According to NASA reports, the Earth's temperature has been steadily increasing since 1900. The temperature between 1961-1990 was 0.2°C warmer than 1900, and 1998 saw temperatures reach nearly 0.6°C warmer than 1961-1990. Most recently in 2015, temperatures were 0.88°C above 20th century averages. The document goes on to define global warming as the increase in average temperatures due to human activity, and lists some of the main causes as the greenhouse effect, deforestation, agriculture, and industrial pollution.