We spend a lot of time developing a computer model of the climate here on this blog. We end up with a hypothesis: doubling the atmospheric carbon dioxide concentration will warm the Earth by around 0.9°C. But this hypothesis is unproven. It is pure speculation.
Scientific method proceeds like this: hypothesis, null hypothesis, experiment. In the hypothesis stage, we are wondering how the world works. We make a set of assumptions, which together we call a hypothesis. We perform calculations so as to work out the implications of our assumptions. If we use a computer to perform intense and repetitive calculations, we call the computer program a computer model.
It does not matter how many conflicting hypotheses we come up with, because in the next stage of the scientific method, we put our personal hypotheses aside and adopt a common hypothesis, which is the assumption that we don't know what we are talking about. We call this assumption the null hypothesis. If we are studying the effect of aspirin on blood pressure, we assume there is no such effect. If we are studying the effect of human carbon dioxide emissions upon the climate, we assume these emissions have no effect.
The null hypothesis becomes the official position of science until it is proved wrong by observations of nature. We can all work together without arguing because we have agreed that the null hypothesis is the official position of science. High energy physicists worked for twenty years to design and build the Large Hadronic Collider to disprove the null hypothesis that there was no such thing as a Higgs Boson. At no point was there any need for physicists who did believe the Higgs Boson to argue with those who did not. At no point was there any need to conduct a survey of physicists to determine whether they believed in the Higgs Boson or not. At no point did any physicist claim that their computer model of the Higgs Boson, which agreed perfectly with past observations, proved that the Higgs Boson existed. The null hypothesis is what makes it possible for people from all over the world, with differing opinions, to work together towards a common goal.
Our experiments are designed to disprove the null hypothesis. In the case of aspirin and blood pressure we might devise a double-blind study with a control group in order to see if taking aspirin has an effect upon blood pressure that is statistically significant. By statistically significant we mean very unlikely to occur at random if the null hypothesis were true. If our null hypothesis about the roll of a particular six-sided die is that the result of the roll is random, we roll the die thirty times. If we get thirty rolls of five, this disproves our null hypothesis. Such an outcome is very unlikely under the null hypothesis. The only hypothesis consistent with our observations is that the die almost always comes up five.
Thus our position in climate science must be: human carbon dioxide emissions have no effect upon the climate. To further the field of climate science, we must try to disprove this null hypothesis with observations of nature. We can look at the temperature of the world, its atmospheric CO2 concentration, cloud cover, extreme weather, and lapse rates. In each such study, we are looking for observations that disprove our null hypothesis. The fact that our observations are consistent with some other hypothesis that we like better is irrelevant. We must disprove the null hypothesis first.
The world warming up or cooling down while we are emitting CO2 does not disprove our null hypothesis. A look at the world's climate history shows the world warming, cooling, and staying the same in roughly the same proportions. The chance of the world getting warmer while we emit CO2 is roughly one in three. If we believe the world has warmed up by 1°C in the past century, this does not contradict our null hypothesis. In order to disprove our null hypothesis, we need observations of nature that prove beyond reasonable doubt that our null hypothesis is false. Usually, in science, we perform experiments to disprove our null hypothesis. But in the case of the Earth and its climate, such experiments are most likely impractical and unethical. We might be tempted to argue that we are entitled to disprove our null hypothesis by some other means, such as common-sense reasoning, or plausible computer models of the atmosphere. Or we might argue that it is best to believe in anthropogenic global warming just to be on the safe side. But no such dismissal of the null hypothesis can be called scientific.