In one way, the IPCC forecasts represent the best-case scenario because they are smooth. There are no surprises. The climate record from Greenland shows a period of stability throughout the Holocene, the past 10,000 years when civilization and agriculture developed (Fig. 12.13). Prior to this, during the last ice age 20-80 kyear ago, the climate of Greenland was not so stable. Temperatures varied wildly in 1000 year swings called Dansgaard-Oeschger events, sudden warmings of 10-15°C within a few years, followed in general by more gradual coolings check. The record is punctuated by what are known as Heinrich events, times when an ice sheet collapses into the ocean within a few centuries. These climate events are most intense in the high Northern latitudes, but their effects are felt as far away as New Zealand and Antarctica.
Holocene stable climate
Holocene stable climate
0 20 40 60 80 100
Age (thousand years before present) Fig. 12.13 Temperature reconstruction from Greenland GISP II ice core.
Scientists are still trying to figure out the reasons for all of the variability in the glacial climate. The circulation in the North Atlantic is one player. The Atlantic carries a considerable amount of heat into the high latitude Northern hemisphere. The warm surface waters in high latitude cool and sink, making way for more warm surface flow, bringing in more heat.
During glacial time, climate seemed able to rearrange itself into a different configuration. In this configuration, ice covered much of the North Atlantic. The overturning circulation in the North Atlantic slowed. The ocean carried less heat into the high latitude northern hemisphere. These events were apparently triggered by excess freshwater release into the North Atlantic. There are examples of climate changes from spilling of a dammed lake and by Heinrich event ice sheet meltdowns.
Here's a scenario. The freshwater release created a layer of low-salinity water at the surface of the ocean. Water is less dense when it has less salt dissolved in it, so the lower salinity surface water could cool down all the way to freezing without becoming dense enough to sink. Now sea ice becomes a player in the drama. The albedo of the ice acts to cool the climate still further, at least in the northern high latitudes. The climate of Greenland is particularly sensitive to this climate rearrangement because it loses its maritime influence. Air temperatures over land or ice plummet to values much lower than would be possible over water.
The abrupt climate change scenario for the future that people have been fretting about may be a cousin to this cold configuration of glacial climate. There are two possible sources of excess freshwater to the North Atlantic. One is precipitation, which is expected to increase, because of rising temperatures and water vapor saturation and all that. The other source people think about is meltwater from Greenland. Perhaps the Greenland ice sheet collapses like a modern-day Heinrich event, releasing its freshwater into the North Atlantic over a century or so. In models, the overturning circulation in the North Atlantic can be stopped or greatly slowed by an excess input of freshwater. In coupled atmosphere ocean climate models, the overturning circulation in the North Atlantic slows down as you increase atmospheric CO2. If we were to simply draw a parallel between the past and the future, the result could be a paradoxical cooling in the northern high latitudes. Climate models do not predict a huge climate impact from a North Atlantic overturning shutdown. However, climate models have a tendency to underpredict the swiftness and the severity of past climate variability that we know is possible in the real world. This may be because positive feedbacks in the real world are difficult to capture in models.
An abrupt climate shift would be a surprise, like the ozone hole. Once the climate rearranged into some different mode, it could conceivably remain locked in that mode for centuries.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.