Bicentennial Volcanic Activity Cycles and Their Long-Term Impact on Northern Hemisphere Climate

Six up-to-date reconstructions of hemispheric and global temperatures and two indices of volcanic activity were analyzed using both Fourier and wavelet approaches over time intervals of up to 1500 years. A cyclicity with a period of 188-250 years was found to be present in both the Northern Hemisphere temperature and volcanic activity. These cycles were found to be negatively correlated with the correlation coefficients, reaching values of -0.61--0.68 over the entire time interval. The maximum correlation coefficient is reached when volcanic variations lead to temperature variations by 20 years. The combined probability of the null hypothesis-the proposition that in the Northern Hemisphere, there is no real association between the bicentennial change in volcanic activity and temperature and that the revealed correlations arose purely by chance-was found to be no more than 1.2 x 10-2 over the entire time interval and less than 10-2 over the time interval of AD 1270-1980. The effect was weaker for the global temperature and was not detected in the Southern Hemisphere. It is shown that the identified bicentennial temperature variation could have made a certain contribution to the warming in the first half of the 20th century. However, this cycle has probably been in decline for the last 40-50 years and the decline should continue for the next few decades. Possible origins of the revealed bicentennial correlations are discussed, and guidelines for further research are proposed.