In my weekend links, I highlighted a new study published in Science. It is behind a pay wall, but has been well covered by Climate Central and Climate Progress. The study provides us with a good excuse to revisit the whole topic of climate risk.
The paper in question is an empirical study of past permafrost thaw at different temperatures. To summarise the conclusion, stalactite and stalagmite growth in Siberian caves suggests that significant permafrost thaw took place 400,000 years ago when the global mean temperature was around 1.5 degrees Celsius higher than it is today. This level of specificity is new, and is important for not being model based. Incidentally, we have an accurate temperature record going back around 800,000 years through the study of ice cores.
When such studies come through, I think it always useful to place them within a risk component analysis framework. As a reminder, risk is best defined as probability times effect—or more specifically probability times net harmful effect. It is also worth recalling that we should not get sidetracked by the accusation that such studies lack certainty. The human condition is one of decision-making under uncertainty. As individuals, the only real certainty we have in our lives is death.
Keeping these points in mind, climate change can be viewed as a chain of risk components, each of which has its own probability distribution. We move from emissions, through atmospheric concentrations (the study of the airborne fraction that I blogged about here), to climate sensitivity, until finally we arrive at the net harmful effects on both individuals and societies.
From a risk perspective, most of the fighting between the ‘climate skeptic’ and consensus science communities takes place within the ‘radiative forcing and global climate’ box; i.e., the climate sensitivity debate. The working consensus proposed by the Intergovernmental Panel on Climate Change (IPCC) is that global average surface warming will rise by 2°C to 4.5°C—with a best estimate of about 3°C—following a doubling of CO2 from pre-industrial levels. Moreover, the IPCC state that it:
…is very unlikely to be less than 1.5°C.
For reference, the ‘lukewarmer skeptics’ like Richard Lindzen and Roy Spencer espouse a sensitivity of 1 degree Celsius or so. Note, the IPCC does not say such claims are not true, but just are ‘unlikely’.
My gripes at this stage with the ‘climate skeptic’ community from a risk management perspective are three-fold:
First, we are talking about probabilities with respect to an underlying distribution of outcomes. It is highly unlikely that one ‘climate skeptic’ endorsed study will suddenly come along and fix a particular distribution as ‘the one’. Getting our heads around the ‘true’ climate sensitivity is going to be a long hard struggle and likely take years, or more probably decades. Dana Nuccitelli has a good post in Skeptical Science on such issues here.
Second, Lindzen and co’s low sensitivity number is ‘a best estimate’. That is, the centre of a distribution. So even if the best estimate is below 1.5 degrees Celsius, it could have a tail of less likely outcomes out to 3 degrees and beyond. So even in a Lindzen world, we can’t all go about our business safe in the knowledge that we won’t get whacked in the future by dangerous climate change. The severe risk remains.
Third, and most important, climate sensitivity is but one risk component within the overarching concept of climate risk. Even with low sensitivity, there are manifold paths to a climate change dystopia. We could keep going on an exponential emissions path, and start pumping out 15 or 2o billion tonnes of carbon a year (rather than 11 billion tonnes as now)—a path that would eventually take us to a trebling of CO2 levels. Or the terrestrial and ocean carbon sinks could become saturated, with the consequence that the annual rise in CO2 levels would accelerate despite flattening emissions. Or the impacts themselves could be much harsher than currently predicted; worse-than-expected drought, for example, could smash the existing globalised food chains, so leading to societal collapse in many food importers.
This takes us back to the original report I started the post with. Unfortunately, permafrost carbon releases are not incorporated in climate sensitivity calculations. Skeptical Science’s Andy Skuce did an overview of permafrost-related papers in October 2012. Andrew MacDougall, one of the authors of a Nature Geoscience paper quoted in the post, was good enough to clear up this point in the comments section:
The permafrost feedback is not included in any of the CMIP3 or CMIP5 climate models. However, CO2 from permafrost does not contribute to climate sensitivity. Climate sensitivity is defined as “to the equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2concentration (IPCC glossary of terms)”. This definition specifically leaves out carbon cycle feedbacks. The climate sensitivity is determined by the direct radiative effect of CO2 and feedbacks from non-CO2 systems (ex. albedo, clouds, water vapour).
Climate change really is a super-wicked problem: risk keeps emerging from so many sources—like a second-rate zombie movie. And permfrost thaw is a big, fat risk. A risk that falls into the ’emissions’ box in the flow diagram above, even if it is only indirectly human-induced.
When it comes to emissions, I am a great fan of Myles Allen’s Trillion Tonnes of CO2 concept (www.trillionthtonne.org) as a good means of ordering one’s thoughts. Here is where we are on the way to one trillion tonnes, which is currently our best estimate of what will determine dangerous climate change if we assume the IPCC’s climate sensitivity number is about correct:
Obviously, this is purely a ranging shot of risk; a ranging shot that says we can emit another 433 billion tonnes of carbon before getting into real trouble (we are currently emitting around 11 billion tonnes of carbon per annum via fossil fuels and land-use change, see here, a number that rises every year). To put the new Siberian study in the context of the trillionth tonne, the Andrew MacDougall and colleagues paper quoted by Andy Skuce’s post in Skeptical Science suggests permafrost thaw could add 174 billion tonnes of carbon by 2100.
The new Siberian cave paper doesn’t give any specific information around the amount of permafrost carbon emissions, but rather focuses on the timing. In short, it shows a mechanism for morphing 2 degrees of dangerous global warming into much higher levels of extremely dangerous warming. In other words, the Siberian cave data suggest that when we get up to around 2 to 2.5 degrees Celsius of warming (we have already warmed about 0.8 degrees and the new paper’s threshold for large permafrost carbon emissions is 1.5 degrees on top of this)—a level of warming that will likely be reached if we emit a trillion tonnes of carbon—then permafrost thaw will start pushing out a lot more carbon into the atmosphere. And that means further temperature rises.
We already knew that warming would continue to increase if we were to cease emissions overnight. But now we have likely confirmation that emissions themselves could continue to rise significantly once a certain temperature tipping point has been reached—even if we switched off every human-induced source of coal, gas and oil-related carbon emissions.
In many ways, the new paper shines a light on our ignorance. We may have a better idea of when large permafrost emissions will kick in, but we don’t know how quickly they could accelerate. From a risk perspective, we are like chimpanzees wondering out of the jungle into the open savanna. There are certain threats, like snakes, with which we are already acquainted; but others, like lions, with which we are not. The ‘climate skeptic’ tells us to discount the risk from lions since we have no experience of them; a prudent chimpanzee would disagree.
Risk and Well-Being 