Modeling and Observing Climate Change


December 18, 2007


Current climate models work by attempting to simulate the weather from first principles. In effect they are weather forecast models run for decades to centuries and in some cases millennia, typically representing scales of 200km or greater. Scales below this need to be parameterized in terms of the large scale flow, and these parameterizations lead to uncertainty in future warming.

Observational datasets of climate change are created by combining observations originally made for other purposes. Observing practice has not remained constant over the 20th (or earlier) centuries, and this introduces bias which if not corrected leads to an incorrect view of 20th century climate change. Two examples will be discussed: the record of surface temperature and the upper-air radiosonde network.

Using models and observations I will illustrate how the 4th IPCC assessment report reached the conclusion that “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations”.


Simon Tett

Professor Simon Tett did his PhD at the University of Edinburgh in the use of parallel computers for Numerical Weather Prediction. In 1991 he joined the Hadley Centre where he did research which showed that human emissions of carbon dioxide were likely to be responsible for 20th century warming. After this he managed a team of scientists who created datasets of historical climate change. Since July 2007 has been Chair of Earth System Dynamics and Modeling at the University of Edinburgh. He has published more than 50 peer-reviewed papers, won the Norber-Gerbier WMO prize twice (1997, 1998), a NOAA price for best scientific paper (1998), the LG Groves prize for Meteorology (2006) and gave the Margary lecture to the Royal Meteorological Society in 2007. He contributed to the last three IPCC assessments. He is a member of the scientific steering committee of NERC’s RAPID thematic program and a National Centre for Atmospheric Science Principal Investigator. Tett’s scientific interest is the testing of climate models against observations of change in order to better constrain the future. To this end he has set up and carried out many coupled climate model simulations, caused the development of observational datasets and used them to test climate models.Tett, with others (Tett et al, 1999), showed that 20th century climate change could only be explained by a combination of natural and human factors with greenhouse gases causing warming offset by other forcings. He then initiated and contributed to research showing that given natural and anthropogenic forcings a state-of-the-art climate model could reproduce the observed 20th century temperature change (Stott et al, 2000). Following this he computed, with uncertainties, the relative contribution of different natural and anthropogenic forcings to 20th century climate change (Tett et al, 2002). While leading a team of scientists who produced and analyzed datasets of observed climate changes with comprehensive uncertainties (Thorne et al, 2005; Rayner et al, 2006 and Brohan et al, 2006) he carried out and analyzed simulations of the last 500 years (Tett et al, 2007; Gregory et al, 2006). This work showed that in the model world, natural and anthropogenic forcings have different climate sensitivities, natural forcings significantly amplify climate variability, that much of 19th century sea-level rise was a delayed response to Tambora and that human emissions of carbon dioxide had a significant effect on tropical temperatures in the 19th century. Results from these simulations were also used to test methods to reconstruct past climate (von Storch et al, 2006).Professor Tett has two school age children and two slightly older cats. He lives in Edinburgh and his interests, outside climate research, include opera, cycling, walking in the countryside and complicated board games. He used to think of himself as technically able but now realizes he knows nothing.