Take a Breath – Correlation, Causation, Carbon, and Common Sense

Or: Curses, Soiled Again!

As the world warms due to our seemingly inexorable increase of greenhouse gases, there will be positive (amplifying) and negative (dampening) feedbacks that respectively exaccerbate and mitigate our perturbation of the climate system. We know that significant positive feedbacks exist due to solid theoretical and empirical evidence. Likewise we know that although there are negative feedbacks and we’re not in any danger of seeing runaway warming or cooling, none exist that will magically prevent climatic changes on the order of at least 6°C, as we drive and hold GHGs to levels that haven’t been seen for millions of years.

The “biggest”- in terms of importance from a policy perspective- feedbacks have been traditionally (e.g. the Charney report) the ice-albedo and water vapor feedbacks. I’m not aware of any who seriously deny the existence of the former, and while there had been some arguing over the latter, that seems to have largely died down (e.g. here, here). Ice-albedo and water vapor have been considered the most important feedbacks from a policy standpoint not just because of their magnitudes (responsible each for roughly as much warming per doubled pre-industrial CO2 level as the CO2 increase itself), but also because of their speeds- they “kick in” relatively quickly, on time scales (decades to centuries) that policy makers are concerned with. Other feedbacks involving large changes to the ice sheets and vegetation have been described as “slow” feedbacks- those which have more to do with long term equilibrium changes than what we’ll see by end of century, and are sometimes ignored from a policy perspective and left out of simple modeling exercises. And they’re certainly not the only two significant feedbacks that are treated this way. That might sound surprising, but even relatively simple model exercises can tell you a great deal about planetary energy balance- i.e. you don’t need to have every feedback explicitly treated in a model in order to give policy-level recommendations like “tripling pre-industrial CO2 will have a profound global warming effect with ensuing climatic changes”.  In any event, the complexity of climate models and their development over time is beyond the scope of this post. I’ll leave RealClimate to do the necessary oversimplification:

Initially (ca. 1975), GCMs were based purely on atmospheric processes – the winds, radiation, and with simplified clouds. By the mid-1980s, there were simple treatments of the upper ocean and sea ice, and clouds parameterisations started to get slightly more sophisticated. In the 1990s, fully coupled ocean-atmosphere models started to become available. This is when the first Coupled Model Intercomparison Project (CMIP) was started. This has subsequently seen two further iterations, the latest (CMIP3) being the database used in support of much of the model work in the IPCC AR4. Over that time, model simulations have become demonstrably more realistic (Reichler and Kim, 2008) as resolution has increased and parameterisations have become more sophisticated. Nowadays, models also include dynamic sea ice, aerosols and atmospheric chemistry modules.

Many current studies also attempt to use a more detailed carbon cycle (i.e. using coupled carbon climate models). The carbon cycle obviously plays an enormous role in regulating the climate, and effectively simulating it in modeling studies can have significant impacts on the amount and rate of warming we can expect to see as anthropogenic warming continues.

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Coupled carbon climate models all show a positive feedback from the terrestrial carbon cycle in response to anthropogenic warming. This is primarily a result of net decreases in primary productivity and increased soil respiration. However, the real world effect of warming on the terrestrial carbon flux is complex, with small scale physical experiments noting that this somewhat idealized model process can be complicated by extended growing seasons, enhanced nutrient availability, shifted species composition, altered ecosystem-water dynamics, etc. (e.g. here).

As an example, there had been some hope (and provisional evidence) that increased nitrogen fixation in response to soil warming could offset the carbon lost at least in the near term (e.g. here).

Unfortunately, it looks like this may not be the case. In a paper out in today’s Nature entitled “Temperature-associated increases in the global soil respiration record”, Allison Thomson and Ben Bond-Lamberty find that the global soil carbon flux has been increasing with the rise in temperature over the last several decades.

