---
title: "Climateprediction.net"
chunk: 3/4
source: "https://en.wikipedia.org/wiki/Climateprediction.net"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:11:42.306440+00:00"
instance: "kb-cron"
---

=== Explanation ===
Climate sensitivity is defined as the equilibrium response of global mean temperature to doubling levels of carbon dioxide. As of June 2025, current levels of carbon dioxide are 426 ppm and grew by 3.72ppm in the last year, compared with preindustrial levels of 280 ppm. 
Climate sensitivities of greater than 5 °C are widely accepted as being catastrophic. The possibility of such high sensitivities being plausible given observations had been reported prior to the climateprediction.net experiment but "this is the first time GCMs have produced such behaviour".
Even the models with very high climate sensitivity were found to be "as realistic as other state-of-the-art climate models". The test of realism was done with a root mean square error test. This does not check on realism of seasonal changes and it is possible that more diagnostic measures may place stronger constraints on what is realistic. Improved realism tests are being developed.
It is important to the experiment and the goal of obtaining a probability distribution function (pdf) of climate outcomes to get a very wide range of behaviours even if only to rule out some behaviours as unrealistic. Larger sets of simulations have more reliable pdfs. Therefore, models with climate sensitivities as high as 11 °C are included despite their limited accuracy. The sulfur cycle experiment is likely to extend the range downwards.

=== Piani et al. (2005) ===
Published in Geophysical Review Letters, this paper concludes:When an internally consistent representation of the origins of model-data discrepancy is used to calculate the probability density function of climate sensitivity, the 5th and 95th percentiles are 2.2 K and 6.8 K respectively. These results are sensitive, particularly the upper bound, to the representation of the origins of model data discrepancy.

== Use in education ==
There is an Open University short course and teaching material available for schools to teach subjects relating to climate and climate modelling. There is also teaching material available for use in Key Stage 3/4 Science, A level Physics (Advanced Physics), Key Stage 3/4 Mathematics, Key Stage 3/4 Geography, 21st Century Science, Science for Public Understanding, Use of Mathematics, Primary.

== The original model ==
The original experiment is run with HadSM3, which is the HadAM3 atmosphere from the HadCM3 model but with only a "slab" ocean rather than a full dynamic ocean. This is faster (and requires less memory) than the full model, but lacks dynamical feedbacks from the ocean, which are incorporated into the full coupled-ocean-atmosphere models used to make projections of climate change out to 2100.
Each downloaded model comes with a slight variation in the various model parameters.
In the initial "calibration phase" of 15 model years, the model calculates the "flux correction"; extra ocean-atmosphere fluxes that are needed to keep the model ocean in balance (the model ocean does not include currents; these fluxes to some extent replace the heat that would be transported by the missing currents).
In the "control phase" of 15 years, the ocean temperatures are allowed to vary. The flux correction ought to keep the model stable, but feedbacks developed in some of the runs. There is a quality control check, based on the annual mean temperatures, and models which fail this check are discarded.
In the "double CO2 phase", the CO2 content is instantaneously doubled and the model run for a further 15 years, which in some cases is not quite sufficient model time to settle down to a new (warmer) equilibrium. In this phase some models which produced physically unrealistic results were again discarded.
The quality control checks in the control and 2*CO2 phases were quite weak: they suffice to exclude obviously unphysical models but do not include (for example) a test of the simulation of the seasonal cycle; hence some of the models passed may still be unrealistic. Further quality control measures are being developed.
The temperature in the doubled CO2 phase is exponentially extrapolated to work out the equilibrium temperature. Difference in temperature between this and the control phase then gives a measure of the climate sensitivity of that particular version of the model.