RealClimate logo


Technical Note: Sorry for the recent unanticipated down-time, we had to perform some necessary updates. Please let us know if you have any problems.

FAQ on climate models: Part II

Filed under: — gavin @ 6 January 2009

This is a continuation of a previous post including interesting questions from the comments.

More Questions

  • What are parameterisations?

    Some physics in the real world, that is necessary for a climate model to work, is only known empirically. Or perhaps the theory only really applies at scales much smaller than the model grid size. This physics needs to be ‘parameterised’ i.e. a formulation is used that captures the phenomenology of the process and its sensitivity to change but without going into all of the very small scale details. These parameterisations are approximations to the phenomena that we wish to model, but which work at the scales the models actually resolve. A simple example is the radiation code – instead of using a line-by-line code which would resolve the absorption at over 10,000 individual wavelengths, a GCM generally uses a broad-band approximation (with 30 to 50 bands) which gives very close to the same results as a full calculation. Another example is the formula for the evaporation from the ocean as a function of the large-scale humidity, temperature and wind-speed. This is really a highly turbulent phenomena, but there are good approximations that give the net evaporation as a function of the large scale (‘bulk’) conditions. In some parameterisations, the functional form is reasonably well known, but the values of specific coefficients might not be. In these cases, the parameterisations are ‘tuned’ to reproduce the observed processes as much as possible.

  • More »

FAQ on climate models

Filed under: — group @ 3 November 2008 - (Svenska)

We discuss climate models a lot, and from the comments here and in other forums it’s clear that there remains a great deal of confusion about what climate models do and how their results should be interpreted. This post is designed to be a FAQ for climate model questions – of which a few are already given. If you have comments or other questions, ask them as concisely as possible in the comment section and if they are of enough interest, we’ll add them to the post so that we can have a resource for future discussions. (We would ask that you please focus on real questions that have real answers and, as always, avoid rhetorical excesses).

Part II is here.

More »

Start here

Filed under: — group @ 22 May 2007 - (Slovenčina) (Polski)

We’ve often been asked to provide a one stop link for resources that people can use to get up to speed on the issue of climate change, and so here is a first cut. Unlike our other postings, we’ll amend this as we discover or are pointed to new resources. Different people have different needs and so we will group resources according to the level people start at.

For complete beginners:

NCAR: Weather and climate basics
Center for Climate and Energy Solutions: Global Warming basics
Wikipedia: Global Warming
NASA: Global Warming update
National Academy of Science: America’s Climate Choices (2011)
Encyclopedia of Earth: Climate Change Collection
Global Warming FAQ (Tom Rees)
Global Warming: Man or Myth? (Scott Mandia, SUNY Suffolk)
Oxford Begbrooke: Climate Basics

There is a new booklet on Climate Literacy from multiple agencies (NOAA, NSF, AAAS) available here (pdf).

The UK Govt. has a good site on The Science of Climate Change (added Sep 2010).

The portal for climate and climate change of the ZAMG (Zentralaanstalt für Meteorologie und Geodynamik, Vienna, Austria). (In German) (added Jan 2011)

Those with some knowledge:

The IPCC AR4 Frequently Asked Questions (here (pdf)) are an excellent start. These cover:

RealClimate: Start with our index

Informed, but in need of more detail:

Science: You can’t do better than the IPCC reports themselves (AR4 2007, TAR 2001).

History: Spencer Weart’s “Discovery of Global Warming” (AIP)

Art: Robert Rohde’s “Global Warming Art

Informed, but seeking serious discussion of common contrarian talking points:

All of the below links have indexed debunks of most of the common points of confusion:

Please feel free to suggest other suitable resources, particularly in different languages, and we’ll try to keep this list up to date.

Does a Global Temperature Exist?

Filed under: — rasmus @ 25 March 2007 - (Português)

Does a global temperature exist? This is the question asked in a recently published article in Journal of Non-Equilibrium Thermodynamics by Christopher Essex, Ross McKitrick, and Bjarne Andresen. The paper argues that the global mean temperature is not physical, and that there may be many other ways of computing a mean which will give different trends.

The common arithmetic mean is just an estimate that provides a measure of the centre value of a batch of measurements (centre of a cloud of data points, and can be written more formally as the integral of x f(x) dx. The whole paper is irrelevant in the context of a climate change because it missed a very central point. CO2 affects all surface temperatures on Earth, and in order to improve the signal-to-noise ratio, an ordinary arithmetic mean will enhance the common signal in all the measurements and suppress the internal variations which are spatially incoherent (e.g. not caused by CO2 or other external forcings). Thus the choice may not need a physical justification, but is part of a scientific test which enables us to get a clearer ‘yes’ or ‘no’. One could choose to look at the global mean sea level instead, which does have a physical meaning because it represents an estimate for the volume of the water in the oceans, but the choice is not crucial as long as the indicator used really responds to the conditions under investigation. And the global mean temperature is indeed a function of the temperature over the whole planetary surface.

More »

Has Pacific Northwest snowpack declined? Yes.

Filed under: — eric @ 20 March 2007

There has been a bit of a flap here at the University of Washington over the state of the snowpack in United States Pacific Northwest region. The Seattle city mayor, Greg Nickels (a well known advocate for city-based CO2 reduction initiatives) wrote in an Op-Ed piece in the Seattle Times that

The average snowpack in the Cascades has declined 50 percent since 1950 and will be cut in half again in 30 years if we don’t start addressing the problems of climate change now. That snow not only provides our drinking water, it powers the hydroelectric dams that keep our lights on.
More »


Switch to our mobile site