A new assumption about carbon budgets reveals climate scientists have been vastly underestimating (by a factor of 2) the amount of carbon absorbed by the ocean for decades. Every past carbon budget estimate has been twice as wrong as the current estimate.
When it comes to the ocean heat fluxes and source vs. sink carbon budget estimates, climate scientists have been providing little more than educated guesses for decades.
For example, climate models have long suggested the ocean heat fluxes may only vary around 1 W/m². But “objective” analyses of oceanic latent heat flux (LHF) using different assumptions (equations) reveals fluxes were likely closer to 10 W/m² during 1981-2005 (Yu and Weller, 2007). So our modeled guesses were off by a factor of 10 compared to newer analyses.
Image Source: Yu and Weller, 2007
Ocean carbon sink processes not understood and driven by natural variability
McKinley et al. (2017) analyzed ocean carbon sink estimates and was willing to admit that due to a lack of observation, we lack a “detailed, quantitative, and mechanistic understanding of how the ocean carbon sink works…”
In addition, because internal variability in oceanic carbon uptake is so massive and largely unobserved, we cannot yet detect an anthropogenic influence.
McKinley and co-authors go so far as to acknowledge the “change in CO2 flux over 10 years (1995-2005)…is due almost entirely to the internal variability” because in most ocean regions “the forced [human-induced] trends in CO2 flux are too small to be statistically significant” and the “variability in CO2 flux is large and sufficient to prevent detection of anthropogenic trends in ocean carbon uptake on decadal timescales.”
Image Source: McKinley et al. (2017)
The Southern Ocean absorbs more than 10 times less carbon than previously thought
The Southern Ocean is where the largest portion of anthropogenic carbon (from our emissions) is said to be absorbed, or Earth’s largest oceanic CO2 sink.
Just 2 years ago, a carbon uptake analysis (Gray et al, 2018 with a Physics Today press release) that utilized estimates from biochemical floats instead of estimates from ships suggested the exact opposite of what had been previously thought. Instead of absorbing close to 1 petagram of carbon (PgC) per year, the Southern Ocean is barely even a carbon sink at all – just 0.08 PgC of yearly absorption. In fact, large regions of the Southern Ocean near Antarctica are a net source of CO2 to the atmosphere.
In other words, when estimates are float-based rather than ship-based, one estimate is more than 10 times different than the other.
Image Source: Physics Today
The global ocean absorbs 2 times more CO2 than previously thought
And now a new study (Buesseler et al., 2020) has scientists insisting that all of our previous estimates of global ocean carbon uptake are substantially wrong because we’ve been measuring from a fixed depth rather than varying depths.
Previously scientists had been using flux estimates from the “canonical fixed 150-m depth.” The new-and-improved way to assess carbon uptake is from varying but often much shallower depths: the euphotic zone (Ez). This is the section of the upper ocean layer that sunlight is able to penetrate, and it can “vary from less than 20 m to almost 200 m” in depth.
When we use the Ez to estimate carbon absorption versus export, the absorption changes from 2.8 petagrams of carbon (PgC) per year to 5.7. So the global ocean sink can be more than doubled just by varying the depth of measurement rather than using a fixed depth.
So, up to this point, scientists’ past guesses about ocean carbon uptake have been emphatically wrong. We can be assured, though, that our current guesses are anywhere from 2 to 10 times less wrong than the last ones.
10 responses to “Scientists Just Discovered Their Past Carbon Budget Guesses Have All Along Been Twice As Wrong As They Thought”
Having logged hundreds of dives I can clearly attest that there is a massive amount more life in the top 20 m or so whereas by the time one gets to 75m, the deepest I have gone, it is getting dark and there is little to see.
Using 150m as the absorption level for Co2 smacks to me of stupidity or an attempt to pick a level where it will be low. Any diver could tell you that life in the upper 20m – 30m will be massively taking up CO2 vs 150m
This series of articles is profoundly unsettling: There is NO consensus about ocean uptake of CO2;-} Worse, one gets the impression climatologists have made little effort to date to quantify ocean uptake even though oceans have long been thought to be earth’s biggest CO2 sink.
Combining the above with woefully inadequate ocean temperature data (too few years, poor geographic scope, poor knowledge of temperature change over time by depth … ) causes an insurmountable GIGO problem for climate models. GIGO, of course, stands for Garbage In / Garbage Out.
I will squelch the urge to conclude by asking “What else could go wrong with climate modelling?” to avoid flooding this website with replies;-}
Better late than never. More of an overdue admission than a revelation:
So much for “The science is settled.”
We know approximately how much CO2 mankind adds to the atmosphere each year, from using fossil fuels and making concrete, thanks to the financial bean-counters: It’s currently about 10.5 PgC per year (which is equivalent to about 5 ppmv CO2).
We also know the approximate rate at which atmospheric CO2 levels are rising, thanks to precise measurements at Mauna Loa, Cape Grim, Barrow, Cape Matatula, Hateruma, Mace Head, the South Pole, and other places. It varies considerably from year to year, but averages about 2.5 ppmv per year.
The difference between those two figures is the net rate at which nature is removing CO2 from the atmosphere: currently about 2.5 ppmv per year.
If the oceans were really removing 5.7 PgC per year, it would mean that we’re missing a major carbon source, because 10.5 – 5.7 = just 4.8 PgC, which is only 2.2 ppmv CO2 per year — and we know that the biosphere is also removing a lot of CO2 from the atmosphere.
Or else Buesseler is wrong, and the usual estimate of about 2.8 PgC/year is closer.
And why would it not be the case that we’re missing major natural CO2 emissions sources? That wouldn’t be surprising in the slightest. There is so much we don’t know about both the Earth’s carbon sources and sinks.
After all, the oceans contain 50 times more CO2 than the air does. This means there has been a 40% change in 1/50th (1-2%) of the available ocean-atmosphere CO2 since 1750. Why would change in 1-2% of the ocean-air carbon drive what happens to the constitution of the 98%? It would be the other way around: ocean CO2 would drive changes in atmospheric CO2.
If the oceans were really removing 5.7 PgC per year, it would mean that we’re missing a major carbon source,
I’ve told you elsewhere the major source is ocean outgassing, which occurs at or above 25.6C (~26C, see second link for current boundary), and is also controlled by local dissolved CO2 which varies.
Atmospheric CO2 increased from solar-warmed tropical outgassing.
The fluxes into the atmosphere are estimated to be ~820 Gt per year: 770 Gt from oceans and biosphere, and 40Gt from humans, or 3.5% anthropogenic. Those natural sources/sinks have an error range of 2 to 22%, so that changes in the natural sources stimulated by rising temperatures overwhelm human contributions. 2020 is a good example of lower FF emissions not showing up Moana Loa measurements.
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From the effects of desert dust on the ocean surface, to underwater volcanoes affecting water temperature and chemistry — we just don’t fully understand.
So how many underwater volcanoes are there, where are they, and how do they each change the chemistry of the local waters along with the oceans and seas in general. We have no long term records showing us how the oceans have changes over say the last 300 years or longer, so we can not say with any certainty what has happened recently is influence by man or (more likely) natural events.
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