Permafrost - a game changer in the carbon cycle?
A few weeks ago, professor Viv Jones gave a lecture about the Arctic and its carbon cycle. Before attending this lecture, I was aware that some methane was stored in the frozen ground in the Arctic and the seabed under the Arctic, but I never realised just how much.
Professor Jones mentioned that 500 - 2500 Gt (1015g) of 'frozen' methane (methane clathrates) locked up in marine sediments in Arctic coastal waters, could be released into the atmosphere as a consequence of global warming. To put that in perspective, global methane emissions according to the IPCC are between 0.542 and 0.852 Gt per year.
To explore this potential danger a bit more, we will zoom in today on one of the main methane reservoirs of the Arctic: the permafrost.
A few years ago, scientists feared a tipping point, at which the positive feedback of GHG release from the permafrost would result in unstoppable global warming. However, when scientists from the Permafrost Carbon Network published a review study in Nature in 2015, the view of the 'carbon explosion' changed. The study by Schuur et al. considers that a large amount of carbon will be released from the permafrost if warming continues, but the release will be gradual - giving us time to adapt to global warming to the consequent warming.
Based on long-term laboratory incubations, dynamic models and new field observations, Schuur et al. conclude that 5-15% of the carbon in the permafrost (130-160 Pg C) could be released during this century. This amount of carbon, mainly released as CO2, would not trigger abrupt climate change. The study was innovative in its use of such extended long-term laboratory experiments and their models which were more complex than any produced previously.
Models of permafrost thawing and subsequent carbon release still vary a lot and are hardly ever consistent with field observations. The dynamics of permafrost thawing are complex and a lot of factors have to be taken into account, which are often left out: what time does it take microbes to process carbon into carbon dioxide and methane? What are the dynamics of carbon transfer within the soil? The carbon pool in models is often only measured in the first 3m depth, giving about half the amount of carbon as field measurements do.
The substantial impact of these factors is demonstrated in this new study in Nature, published last week. In the study, Koven et al. look at the positive carbon feedback to warming and try to capture the efficiency of microbes in their model. They introduce the parameter tau (τ), the turnover time, which gives an indication of the time it takes to process the total carbon stocks by respiration by microbes. They find that the turnover time is more sensitive to changes in temperature at cold temperatures (like the carbon in permafrost) than at warmer temperatures (fig. 3). When the permafrost warms, microbes become more efficient and convert more carbon to CO2 and methane. So, apart from the temperature feedback of increased permafrost thawing, there is also a feedback in the efficiency of respiration by microbes.
These uncertainties in permafrost dynamics are the reason why permafrost carbon emissions are often omitted from climate models, such as in the IPCC AR5. Taking GHG release of permafrost into account means that we would need to further reduce our emissions from fossil fuels and land use changes if we want to stay under 2°C warming.
However, observations in the Arctic reveal that 8 Tg (1012g) yr-1 methane is released to the surface. To put that into context, fossil fuels cause an increase of circa 105 Tg yr-1 .
So for now, permafrost emissions won't kill us. But nobody really knows what will happen in the future and we are reliant on further scientific research to tell us. If you want to stay up to date with the research, Nature and Nature GeoScience both have a collection of selected articles on permafrost.
Professor Jones mentioned that 500 - 2500 Gt (1015g) of 'frozen' methane (methane clathrates) locked up in marine sediments in Arctic coastal waters, could be released into the atmosphere as a consequence of global warming. To put that in perspective, global methane emissions according to the IPCC are between 0.542 and 0.852 Gt per year.
To explore this potential danger a bit more, we will zoom in today on one of the main methane reservoirs of the Arctic: the permafrost.
Permafrost is ground that has been frozen for at least two years in a row ('permanently frozen'), but can be frozen for thousands of years and can extend to almost 5000 ft underground. Permafrost is mainly found in the Arctic and takes up 24% of the land surface of the Northern Hemisphere (fig. 1).
