evidence: Overwhelming scientific consensus finds human activity primarily responsible for climate change. [...] The amount of solar energy received by the earth goes up and down in cycles, but overall there is no net change since the 1950s. There has, however, been a big increase in global temperatures that is too large to attribute to the sun. For this reason, NASA and other scientists say the sun is not responsible for global warming. [28] The sun has had only a minor effect on the Northern Hemisphere climate over the past 1,000 years, and global warming from human-produced greenhouse gases has been the primary cause of climate change since 1900. [26] A study found that solar activity could not have contributed to more than 10% of the observed global warming over the 20th century. [27] [...] A study published in Energy & Environment wrote, "variations in solar activity and not the burning of fossil fuels are the direct cause of the observed multiyear variations in climatic responses." [69] In a study by Willie Soon, PhD, Physicist at the Harvard-Smithsonian Center for Astrophysics, a strong correlation between solar radiation and temperatures in the Arctic over the past 130 years was identified. [70]