The day after tomorrow – don’t prepare for the ice age just yet.

The Day after Tomorrow, a Sci-Fi disaster film released in 2004 sought to portray what would happen if one of Earth’s largest heat transport systems shut down. The film is extremely dramatic and shows huge shifts in Earth’s climate over a matter of weeks after the opening scene. Scenes which were so dramatic that NASA science consultants labelled them too ridiculous to occur, so if you watched the film and were worried about the impending ice-age, then don’t be! But, I would like you to come with me as we discover what real process was at that heart of this Hollywood-representation of climate change. Lets go!

What is the Thermohaline Circulation (THC)?

Before we dive into what happened in The Day After Tomorrow, I want to share some knowledge about the thermohaline circulation or THC, as scientists love using acronyms. The THC is a global conveyor belt of overturning water and is controlled by temperature (Thermo-) and salinity (haline), so in general, it’s a global ocean current that is driven by density differences. As you’re probably aware, salty water is more dense than fresh water, it’s the reason you can go to the dead sea and easily float. The dead sea is almost 10 times saltier than typical ocean water and this makes it dense and easy to float in.

So the THC relies on the denser nature of saline water to help drive it. The current itself travels from the north Atlantic down to Antarctica, around through the Indian ocean and across the Pacific. It literally is a huge, global, ocean current. As I mentioned above, it has two main drivers, temperature and salinity. “Deep water formation” refers to sea-ice regions where water is sinking deep into the ocean due to increased salinity. This is where the circulation begins, salt from the ocean water cannot be incorporated into the ice, in the same way that salt will not evaporate with water, it stays behind. Building up the salt content in the water (from making sea ice) will result in an increase in density and this will cause the denser water to sink down.

Thermohaline_Circulation_2
The Thermohaline Circulation in all its glory! It covers most of the globe and has localised centres of deep water formation in the northern and southern polar regions. Credit: NASA, with modification by Robert A. Rhode.

As the deep water is formed and sinks, this creates a depression in the ocean surface and your intuition says that if you depress a water surface, then other water quickly comes to fill the hole. So this sinking pulls surface water from the equatorial regions, cools it and then eventually sinks it in an ongoing process. This, in essence, is the Thermohaline circulation.

Now, why is the THC important? Well, the ocean is the biggest heat sink we have, it accounts for approximately 70% of the surface of Earth. Plus the ocean typically has an albedo of about 0.06, this means it only reflects 6% of the incoming radiation from the sun. So it absorbs almost 95% of that radiation, making it a major and important source of heat redistribution. The THC has different roles, one is to transport that heat to polar regions (generally away from the equator) and this helps regulate the formation of sea ice and provides heat for Europe and North America. But also the circulation helps to upwell nutrient-rich deep water to aid the growth of ocean biota. So it’s a life-giving circulation.

How does the THC relate to The Day After Tomorrow?

The THC was at the heart of The Day After Tomorrow, Dr. Jack Hall (the protagonist’s father) is a palaeoclimatologist, meaning he studies past climate by looking at proxies such as ice cores. This is the reason he was in Antarctica in the opening scene when he witnesses a Rhode Island-size (~3000 square km) chunk of Antarctic ice-shelf break free. This film wastes no time and the following scene sees Dr. Hall expressing his concerns to the American vice president and ambassadors of many countries at the United Nations conference on global warming in New Delhi. It is here that he introduces the North Atlantic current which is part of the larger Thermohaline circulation. The North Atlantic current is a west bound current originating from where the Gulf Stream turns north and is a major supply of equatorial heat to North America and Europe.

He goes on to talk about palaeoclimate research and modelling that he and his team have been doing and found that in the past, the climate has experienced runaway warming events which eventually have triggered an ice age. He then goes on to explain that America has a temperate climate, thanks to the THC and that global warming is affecting the ocean circulation:

“Eventually it (the THC) will shut down, and when it does, there goes our warm climate.” – Dr. Jack Hall (The Day After Tomorrow resident scientist).

Shortly after this, ocean buoys which are recording sea surface temperature begin to record dramatic drops in temperature. The following scenes depict severe storms such as softball sized hail and extreme typhoons. Later, Dr. Hall finds himself in another meeting with NASA scientists from the National Oceans and Atmospheric Administration (NOAA). They are discussing what may be causing these severe storms and weather patterns, one idea thrown around was erratic solar behaviour. This was quickly shut down with the confirmation that solar activity had been stable at the time. Dr. Hall then suggests maybe it’s the North Atlantic Current (NAC). Remember, when he mentions the NAC, he is referring also the larger picture which is the THC. He exclusively mentions the NAC, as that is the current which directly affects America. It is at this point he says they’ve hit a “critical desalinisation point”, which is referring to too much fresh water input into the ocean. Probably the result of the huge chunk of ice from the opening scene.