Estimated annual global Rs. The dashed line indicates results outside the time period covered by main data set, S1 (1989–2008), but within the period covered by the entire RS database, S0 (1961–2008), and should be considered speculative. The grey region shows the standard deviation of the Monte Carlo simulations (N51,000).

Now, as the title of the paper indicates, Thomson and Ben Bond-Lamberty did not attempt an attribution study. Rather, they compiled a global record of soil respiration of carbon, and found that not only was it on the rise, it was strongly correlated with the rise in temperature as humans have been warming the planet.

Of course plenty of naysayers might chime in at this point to state the obvious- correlation is not proof of causation. Yes, the authors themselves take great pains to make this clear, and I’m certainly not claiming otherwise. However, the bottom line is this:

This isn’t “definitive evidence” that the positive warming-soil carbon feedback has already kicked in. But it’s in line with what we expect from the carbon cycle, sooner or later. If more data and/or further analyses attribute the increase to something else or indicate that there isn’t really a trend to speak of yet, that won’t “debunk” the reality that our best evidence says we should expect such a feedback to amplify anthropogenic warming as we force the planet to a hotter and hotter equilibrium.

Map of predicted global soil respiration for 2008

Think of it this way. Recently, there have been claims that the incredible reduction in Arctic sea ice over the last several decades is not due to warming, but rather wind. While it is true is that wind patterns have been responsible for much of the variance in ice and has contributed to its decline, the studies cited by denialists actually state that wind has been responsible for a mere third of the reduction in sea ice.

But here’s the thing- a seasonal ice free Arctic is inevitable if we warm the planet enough. No amount of wind variability will change that. Likewise, even though an attribution study hasn’t been performed here and another process might be responsible for Thompson and Bond-Lamberty’s carbon rise, unchecked warming will eventually engage a warming-soil respiration feedback.

It’s important to not jump on studies like these (or the recent methane paper) as evidence of some doomsday/point of no return scenario. But likewise it’s important to remember that there’s no such thing as a free ride in the climate system. And if we decide to push warming high enough, for long enough, we’re going to end up with a fare that we’re not actually capable of paying.

Unfortunately, it looks like this may not be the case. In a paper out in today’s Nature entitled “Temperature-associated increases in the global soil respiration record”, Allison Thomson and Ben Bond-Lamberty find that the global soil carbon flux has been increasing with the rise in temperature over the last several decades.

Estimated annual global Rs. The dashed line indicates results outside the time period covered by main data set, S1 (1989–2008), but within the period covered by the entire RS database, S0 (1961–2008), and should be considered speculative. The grey region shows the standard deviation of the Monte Carlo simulations (N51,000).

Now, as the title of the paper indicates, Thomson and Ben Bond-Lamberty did not attempt an attribution study. Rather, they compiled a global record of soil respiration of carbon, and found that not only was it on the rise, it was strongly correlated with the rise in temperature as humans have been warming the planet.

Of course plenty of naysayers might chime in at this point to state the obvious- correlation is not proof of causation. Yes, the authors themselves take great pains to make this clear, and I’m certainly not claiming otherwise. However, the bottom line is this:

This isn’t “definitive evidence” that the positive warming-soil carbon feedback has already kicked in. But it’s in line with what we expect from the carbon cycle, sooner or later. If more data and/or further analyses attribute the increase to something else or indicate that there isn’t really a trend to speak of yet, that won’t “debunk” the reality that our best evidence says we should expect such a feedback to amplify anthropogenic warming as we force the planet to a hotter and hotter equilibrium.

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13 responses to “Take a Breath – Correlation, Causation, Carbon, and Common Sense

  1. Do you mean CAUSING rather than “preventing” in the first paragraph?
    ["we’re not in any danger of preventing runaway warming or cooling"]

  2. Pingback: Länkar 2010-03-25

  3. Over the last hundred years or so, artificial drainage (primarily sub-surface drain tiles) has undoubtedly released more soil carbon than warming temperatures. I wonder how (or whether) that and future drainage are addressed in soil carbon studies.

