Well, I suspect this would only be an important effect if there are more earthworms on the planet now than pre-climate change. Whereas, while there is some expansion into former glaciated lands in N America, humankind otherwise has sealed off much former good worm habitat with millions of hectares of worm-proof structures via urbanisation and roading.
Lubbers et al., (2013) Greenhouse-gas emissions from soils increased by earthworms. Nature Climate Change, doi: http://dx.doi.org/10.1038/nclimate1692
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