Under this funding, the Principal Investigator (PI) will study oscillations in the global record and their effect on the deduced anthropogenic warming trend. In particular, the relationship between global variability at a 70-year scale and the Atlantic Multi-decadal Oscillation will be examined. The deduction of the anthropogenic trend in data is often complicated by the presence of this large amplitude low-frequency variability, since its trough-to-peak excursion is one half of the observed warming trend for the past 150 years. When it is added to the underlying anthropogenic warming trend, it creates the accelerated warming seen since 1978 and the cooling experienced in the 1970s. The early twentieth century warming was probably largely caused by the previous positive phase of this oscillation. The global oscillation appears to have its origin in the sea surface temperature variability of the North Atlantic, as a manifestation of a high frequency variability of the thermohaline circulation, but the mechanisms involved are not yet well established.
Current data analysis methods may not be adequate for studying such low-frequency variability given the limited time span of the observation. New methods are for analyzing the global data record, to provide a quantification of errors due to the fact that even the longest quasi-global climate records (1850-present) are not long enough. On the other hand, a longer instrumental record at Central England (1659-present) and several even longer ice-core data records from Greenland will be studied to establish the existence of this oscillation in the pre-industrial era. The global temperature and sea-level pressure will be studied in more detail to understand the propagation of the signal from the Atlantic to the Pacific. Correlation studies will be performed with the Arctic ice-melt to determine if the latter is involved as a negative feedback necessary to establish an oscillation.
The project will establish the significance of multidecadal natural variability in the North Atlantic and thus improve the detection and attribution of climate change. This is an important problem in climate science that has substantial broader impacts.
