Storms and hurricanes

Trenberth began some fundamental work related to changes in extremes with climate change in 1998. Until then, the focus of the scientific community had been mainly on changes in average temperatures and precipitation. Trenberth pointed out that the intermittent nature of precipitation mandated attention to intensity, frequency, duration, and type as well as amount.^[16] All storms reach out and gather in the available water vapor, which fuels the storm. Therefore, increases in water vapor in the atmosphere with higher temperatures will lead to greater intensity but less frequency of storms. This is because the total amount of water vapor is controlled by surface evaporation, not temperature.^[16][17] The prospects are therefore for more severe storms.  

Until 2004, little attention had been paid to hurricanes and tropical storm changes, but the summer of 2004 was when four hurricanes made landfall in Florida. The question was whether there was a human global warming role in the activity and thus the damage. To Trenberth it was obvious that there was, and he spoke up when official NOAA statements on hurricanes attributed it all to natural climate variability. Trenberth participated in a tele-news conference, set up by Harvard University, and cautiously suggested that global warming was undoubtedly playing some role.^[13]: 75 This led to a major outcry from some hurricane meteorologists, and extensive criticism for example by Christopher Landsea, an American meteorologist.^[18] 

As a response, Trenberth published further research on this topic in mid 2005.^[19]  Coincidentally, a record breaking hurricane season began shortly after (still in 2005) in which Hurricane Katrina caused all kinds of devastation in New Orleans. Two important studies who supported Kevin's research findings came out shortly thereafter: One by Kerry Emanuel,^[20] and another led by Peter Webster.^[21]  Further details on natural variability were provided in a publication by Kevin Trenberth and Dennis J. Shea in 2006.^[22] Trenberth further explained the concept to a broader audience in an article on hurricanes and climate change in Scientific American in 2007.^[23] It has the short and snappy title: "Warmer Oceans, Stronger Hurricanes".

Short-term climate 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 700 m depth.^[24]

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.^[25]

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.^[26] 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."^[27] Trenberth's explanation attracted wide attention in the press.^[27][28][29]

Hacked e-mail controversy in 2009

Kevin Trenberth was "one of the victims in “Climategate” where hacked emails from climate scientists were distorted by climate-change deniers to sow confusion."^[30] In the Climatic Research Unit email controversy, an unlawfully disclosed email from Trenberth about one of his publications from 2009 was widely misrepresented; he had written, "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." In that 2009 paper, "An imperative for climate change planning: tracking Earth's global energy",^[31] Trenberth had 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.

Trenberth has stated later: "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."^[32]

Public stance on climate change

For decades, Kevin Trenberth has been outspoken about climate change and the urgency to take action. One of his key messages has been: "It’s real, the problem is cumulative, and we’re causing it. Today’s blanket of greenhouse gases would disperse only over centuries. Cutting emissions is the most important of all possible responses."^[30] And "we also have to build resilience to the new extremes".^[30]

Selected books