In this interview with Special Topics correspondent Simon Mitton, Dr. Simon Tett of the UK's Hadley Centre for Climate Prediction and Research discusses his work on modelling climate change, and the impact of his highly cited papers. Dr. Tett ranks among the top 20 researchers in the field of global warming over the past decade. His most-cited paper, "Climate response to increasing levels of greenhouse gases and sulfate aerosols," (J.F.B. Mitchell, T.C. Johns, J.M. Gregory, S.F.B. Tett, Nature 376 [6540]: 501-4, 10 August 1995) ranks at #2 on our list, with 305 citations. Dr. Tett's work can be found in the field of Geosciences in ISI Essential Science Indicators Web product.

  You did your postgraduate training at Edinburgh University where you worked in the departments of physics and meteorology. How did you come to choose Edinburgh, and why?

When I was completing my degree in Mathematical Physics in 1987 I was getting interested in climate. In particular I was interested in why the Earth was the temperature it was. It was much warmer than you'd expect given the distance from the sun and the planetary albedo. The answer was "the greenhouse effect." I was also interested in software—at least the scientific side of the problem. So I did a Ph.D. in "Parallel algorithms for atmospheric modelling," which was all about how you could use parallel computers to do weather prediction and climate modelling. I think I did something right because the models I run today are running on a parallel computer. My Ph.D. was in two departments (Meteorology and Physics) and I could have fallen between the gaps, but I had two excellent supervisors: Bob Harwood and Richard Kenway.

  After you had completed the doctorate you went to work for the Hadley Centre, and you've been there ever since. Why did you choose to work for the research arm of the Met Office?

The Hadley Centre opened in 1990, and its mission is to provide the UK with an up-to-date expert assessment of natural and anthropogenic changes in global and regional climate. The specific areas of research are to understand the physical, chemical, and biological processes within the climate system; to make state-of-the-art models to represent them; and to use the models to simulate the changes of the last 100 years and make predictions for the next 100 years. In 1990 the Hadley Centre did a special recruitment. The advert in New Scientist sounded very exciting and I knew I wanted to work in the area. I was very pleased to be offered a post in the climate change group and joined in early 1991.

  Your paper (S. Tett, "Simulation of El-Niño/Southern Oscillation-like variability in a global AOGCM and its response to CO2 increase," Journal of Climatology, 8 [6]: 1473-1502, 1995) marks the start of your interest in comparing models with real data, correct?

Yes, when I tried to simulate the El-Niño/Southern Oscillation, I noticed something odd: apparently no one working on the models was bothering to look at the data to see what had gone on in the past. This stimulated my interest in the natural variability of the climate, and the use of the observed variability to benchmark the models. The 1995 paper shows how El-Niño responds to a carbon dioxide increase in the Atmosphere Ocean General Circulation Model. By this stage, too, a lot of us here were deeply embroiled in many aspects of climate change.

  Your most-cited paper, "Climate response to increasing levels of greenhouse gases and sulphate aerosols," has had a huge impact. What's this paper all about?

All attempts at detecting and attributing climate change signals need a reliable observed data set and simulations with mechanisms that drive climate change included. In a nutshell, this paper is important because it was the first study to investigate the effect of sulphate aerosols in a general circulation model of the climate system. The general circulation model we employed had 20 layers in the ocean and 19 for the atmosphere. The experiments simulate the climate back to 1860 (which is when the global records of surface temperature became reliable), and they are projected forward to 2050. We found that the greenhouse-gas forcing increased slowly from 1860-1960, but then accelerated markedly. After 1970 our model with greenhouse gases alone begins to depart significantly from the observations. However, when we included sulphate aerosols, which have a cooling effect, the model agreed with the data from the 1930s and onwards. The rapid warming that has taken place since 1970 is, according to the model, attributable to a heating effect from greenhouse gases and a cooling effect from sulphate aerosols. Fundamentally we showed that climate models cannot simulate the observations unless forcing factors additional to greenhouse gases are included.

At the end of the paper you mention the need to consider tropospheric ozone in the models.

Yes, and that was the next step: feeding in the ozone, which enabled us to get the stratospheric cooling and tropospheric warming correct. In 1996 Ben Santer, myself, and other colleagues published a paper ("A search for human influences on the thermal structure of the atmosphere," B. D. Santer, et al., Nature 382 [6586]: 39-45, 4 July 1996) that was to win us NOAA's "Best Scientific Paper" award in 1998. But despite our successes I was still feeling that we hadn't got to the bottom of whether or not natural forcing could explain everything. We ran simulations that included the effects of volcanic aerosols and changes in solar irradiance. We published those findings in papers in 1999 ("Causes of twentieth century temperature change near the earth's surface," Tett et al., Nature 399 [6736]: 569-72, 10 June 1999) and 2001 ("Attribution of twentieth century temperature change to natural and anthropogenic causes," Stott et al., Clim. Dyn. 17 [1]: 1-21, January 2001). Read a Special Topics interview with Climate Dynamics.

  What's your current thinking on the causes of climate change?

We've got a paper in press with the Journal of Geophysical Research on the natural and anthropogenic contributions to 20th century temperature change. This describes experiments with the third-generation Hadley Centre Coupled Model (HadCM3) . We find that the warming which took place early in the 20th century can be accounted for through natural variability. That warming is best explained by changes in solar brightness, a scarcity of explosive volcanoes, and internal climate variability. But in the second half of the 20th century there's no way that nature alone can account for the warming, and human activity is the major driving force.

  How does your work relate to the Reports produced by the Intergovernmental Panel on Climate Change?

In the Second Assessment Report, I was a contributing author to three of the chapters dealing with the science of climate change. That Report had quite a controversial closing statement in chapter 8, where we said, "the balance of evidence suggests a discernible human influence upon global climate." I was a contributing author to chapter 12 of the third assessment report, which made a stronger statement on the detection of climate change.

  What is your current research program?

I am currently trying to do too many things at once! I have been setting up and running a simulation of the last 500 years using HadCM3. One of the problems with all detection and attribution studies (see various papers) is that they rely on simulations of climate variability rather than observations. The real world also included "forcings" such as volcanic eruptions, solar irradiance changes, and orbital changes. Proxy data such as tree rings and corals may allow validation of model variability by comparing these "observations" with simulations. I am a member of the steering committee for a thematic program on rapid climate change that has some interest in that area. The program has a budget of $28 million and a challenging agenda!

I am completing a paper on a comparison between the simulation and satellite measurements of lower tropospheric temperatures. The simulation fails to capture what is observed and I explore various possibilities for this. Many other colleagues and I are investigating simulated and observed changes in ocean temperature.

Alas my attention keeps being pulled away by the time commitments of my new job! 

  What's been the most difficult moment in the course of your professional career?

Moving jobs in the Hadley Centre. I had been working in modelling climate change for 10 years when in early 2001 I moved jobs to head a team who were responsible for developing data sets of observed climate and monitoring climate. I had a lot to learn!

  What big question do you want to answer in the next 10 years?

What is happening to the climate now and what might happen to it in the future? I believe that the best way to answer this is to look at historical climate change using models and a variety of observations.

Dr. Simon Tett
Hadley Centre for Climate Prediction and Research 
Berkshire, United Kingdom

At the Hadley Centre, which is part of the Met Office, Dr. Tett is the managing scientist for data development and applications. He is also manager for the Centre's "Climate monitoring and dataset development" theme, which concentrates on the development of datasets of observed climate variability and change, and monitors climate. His personal research aim is making quantitative comparisons between simulated and observed climate change.