One of our major aims as Earth System scientists is to predict how the Earth will respond to changes and perturbations in the future. The models and concepts that are used to do this are based on physical laws and information obtained from observations of the past. New paleoclimate records are crucial for a test of our current understanding. This process of testing is particularly important for the credibility of the science: by making predictions that can be tested against observation, we make it clear that our science is reaching the "Popperian" threshold [Popper, 1963], requiring that hypotheses be falsifiable. The imminent appearance of an ice core record of atmospheric CO2 covering the last 740,000 years prompted some of us to issue a challenge at the end of last year [Wolff et al., 2004] - for the modelling community to predict, based on current knowledge, what the record will look like. In this article we describe the submissions to the challenge, so that the concepts underlying the predictions can be explained and discussed when the measured data appear later this year.