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.
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.
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.
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.