Exmouth, Australia
Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level. “The question now,” said Presidential Science Advisor John Holdren recently, “is whether we can avoid catastrophic interference.” The most conservative estimates suggest a further 1 to 2°C of global temperature rise this century, even if massive reductions in greenhouse gas emissions were to occur. It is because climate is locked into this temperature trajectory, at least in the near-term, that understanding climate and ice sheet dynamics under warmer conditions is urgent.
The goal of this project, Sea Change, is to increase public awareness and understanding of this critical issue by engaging science-attentive audiences in the work of Boston University Professor Marueen Raymo’s PLIOMAX project and its efforts to increase the accuracy of sea level estimates during the mid-Pliocene warm period.
It is widely recognized that greenhouse-gas-induced warming over the next few centuries could lead to a significant rise in sea levels. Scientists are debating what a “safe” level of atmospheric greenhouse gases would be, with obvious ramifications for social and political organizations, as they become increasingly focused on legislating solutions aimed at altering human activities to protect the planet from danger. The scientific community relies heavily on the predictive capabilities of general circulation models (GCMs) to assess likely future warming scenarios. However, the results of these models can vary greatly between research groups, especially with respect to their ability to model different components of the climate system, such as ice sheets and thus future sea level. Warming affects sea level because water expands and takes up more volume when it’s heated, and also because changes in the volume of ice sheets (either positive or negative) will affect the amount of water in the oceans. (Until the last decade it was not certain that the net impact on ice sheets of warming would be negative, as it was thought that increased precipitation in the form of snow at high elevations could outweigh the amount of ice lost to melting. It is now generally acknowledged that warming is causing and will continue to cause a net loss of ice and thus a rise in sea level.)
To get at the problems of “knowing, which models are best” or whether they are getting ice “right,” the modeling community puts significant effort into calibrating models against past climate changes. And, since historical records of climate change are so short (typically less than 150 years) and contain only modest changes compared to the magnitude of changes predicted in the future, longer proxy records of climate change in the deeper past are often used in these experiments. In particular, climate modeling groups—including NASA-GISS (NASA Goddard Institute for Space Studies), NCAR (the National Center on Atmospheric Research), and the UKMO (the UK Meteorology Office)—have all run GCM experiments focused on the mid-Pliocene, attracted by its obvious relevance to predicting the impacts of increases in carbon dioxide levels. They have focused on the mid-Pliocene “climate optimum,” between about 3.3 and 2.9 million years ago, a period that was the last time Earth’s climate was warmer than the Holocene (the period since the last ice age) for a significant length of time (this time period was just prior to the onset of the periodic glaciation of North America and Scandanavia). Mean global temperatures are estimated to have been elevated by as much as 3°C with respect to modern values, possibly driven by enhanced CO2 level. This degree of warming is in the middle of the range of what the IPCC predicts will take place later this century.
However, despite widespread documentation of warmer conditions at that time, so far the geologic community has failed to reach a consensus on one critical attribute of the mid-Pliocene interval, namely, the maximum sea-level position, which, of course, is closely related to ice volume. Without a firm knowledge of this important parameter, efforts at evaluating the fidelity of climate and ice model experiments focused on this time period will be doomed to uncertainty.
PHOTO GALLERY OF EXMOUTH, AUSTRALIA
- View Dan's Photographs of Exmouth, Australia
