T = average temperatures of the lower atmosphere and the earth's surface. tCO2 = total CO2 content of the atmosphere. aCO2 = CO2 produced by people's activities. F = burning of fuels and deforestation 1. In the last 200 years or so, there has been a more or less coincident and continual increase in F, aCO2, tCO2 and T. Is it a 'fact' that over the last 200 years or so, F, aCO2, tCO2, and T have all risen? All the observations have been grouped together. If any one of the observations is rejected, the hypothesis loses its foundation. For example, if there has been no increase in T, no global warming hypothesis is needed. If there has been no increase in tCO2, then an increase in T cannot be due to the Greenhouse Effect. Etc. 2. Hypothetically, the increase in T could be due to an intensification of the Greenhouse Effect, caused by an increase in tCO2. Is the hypothesis of a causal relationship between tCO2 and T via the Greenhouse Effect accepted by the scientific community? That there is a cause-and-effect relationship between CO2 content and atmospheric temperature (a 'Greenhouse Effect') is an explanation, not a 'fact'. If that explanation (the validity of the Greenhouse Effect) is not accepted, other causes for any increase in T must be sought. 3. Because T and tCO2 have increased over the same time period, it is not just a hypothesis, but a fact, that the rise in T has been due to an intensification of the Greenhouse Effect brought on by an increase in tCO2. Is it accepted by the scientific community that the increase in T is in fact due to an intensification of the Greenhouse Effect? Even if it is accepted that there has been a coincident rise in T and tCO2, and that the Greenhouse Effect is a valid hypothesis (a rise in CO2 will cause a rise in T), it is still possible that most of the observed T increase is due to other causes. Relegation of the Greenhouse Effect to a minor role greatly weakens the hypothesis of anthropogenic global warming. 4. Because the increase in tCO2 took place over the same time period as an increase in aCO2, it may be concluded that the enhanced Greenhouse Effect and the increase in T it causes are due to people's activities. If there has in fact been an intensification of the Greenhouse Effect, is it accepted that it has been principally due to the activities of people? If an intensification of the Greenhouse Effect due to an increase in tCO2 has occurred, and it resulted in all or most of the increase in T, it is still possible that natural variations in tCO2 have been more important than CO2 produced by people's activities. In that case, the hypothesis of anthropogenic global warming is not supported.