E. Linacre and B. Geerts
Global climate models coupling the atmosphere and the oceans do replicate well certain climate changes, in particular changes in temperature observed over the last century and the last few decades, if the following observed variants are incorporated (1):
The first three of these are largely due to activities of mankind, the last two are natural. The application of statistical tests to temperature measurements during this century lead to the conclusion that the warming is man-made with a confidence of 99% (2). The global mean temperature trend between 1975-1997, and even regional variations in this trend, can be explained very well by observed changes in greenhouse-gas concentrations (3), in other words greenhouse gas emissions are by far the leading cause of climate change during the last quarter of the 20th century. The last decade of the 20th century (especially the years 1990, 1993 and 1997) is almost certainly the warmest since 1400 (3).
The match between observed and modeled temperature change during the 20th century cannot be explained by natural climate variability, as shown by the differences between three long control runs (1). This is an important point, because the global-mean temperature variations between 1400-1900 correlate well with the record of volcano activity (which is uncertain) and, especially, changes of solar brightness (which are well known) (3).
In fact, it is possible that the temperature anomalies now known as the Medieval Warm Period and the Little Ice Age are entirely attributed to the inherent, internal variability of the global climate, i.e. not to any external forcing mechanism. This was shown by a numerical simulation of the climate for 500 years under a constant solar brightness, starting with various realistic initial conditions (4). One of the ten runs reveals spatial differences and temporal changes of temperature remarkably like those actually observed. Sceptics of the greenhouse warming theory have grasped this argument to say that enhanced greenhouse gas concentrations may not be the cause of the global warming observed during the last few decades, but merely enhancing factors. Clearly this opinion is flawed, as shown by GCM simulations for 1963-1995 (1).