If you look at the graph of global temperature of the last century, in the eyes rush failure in the area of 1960-ies. About this cool period, said many. Some suggest variations in emissions of aerosols (small particles that reflect sunlight. The others nod on the circulation of the Atlantic ocean, the cycle of change which is called the Atlantic multidecadal oscillation (AMO). The situation is complicated by the fact that the first may affect the second and the effect of both is accumulated.

The surface temperature of the North Atlantic in September 2001 (image Ronald Vogel, SAIC for NASA GSFC).

Surface waters that flow into the North Atlantic, cool down, closer to the Arctic, plunge into the bottomless pit and returns back to the South. It is believed that the AMO strengthens this process, resulting in a "conveyor belt" that is accelerated, slowed down. Since the distribution of cold and warm water on the surface changes, AMO can have an impact on regional and global climate (just as it makes the southern oscillation in the Pacific ocean).

AMO intensively investigated, but questions remain. This is partly due to the fact that in contrast to El Nino and these La Nina years of its cycle takes much more time, and therefore in the entire history of observations have been unable to accumulate enough data. The study paleoclimatological markers showed that AMO changes over time - sometimes it takes about 60 years, and sometimes placed in twenty.

The authors of the new article offer a fun way to increase the data set due to deeper penetration into the past - to 1659, when in Central England began systematic observations of temperature in connection with the invention sealed liquid thermometer. Of course, these observations are regional, not global in nature. But, comparing them with measurements of global temperatures, rising to 1850, Ka-kit Tung and Zsenso Zhou from the University of Washington (USA) concluded that the testimony of Central England very well agree with them.

It turned out that, as a rule, AMO held from 50 to 80 years and, apparently, took part in all major climatic changes of the last 400 years. The little ice age of the XVI-XIX centuries was probably caused by a decline in solar energy and a number of strong volcanic eruptions, but, according to the researchers, AMO first contributed to the prolongation of a cold snap, and then the end.

Experts attribute the bulk of the warming in the first half of the XX century with AMO, leaving anthropogenic greenhouse gas emissions and increasing solar activity secondary role. In the same way they explain the cooling in the area of the 1960-ies and partly due to warming in the 1990-X. In fact, removing the calculated contribution of the AMO, the authors came to the conclusion that from about 1910 anthropogenic warming was relatively linear - around 0.08'c decade. This is about half of the estimates obtained by those who tried to isolate the contribution of natural variability in the warming of the past 30 years (for which data is available satellite observations).

That Delworth of the National Board of oceanic and atmospheric research (USA), also an expert on AMO, strongly doubts the conclusions colleagues. Of course, first of all, there are questions as to the legality of the use of historical records of Central England as an indicator of global temperature. For example, because AMO warming in the North Atlantic may come back to haunt cooling in the southern ocean, and they compensate each other. Besides an accurate assessment of the contribution of the AMO in global temperatures - employment is very ungrateful, because the phenomenon itself is not fully studied and understood.

The study is published in the journal Proceedings of the National Academy of Sciences.

Prepared according to Ars Technica.