    • via consequently increased oxygenation and temps presumably? Do you have references, or estimates of the amount of land tiled? Thanks.

      Had just seen this–thanks for posting on it tb.

      • Yes, as the drained soil is aerated (particularly if it is subsequently tilled) microbial activity liberates C as carbon dioxide. I have no links but do have plenty of personal experience with tile lines that were waist-deep a century ago but are now near the surface. This represents hundred of tons per acre that has simply vanished from this muck soil. The effect on soils with less native organic matter will be less dramatic; but, as a rule, any time wet soils are drained they lose carbon.

  4. I think you mean “tiled” not “tilled”; the latter has released far more carbon globally than drainage tiling has.

    Granted the process you describe certainly occurs and is important, but one would need some estimates of global area tiled and C loss rates therefrom before concluding anything about relative magnitudes of loss.

    Also, wrt to depth to tiles, are you not observing erosional losses there? Geographically, where is this? Am curious because I hale from an area that was originally a vast hardwood swamp that was drained in the late 19th C for agriculture (and hence is both tiled and tilled).

    • oops–no I see you did in fact mean tilled there, sorry.

      • I am located near the distal boundary of the last two ice sheets in Ohio. Our soils are glacial till and outwash. We have everything from peat/much to gravel. The tile I referenced above is in a depositional area, so erosion is out of the question. Wind erosion is not a major issue in this low spot and it rarely sees anything but shallow tillage. But parts of it are well over 50% organic matter and will burn if placed in a fire, though it will not sustain a fire by itself. My grandfather remembered the men digging in the tile around 1910 and ran up and down the ditches they made. Today that tile is no more than 8″ deep.

      • Oops…peat/much should read peat/muck.

      • Very interesting–thanks dko.

  5. A. Semczyszak

    1. Look at it from another angle:
    Excerpts from the works of Ben Bond-Mony, Allison Thomson, and their comments:
    “The increase in carbon dioxide given off by soils, about 100 million metric tons per year since 1989, won’t contribute to the greenhouse effect unless it comes from carbon that had been locked away out of the system for a long time, such as in ARCTIC TUNDRA [1 / 5 land area - arctic soil detritus is a great weight - most of the ( in arctic soil) accumulated 21,6 kg C x m -2] This analysis could not distinguish whether the carbon was coming from old stores or from vegetation growing faster due to a warmer climate. But other lines of evidence suggest warming is unlocking OLD CARBON, according to the researchers.”
    “We estimate that the global RS in 2008 (that is, the flux integrated over the Earth’s land surface over 2008) was 98 ± 12 Pg C”
    “The scientists also calculated the total amount of carbon dioxide flowing from soils, which is about 10-15 percent higher than previous measurements. [Researchers have estimated that soil respiration accounts for 77 Pg of carbon released to the atmosphere each year. (Wikipedia - Raich J, and Potter C.,1995)].”
    “We find that the air temperature anomaly (the deviation from the 1961–1990 mean) is significantly and positively correlated with changes in RS.”

    2. … and the drawing: http://i41.tinypic.com/j64tty.png; compare with this: http://www.jennifermarohasy.com/blog/archives/Jan%20Pompe_co2%20and%20temp2.gif

    Excellent correspondence between changes in: annual mean CO2 growth, annual mean temperature (especially land), annual global RS, may indicate that this NATURAL warming [Increase in temperature - a cycles of Millennium, a cycles of 4.2 thousand year = further - North - THC extension for NH + AMO ...] is responsible for the increased concentration of CO2 in the atmosphere, rather than anthropogenic emissions.

    “Using the complete set of data collected from the studies, the team estimated that the carbon released in northern — also called boreal — and Arctic regions rose by about 7 percent [!!!]; in temperate regions by about 2 percent; and in tropical regions by about 3 percent …”

  6. Pingback: Zero Carbon Society » Curse of the Climate Zombies

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