There are two ways in which global warming can become a myth or a "global" rather than "urban" legend. The first way is in the marketing schemes of those who wish to convince the public that global warming is not truly occurring. One group is pushing the anti-global warming position for extremist political purposes. Another group is pushing the anti-global warming position because it would mean major changes in the way that things are produced and sold.
This provides ways for the major polluters to dodge the responsibility for the increased particulate matter, ocean and other biome pollution, and other profitable activities that contribute to changes in our atmosphere that contribute to global warming. In summary, myths are easily created when people are convinced to believe that they are myths.
The other way that global warming can be a "myth" is from lack of understanding of what global warming or cooling truly is. What is being measured and spoken of is the entire atmosphere of Earth. All of it is being spoken of when discussing climate. Localized and specific weather events, such as record cold temperatures in a specific region of the planet during what has been described as a period of global warming, are just that: weather.
As a result, when discussing climate, record heat in London does not signify global warming any more than record snows in New York signify global cooling. These are weather events, not changes in the climate of the whole planet!
Weather and climate are two different things: Think of all of the atmosphere versus what is going on in a specific place at a specific time. Think of climate as what is expected and weather as what actually happens.
Climate, or the condition of the entire Earth's atmosphere is measured in many ways and for many reasons. There are differing focuses, such as the ocean's ability to take in carbon dioxide and to produce most of our oxygen. Other focuses may involve the carbon that we introduce to the atmosphere, or the particulate matter that humans and volcanoes spew forth, or the fact that we simply exist to farm, desertify, build massive cities and so on, or the fact that massive forest fires occur.
It is a fact that 6.7 billion people will impact the environment and atmosphere. It is a fact that natural events and cycles contribute to the changes in climate. But it is difficult, if not impossible to measure the entire atmosphere right now with enough thoroughness, or over long enough time to come up with a guaranteed and complete picture of the cause of global warming.
Finally, global warming is not something that is taken as a "snapshot" of our climate, at one period of time. The factors that go into global warming go back for as far as measurements exist. As new technologies allow views, sampling, testing, and the aggregation and interpretation of data, the situation becomes very complex, indeed.
Global warming and cooling is something that is based on measurements, records and observations which cover at least ten years, and preferably hundreds of years! Then, the aggregated data is used to create averages or baselines. Then the latest measurements are compared to determine if the climate temperature is flat, cooling, or in a warming trend.
There is and will be much disagreement as scientists compare their way of arriving at conclusions with other scientist's ways of arriving at conclusions. The fact is that there was a flat global warming trend in the early 2000s, with a cooling year in 2008. But the trend is toward overall global warming in the future, according to most who study our climate.
Neither global warming, no change, or global cooling are myths. These things do happen as our atmosphere, all of it, goes through it's changes.