Eventually the weather patterns become more extreme, with huge storm surges hitting New York city and eventually freezing over and Europe also suffers during this time. I’m not going to go into each and every detail but I will make a general remark, which is, these scenarios are extremely overdramatised. But you probably already knew that!

What would really happen if we shut down the THC?

This is quite a researched question and it is not a trivial one. The climate system is complex and has many variables and we simply don’t understand enough about every single variable to be able to make 99% accurate models. Though the models still do pretty well.

If we take the opening scene where the ice sheet breaks off and goes into the ocean. This would dump a huge amount of fresh water into the waters of Antarctica and yes, very well could make the THC less efficient. But here’s the thing, nowadays, the collapse of the THC is regarded as a “low probability – high impact” risk, which basically means it’s unlikely to happen, but if it did, it would cause big problems. It’s believed that the effect of global warming on the THC would not result in a total collapse, but rather a weakening of ~20-50%. Climate modellers found that by adding more variables, the weakening of the THC became less and less. The study Gregory et al., 2005, published in the journal Geophysical Research Letters, compared 11 different climate models to try and assess the response of the THC to global warming. They found that over a 140 year period, all models showed a weakening in the THC by 10-50% but no models showed a complete collapse. Another very interesting point, is that no model showed cooling at all, all of them showed gradual warming! Why is this? Well cooling from a weakened THC is simply smaller than the warming due to the greenhouse effect, this mean there would be a net warming.

Ok, but that’s just a weakening, even though we’ve now established that turning the THC off is probably not likely, what would happen if it did get turned off? Would we see the events depicted in The Day After Tomorrow? Wood, Vellinga and Thorpe (2003) is a paper published in Philosophical Transactions of the Royal Society and basically aimed to show what happened when you shut off the THC completely by 2050. The models are treated in a similar way to what i’ve already described, fresh water is added and the mean density lowers, which prevents the formation of deep water and halts the THC. Unfortunately there are discrepancies amongst models with varying results. Though, what seems to be clear from this study is that instantly shutting down the THC would result in cooling in the Northern Hemisphere, namely Europe and America, but then over decades, the greenhouse effect takes over and keeps on warming. But again, the complete collapse of the THC is regarded as a low possibility, it’s more likely the THC will weaken over time.

So did The Day After Tomorrow get it right?

I think you know the answer to this, and the answer is yes, they did get it right. Ah, I mean, no, they definitely got it wrong. But it’s Hollywood, they need to pack it full of drama to get us to watch it. The main area in which they were wrong, was the time scale and severity of the weather patterns, and of course the shut down of the THC.

Firstly, we’ve established that the THC would not likely shut down in the first place, it will be a gradual weakening of 20-50% over decades.

Secondly, the extreme weather events such as soft ball-size hail and huge tornadoes that destroyed whole cities are not likely to occur on such a short time scale. Though a point that they do get right, is that global warming will see more erratic and severe weather events. The reason is that a warmer planet means more evaporation, more evaporation means more energy in the atmosphere, voilà, this leads to stronger storms. But don’t expect a Day After Tomorrow-sized tornado to sweep through your city anytime soon…

Thirdly, the ice-age they refer to would likely not occur as a result of the weakening of the THC.  This is because the greenhouse effect is still present and the melting of carbon-dioxide-rich ice would just add to the warming. So it’s likely the planet would get hotter before it would get any cooler.

There are so many other things one could discuss with regards to this film. But hopefully this has shed some light on the role of the THC. The consequences of global warming will surely be the topic of many a posts to come.

References and further reading:

US Department of Commerce, National Oceanic and Atmospheric Administration. (2013, June 01). Currents. Retrieved May 30, 2017, from http://oceanservice.noaa.gov/education/tutorial_currents/05conveyor1.html

What is the thermohaline circulation (THC)? (n.d.). Retrieved May 30, 2017, from http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html

Rahmstorf, S., Shifting seas in the greenhouse?Nature, 1999. 399: p. 523-524.

Global warming and thermohaline circulation stability, Richard A. Wood, Michael Vellinga, Robert Thorpe, Phil. Trans. R. Soc. Lond. A

Gregory, J.M., K.W. Dixon, R.J. Stouffer, A.J. Weaver, …, I.V. Kamenkovich, …, A. Sokolov, et al. (2005): A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration. Geophysical Research Letters, 32: L12703 (http://dx.doi.org/10.1029/2005GL023209)

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