Answer: Both. It depends on your historical time frame.
With a global average surface temperature of 79 degrees Fahrenheit, 2008 was the coolest year since 2000, according to climatologists at the NASA Goddard Institute for Space Studies (GISS). But it’s also the ninth-warmest year since 1880, so it’s probably not time to invest in a ski resort just yet.
Including the 2008 dip, the 10 warmest years on record (since 1880) have all occurred between 1997 and 2008, according to NASA.
The NASA scientists attribute 2008′s relatively lower temperature to a cooler Pacific Ocean, due to a strong La Nina pattern in the first half of 2008. La Nina and El Nino are opposite phases of a natural oscillation of upwelling and subsequent temperatures in the equatorial Pacific Ocean.
2008 temperatures in the United States were cooler than any other year this decade, but, as illustrated on the map below, other parts of the world such as Eurasia and the Arctic were exceptionally warm.
Director of GISS James Hansen predicts that because a shift to El Nino is expected to start this year or next, it “still seems likely” that we’ll see a new record high for the average global surface air temperature in that time frame.