Temperatures go up sometimes and down other times. However, over the last 100 years, it appears that they have gone up more than they have gone down. Because of these year-to-year fluctuations, climate scientists tend to think in terms of moving 5-year averages, rather than an individual year's average global temperature. With that in mind, the 5-year moving average of the global average temperature has increased by about 0.7 degrees Celsius over the last 100 years
There is a well-known phenomenon known as the Urban Heat Island (UHI) effect. Simply stated, cities are warmer than rural areas. You can see this on your nightly news weather forecast, when they tell you that the temperature is X in the city, and Y in some remote location outside the city. Invariably, X is greater than Y. Cities generate huge amounts of heat, from air conditioner exhausts, millions of cars going up and down the roads, factories and plants, buildings that absorb heat during the day and release it at night, and for that matter, millions of 35-degree-C human bodies. All this adds up and makes the cities as much as 3 degrees C warmer than they would otherwise be. So how does that effect the so-called global warming. Well, the UHI effect was well known even 150 years ago, when man first started wide-spread observations of temperature. And they did not want their records to be influenced by this "artificial" effect. So they placed their temperature observation stations outside of the cities so they would not be influenced by the UHI effect. However, in the last 150 years, most of these cities have increased in population and, more importantly, size, and have expanded so much that those old observation stations are now well within the city limits, and subject to the very UHI effect that they were trying to get away from. For example, 100 years ago, Fairfax County, Virginia was an almost completely rural, undeveloped area. Now it's a suburb of DC, with just as many people, cars, buildings, industry, etc, as any large city. So, if you have a temperature observation station in the middle of Fairfax County, and 100 years ago, the average annual temperature was, say 20 degrees C, and this year, the average temperature was 22 degrees C, then you can say that the temperature there went up by 2 degrees over the last 100 years. But how much of that 2 degrees was due to "global warming", and how much of it was due to the UHI?
One global warming (skeptic) researcher has undertaken the task of locating every temperature observation station in the US and investigating it for possible local environmental effects that would bias the readings of those stations. Of the hundreds he has investigated, about 1/4 of them have activity in their immeditate vicinities that are well known to increase local temperatures. One picture he posted on his website showed an observation station within 20 feet of a large apartment building. It was also within 10 feet of an asphalt parking lot (asphalt absorbs huge amounts of heat - if you don't believe it, try walking on it barefoot in the summer), not to mention all the cars parked there that are radiating away the heat they build up while operating. Less than 5 feet away from the station was a window-unit air conditioner, which anyone can tell you puts out lots of warm air. Another skeptic researcher has investigated the changes in painting of temperature observation stations over the years. You see, the instruments that record weather data must be shielded from direct sunlight, wind, rain, and other environmental factors that might affect their readings. So these instruments are contained in boxes. The boxes are painted white to reflect sunlight, to minimize the absorption of heat into the box. However, the specific type of paint has changed over the last 100 years. This researcher has confirmed that the paint used today is significantly less reflective than that used 100 years ago, meaning that the boxes are absorbing more heat now. How big a difference does that make in the "observed" temperature?
The Arctic Climate Impact Assessment, an international project of the Arctic Council ( a high-level intergovernmental forum) and the International Arctic Science Committee (IASC), created for evaluating and synthesize knowledge on climate variability, climate change, and increased ultraviolet radiation and their consequences, have arrived to the conclusion that the world temperature is increasing in a way that may be considered alarming. Their measurements have been taken at points of the globe were no human or other interference can affect results, representing one of the most credible studies.