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A planet at risk

Published on November 15th, 2013 | by Emily Corbett

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# Climate sensitivity modelling

By Nathan Eizenberg, Monash University

How sensitive is our planet’s climate to change? How important is the hydrological cycle to climate change? What would happen if the sea stopped moving?

These are some of the questions I tackled when I embarked on my research project in Climate sensitivity modelling with Dr Dietmar Dommenget.

We decided that it would be interesting to investigate climate sensitivity and attempt to model how sensitive the planet’s surface is to changes in temperature. More specifically, we were interested in how the different processes in the climate affected how sensitive our planet is.

Below is a table of the experiments we conducted. The method was simple; successively “turn on” processes within a climate model and compare the output after a simulated 30 years.

 Exp Processes included in model 1 Thermal+ Solar radiation processes with non-constant cloud cover and water vapour in atmosphere. 2 Above + Ice-albedo feedback 3 Above + Local hydrological cycle for latent heat, water vapour 4 Above + Diffusion of heat and water 5 Above + Advection of heat and water vapourComplete model without deep ocean 6 As 9 + Deep Ocean circulation

We used the Globally Resolved Energy Balance (GREB) model developed by Dommenget, D., and J. Floeter in 2011. The results are best interpreted graphically. Using a program called GrADS, we mapped the surface temperature as a function of longitude and latitude to see which areas are most sensitive under each experiment.

A relatively simple model (Experiment 1) produced a simple sensitivity response graph above that is less sensitive to change near the equator and is more sensitive near the poles.  However if we jump to experiments 3 and 4 we can answer an important question…

How important is the diffusive heat and water vapour transport in climate sensitivity?

Diffusion is worth looking up but it basically means the spreading out of something, in our case, heat and water vapour. This video shows an example of diffusive transfer with colours in a photographic image.

Figure 3: Sensitivity response graph Exp 3

Figure 4: Sensitivity response graph Exp 4

Figure 4: Difference sensitivity graph (Exp 4 – Exp 3)

This is a difference graph created by taking the difference between two responses (Exp 4 – Exp 3). This representation enables us to isolate the effect of introducing the new process and depict it graphically. The difference between experiment 4 and experiment 3 is the introduction of heat and water transport by diffusion. We can see from the response pattern that the mid latitudes became more sensitive, as well as any area over oceans.

This is understandable! Near the equator, the sun has maximum solar radiation and hence there is lots of heat to diffuse to other areas making it more sensitive to heat diffusion! Over oceans there is maximum water vapour from evaporation, this is why we can see an increase in sensitivity over ‘wet’ parts of the globe when we introduce water vapour diffusion. Land is considered ‘dry’ and so doesn’t have much water vapour to spread. On our graph above areas over land showed very little change in sensitivity.

Although this is only one experiment in my project, the same analysis was applied to all other experiments. Generally, the same trend appeared and oceans proved to be all the more important to our climate. One implication of this is the vitality of understanding climate processes such as the El Niño – Southern Oscillation, that have a strong oceanic component.

However, don’t take my word for it! Discover for yourself how and why the climate is changing. The Monash Simple Climate Model is a user friendly website that allows anyone to use the GREB model and conduct their own experiments.

My name is Nathan Eizenberg and I am a science student at Monash University in Melbourne, Australia. I undertook a summer research project instead of having a normal summer vacation to try and see what it is like working in scientific research. I am completing an academic minor in Atmospheric Science and recently took a subject with a very inspiring professor, Doctor Dietmar Dommenget, so I decided to take a project with him and study climate modelling.

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