The Arctic ice sheets are melting. Many already understand that is will lead to sea-level rise, but there is another effect that this may have on the world. In the past the melting of large masses of ice in the Artic Ocean has happened before and they had massive effects on the climate. Europe cooled dramatically, especially in winter, and Asia showed signs of desertification. Climatologists believe that these temporary climatic shifts may happen again now that the Arctic ice sheets are melting due to human driven climate change.
You may wonder how melting ice sheets may make parts of the world cooler. Some might know of the albedo effect. The white of the ice reflects the energy of the sun back into space. As the ice melts there is less white to reflect the energy and more heat remains on Earth. If the heat is increasing, then how can parts of the world become cooler? This is because the chain reaction that leads to potential cooling isn’t linked to the amount of ice left, but the amount melted. When ice melts, fresh water is added to the ocean and this influences how the ocean functions.
Oceanic circulation is often called a conveyor belt by oceanographers. Ocean waters rise in the Northern Pacific Ocean and the Indian Ocean. Then they travel across the surface to eventually sink again in the North Atlantic Ocean. From there they travel across the bottom of the ocean back to the Pacific and the Indian Oceans. Figure 1 shows this circulation. It takes centuries for waters to travel along the entire conveyor belt, but if the circulation gets disturbed the effects may be noticeable quite a bit faster. This may be a good hundred years, but that is quite fast on a geological time scale.
Figure 1: This map shows a simplified image of the Great Ocean Conveyor Belt. Arrows indicate the direction in which the water travels. Blue indicates deep water currents and red indicates surface water currents (Source: IPCC 1996, based on Broecker (1987)).
So what happens if a lot of fresh water from icebergs gets added in the area where the waters sink? To answer this question we need to know the mechanisms behind the sinking of these waters or, as it is also called, the North Atlantic Deep Water formation. The warmer waters that arrive from the south are saltier without being heavy. This is due to the different properties water can have at different temperatures. As the waters travel north they become colder. The higher salt content causes the waters to become heavier and eventually sink. Much like water evaporates more when the air is warm and then falls down as rain when the air cools. When fresh water is added to these waters their salt concentration decreases and therefore the speed at which they sink decreases as well.
Scientists have measured a slowing down of North Atlantic Deep Water formation in certain places in recent years. They have also observed mass melting of icebergs in the distant past. These are called Heinrich events and can be observed in oceanic sediment by the large rocks that fell as the icebergs melted. Both computer models and natural records from prehistoric times show a chain of events caused by this melting and subsequent slowing and/or stopping of the North Atlantic Deep Water formation. Figure 2 shows a simplified map of climate anomalies from an event that happened 8,200 years ago. Lands near the North Atlantic cooled down, especially during winter, Africa and Asia saw decreases in rain, and the Americas suffered from increased winds.
Figure 2: Summary map of climate anomalies during the 8k event (from Alley & Argustsdottir (2005)).
But how can one part of this circulation cause effects as far away as India? Lets go back to the name ‘conveyor belt’. If you stick a screwdriver in the place where the conveyor belt at a supermarket cash registry disappears down, the whole belt will stop moving. The same applies to the oceanic conveyor belt to a degree. If no more water sinks in the North Atlantic Ocean the rest of the system will slow down and stop as well. The oceans have a massive influence on the climate of the world, especially on landmasses near these oceans. Warm waters from the equatorial region move north and release their heat (also shown in figure 1). During winters this means that temperatures do not get as low as they could. If no more warm waters move north this influx of warmth is also gone. North America and Europe suffer much colder winters in this scenario. Models and prehistoric records have shown the formation of sea ice in Europe in these conditions. In Africa and Asia monsoons are very important for the formation of rain. Monsoons form due to the balance of ocean temperatures and land temperatures. The slowing and/or stopping of the ocean conveyor belt will change this balance and the characteristics of the monsoons will change, leaving Africa and Asia drier than before.
However, we shouldn’t panic and start worrying about apocalyptic winters in Europe or massive crop failures in Africa and Asia just yet. The Earth’s climate is a system about balances. A balance can be upheaved, but as long as certain thresholds aren’t passed the balance will be restored. The slowing of North Atlantic Deep Water formation that has been measured in certain spots recently doesn’t mean that the whole system will be messed up. In other areas the waters are still sinking at regular speeds and scientists haven’t measured any of the other waters moving towards these sinking spots slowing down. Even if it came to it, the conveyor belt can restart and speed back up again, as it has in the past. This article isn’t written for the purpose of scaremongering. It is a look at a truly tiny part of the massive system of checks and balances that influence the climate on Earth. How complicated it is to predict how the human driven climate change will effect the Earth in the long term. There is a reason why we changed from talking about Global Warming to Climate Change. In the past Heinrich events and the climatic changes linked to it were most often observed at the end of ice ages. The Earth was warming, but would temporarily cool down again. However, the forcing behind the warming was too strong and the Earth would continue to warm. If the forcing had not been as strong, the Earth could have gone back to the way it had been before (the ocean conveyor belt never stayed still). This is why we talk about thresholds and tipping points. The Earth can regain her balance. It will take a long time, but we can help her along the way.
If you want to read more about the North Atlantic ‘conveyor belt’ and the associated climatic changes a good place to start is: Alley, R. B. (2007). Wally was right: Predictive ability of the North Atlantic" Conveyor belt" hypothesis for abrupt climate change. Annual Review of earth and Planetary sciences, 35(1), 241-272.
Alley, R. B., & Ágústsdóttir, A. M. (2005). The 8k event: cause and consequences of a major Holocene abrupt climate change. Quaternary Science Reviews, 24(10-11), 1123-1149.