Especially the Arctic is very vulnerable for changes in climate. As permafrost is thawing, billions of tons of carbon, stored in the soil are being released. This carbon is decomposed by microbes to methane and CO2, warming the Earth more, causing further thawing of the permafrost: a positive feedback (fig. 2).
As you can imagine, permafrost became a dangerous word for everyone and it didn't escape the newspapers: The New York Times, The Guardian, The BBC, have all reported on the potential danger.
But nobody really knows how much and how fast carbon could be released from the permafrost.  |
Figure
1 | a) Soil organic carbon
map (kg C m-2), measured in the first 3 m
of the permafrost. Black dots show field sites where the measurements are
taken. b) Map of the distribution of permafrost. Continuous permafrost(>90%)
is shown in purple, discontinuous permafrost (<90%) is pink.
|
 |
| Figure 2 | Positive feedback of greenhouse gas release in the permafrost cycle. Permafrost thaws, releasing carbon dioxide and methane, resulting in atmospheric warming and more permafrost thawing. |
As you can imagine, permafrost became a dangerous word for everyone and it didn't escape the newspapers: The New York Times, The Guardian, The BBC, have all reported on the potential danger.
A few years ago, scientists feared a tipping point, at which the positive feedback of GHG release from the permafrost would result in unstoppable global warming. However, when scientists from the Permafrost Carbon Network published a review study in Nature in 2015, the view of the 'carbon explosion' changed. The study by Schuur et al. considers that a large amount of carbon will be released from the permafrost if warming continues, but the release will be gradual - giving us time to adapt to global warming to the consequent warming.
Based on long-term laboratory incubations, dynamic models and new field observations, Schuur et al. conclude that 5-15% of the carbon in the permafrost (130-160 Pg C) could be released during this century. This amount of carbon, mainly released as CO2, would not trigger abrupt climate change. The study was innovative in its use of such extended long-term laboratory experiments and their models which were more complex than any produced previously.
Models of permafrost thawing and subsequent carbon release still vary a lot and are hardly ever consistent with field observations. The dynamics of permafrost thawing are complex and a lot of factors have to be taken into account, which are often left out: what time does it take microbes to process carbon into carbon dioxide and methane? What are the dynamics of carbon transfer within the soil? The carbon pool in models is often only measured in the first 3m depth, giving about half the amount of carbon as field measurements do.
The substantial impact of these factors is demonstrated in this new study in Nature, published last week. In the study, Koven et al. look at the positive carbon feedback to warming and try to capture the efficiency of microbes in their model. They introduce the parameter tau (τ), the turnover time, which gives an indication of the time it takes to process the total carbon stocks by respiration by microbes. They find that the turnover time is more sensitive to changes in temperature at cold temperatures (like the carbon in permafrost) than at warmer temperatures (fig. 3). When the permafrost warms, microbes become more efficient and convert more carbon to CO2 and methane. So, apart from the temperature feedback of increased permafrost thawing, there is also a feedback in the efficiency of respiration by microbes.
 |
| Figure 3 | The turnover time (carbon stocks to net primary productivity) plotted against the annual mean air temperature. The lower the air temperature, the steeper the regression line. |
These uncertainties in permafrost dynamics are the reason why permafrost carbon emissions are often omitted from climate models, such as in the IPCC AR5. Taking GHG release of permafrost into account means that we would need to further reduce our emissions from fossil fuels and land use changes if we want to stay under 2°C warming.
However, observations in the Arctic reveal that 8 Tg (1012g) yr-1 methane is released to the surface. To put that into context, fossil fuels cause an increase of circa 105 Tg yr-1 .
So for now, permafrost emissions won't kill us. But nobody really knows what will happen in the future and we are reliant on further scientific research to tell us. If you want to stay up to date with the research, Nature and Nature GeoScience both have a collection of selected articles on permafrost.
Comments
Post a Comment