How much methane is released into our atmosphere from freshwaters depends on the balance between its production and consumption by two distinct groups of microbes. While we already know that microbes are stimulated by warming i.e., they respire harder at higher temperatures, it is far more challenging to determine how global climate warming will affect the balance between methane production and consumption in the long term. Here we used a unique natural experiment spanning the northern hemisphere to test the effect of warming on methane emissions over centuries to millennial time scales. We used collections of geothermally warmed streams, spanning parts of Alaska, Greenland, Iceland, Svalbard and Kamchatka. The studied streams are indirectly warmed through the bedrock and so are distinct from the harsh environment of their cousins in Yellow Stone Park, for example. Our natural experiment enabled us to measure methane production, consumption and emission and the associated microbes in an environment where they have had plenty of time “to adjust” to warmer conditions, thereby mimicking the long-term effect of climate change. We show that while warmer methane consuming microbes do work harder, they cannot fully counter the extra methane being produced with warming. Our study thus describes a seemingly inevitable increase in methane emissions as Earth continues to warm, building a positive feedback loop into climate change.
The remoteness of our high-latitude study sites ensured we could separate the effect of temperature from, for example, human pollution on methane emissions – but that remoteness came with challenges. While Iceland is a short hop from Heathrow, our sites in Kamchatka, Alaska and Greenland were not just “down the road” and simply getting to these remote locations required combinations of small planes, boats, cars and sturdy hiking boots. Navigating customs to get our equipment crates and samples through could be a challenge, as was the acquisition of chemicals, such as ethanol, nitrogen gas and liquid nitrogen. Although a previous expedition had broadly scouted our sampling locations, the precise location of individual streams was often a bit vague as their GPS coordinates had not been recorded! Rather, in Alaska, we were left to pursue a “treasure map” with streams marked by "landmarks” such as “Mad dog house” and “Man at airport's house”! The physical demand of trekking several miles on rough terrain to sample a stream with heavy field kit was also intense.
Besides, working in the Arctic comes with its own challenges. Fingers go numb when working downwind from a glacier in the rain in Iceland. Mosquitos and black flies make life miserable, as every bit of exposed skin is attacked. Bear encounters are a real threat in Alaska, Kamchatka and especially Svalbard. To protect us from polar bears, we had polar bear guards and received rifle training for extra protection. Luckily, we only encountered bears from a safe distance. In Kamchatka, we were stranded on a dirt road with a flat tyre (one of many) while bears had been spotted in the area, but our loud singing must have kept them at bay. In Greenland and Svalbard, a boat was needed to reach our sites but a miscalculation of tide times - was it the Captain? - in Greenland meant being stranded for hours in the cold on a remote beach with minor provisions and limited heat source. On our next outing, we made sure to bring tents and provisions! In Svalbard, we had to navigate our small sailing boat through fields of ice sheets, while on the return journey we were forced to shelter for 8 hours from a vicious storm.
At times it could have almost felt too much, but to balance the challenges, we also experienced some wonderful moments. For a group of young researchers, brought together from different disciplines, unknown to each other before travelling to these remote places, and working together under strict time constraints and stressful conditions, somehow, it all worked. The surroundings were often spectacular - from volcanos in Kamchatka to overturning icebergs in Greenland and we often caught sight of incredible wildlife, such as whales, polar bears and curious Arctic foxes. Visiting the Arctic between June and September allowed us to experience both the midnight sun -which plays with your sleep- and the northern lights, which were just beautiful. The locals were welcoming, sharing their stories and teaching us a lot about living in these regions and how climate change is affecting their way of life. We rose to the challenges offered by these remote field settings to deliver research we hope raises awareness of the effects of climate warming on greenhouse gas emissions more highly.