Their study highlighted interesting findings , such as confirming anecdotal evidence that there had been a strong El Niño in 1791 , and finding that in 1816 the " Year Without a Summer " in Eurasia and much of North America had been offset by warmer than usual temperatures in Labrador and the Middle East . It was also an advance on earlier reconstructions in that it went back further , showed individual years , and showed uncertainty with error bars . " Global-scale temperature patterns and climate forcing over the past six centuries " ( MBH98 ) was published on April 23 , 1998 in the journal '' Nature '' . In it , " Spatially resolved global reconstructions of annual surface temperature patterns " were related to " changes in greenhouse-gas concentrations , solar irradiance , and volcanic aerosols " leading to the conclusion that " each of these factors has contributed to the climate variability of the past 400 years , with greenhouse gases emerging as the dominant forcing during the twentieth century . Northern Hemisphere mean annual temperatures for three of the past eight years are warmer than any other year since ( at least ) AD 1400 . The last point received most media attention . Mann was surprised by the extent of coverage which may have been due to chance release of the paper on Earth Day in an unusually warm year . In a CNN interview , John Roberts repeatedly asked him if it proved that humans were responsible for global warming , to which he would go no further than that it was " highly suggestive " of that inference
Year Without a Summer
Year Without a Summer
Michael E. Mann