Glacial meltwaters buffer carbon emissions
October 25, 2019 11:16 AM   Subscribe

Glacial rivers absorb carbon faster than rainforests, scientists find (The Guardian). "In fact, during the 2016 ablation season (a relatively low melt year), the glacial rivers in the Lake Hazen watershed consumed, on a per square meter basis, about half as much carbon daily [...] as the Amazon rainforest [...]. During the 2015 ablation season, when glacial melt was ∼3 times that in 2016, CO2 consumption rates in the glacial rivers were, on average, twice that of the Amazon rainforest, with maximum daily rates of up to 40 times higher on a per square meter basis [...]." Original paper (PNAS, academic paywall).

The cold, turbid, mineral-rich water released by glacial melting provides a potent buffer against rising atmospheric CO2 levels, becoming more effective with greater rates of melting. This is a bit of good news in the ongoing climate crisis, giving us a little more of a buffer than we thought we had. However, as the glaciers melt, this buffer will be lost, giving yet another reason to stop global heating before we lose the polar glaciers entirely.
posted by biogeo (14 comments total) 12 users marked this as a favorite
But their limited size means on a gross scale, they pull in far less than the sprawling Amazon.
Could anybody with access to the paper find out how much the "far less" is? Does this matter in big terms, or is it just a footnote to the main processes?
posted by clawsoon at 12:05 PM on October 25, 2019 [1 favorite]

They don't do an exact calculation for this as a net phenomenon. Their study covers a single watershed, the Lake Hazen watershed. They estimate that this watershed acts as a net sink for about 1020 Mg of carbon annually. Their per-area comparison with the Amazon basin is based on calculations from another paper, which reports a value of 0.38 Pg-C annually (which is 30% lower now than it was in the 90's due to deforestation). This means that the Lake Hazen watershed provides roughly 0.27% of the total carbon removal from the atmosphere that the Amazon basin does. There are fairly large error bars on that number since the amount of carbon captured by glacial meltwater seems to vary by at least a factor of 4 from one year to the next depending on the amount of glacial melting, and the amount of carbon captured by the Amazon is declining every year. But I think it's safe to say we're looking at this one arctic watershed providing less than 1% but more than 0.1% of the carbon capture that the Amazon does.

How representative this single watershed is isn't completely clear, and I don't have a sense of how many other glacial watershed systems there are in the arctic that would provide similar levels of capture.

It is also worth noting that much of the carbon capture by the glacial systems occurs by chemical weathering of minerals, meaning that that carbon is being at least temporarily removed from the biosphere and has a higher probability of leaving the carbon cycle altogether, as opposed to biotic carbon which will eventually be re-released into the atmosphere via respiration or combustion. (Unless carried to the bottom of the ocean or something in the form of a dead whale.)
posted by biogeo at 12:32 PM on October 25, 2019 [5 favorites]

My hope for the future is that we'll use this buffer wisely, and people in 2140 will sit there reading a book to the kids that's like The Giving Tree but about a glacier.

(and at the end it's starting to refreeze into its original majesty or seeing new little glaciers form nearby, idk i'm not a children's author.)
posted by Tess of the d'Urkelvilles at 12:36 PM on October 25, 2019 [2 favorites]

As glacial ice melts and methane hydrate ice reserves are opened up and begin releasing methane (with ~30x heating capacity as CO2), I'm curious if this is accounted for in offset measurements. I'll have to wait until I get behind my lab's paywall to read it properly.
posted by They sucked his brains out! at 12:54 PM on October 25, 2019 [2 favorites]

But CO2 dissolves in water makes carbonic acid so this just acidifies the water? Not great? Wiki says yes but it’s not a big deal? Ok.
posted by seanmpuckett at 2:05 PM on October 25, 2019

The edge glacial meltwater has is that the CO2 can bind with the finely ground basic minerals after having become carbonic acid through an accelerated version of weathering. It's a fortunate bit of negative feedback in the system but it won't be enough and, of course, it won't last forever.
posted by sjswitzer at 2:23 PM on October 25, 2019 [2 favorites]

My understanding is that the mechanism of capture isn't just that the CO2 is dissolving in the water, which wouldn't really help that much. Rather, after dissolving it reacts, as carbonic acid, with minerals in the water and is itself mineralized. This is the source of the surprise for this research. It seems that the general wisdom is that fresh water ecosystems are sources, rather than sinks, of atmospheric carbon, due to biological activity. But the high turbidity and cold temperature of these meltwater systems mean there's comparatively little life in them, and the high turbidity also means there's lots of minerals for the CO2 to react with. These factors reverse the usual trend and cause these meltwater systems to act as carbon sinks.
posted by biogeo at 2:24 PM on October 25, 2019 [4 favorites]

It's a fortunate bit of negative feedback in the system but it won't be enough and, of course, it won't last forever.

