The Models Don't Have Clouds
Posted on: April 25, 2006 10:53 PM, by coby, A Few Things Ill Considered, science blogs
This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.
Objection:
Clouds are a very large negative feedback that will stop any drastic warming. The climate models don't even take cloud effects into account.
Answer:
All of the Atmospheric Global Climate Models used for the kind of climate projections reported on by the IPCC take the effects of clouds into account. You can read a discussion about cloud processes and feedbacks in the IPCC TAR.
It is true however that clouds are one of the largest sources of uncertainty in the GCMs. They are very complicated to model because they have both positive feedbacks, preventing surface heat from escaping back into space and negative feedbacks, reflecting incoming sun light before it can even reach the surface. The precise balance of these opposing effects depends on the time of day, the time of year, the cloud's altitude, the size of the water droplets and/or ice particles forming the clouds, the latitude, the current air temperature and the cloud's size and shape. On top of that, different types of clouds will interact, amplifying or mitigating each other's effect as they co-exist in different layers of the atmosphere. There are also latent heat considerations as water vapour condenses during cloud formation and precipitation events and as water droplets evaporate when clouds dissipate.
The ultimate contribution to global temperature trends is very uncertain, but according to the best estimates is likely to be positive over the coming century. There is no indication anywhere that any kind of cloud processes will stop greenhouse gas driven warming, and this includes observations of the past as well as modelling experiments.
This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.
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