Q&A with National Center for Atmospheric Research Climate Scientist Kevin Trenberth

Kevin Trenberth is one of the leading climate scientists in the world. Along with his colleagues at the Intergovernmental Panel on Climate Change (IPCC) and former Vice President Al Gore, Trenberth is a recipient of the 2007 Nobel Peace Prize. Recently, Trenberth has been working with researchers at NCAR to better understand how the Earth system transfers energy between the atmosphere and the ocean. This subject is of particular importance today as a controversial talking point of those skeptical of action on climate change gains steam. Trenberth has much to say on the matter.

Q: Skeptics like to point out that if you look at the annual global average temperature data since 1998, there’s been no warming. What do you make of this talking point?

A: That is very much cherry picking the start date as to when you are looking at trends. 1998 is the warmest year in the 20th century. The skeptics are apt to use that as a starting point and say, alright let’s start with 1998 and look and see what the trend is since then. Statistically, there’s no good reason for doing that.

It’s also very narrowly configured towards the global mean surface temperature, which is just one small part of what’s going on with the planet and with global warming. Global warming means much more than just global mean surface temperature rise. It’s also melting sea and land ice, sea level rise, the warming of the ocean, the warming of land, and all of the other changes that are going on in association with global climate change.

Q: So where is the warming going?

A: It’s not as if global warming has stopped, it’s just that it’s being dumped into different parts of the climate system. If you look at the Arctic, for instance, Arctic sea ice was at a record low in 2012. There’s been a 40% decrease in Arctic Sea ice. Similarly, if you look at global sea level, which is perhaps the best single indicator of global warming, the rate has remained remarkably steady at around 3.2mm/year.

Q: What role is natural variability playing in all of this?

A: Natural variability seems to have been quite strong in the last decade or so, focused in the Pacific Ocean in what we call the Pacific Decadal Oscillation (PDO). The PDO is playing an important role in all of the weather patterns that we’re seeing around the world.

Q: Is it possible that global surface temperatures could start increasing again soon?

A: If you look at the past record you can see that. There’s often not that much change in the global mean temperature for several years, and then suddenly it jumps by 3/10 of a degree C or something like that. But then it never really goes back down to the previous level. So it often seems to go more like a staircase, it goes along and it jumps a bit and it goes along and then jumps a bit. It does relate to when the natural variability is going in the same direction as the climate change wants, then that’s when you get this step in the staircase. It’s not like it’s a linear trend, where it’s gradual relentless warming one year after the next. I think that’s something that people expect, but in fact, that’s not the way the system works.

Q: So are the models wrong?

A: Most of the model’s projections for climate change have been looking out to 2100, not predicting individual years. But the pressure is on now to do shorter term and more regional projections. Some of that is inherently not possible due to natural variability. It’s a misrepresentation of what the climate science has said in the past and the IPCC has said, to claim that the hiatus is not predicted. The system was never designed to predict it in the first place.


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