If a scientist wants to show that global warming is a real threat she shows temperature data from roughly 1850 to today (e.g., here; scroll down a bit). There are beautiful data from Britain from 1850 to today showing a nearly linear increase.

If a scientist wants to show that global warming is no threat he shows the data from Austria from 1700 to today. They look V-shaped with the lowest point at about 1850 and a nearly linear decrease from 1700 (temperatures about as hot as today) to 1850. (sorry I have no link to these data)

Global warming in the last decades is an empirical fact and not disputed among scientists. The reasons for the increase (and therefore the prediction of the future trend) are debated.

The pessimists (majority) think that the contribution of human activity to warming is significant compared to other contributions as sun activity, volkanoes etc. They think that in the worst case there could be a runaway greenhouse effect in which the balance of earth temperature could be disturbed for bad.

The optimists (minority) think that the contribution of human activity is either too small compared to natural causes of temperature fluctuation or that it will be cancelled out by a built in stabilising process (like, e.g., with rising temperature more water will evaporate and more clouds will cooll down the earth again). They think that in the worst case human activity will artificially trigger the onset of the usual natural fluctuations of temperature level.

Who's right? We might know in about 200 years. Therefore, for the question of which action to take today, science is not the correct partner to ask. There are theories, models, conjectures, but no decisive answer as much as everybody would like to have it.

We can make two possible errors. We can follow the pessimists and they turn out to be wrong and we can follow the optimists and they turn out to be wrong. Ask yourself and the politicians which of both errors would have worse consequences. You have not only to consider what is the probability of a scenario you also have to consider the gains and losses in case it happens

As a scientist I am with the minority in this case, that is I consider it most likely that the temperature regulating system is a system with negative feedback and will therefore on the long run prevent an adverse effect of human activity on global temperature.
As a political being I consider the costs of being wrong with my opinion as too high for not to act. It is worth and has very small costs going with it when we start thinking about alternative ways of energy production and about using the nonrenewable resources of the earth less and less. It is an investment with low costs, a potentially very high benefit, and a positive spin-off in other domains as e.g. pollution.

So though I think it will probably have no influence on global temperature, I'm for protective measures against.

Wolfgang