Why are our land and oceans projected to absorb less carbon dioxide?

11 August 2021

Over the past couple of days, we’ve been taking the time to go through the comprehensive Sixth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC), and boy, is there a lot in there. 

What was published on Monday was just the first in a series of updates coming from the international scientific body – there’s more to come next year particularly when it comes to adaptation and mitigation. 

One thing (among many terrifying things) that caught our eye was this line: 

Under scenarios with increasing carbon dioxide emissions, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of carbon dioxide in the atmosphere.” 

To put that in more plainspeak: If the amount of carbon dioxide in our atmosphere continues to rise, our land and oceans will be less effective at absorbing carbon dioxide. This is – to put it mildly – a huge problem. 

Okay, sounds scary. How hard is our land and oceans working as we speak to absorb greenhouse gases? 

Short answer: very hard. 

Currently around roughly half of carbon dioxide emissions are stored by plants, soils and oceans, according to the IPCC. So if we don’t have these natural sinks (i.e. things that absorb carbon dioxide rather than producing it), global temperatures would be considerably higher than they already are. 

When it comes to anthropogenic heat, the ocean is doing the lion’s share of work in that department, as it has stored 90 per cent of excess warming. 

But, like we said, if we continue to see carbon dioxide increase in the atmosphere, that aforementioned trend starts to change. Drastically. 

How so? 

Good question – and luckily we had Emeritus Professor and former IPCC scientist John Sweeney help us get our heads around it. 

So extra carbon dioxide in the atmosphere actually acts as fertiliser for photosynthesis and encourages more vegetation to crop up. In fact we have seen a lot of extra vegetation growth occurring on land over the past two decades and that’s taken out a lot of anthropogenic carbon dioxide emissions from the atmosphere, according to Prof Sweeney. 

However, he stressed, there comes a point where the indirect effects of climate change start to kick in, and they completely negate the effects of any above-mentioned vegetation growth. (Not to mention there’s the pressing issue of deforestation, too). 

Disasters like intense heat, drought and flooding are turbo-charged by a warming climate, and these events essentially wipe out any benefits from additional vegetation. The more carbon dioxide there is in the air that causes additional warming, the more intense these above-mentioned occurrences are – which explains why the IPCC are warning us that current land sinks are projected to weaken as warming ramps up over the coming decades. 

When it comes to the oceans, that’s where things get a little more complicated, according to Prof Sweeney. 

However, “the general consensus here is if you have warmer ocean water, you store less carbon dioxide in that water,” he told The Green News. 

Let’s also not forget that we’ve also got the issue of ocean acidification. In short, this is a process where through the absorption of carbon dioxide, seawater becomes more acidic through a series of chemical reactions.  

Seawater becoming more acidic leads to less carbonate ions in our oceans, which are critical for sea shells and coral skeletons to build their exteriors. Shellfish, like clams, are important to us not only for their immediate value to coastal communities, but they also take carbon dioxide out of the water and mineralise it, acting as effective carbon sinks. 

Mangroves, kelp and seaweed are useful to sequester carbon too and will continue to be, however their absorption is declining due to threats to their habitats. 

Yikes. So one last question – how soon can we expect this anticipated decline? 

Like we said earlier, to a degree this is already happening, but the IPCC said that our land and oceans are projected to see an absorption decrease in the second half of the 21st century. 

But, what’s very important to keep in mind here is that how much these sources can absorb is completely dependent on emissions trends over the coming years. 

In the lowest-emissions projection by the IPCC, land and ocean can absorb 70 per cent of total carbon dioxide emissions up to the end of the century. 

However, in higher-emitting scenarios, these natural sinks could take up as little as 38 to 44 per cent of total carbon dioxide. 

Which is a long way of saying what happens next will determine that exact figure. And that doesn’t just go for natural sink capacity – that’s the climate crisis across the board. 

We’re continuing to take a look at the IPCC report in depth this week. Have anything you’d like us to take a deeper dive into? Let us know at editor@greennews.ie

About the Author

Kayle Crosson