Really, it\'s enough to drive a climate scientist over the edge. In past years, satellite images have shown a perceptible growth in grasses and shrubs in parts of the Arctic, a phenomenon pinned on global warming. But part of the greening could come from lemmings, surprised researchers have found. University of Texas scientists counted plant cover and biomass in a huge area in coastal Alaska where brown lemmings (Lemmus trimucronatus) have been monitored for more than 50 years in a project to understand their boom-and-bust population cycles. On small plots that had been fenced off to exclude the lemmings, certain plant types called lichens and bryophytes had increased, the researchers found. But where the lemmings scampered unhampered, there was an increase in grass and sedge -- curiously, the very same plants that the hamster-like herbivores feed on. The reason for this is unclear. Urine and faeces from the lemmings could be acting as a fertiliser, helping the plants to grow, the researchers suggests. Alternatively, the rodents could be chomping on the dead grass and sedge litter, which encourages new growth. Either way, the findings pose a tundra conundrum. \"We really need to be careful attributing the greening of the Arctic to global warming alone,\" said the lead investigator, David Johnson. \"We have shown that lemmings can promote similar greening, through the increase of grasses and sedges, as warming does in Arctic regions where lemmings are present and go through dramatic population cycles.\" Global warming is still the big suspect, as greening is happening in areas where lemmings do not occur in large numbers. Higher temperatures open up habitat that plants previously found too chilly. Even so, lemmings and other herbivores could play a bigger role than is thought. Indeed, the lemmings may also be helping to fight climate change. Vegetation is a \"carbon sink,\" because it stores carbon dioxide (CO2), the principal greenhouse gas, through photosynthesis. \"It is plausible that herbivores, in some situations, may provide a mechanism for higher plant growth, maintaining these ecosystems as carbon sinks,\" said Johnson. The paper appears on Friday in Environmental Research Letters, published by Britain\'s Institute of Physics.