According to Pew Center global warming is an " increase in Earth's temperature over the period of time".
The Intergovernmental Panel on Climate Change has been investigating the global climate change since 1988. Their job is to publish reports that investigate effects that humans have to change of Earth's climate.
In their publication called "Climate Change 2007: Synthesis Report", they presented the data that supports the idea that our planet is becoming warmer. For example they found that 11 out of 12 years from 1996- 2005 were the hottest years since mid 19th century. They also reported that in the past 50 years temperature has increased twice as fast as in the previous century. They also concluded that "the probability that this is caused by natural climatic processes alone is less than 5%."