The linked article contains many good points (see the extract below), supporting the position that there may be no budget available for more carbon emissions if society wants to take a responsible approach towards climate change risks:
http://www.climatecodered.org/2014/06/carbon-budgets-climate-sensitivity-and.htmlExtract: "… preliminary results by scientists at the California Institute of Technology Jet Propulsion Laboratory presented at the 2013 AGU meeting showed that higher sensitivity models do the best job simulating observed cloud changes. These results are also consistent with Lauer et al. (2010) and Clement et al. (2009), which looked at cloud changes in the Pacific, finding the observations consistent with a positive cloud feedback" (4).
If indeed ECS is more likely at the higher end of the range, this would diminish the remaining carbon budget. Quantifying a carbon budget for a ~4°C mid-point ECS has not been done as far as I can ascertain.
Long-term earth system sensitivityPaleoclimatology (study of past climates) suggests that if longer-term feedbacks of "slow" factors are taken into account, such as the decay of large ice sheets, changes in the carbon cycle (changed efficiency of carbon sinks such as permafrost and methane clathrate stores, as well as biosphere stores such as peatlands and forests), and changes in vegetation coverage and reflectivity (albedo), then the Earth's sensitivity to a doubling of CO2 could itself be double that of the "fast" climate sensitivity predicted by most climate models, or around 6°C (5). These "slow" feedbacks amplify the initial warming burst. A measure of these effects for a doubling of CO2 is known as Earth System Sensitivity (ESS).
Longer-term ESS is generally considered to come into play over periods from centuries to several millennia, depending on how fast is the rate of change in greenhouse gas levels and temperature.
The problem is that rate of climate change now being driven by human actions may be as fast as any extended warming period over the past 65 million years, and it is projected to accelerate in the coming decades. This means that longer-term "slow" events associated with ESS – such as loss of large ice sheets, and changes in Arctic and biosphere carbon stores – are starting to occur now, are happening much more quickly than expected, and likely will proceed at a significant scale in the current hundred years. We face an event unprecedented in the last 65 million years of "fast" short-term and "slow" long-term climate sensitivity events occurring alongside one another in parallel, rather than one after the other in series as is usually the case. Thus, even as some of the "fast" warming is still to be realised due to thermal inertia, some of the "slow" feedbacks are already coming into play:
Evidence from Earth’s history suggests that slower surface albedo feedbacks due to vegetation change and melting of Greenland and Antarctica can come into play on the timescales of interest to humans, which could increase the sensitivity to significantly higher values, as much as 6°C … the slow feedback climate sensitivity has relevance in the Anthropocene era, since ice sheet/vegetation feedback may become significant on decadal-to-centennial timescales of interest to humans (6).
and
Unfortunately, slow feedbacks are amplifying on time scales that humans care about: decades, centuries, even millennia. As the planet warms, for example, ice sheets melt, exposing a darker surface that increases warming. Also warming causes a net release of long-lived greenhouse gases from the ocean and soil. Vegetation changes that occur as climate warms from today's situation will also have a significant amplifying effect, as forests move into tundra regions in North America and Eurasia (7).
The problem is that the IPCC carbon budget analysis assumes that none of these longer-term feedbacks will be materially relevant before 2°C of warming, and so exclude the possibility of large-scale permafrost, methane clathrate or less efficient biological stores (Amazon, tundra etc) making contributions to atmospheric greenhouse gas levels and impacting on the carbon budget.
Thus the IPCC 2013 report notes that "Accounting for ... the release of greenhouse gases from permafrost will also lower..." the target, and that the CMIP5 modelling used for the IPCC's carbon budgets does not include "explicit representation of permafrost soil carbon decomposition in response to future warming". It also notes that "the climate sensitivity of a model may... not reflect the sensitivity of the full Earth system because those feedback processes ["slow feedbacks associated associated with vegetation changes and ice sheets"] are not considered".
Several lines of evidence suggest theses assumptions are not robust. Recent research shows that the Amazon may often be releasing huge quantities of CO2 to the atmosphere, acting not as a carbon sink but as a source (
; and that the seafloor off the coast of Northern Siberia is releasing more than twice the amount of methane as previously estimated and is now on par with the methane being released from the Arctic tundra (9).
In February 2013, scientists using radiometric dating techniques on Russian cave formations to measure historic melting rates warned that a +1.5ºC global rise in temperature compared to pre-industrial was enough to start a general permafrost melt. They found that “global climates only slightly warmer than today are sufficient to thaw extensive regions of permafrost.” Lead researcher Anton Vaks says that: “1.5ºC appears to be something of a tipping point” (10).
In 2011, Schaefer, Zhang et al. warned: "The thaw and release of carbon currently frozen in permafrost will increase atmospheric CO2 concentrations and amplify surface warming to initiate a positive permafrost carbon feedback (PCF) on climate…. [Our] estimate may be low because it does not account for amplified surface warming due to the PCF itself….We predict that the PCF will change the Arctic from a carbon sink to a source after the mid-2020s and is strong enough to cancel 42-88% of the total global land sink. The thaw and decay of permafrost carbon is irreversible and accounting for the PCF will require larger reductions in fossil fuel emissions to reach a target atmospheric CO2 concentration" (11).
This very strong and disturbing finding – that permafrost decay is "irreversible" and requires a lower carbon budget – is not reflected in the IPCC's figuring.
ConclusionClimate change with its non-linear events, tipping points and irreversible events – such as mass extinctions, destruction of ecosystems, the loss of large ice sheets and the triggering of large-scale releases of greenhouse gases from carbon stores such as permafrost and methane clathrates – contains many possibilities for catastrophic failure.
If climate sensitivity is, in reality, at the high end of the range used for the IPCC's carbon budgets, then as a consequence that means that we must adopt a very low-risk of exceeding the target. As the previous post explained, If a risk-averse (pro-safety) approach is applied – say, of less than 10% probability of exceeding the 2°C target – to carbon budgeting, there is simply no budget available, because it has already been used up. The notion that there is still "burnable carbon" is a myth."
