Sack Lunch Seminar (SLS)

Significant Component of Unforced Multidecadal Variability in the Recent Acceleration of Global Warming.


The problem of separating variations due to natural and anthropogenic
forcing from those due to unforced internal dynamics during the twentieth
century is addressed using state-of-the-art climate simulations and
observations. An unforced internal component that varies on multidecadal
time scales is identified by a new statistical method that maximizes
integral time scale. This component, called the Internal Multidecadal
Pattern (IMP), is stochastic and hence does not contribute to trends on
long time scales, but can contribute significantly to short-term trends.
Observational estimates indicate that the trend in the spatially averaged
``well observed" sea surface temperature (SST) due to the forced component
has an approximately constant value of 0.1K/decade, while the IMP can
contribute about +/- 0.08K/decade for a 30-year trend. The warming and
cooling of the IMP matches that of the Atlantic Multidecadal Oscillation
and is of sufficient amplitude to explain the acceleration in warming
during 1977-2008 as compared to 1946-1977, in spite of the forced component
increasing at the same rate during these two periods. The amplitude and
time scale of the IMP are such that its contribution to the trend
dominates that of the forced component on time scales less than 16 years,
implying that the lack of warming trend during the past ten years is not
statistically significant. Furthermore, since the IMP varies naturally on
multidecadal time scales, it is potentially predictable on decadal time
scales, providing a scientific rationale for decadal predictions. While
the IMP can contribute significantly to trends for periods of 30 years or
less, it cannot account for the 0.8C warming trend that has been observed
in the twentieth century spatially averaged SST.