His study looked at the effects of El Nino in the past as well as the effects of the release of gases such as carbon dioxide into the atmosphere by human activities.
The analysis of past El Nino events in the 20th century found that the result was a slightly colder than normal annual average temperature over the 48 contiguous states.
To double check that, the researchers conducted two sets of 50-year computer simulations of U.S. climate, with and without the influence of El Nino. They again found a slight cooling across the nation when El Nino was present.
Then they looked at the effect of the increased greenhouse gases — which are given that name because they can help trap heat from the sun somewhat like a greenhouse traps heat.
They ran 42 different tests using complex computer models to simulate changes in the atmosphere under various conditions and concluded that the "2006 warmth was primarily due to human influences."
While Hoerling's study focused on the United States, NOAA also tracks world climate. Worldwide, 2005 was the warmest year on record, topping 1998, according to the agency.
The research was supported by NOAA's office of Global Programs.
