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Carbon Cycle Feedbacks

Posted on 24 February 2011 by Andy Skuce

The document entitled 'Carbon Dioxide and Earth's Future: Pursuing the Prudent Path', referenced in the "skeptic" scientist letter to US Congress, makes the claim that rising CO2 concentrations have "actually been good for the planet" because of the fertilization effect of CO2. Although it is true that there has been a measurable CO2 fertilization effect, particularly in the tropics (see this video seminar), this is only one factor that will influence the response of the global carbon cycle to climate change. It's instructive to look at some important factors that are not mentioned in The Prudent Path.

Sources of uncertainty

Predictions of the path of climate change over the rest of this century are very  uncertain.  To start with, we don’t know exactly how much fossil carbon we have in reserves of oil, gas, and coal, and we don’t know how quickly the world economy is going to consume them. Burning of these fuels will increase the CO2 in the atmosphere, but the amount of warming that this will produce can't be precisely determined. Climate sensitivity, expressed as a temperature increase for a doubling of CO2, is generally considered to lie in the range 2 to 4.5°C. The main uncertainties in the estimation of the climate sensitivity are the effects of clouds and aerosols. The IPCC AR4 best estimates for various emissions scenarios predict an increase of 1.8 to 4°C (above 1980–1999 temperatures) by 2100. 

“Slow” feedbacks

Large and numerous though these uncertainties are, they do not include the so-called “slow feedbacks”, notably the absorption or release of CO2 of plants and soils on land. Since about 1940, land plants have been absorbing about 1-2 billion tonnes per year of human carbon emissions. The location of this sink is not known exactly, but it is thought to be in temperate forests that are growing back in in the USA, China, and Europe; boreal forests in Russia; and in tropical forests, where enhanced CO2 fertilization and secondary regrowth currently outweigh the effects of deforestation.

The net uptake of CO2 by the world’s forests may not last forever. Bark beetles, probably thriving due to warmer winters, have ravaged forests in Western Canada, as shown on the mountainside in the photograph above. Recent fires in Alaska and Canada (hat-tip) are increasing CO2 emissions and reducing the carbon content of the soils, which produces a weaker carbon sink because the trees grow back more slowly. In the Amazon Basin, severe droughts in 2005 and 2010 have severely damaged large parts of the rain forest, releasing huge quantities of CO2. All these effects are to some degree caused by climate change and, as the climate changes further, we can reasonably expect them to get worse and more widespread.

Another slow feedback is related to thawing of the permafrost and the release of the carbon contained within it. A recent paper by NOAA and NSIDC predicts that by the late 2020's the tundra will turn from a carbon sink to a carbon source and that, by mid-century, the permafrost will be releasing 1.5 billion tonnes of carbon per year; equivalent to today's net sink due the entire terrestrial biosphere. Much of the carbon will be released in the form of methane, which will amplify the greenhouse effect for decades compared to a release of a similar volume of CO2.

Last year, Natalia Shakhova and her co-workers reported extensive methane venting from sediments of the East Siberian Artic shelf. The rapidly warming Arctic climate is melting the permafrost layer below the shallow sea, making the layer permeable and releasing gas trapped below it.

How not to deal with uncertainty

Revealingly, the 130 pages of the  Pursuing the Prudent Path document do not contain the words “beetle”, “fire”, or “permafrost”, even once. Tropical forests are discussed at length, but only in the context of the CO2 fertilization effect; no mention is made of the recent devastating droughts in the Amazon Basin.  The Prudent Path document assumes that the CO2 uptake of the terrestrial biomass will continue indefinitely and even increase, whereas it avoids any consideration of  the possible saturation of the fertilization effect or its limitations in promoting growth in temperate or boreal forests due to nitrogen fixation.

David Archer in his recently published primer The Global Carbon Cycle remarks that the reservoirs of carbon in plants and soils are as charged up today as they can possibly be. He also comments that, in the worst case scenario, elements of the carbon cycle may release as much carbon to the atmosphere as humans have.  Uncertain though this is, it would obviously make climate change much worse. Some of the carbon cycle-climate feedbacks may have started already Surely, the worst way of dealing with uncertainties like these is to ignore them and hope they go away while we continue business as usual. The truly prudent path to follow is the one that takes account of all of the data.

