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Climate Change: Of Carbon Emissions and Carbon Sinks

Lindsay Wilson's picture
, Shrink That Footprint
  • Member since 2018
  • 145 items added with 113,661 views
  • Jun 5, 2013

The world reached a grim milestone recently, with atmospheric concentrations at the historic Mauna Loa observatory hitting the 400 parts per million mark due to ever increasing global carbon emissions.

While this event rightly got the media coverage it deserved, rarely do we stop to appreciate the incredible job land and ocean sinks have played in ensuring this figure isn’t significantly higher.

Using data for all sources and sinks of human carbon emissions over the last 262 years this post highlights just how hard the oceans, plants and soils are working to slow the growth of atmospheric carbon dioxide concentrations.

Global carbon emissions and sinks 

Since 1750 the human race has been responsible for 2,000 gigatonnes of carbon dioxide emissions.

These human carbon emissions have added to the much larger natural carbon cycle and resulted in the growing atmospheric concentrations so well documented by the Keeling Curve.

Using data from the IPCC, GCP, and CDIAC we can quantify where human carbon emissions have come from and where those emissions have gone over the last 262 years:

Carbon emissions and sinks since 1750

Click the image to enlarge.

Human induced carbon emissions since the industrial revolution have totaled almost 2,000 Gt CO2.  The major sources of emissions have been coal* (34%), oil (25%), gas (10%), cement (2%) and land-use (29%).

A few important caveats for these figures are worth discussing.  Coal emissions also include a significant volume of biomass emissions, while dwarfed by coal in recent years biomass emissions were likely dominant until at least 1900 .  For natural gas a small share results from flaring, and oil emissions included a very small share from bio-fuel emissions.  Land use emissions represent the net change in carbon stocks resulting from human activities.  This includes both increased emissions and reduced sink capacity.

The figures for the land, ocean and atmospheric sinks describe where these 2,000 Gt COhave gone.  The three major sinks for human carbon emissions are the atmosphere (44%), the ocean (30%) and the land (26%).

These figures don’t necessarily account for the same molecules of carbon dioxide, but rather how the imbalance created by adding extra human emissions to the natural carbon cycle have affected sink carbon stores.

The carbon dioxide that has been added to the oceans, plants, soils and fungi is the result of both greater emission rates and higher atmospheric concentrations.  This results in a kind of negative feedback where ocean and land sinks absorb more carbon dioxide as more is pumped into the air.

Together ocean and plants sinks have absorbed 56% of human carbon emissions since 1750.  Without these sinks working overtime atmospheric carbon concentrations would already be well over 500 parts per million (ppm).  In the case of the ocean acidification in particular this has not come without a cost.


The importance of carbon sinks

We can take our analysis further by thinking about these carbon emissions and sinks in terms of atmospheric concentrations.

For each 7.8 Gt of carbon dioxide that is added to the atmosphere global concentrations increase by 1 part per million.  The 879 Gt of human carbon emissions that have stayed in the atmosphere since 1750 are thus equal to an increase in atmospheric concentrations of 113 ppm.

This extra carbon dioxide has driven global atmospheric concentrations from 280 ppm in 1750 to 393 ppm by 2012.  Note this is the annual global average as opposed to the more widely reported Mauna Loa figures.

We can use a similar calculation to understand how all human sources of carbon emissions and carbon sinks relate to the atmospheric concentration of carbon dioxide.

I find the result a powerfully simple way of visualizing how human carbon emissions and carbon sinks affect the atmospheric concentration of CO:

The importance of carbon sinks

Click the image to enlarge

The data in this waterfall chart is exactly the same as the previous emissions and sinks analysis, but has been converted to illustrate how emissions and sinks affect atmospheric concentrations.

If all human carbon emissions had remained in the atmosphere over the last 260 years atmospheric carbon concentrations would be 537 ppm, or 257 ppm above where they where in 1750.  If we exclude land-use emissions this figure is 460 ppm.

In terms of a parts per million increase the sources of human carbon emissions are equivalent to the following: coal (+86 ppm), oil (+64 ppm), gas (+26 ppm), cement (+5 ppm) and land-use (+ 76 ppm).

Without land and ocean sinks working overtime concentrations of carbon dioxide would be well beyond the 400 ppm recently observed in Hawaii.  The ocean sink (-76 ppm) and land sink (-68 ppm) have absorbed 56% of human carbon emissions since 1750, keeping global carbon dioxide concentrations ‘down’ to 393 ppm in 2012.

These natural sinks for carbon dioxide have been crucially important in slowing the growth of atmospheric carbon dioxide in the past.


The future of carbon sinks

To keep things as clear as possible this analysis has only looked at cumulative emissions.  The limitation of this approach is that it doesn’t tell us much about the annual rates of carbon emission and sink absorption.

The high level story is pretty simple.  Human kind is emitting more and more carbon dioxide, as falling land-use emissions are dwarfed by emissions from our growing use of fossil fuels.  In reaction to increased emission rates and growing atmospheric concentrations both land and ocean sinks are absorbing more carbon dioxide.  The Global Carbon Budget has an excellent summary of this.

Despite the fact that sinks are absorbing more CO2 the atmospheric concentration is growing at a faster rate than ever.  In the decade from 2000-2009 the atmospheric concentration of carbon dioxide grew at an average rate of 2.0 ppm/yr, higher than any previous decade measured.  To reduce this growth rate global carbon emissions need to decline.  To stop concentrations growing at all would require an immediate reduction in carbon emissions by 55-60%, followed by further reductions in time.

