The soils of the northern hemispheric permafrost region are estimated to contain 1100 to 1500 Pg of carbon. A substantial fraction of this carbon has been frozen and therefore protected from microbial decay for millennia. As anthropogenic climate warming progresses much of this permafrost is expected to thaw. Here we conduct perturbed model experiments on a climate model of intermediate complexity, with an improved permafrost carbon module, to estimate with formal uncertainty bounds the release of carbon from permafrost soils by the year 2100 and 2300 CE. We estimate that by year 2100 the permafrost region may release between 56 (13 to 118) Pg C under Representative Concentration Pathway (RCP) 2.6 and 102 (27 to 199) Pg C under RCP 8.5, with substantially more to be released under each scenario by the year 2300. Our analysis suggests that the two parameters that contribute most to the uncertainty in the release of carbon from permafrost soils are the size of the non-passive fraction of the permafrost carbon pool and the equilibrium climate sensitivity. A subset of 25 model variants are integrated 8000 years into the future under continued RCP forcing. Under the moderate RCP 4.5 forcing a remnant near-surface permafrost region persists in the high Arctic, eventually developing a new permafrost carbon pool. Overall our simulations suggest that the permafrost carbon cycle feedback to climate change will make a significant contribution to climate change over the next centuries and millennia, releasing a quantity of carbon 3 to 54 % of the cumulative anthropogenic total.
Soils of the northern hemispheric permafrost region are
estimated to contain between 1100 and 1500 Pg C of organic matter
The objective of this study is to use the new constraints on the quantity and
quality of the permafrost carbon pool to explore key questions about the
effect of the permafrost carbon pool on climate change. The questions we will
investigate are as follows. (1) How much carbon will be released from permafrost soils
by the
years 2100 and 2300, and what are the uncertainty bounds on these estimates?
(2) Which of the uncertain parameters identified by
For the purposes of analyzing incubation experiments and modelling of soil
respiration, soil carbon is conventionally conceptualized as a small number
of carbon pools each with an characteristic resistance to decay
In general there are two sources of uncertainty in modelling: structural
uncertainty and parameter uncertainty
There are many methods to propagate uncertainty in model parameters into
uncertainty in model outputs
Anthropogenic climate change will not cease in year 2100
The UVic ESCM is a climate model of intermediate complexity with a full three-dimensional ocean general circulation model coupled to a simplified
moisture–energy balance atmosphere and thermodynamic–dynamic sea-ice model
The ocean inorganic carbon cycle is simulated following the protocols of the
ocean carbon cycle model intercomparison project
The version of the UVic ESCM used here is based on the frozen ground version
documented in
A permafrost carbon module was added to the UVic ESCM by
To fix this deficiency, diffusion is carried out with an effective carbon concentration which is related to the actual carbon concentration by
In the present version of the UVic ESCM permafrost carbon is treated as an
entirely separate soil carbon pool. Permafrost carbon is created when carbon
is diffused across the permafrost table. The permafrost carbon can only be
destroyed through simulated microbial respiration. This scheme allows the
properties of the permafrost carbon to be prescribed. Permafrost carbon is
also assigned an available fraction, which is effectively the combined
fraction of the fast and slow soil carbon pools. When permafrost carbon
decays the available fraction is reduced by the appropriate amount. The
available fraction is increased as a function of time and soil temperature
with a permafrost carbon transformation parameter determining the rate of
change. This scheme effectively slowly transforms the passive fraction of the
permafrost carbon into the slow soil carbon pool where it can be respired to
CO
Figure
Comparison of the estimated soil carbon density in the top 3 m of soil in the northern
hemispheric
permafrost region from
The saturation factor from Eq. (
We have chosen to perturb four parameters that describe the permafrost carbon
pool: (1) the quantity of soil carbon in the top 3 m of soil in the
permafrost region, taken from
Besides the parameters we have chosen to perturb, many other parameters in the
UVic ESCM could affect the magnitude of the release of carbon from permafrost
soils. In particular parameters from the Triffid dynamic vegetation model
that control net primary production determine the input of carbon into the
soil, and therefore the net change is soil carbon in response to warming.
