Now I have a few full runs so I have stuff to compare.

It seems to me that the phase 3 precipitation graph always starts way too low

From my understanding it shouldn't be too much different from the last year of phase 1 and the first year of phase 2, that runs parallel to phase 3

As both phase 2 and phase 3 have the same world to start with, the doubled CO2 cannot cause such a change for phase 2 yet, especially as it is <b>lower</b> than phase 2 but goes <b>higher</b> in the second year already.

Can it really behave like this, are the values wrong or is just the graph plottet wrong? Or did I misunderstand something?

This is explained in the first result section - unfortunately unavailable at present.

As the atmosphere warms up it can hold more water vapour. To get to having more water vapour, evaporation increases and precipitation decreases. Once it gets nearer to the atmosphere's water carrying capacity precipitation increases because there is more evaporation.

I think the answer on the first results page talks about the energy balance situation and is almost certainly a better answer.

Hope the above is ok as a simple answer.

Yes, I understand this connection.

I couldn't search the main site and the other forum as the server seems to be unavailable this weekend - but of course if there is such a question, it needs to be asked ;-)

Thanks :-)

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Someone posted you can still look at <a href="http://www.climateprediction.com">http://www.climateprediction.com</a>

So here is the given answer:

<b>Why does the precipitation rise when carbon dioxide is doubled?</b> As the temperature of the air increases, for a fixed amount of water vapour in that air the relative humidity will decrease, so warm air can hold more water vapour before it saturates than cold air. However, this doesn’t determine how fast moisture circulates through the water cycle (you can read more about this here - follow the links to 'weather' and then 'water cycle'). The intensity of the water cycle is controlled at a global level by how fast water can condense rather than by how much water vapour there is in the atmosphere. As water vapour condenses to form clouds, it releases latent heat. If nothing removed this heat, the air would warm up and would be able to hold more moisture, so the condensation would stop. What actually happens is that the atmosphere gets rid of this heat, mostly in the form of longwave radiation. As the atmosphere warms up, outgoing longwave radiation increases (click here to read more about this) which allows more cloud droplets to form and so the whole water cycle intensifies.

<b>Why does the precipitation fall initially when carbon dioxide is doubled?</b>
Initially, the carbon dioxide insulates the atmosphere, trapping longwave radiation. The amount of longwave radiation lost to space falls. Less cloud droplets can form because the atmosphere cannot get rid of the energy released by condensation fast enough.
Eventually, in most models, the temperature of the Earth increases and, as it does so, the outgoing longwave radiation increases again, compensating the direct insulating effect of increasing carbon dioxide. This makes future precipitation changes so much more uncertain than temperature changes: in some models, there might even be a net reduction in rainfall following a doubling of carbon dioxide for much more than a year or two.
You can read more about this in the Allen, Ingram and Stainforth Nature paper which is on our publications page.