In any future emissions scenario the reaction of our carbon sinks will play a key role in controlling atmospheric carbon concentrations.  Hopefully sink absorption will continue to moderate the growth rate of atmospheric carbon, but this is not certain.  Plenty of research warns of the dangerous possibilities of sinks becoming sources of emissions.  These are the risks of positive feedbacks from things like drought, fire, peat-land dehydration, permafrost melt and out-gassing oceans.

Whenever we talk about tackling the carbon problem it is worth remembering the incredible job the land and ocean sinks do in slowing the growth of atmospheric carbon.  This is a good reminder that reducing land use emissions and protecting carbon sinks are also part of the solution.

I K's picture
I K on Jun 6, 2013

Good post, it seems like more than half of what has been emmited to date was scrubbed by nature or more accurately chemistry.

if you plot a graph of how much the sinks absorb vs atmospheric concentration id wager you would find the higher the concentration the more is absorbed just like in any partial pressure experiment you conduct

what this means is as the concentration of CO2 increases so does the amount the sinks absorb per time period. as such there is an upper limit to ppm we can reach at a given butn of fossil fuels.


I K's picture
I K on Jun 6, 2013

If you would be so good as to calculate the absorption percentage over the last decade, and the decade before and the one before that etc vs concentration during that decade that would be a good graph.

Would be nice to see if it is a linear or power function and you can plot it forward to get an estimate of how much the sinks may absorb at say 500ppm

Lindsay Wilson's picture
Lindsay Wilson on Jun 6, 2013


No time for graphs but I know the data:

For the time series: 60s, 70s, 80s, 90s, 00s

Airborne fraction is: 38%, 46%, 49%, 39%, 46%

Atmospheric growth: 0.8, 1.3, 1.7, 1.5, 2.0 ppm/yr

So although sinks are taking up more CO2 in absolute terms, the airborne fraction is reasonably stable, thus concentrations are rising in line with rising global carbon emissions.

Sink uptake is a function of both emission rates and atmospheric concentrations.  While there is some reason to be optimistic land sink uptake will continue to grow there is more pessimism surrounding the future of ocean sinks. Either way, there is considerable uncertainty about future sink function, which is a yet another reason for drastic action to cut emissions. 

Max Kennedy's picture
Max Kennedy on Jun 6, 2013

Though atmospheric carbon gives rise to a cycle of increased temperature, which is bad, it is important to remember that ocean uptake of CO2 increases acidity which has major effects on the biotic communities in the ocean.  This in turn feeds back into decreasing our food supply and into the decreasing the oceans capacity to absorb more CO2.  This is also a bad thing.

Lindsay Wilson's picture
Lindsay Wilson on Jun 6, 2013

Yep.  I didn’t have space to get into in the post so only briefly referenced it:

In the case of the ocean acidification in particular this has not come without a cost.’

Max Kennedy's picture
Max Kennedy on Jun 6, 2013

Really? Only if the sinks are infinite which they aren’t.  There is a carrying capacity which your comment totally disregards!

Lindsay Wilson's picture
Lindsay Wilson on Jun 6, 2013

Fair enough.  But again, I’ve link to a bunch of research about the risks of sink degradation, at the bottom.

‘.  Plenty of research warns of the dangerous possibilities ofsinks becoming sources of emissions.  These are the risks of positive feedbacks from things like drought, fire, peat-land dehydration, permafrost melt and out-gassing oceans.’

Can’t cover everything, it was already to long

Max Kennedy's picture
Max Kennedy on Jun 6, 2013

Yep, I got that from your article, I K didn’t understand the concept though and my comment was regarding his post.

I K's picture
I K on Jun 6, 2013

For all intents the oceans and earth are an infinite sink for any plausible amount of carbon dioxide we are likely to burn.

Anyway who wants to post a graph of ocean co2 levels over the decades

Lindsay Wilson's picture
Lindsay Wilson on Jun 6, 2013

I see.  I’m just getting the hang of it here

Lindsay Wilson's picture
Lindsay Wilson on Jun 6, 2013

Plenty of smart people are less relaxed about this issue 


Max Kennedy's picture
Max Kennedy on Jun 6, 2013

Difficult to have an intelligent discussion with that level of willful ignorance.  It is exactly why once numerous species, such as the cod, passenger pigeon, bison etc… are mega depleted or gone….there is so much we can’t do any damage.  Yeah, right.  For people still able to think, look around, we’ve changed the world as much as a supervolcano and not in a good way either!

Max Kennedy's picture
Max Kennedy on Jun 6, 2013

Thank whatever powers you believe in for that.  Unfortunately the obstruction of the head in the sands will likely cause no end of grief anyway.


I K's picture
I K on Jun 6, 2013

More or less grief than running around like a headless chicken shouting we must do something? 

Rick Engebretson's picture
Rick Engebretson on Jun 7, 2013

I was once a scientist. Then there was a TV show called “The Waltons” so I moved my young family to the country to live like them; under control.

But the forest has been growing too fast to even burn for firewood heat in Northern Minnesota in a poorly insulated house.

The real farmers out here do one of two things; get big cattle to chew biomass into submission, or spray it and hope to get some cash crop.

You are absolutely right; land carbon sinks (eg. trees) are in overdrive. And it is not some invisible carbon storage system. And it must be maintained or it will burn catastrophically.

I’ve been reading economics and theory and data for years on TEC. What we really need are some big, strong farmers and loggers (instead of big strong firefighters and little guys in cubicles calling themselves the experts).

It would have been nice if agriculture/forestry and oil worked together to help recycle this biomass sink carbon into more useful fuels. I have never seen a sincere inclination, however.

So you are completely right. I hope it buys you a cup of coffee.

Lindsay Wilson's picture
Thank Lindsay for the Post!
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