Modeling abrupt climate change

In their efforts to predict future climate change and possible effects of global warming, scientists at NASA are looking at well-documented events from the past as "blueprints" for the future. They believe that by focusing on a past event and entering relevant data and associated proxy data into a computer model, if the model is developed correctly and its output matches the actual conditions that resulted in the past, then that working model can be used with today's observations to predict future scenarios. According to Gavin A. Schmidt, a researcher at

NASA's Goddard Institute for Space Studies (GISS), "We only have one example of how the climate reacts to changes: the past. If we're going to accurately simulate the Earth's future, we need to be able to replicate past events."

According to Allegra LeGrande and Gavin Schmidt, both scientists at NASA, varying specific components of a model can generate future scenarios. For instance, by varying the level of CO2 in various iterations of a model, scientists can gain a clearer picture of what the Earth's environment would be like under those conditions. Likewise, a model can be run repeatedly and changes can be observed by varying the atmospheric temperature, the amount of precipitation, the flow of ocean currents, the percentage of ice cover, the percentage of freshwater, and any other variable related to global warming and climate change.

When designing such a model, scientists try to find a past event that lends itself to the creation of a solid, reliable model. To accomplish this, a model must have widespread and clear data, it must be an event that actually caused a change in the climate in the past, and it must be of a limited, defined duration. In line with these requirements, scientists at NASA decided to model abrupt climate change. The past event they chose on which to base the model was an abrupt cooling event that occurred across the Northern Hemisphere roughly 8,200 years ago. Fortunately, this event has been well documented with many different types of paleoclimate records as well as a significant geologic event—the catastrophic draining of the prehistoric glacial Lakes Agassiz and Ojibway in Canada. They both drained into the Hudson Bay at about the same time, flushing the North Atlantic with an overload of freshwater.

Climatologists believe this, in turn, disrupted the flow of the meridional overturning circulation (commonly referred to as the MOC) in the North Atlantic. Also referred to as the global conveyor belt, the MOC is critical for the distribution of warmth in the Atlantic. The current is an enormous continuous loop that brings warm water up from the equator to the polar regions. This warm water is what keeps western Europe's climate mild, considering how far north in latitude it is. When the current reaches its northernmost point and cools, it becomes dense, sinks, and begins its journey back toward the equator, where it is warmed again. The salinity level also plays an important role in the overturning of the current.

The problem with adding large amounts of freshwater to the ocean—by emptying a lake or through the melting of ice caps or glaciers—is that it decreases the salinity of the ocean water and slows the overturning process at high latitudes. Slowing the process slows the entire conveyor belt, which means that warmth from the equator is not brought into the Northern Hemisphere.

Using this information, NASA developed an ocean-atmosphere model that was analyzed by the Intergovernmental Panel on Climate Change (IPCC) in 2007 for their fourth assessment report. The model they developed, called a GISS ModelE, took the hypothesized cause (the draining of the lakes) and tried to reproduce the response. They accomplished this by adding calculated volumes of freshwater over predetermined intervals. To make the model as consistent as possible, they added tracers into the model, such as water isotopes, methane, dust, and other aerosols to account for climate proxies of the past, such as ice cores, cave records, and ocean and lake sediments.

Once the model was run, the scientists determined that the addition of the freshwater from the two ancient lakes to Hudson Bay was indeed enough input to slow down the conveyor belt current and cause a definite cooling in the Northern Hemisphere. In fact, it could slow the rate of flow of the MOC from 30 to 60 percent and cause a cooling of 3.5 to 5.5°F (2-3°C) in the North Atlantic as well as shifts in rainfall bands to the south in both the North Atlantic and the Tropics.

The results of this test are significant to the issue of global warming today and the idea of freshwater forcing. According to Allegra LeGrande and Gavin A. Schmidt of NASA, freshwater forcing could happen from the atmosphere heating and the ice caps and glaciers melting and draining into the ocean, ocean temperature warming, and increased rainfall.

"The flood we looked at was even larger than anything that could happen today," LeGrande reported. "Still, it's important for us to study because the real thing occurred during a period when conditions were not that much different from the present day." "Hopefully, successful simulations of the past such as this will increase confidence in the validity of model projections," added Gavin Schmidt.

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