In cold periods, the methane concentration was typically between 0.3 and 0.4 parts per million, whereas in previous warm periods it was 0.6 — 0.7 ppm. (environmentalresearchweb.org)
He said it's not that simple, explaining that it's the relative ranking of probabilities of last year and previous warm years that leads to the conclusion that 2014 tops the list. (dotearth.blogs.nytimes.com)
The null hypothesis is that whatever caused previous warm spells is operating now. (wattsupwiththat.com)