Continuing that thought... It's negative feedback loops like this that keep the climate relatively stable over time. They work a bit like rubber bands. And like rubber bands they work fine until they snap.
posted by sjswitzer at 2:30 PM on October 25, 2019 [3 favorites]

To me, this is bad news because here we have an unlooked for and powerful buffer system which has been operating apparently at something like peak capacity while the effects of Global Warming were busy passing the worst case scenario milestones set out by climate scientists over the last 20 years.

When this is exhausted, watch out.
posted by jamjam at 2:48 PM on October 25, 2019 [7 favorites]

Are the study's scientists making the comparison to the Amazon or is that just journalists after the fact? I can't tell because of the paywall.

The reason I ask is because whether the Amazon is a net carbon sink is itself a debatable issue. There seem to be papers on both sides, which to me means that if it is a net sink, it probably is pretty small and that the amount of carbon in mature Amazon forests is close to steady state.

To be a net carbon sink, it would mean that increased carbon dioxide in the atmosphere is causing mature Amazon forests to grow bigger -- taller trees, more leaves, more underground fungi. That would be a pretty tough thing to measure, in aggregate.
posted by JackFlash at 5:40 PM on October 25, 2019 [1 favorite]

From general principles, I would not expect biological systems to be reliable carbon sinks. Their carbon is always "available" in some sense. Destroying a carbon reservoir like the Amazon is terrible, but the fact is that it was only a reservoir. Pumping geological carbon into the atmosphere can only be counteracted, long term, by geological sinks. Otherwise, we're relying on "mere" reservoirs, which can always and easily be breached.
posted by sjswitzer at 6:16 PM on October 25, 2019 [1 favorite]

(NB: Many biological systems send carbon to the geological deep. A big shout-out to our mollusk friends.)
posted by sjswitzer at 6:28 PM on October 25, 2019 [2 favorites]

From general principles, I would not expect biological systems to be reliable carbon sinks.

This is true in a steady-state sense, but the carbon cycle is a far-from-equilibrium system. A diagram from NASA's Earth Observatory gives rough numbers for the total size of the various major parts of the carbon cycle, and this animation (which I think I originally saw via Metafilter) gives a nice sense of the way things flow between compartments.

Climate change occurs because of the increase in the amount of carbon in the atmospheric compartment, but carbon is constantly flowing both into and out of the atmosphere. The terrestrial biosphere (including the soil) represents a critical sink as part of this dynamical system. It's like pouring water into a leaky bucket: as the water level rises, the flow rate through the leak increases until a dynamic equilibrium is reached. Increase the flow rate into the bucket (an analogy for burning fossil fuels) and the water level rises (an analogy for increased atmospheric CO2) but re-establishes at a new equilibrium. Partially block up the hole in the bucket (an analogy for deforestation, which decreases carbon fixation via photosynthesis) and the water level also rises but can still establish a new equilibrium (as the flow rate into other compartments increases in compensation). Do both at the same time, and the bucket may overflow (an analogy for irreversible climate change).

We have a pretty good handle on how much carbon we're dumping into the atmosphere by burning fossil fuels, but this alone isn't enough to tell us how atmospheric CO2 levels will change. We also need to know at what rates carbon is being exchanged with other compartments in the cycle, and this is a bit more complex. This research helps show that there's an additional mechanism for removing CO2 from the atmosphere that we didn't know about. Yes, that carbon is still part of the carbon cycle, which it wouldn't have been if we hadn't extracted it and burned it. Ultimately we need to just stop doing that, there is no other long-term solution. But as a carbon reservoir, the biosphere is several times larger than the atmosphere, and it is capable of sinking (or, as we burn it, sourcing) quite a lot of carbon. I do think that abiotic carbon that's locked up as minerals is much slower to return to the atmosphere, though.
posted by biogeo at 7:54 PM on October 25, 2019 [2 favorites]

By the way, it looks like the original article is open access:

(at least it is for me)
posted by piyushnz at 9:17 PM on October 25, 2019

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