 The usual use of the word for climate model is a piece of computer code, a list of instructions to a computer that encapsulate our very best understanding of the way that the atmosphere of our planet and the ocean work in a physical sense. The basic underpinning laws that climate models are built from includes the basic newtons, laws of motion, conservation energy, conservation of mass, basic physical principles that physicists discovered a very long time ago. So all these models share lots of characteristics and of course the physics of the models are very very similar because they're all based on the fundamental same equations of fluid dynamics, the same equation that we knew that equation since hundreds of years. It's the way that climate models work is that they divide the world up into a series of boxes, so it's very like Lego if you like. The model is like if you would construct your world out of little Lego blocks and it's basically the size of the Lego blocks. So you can buy a really expensive Lego Star Wars ship, my son has those. They're very big, expensive takes a poor parent's two days to build them and they have lots of detail or you can buy a little car for a three year old which doesn't have that much detail but it's made out of big big blocks. There's probably no parts of a climate model of a modern climate model that still reflects what was done in the 70s, almost everything's been rebuilt or rewritten I think. The resolutions increased from something around 700 by 500 kilometres to 100 by 100 kilometres. The detail in the vertical has increased dramatically, oceans have been properly and fully coupled. The land surface has been completely revised to incorporate a whole suite of processes. Sea ice models have improved dramatically, cloud parameterisations have improved, we've resolved most of the water vapour feedback problem. It's like asking what's the relationship between a Formula One Grand Prix car in 2014 compared to 1970 and the answer is there probably isn't a single widget that's shared. The resolution is getting higher, in other words the boxes are getting smaller and smaller but the computer power is also increasing and so we're able to simulate sort of the same amount of time if you like with one of these models. There's a whole variety of reasons why we're confident in the skill of climate models for the problems that they were designed for. First of all they're built upon physical principles and those physical principles are known unless of course Newton was stupid which I don't think he was. So we have basic fundamental theory not sourced from climate science but sourced from basic chemistry, basic physics, basic biology and applied mathematics that says the core of climate models is sand. Secondly of course they use routinely in other applications like weather forecasting and so we have effectively can evaluate a lot of our science routinely and weather forecasting is becoming increasingly accurate irrespective of what some of your listeners might think if they actually write a diary of a five-day forecast and then check off how those five-day forecasts evolved they'll find that they are shockingly accurate nowadays. So that's the second test. Thirdly we routinely test our models against observations over the last century and earlier and they do extremely well in that in that respect and finally we can test our models against perturbations so we can for instance simulate a volcanic eruption for example and check that the climate models respond appropriately to what a volcano does to the atmosphere. So there are multiple lines of evidence and they all point to the climate models being reliable for what they were designed.