In a 2009 paper, "An imperative for climate change planning: tracking Earth's global energy", Trenberth discussed the distribution of heat and how it was affected by climate forcing, including greenhouse gas changes. This could be tracked from 1993 to 2003, but for the period from 2004 to 2008 it was not then possible to explain the relatively cool temperatures of 2008. In the Climatic Research Unit email controversy, an unlawfully disclosed email from Trenberth about this paper was widely misrepresented; he wrote, "The fact is that we can't account for the lack of warming at the moment and it is a travesty that we can't." Trenberth has stated: "It is amazing to see this particular quote lambasted so often. It stems from a paper I published this year bemoaning our inability to effectively monitor the energy flows associated with short-term climate variability. It is quite clear from the paper that I was not questioning the link between anthropogenic greenhouse gas emissions and warming, or even suggesting that recent temperatures are unusual in the context of short-term natural variability." In a 2013 scientific paper in Geophysical Research Letters, Trenberth and co-authors presented an observation-based reanalysis of global ocean temperatures. This proposed that a recent hiatus in upper-ocean warming after 2004 had seen the long-term increase interrupted by sharp cooling events due to volcanic eruptions and El Niño. Despite this, ocean warming had continued below the 700m depth. In a second 2013 paper, Trenberth and Fasullo discussed the effect of the 1999 change from a positive to negative phase of the Pacific Decadal Oscillation. This was associated with a change of surface winds over the Pacific which had caused ocean heat to penetrate below 700m depth and had contributed to the apparent global warming hiatus in surface temperatures during the previous decade. In an interview, Trenberth said, "The planet is warming", but "the warmth just isn't being manifested at the surface." He said his research showed that there had been a significant increase in deep ocean absorption of heat, particularly after 1998. He told Nature that "The 1997 to '98 El Niño event was a trigger for the changes in the Pacific, and I think that's very probably the beginning of the hiatus". He said that, eventually, "it will switch back in the other direction." Trenberth's explanation attracted wide attention in the press. Trenberth received the 2017 Roger Revelle Medal from the American Geophysical Union for his work on climate change issues.
