Scientific Theme: Advanced Modeling and Observing Systems

Complementary Research: Innovative Research Program
Innovative Research Program
Possibility of Abrupt Climate Change in the Next Few Decades
Investigator: Prashant D. Sardeshmukh (CIRES/NOAA ESRL)
Objective. We propose to perform several 100-year integrations of two global atmospheric general circulation
models (GCMs) with prescribed ocean sea surface temperatures (SSTs) in the tropics and with coupling to a simple
mixed-layer ocean model elsewhere. The prescribed tropical SST fields will incorporate a hypothesized continuation
of the warming trend observed over the past 50 years, including the continued warming of the tropical Indian,
western Pacific, and Atlantic oceans. For reasons explained below, we suspect that the global climate system is close
to a ―tipping point‖ with regard to the continued warming of especially the Indian Ocean. We believe that our GCM
experiments, conducted with two different GCMs to generate confidence in the results, will provide a clear
assessment of the likelihood of this tipping point being crossed in the next several decades, with potentially dramatic
consequences for the climates of North America, Asia, and the Arctic.
Background. There is growing evidence that much of the global and regional climate changes over the next century
associated with anthropogenic forcing will be mediated, or at least be strongly influenced, by tropical SST changes.
The tropical oceans have already warmed substantially over the last 50 years (see figure below). Recently we
demonstrated that this warming is outside the range of natural ENSO SST variability at almost all tropical locations.
The warming of the Indian, western Pacific, and south Atlantic Oceans is especially significant in this regard. At
present, it is not clear to what degree this warming is associated with anthropogenic forcing or with natural decadal
and longer term variations of the climate system. The answer to this question has a large bearing on how this
warming pattern will evolve over the next several decades and what its worldwide impacts will be. Most climate
models involved in the IPCC Fourth Assessment of projected climate changes misrepresent important aspects of
tropical climate dynamics, including ENSO, the long-term oceanic subsurface controls on ENSO, and the oceanic
―dipole‖ dynamics of the Indian and Atlantic Oceans. These deficiencies greatly limit our confidence in their ability
to predict the future evolution of the warming pattern. This is unfortunate because there is growing evidence that
regional climate changes around the globe will be highly sensitive to the precise pattern of the tropical SST
warming. This sensitivity arises mainly from the generally opposite global circulation responses to deep atmospheric
convective heat sources over the west Pacific and Indian Oceans.
Observed warming of the Tropical Sea Surface Temperatures 1951-2000.
Importance. The opposite sensitivity of many aspects of the global climate to strong tropical Indian and west Pacific
SST warming (strong enough that it leads to enhanced deep atmospheric convection in both basins) raises the
possibility of a climatic ―tipping point.‖ The Indian Ocean warming has thus far not been strong enough to enhance
the deep convection there, but if it continues, then at some point it will certainly trigger atmospheric deep
convection and rapidly force an opposite global response to that already being forced by the west Pacific warming
(which, among other effects, has been implicated in the extended four-year 1998-2002 drought over the
Southwestern United States). This rapid change will be felt most dramatically in the Pacific-North American (PNA)
sector, but also in the high Arctic latitudes. We believe this to be a serious possibility, and would like to assess it
quantitatively in a realistic modeling environment.
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