Looking for evidence of global warming

According to the global warming theory, the temperature on Earth has increased on average since the end of the 19th century. During the last century until now, the temperature supposedly has gained about 0.8 degrees. Apparently, most of this warming has happened in the last two or three decades. In the future, we can expect the temperature to rise even more, leading to droughts, heat waves, and other unpleasant phenomena. Obviously, some regions will be hit harder than others. Several solutions have been proposed, including reduction of greenhouse gas emissions and geo-engineering by spreading special gases in the atmosphere in order to reflect more sunlight.

The data we downloaded from the Dutch Meteorological Institute, KNMI, is not sufficient to prove whether global warming is real or not, but we can certainly examine it further. For instance, we can check whether the temperature in De Bilt (that's where the data was collected) in the first half of...