Further information

This excellent video seminar given by Stephen Pacala at Stanford University is well worth watching to learn more about climate change, CO2 fertilization and  the world's forest inventory. (hat-tip rustneversleeps

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Comments 1 to 9:

  1. We know that carbon uptake by plants could't happen fast enough in the past to prevent severe climate change, why on earth would you think it could happen now with the greatly reduced, fragmented, damaged forest remnants we have; with the further damage to those forests being caused by the changing climate; and with the continuing short-sighted effects of logging, clearing, wood-chipping, recreational use, prescribed burning. It is also worth noting that it takes hundreds of years for a forest to mature, ie to have taken up the maximum that can be absorbed by vegetation in that area given the parameters of topography, soil, moisture, nutrients and so on. We don't have hundreds of years.
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  2. Oh, and before any arrive, some interesting material coming to light eg and about the existence of highly sophisticated "denier bot" operations, able to swamp any climate change threads with denier diversions, while appearing to be genuine individuals with unique IPs, backgrounds, internet histories. I'm sure all of us have had our suspicions from time to time!
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  3. David Horton @ #1 - paradoxically, I've heard that some areas are substantially increasing their logging rates because they know the pine beetle is going to kill most of the trees in the next few decades, and they want to harvest the timber before it gets ruined. Yet again, though, as this article points out, it seems wherever there is uncertainty, the deniers assume without question that the error is on the high side, and the actual value is at the bottom end of the uncertainty range. We're gonna be in one heck of a pickle if the actual values turn out to be at the top end of the range. Well, if nothing else, at least the unrest across the arab world may help promote alternatives to fossil oil. It's going to be a painful process to get through, though (and has already been painful for the folks in those countries, with more pain to come as their oil-dependent economies start to falter - I saw a report [free reg req'd, I think] this morning that an analyst has given a 25% chance for revolution in Saudi Arabia, which holds 20% of world reserves)
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  4. An ounce of prevention is worth a pound of cure. It held true in the past and it holds even truer today.
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  5. Speaking of permafrost. NOAA Hot on Methane’s Trail A new study led by Kevin Schaefer, Ph.D., and a team of scientists from NOAA-funded Cooperative Institute for Research in Environmental Sciences and the NOAA Earth System Research Lab now predicts a 29 to 59 percent decrease in permafrost by 2200. Published in the journal Tellus B, the study estimates a large release of carbon – in the form of carbon dioxide and methane – from thawing permafrost over the next century, though much is still unknown about how these emissions will accelerate climate warming. Scientific climate projections do not currently account for carbon emissions from permafrost, but the study concludes that the effect is “strong enough to warrant inclusion in all projections of future climate.” It will certainly be interesting to see how much the inclusion of methane in future projections will change the range of outcomes.
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  6. The actual GHG values from gases released from melting permafrost are almost a lock to be worse than projected. Schaefer et al 2011 presumes (for purposes of ready calculation, I believe) 100% of emissions to be in the form of CO2, while in actuality much will be in the form of CH4 (methane). Should this stand up to further scrutiny & be confirmed by subsequent studies, this means an effective CO2 doubling from permafrost melt alone by 2200. In addition, Schaefer et al 2011 do not consider methane hydrate releases, which are currently underway in the East Siberian Arctic Sea. Needless to say, no current model begins to take this all into consideration. The edge nears. The Yooper
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  7. Your comments re the photo of Yoho National Park (that the dead trees on the mountain slopes illustrate bark-beetle damage) contradict the wikipedia info for the photo. Poor choice of photos, however doesn't negate the point about warmer winters and the exploding range of this insect: see this RealClimate post for example.
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  8. Andy S draws attention to the fact that as temperatures rise greenhouse gas emissions from sinks where they are now trapped will also rise. He notes that these include CO2 emissions from warming soil and tree destruction, now occurring, to which might be added the oceans as surface temperature rises in the future. More damaging are CH4 emissions, now becoming increasingly evident, caused by the thawing of clathrates and permafrost allowing embedded organic material to resume decaying. Hansen has warned that these emissions could result in runaway global warming and for good reason when one considers that I litre of solid clathrate yields ~165 litres of CH4. Millions of tones of methane clathrates are thought to lie beneath Arctic offshore sediments. They have started melting. Given the threat of these emissions, one is forced to ask why the need to limit average global temperature to no more than 2C above preindustrial temperature by 2100 is measured in terms of CO2 concentration in the atmosphere and not in terms of CO2-e? It is all very well to assert that the cooling effect of aerosols off-sets the effect of other warming greenhouse gas emissions but how can that be true when emissions of the kind described above are increasing? By ignoring their effect are we not underestimating the magnitude and rate of future global warming?
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  9. Hugh@7: the comment you referred to says that the trees were burned by a prescribed fire. On the other hand, this Wikipedia caption to the same picture says Invaded pine tree forest on the slopes of Chancellor Peak in Yoho National Park, Canada. The two comments are not contradictory since Parks Canada has adopted a policy of prescribed burns to manage the beetle infestations. I should add that climate change is not the only factor influencing pine beetle outbreaks, forest management practices, especially fire suppression in the twentieth century, have played a major role. In case anyone is curious, here's a map showing the location of Chancellor Peak and the areas susceptible to pine beetle outbreaks in three of the Canadian Rocky Mountain Parks. According to the BC Ministry of Forests, 16.3 million hectares of forest have been affected by pine beetle infestations. That's roughly the area of Austria and the Czech Republic combined. Map here.
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