However, for this study we have chosen to focus on uncertainty inherent to
the permafrost carbon system instead of taking a global focus implied in
perturbing the whole terrestrial carbon cycle
The quantity of carbon in permafrost soils is controlled by changing the
saturation factor
Arctic amplification can be changed in the UVic ESCM by changing the
meridional diffusivity of the simplified atmospheric model
Probability distribution functions of the six parameters perturbed in this study. Panel
The Latin hypercube sampling, described in the introduction, was used to
create the parameter sets. Each PDF was sampled from 25 equal-probability
intervals and the value selected from each interval was randomly matched to one
of the values selected from each of the other PDFs to create a “cube”
containing 25 parameter sets. This sampling was repeated 10 times to create
10 cubes for a total of 250 model variants. Each of these variants was
spun up for 5000 years under estimated year 1850 forcing to generate the
permafrost carbon pool. Each model variant was forced with historical forcing
followed by each of the four representative concentration pathways (RCPs)
used in the fifth assessment report of the Intergovernmental Panel on Climate
Change (IPCC AR5). The simulations were carried out with prescribed
atmospheric CO
The old permafrost carbon capable version of the UVic ESCM was able to
quantify the previously unaccounted for temperature effect of the permafrost
carbon feedback by comparing model simulations with and without permafrost
carbon
Twenty-five model variants (one cube) were projected 8000 years into the
future under continued RCP 4.5 and 8.5 forcing. For this experiment only the
four permafrost carbon parameters were perturbed, and climate sensitivity and
arctic amplification were held at their model default values. For each
scenario the models were forced with prescribed atmospheric CO
Release of carbon from permafrost soils by year 2100 and 2300 for each RCP scenario. Ranges are 5th to 95th percentiles. All values are in Pg C.
The release of carbon from permafrost soils for each RCP and for each of the
250 model variants is shown in Fig.
Release of carbon from the permafrost region for all 250 model variants (grey lines) and four RCP scenarios. Mean for each scenario shown with think solid line. Fifth and 95th percentiles shown with dashed lines.
Emission of carbon from permafrost soils for each model variant (grey lines) and each RCP scenario. Mean for each scenario shown with think solid line. Fifth and 95th percentiles shown with dashed lines.
The emission rate of CO
Peak emission rate of carbon from permafrost soils for each RCP
scenario. Ranges are 5th to 95th percentiles. All values are in Pg C a
The permafrost carbon feedback's effect on climate change will ultimately be
determined by how large the release of carbon from permafrost soils is
relative to the cumulative fossil fuel emissions
In year 1850 the UVic ESCM has a northern hemispheric permafrost area
(including the Tibetan plateau) of 14.87 million km
Reduction in the size of the northern hemispheric permafrost region
by year 2100 and 2300 relative to year 1850 (14.9 million km
The relative importance of uncertainty from each perturbed model parameter to
the overall uncertainty can be evaluated by computing the correlation
coefficient between the parameter value and the value of some model output
Correlations were also conducted between each model perturbed parameter value and release of carbon from permafrost soils by 2300. By 2300 the importance of the initial available fraction has decreased and has an R value of 0.36, the correlation with permafrost carbon transformation rate has increased to an R value of 0.43, and the correlation with climate sensitivity has increased to 0.64. The correlations with initial quantity of carbon in the permafrost region, permafrost carbon decay rate, and arctic amplification remain weak by year 2300, at 0.13, 0.02, and 0.11 respectively. These results demonstrate that the relative importance of uncertainty in parameters changes depending on the time frame of interest.
The low sensitivity of the release of carbon from permafrost soils to the
value of Arctic amplification appears counterintuitive. However, most of the
carbon held in the permafrost region is held in the region's southern extent
(Fig.
