Global warming is a trend increase in the average temperature on earth over the past 100 years. It is calculated according to weather stations around the world, in Europe, these stations have been around for about 150 years. But in General, around the world we have data for the last 100 years, and this calculation shows that the temperature rises, and increases trendy, there is a significant trend and not just fluctuations around some long-average temperature.
At the same time, it should be noted that this temperature is not rising smoothly, and that is the so-called "steps" and sometimes goes on a "plateau". For example, in the past 30 years have seen a very noticeable growth in the temperature, especially in a strong rise occurred in 90-e years, and since 2000 this process has slowed down. Similar things happened before, that is to say that the temperature is rising "steps".