paul Durack notes, “Prior to 2004, research has been very limited by the poor measurement coverage. By using satellite data, along with a large suite of climate model simulations, our results suggest that global upper-ocean warming has been underestimated by 24 to 58 percent. The conclusion that warming has been underestimated agrees with previous studies, however, it’s the first … estimate [of] how much heat we’ve missed.”
This is a lot of heat. From the paper, “For perspective, these adjustments represent more than double the 1970‐2004 heat storage change for all non‐ocean (terrestrial, cryospheric and atmospheric) heat reservoirs combined…”
What does this mean?
It may mean that there is more heat added to the Earth’s surface than we thought there was, which means that any empirical estimates of the effects of global warming would need to be increased. But the real meaning may only be understood when we have a better handle on what happens to heat within the ocean, as shallow waters interact with deeper waters, and how the ocean as a system interacts with the atmosphere. And this heat, at this depth, does interact with the atmosphere. Despite the rather spectacular nature of this finding, its greatest significance is probably that it is a major step towards quantifying what may be the biggest single unknown related to climate change: what is happening in the ocean. It may, though this is subject to revision, increase the higher end of the estimate of “climate sensitivity” which is a measure of how much the surface of the earth will warm given a doubling of pre-industrial levels of atmospheric CO2. In that sense, this is potentially unpleasant news.
I have an FAQ on this research that I’m not sure I can provide a link for, but I’ll past the part that addresses the importance of the research, in the view of the authors:
What are the implications of long-term underestimates to ocean warming?
Quantifying how much heat is accumulating in the Earth system is critical to improving our understanding of climate change already underway and to better assess how much more we can expect in decades and centuries to come. Our key result is that the warming of the global ocean in recent decades has been substantially underestimated. These findings will likely lead to a revisit of previous sea-level and climate sensitivity estimates, and to a re-examination of how scientists deal with poorly sampled aspects of the climate system. A key lesson to be learned from our work is that observing the global ocean is critical, and that prior to the recent improvement in global coverage of ocean observations, a substantial and very important part of the global climate system was under-observed. In order to better understand past and future climate changes it is imperative that the global ocean is adequately observed, as it plays a critically important role in the Earth’s climate and its change.
I have a feeling there will be a lot of discussion of this over the next few days.
