A New Source of Atmospheric Methane

Helen - Well let's stick with changing climate and the changing world around us. As the world warms the permafrost is beginning to melt, which is allowing bacteria to change carbon-rich material laid down over 30 000 years ago into the greenhouse gas methane. But how much gas is being produced? Well it's very difficult to quantify because the bubbles come out of thaw lakes, but Katey Walter from the University of Alaska, Fairbanks has used bubble traps to work out how much methane is emerging. It's enough to increase the methane contribution from the northern wetlands by up to 63%.

Katey - This work is all about quantifying a new source of atmospheric methane which was previously not recognised as a large and significant source, and that is bubbling from thaw lakes, lakes where the permafrost is melting and the lakes continue to expand as they melt into that permafrost, that's where they get the name thaw lakes.

Chris - So how have people tried to measure this in the past, or haven't they?

Katey - In the past scientists have measured methane emissions from lakes in two ways, they measure the diffusive emission where methane moves along a concentration gradient, from the sediments into the atmosphere, and they've done that by just measuring the concentration of methane in the surface water of the lakes. Another source of methane from lakes is bubbling, and that's a much more difficult source of methane to quantify because bubbling is very rare both in space and time.

Chris - So what have you done to get these accurate quantifications of them?

Katey - We have the excellent opportunity in Siberia to study bubbling because when the ice forms on the lakes in Autumn it's like putting a piece of Saran Wrap across the surface of the lakes, it traps the bubbles in place as they wobble to the surface and then they freeze into place in the ice. And we can walk across the ice and map out the distribution of point sources and hot spots.

Chris - So you walk out on the ice, you can see where the bubbles are coming up. But then how do you physically work out how much gas is there?

Katey - We've constructed bubble traps out of greenhouse plastic and copper wire and we place those either under the ice or in the summer when there is no ice we just place them floating under the water surface and each trap captures the bubbles that come up continuously. And so we would go out every day and measure the volume of bubbles that had collected.

Chris - So in the grand scheme of things how much methane is this actually contributing to the global environment?

Katey - Well, scaling up, the type of Siberian lake that we were studying, we estimate that methane emissions from these lakes is about 3.8 teragrams per year. Now, these lakes are only a portion of the northern lakes in general so if bubbling is something that happens everywhere then this could be an even much larger phenomenon than just the scope of our Siberia work. And now we see that just adding this small portion of Siberian lakes to the northern wetland emission estimate it increases it by up to 63%, ten to 63%.

Chris - So what are the implications if you add this to the global warming equation, then?

Katey - This is a new positive feedback to global warming. Methane is a very strong greenhouse gas and so as methane is being produced it is trapped in the atmosphere, increasing atmospheric warming which then enhances the thaw and the expansion of these lakes further. So today there are still about 500 gigatons of carbon remaining in this unique type of Siberian permafrost and it's projected that during the next century the majority of that will degrade and that can release tens of thousands of teragrams more carbon into the atmosphere.

Chris - So should this provoke a rethink of what we think is actually likely to happen in terms of global warming in the future, then?

Katey - Well, one component of the general circulation models that is missing is permafrost degradation, and especially with regards to the large pools of carbon that are stored in permafrost. That carbon content is still poorly known let alone these positive feedbacks to climate change that can happen from permafrost degradation. So yes, we do have a lot of rethinking and incorporation of these new sources.