Overall these results are encouraging as the most important factor for
determining release of carbon from permafrost soils in the next century, the
size of the permafrost carbon fast and slow pools, can be measured with
incubation experiments
Cumulative emissions from permafrost soils relative to diagnosed compatible emissions for each model variant (grey lines) and each RCP scenario. Mean for each scenario shown with think solid line. Fifth and 95th percentiles shown with dashed lines. Note that under scenarios with lower emissions permafrost carbon emissions are larger relative to fossil fuel emissions.
Correlation between release of carbon from the permafrost region in year 2100 under RCP 8.5 and value of perturbed model parameters. Red line is line of best fit and R is correlation coefficient.
Correlation between release of carbon from the permafrost region and change in global temperature at years
2100, 2200, and 2300 CE. Red line is line of best fit and
Climate change mitigation targets are often framed in terms of some global
temperature change threshold not to be breached
The evolution of global mean temperature and atmospheric CO
Evolution of CO
The response of the permafrost carbon pool to millennia of anthropogenically
enhanced temperatures varies by scenario followed. Under RCP 8.5 the pool
monotonically declines with time, with the rate of decline varying by
parameter set (Fig.
Evolution of permafrost soil carbon pool under continued RCPs 4.5 and 8.5 forcing until common era year 10 000 (8000 years into the future). Vertical black line indicates change in horizontal scale. Under RCP 4.5 forcing the permafrost carbon pool undergoes a recovery in the late third millennium and under RCP 8.5 forcing declines toward a near-zero value.
The release of carbon in these simulations is smaller than the previous
estimate using an earlier version of this model
The study most similar to the present study is that of
Difference between soil carbon density in the Northern Hemisphere between 1875 and 5250 CE under continued RCP 4.5 forcing. A large permafrost carbon pool has developed in the high arctic by year 5250.
There are many processes that affect the thaw of permafrost and decay of
permafrost carbon that are not accounted for in the UVic ESCM. The UVic ESCM
has permafrost carbon only in the top 3.35 m of soil and therefore does not
account for the substantial quantity of carbon held below 3 m in deltaic
deposits 91
The quantity of carbon held in the northern hemispheric permafrost region is
enormous but incubation experiments conducted on samples of this organic
matter show that most of it is highly resistant to decay
Here we have used a perturbed physic ensemble to place an uncertainty constraint on the release of carbon from permafrost soils. We find that by 2100 the permafrost region may release 56 (13 to 118) Pg C under RCP 2.6, 71 (16 to 146) Pg C under RCP 4.5, 74 (15 to 154) Pg C under RCP 6.0, and 102 (27 to 199) Pg C under RCP 8.5, with substantially more to be released under each scenario by 2300. Of the six parameters perturbed the simulations are most sensitive in year 2100 to uncertainty in the size of the non-passive soil carbon pools and the equilibrium climate sensitivity. Additionally, by 2300 the transformation rate of the passive pool into carbon susceptible to decayed has become important. The simulations are insensitive to uncertainty in Arctic amplification, slow carbon pool overturning time, and the initial quality of carbon in the permafrost region. Our results suggest that a well-designed field campaign and set of incubation experiments intended to better constrain the size of the fast and slow carbon pools in permafrost soils could substantially reduced the uncertainty in the strength of the permafrost carbon cycle feedback. Contingent on our model structure being reflective of the natural world.
We have also projected a subset of a model variants 8000 years into the future, with simulations conducted to the year 10 000 CE under continued RCP 4.5 and 8.5 forcing. These simulations suggest that if permafrost survives in the high arctic, a new permafrost carbon pool may develop leading to a recovery of this carbon pool. Under higher forcing where near-surface permafrost ceases to exist outside Antarctica, the permafrost carbon pool nearly totally decays away over several thousand years. Overall our simulations suggest that the permafrost carbon cycle feedback to climate change will make a substantial contribution to climate change over the next centuries and millennia.
We are indebted to the efforts of the Permafrost Carbon Network for
organizing the collection of data on permafrost carbon quantity and quality.
G. Hugelius graciously provided the data for the map in Fig.