Geoscience Reference
In-Depth Information
(5) Climate models cannot reconstruct past climate.
Past climates are an important test for global climate models and the IPCC Fifth Assess-
ment report has a whole chapter dedicated to palaeoclimatology. The biggest climate shift,
for which we have many palaeoclimate reconstructions, is that of the last ice age, which
ended about 10,000 years ago. A comparison between palaeoclimate data for the most ex-
treme stage of the ice age, which occurred 18,000 years ago, suggests that the global cli-
mate models are rather good. It shows that the AOGCMs used for predicting future cli-
mate can do a good job of reconstructing the extreme conditions of an ice age and can get
sea level close to 120 m lower, and global temperatures 6°C cooler, with atmospheric car-
bon dioxide one-third lower and atmospheric methane halved. One important observation
is that the models are conservative, and they systematically underestimated the extremes
of the last ice age. This means we can assume that the future climate predictions are also
conservative, and thus climate change is very likely to be at the top end of the estimates.
(6) What about Galactic cosmic rays (GCRs).
GCRs are high-energy particles that cause ionization in the atmosphere and it has been
suggested that they could affect cloud formation. GCRs vary inversely with solar variabil-
ity because of the effect of solar wind. This is an excellent example of how climate scien-
ce progresses by gaining new knowledge, testing it thoroughly, and if required adding it
into the climate models. However, there seems to be no correspondence with the vari-
ations of cosmic rays and the global total cloud cover since 1991 or to global low-level
cloud cover since 1994. Together with the lack of a proven physical mechanism and the
plausibility of other causal factors affecting changes in cloud cover, this makes the associ-
ation between GCR-induced changes in aerosol and cloud formation unlikely. Some col-
leagues have found that the evidence showed that connections between solar variation and
climate were more likely to be mediated by direct variation of insolation rather than cos-
mic rays, and concluded that varying solar activity, either by direct solar irradiance or by
varying cosmic ray rates, would have contributed less than 0.07°C warming since 1956, in
other words less than 14 per cent of the observed global warming. Therefore, a review of
the recent and historical literature continues to find that the link between cosmic rays and
climate is tenuous.