There were a few stories in the news recently that reminded me of our previous post on climate change. So I thought it would be a good time to talk about how a warming climate can have an impact on our weather. As I said before, Earth’s climate is an incredibly complex system; when Edward Lorenz made the first computer weather models in 1961, he discovered that a rounding error, (using 0.506 instead of 0.506127), resulted in completely different predictions. But by understanding some of the basic mechanisms that drive earth’s climate, we can begin to understand how trapping more heat in our atmosphere can cause large complex shifts.
A common bad faith criticism of climate science is “if there’s global warming, why is there still snow?” The short answer is that a single snowy day is just one data point and it’s impossible to determine broader trends from single data points. There can still be snow if the planet warms, it will just be less frequent. That said, there are mechanisms by which global climate change can increase the likelihood of severe winter storms under specific circumstances, such as the recent storms that hit most of the central United States as far south as Texas.
Polar Vortex
The polar vortex is one such mechanism. It is a region of stratospheric air that is bound by the polar jet stream. Warm air rises, creating an area of higher pressure in the upper atmosphere. High pressure air tends to move to regions of lower pressure, so air above warm regions moves toward cooler regions. In the northern hemisphere, this means that warmer air from the middle latitudes moves northward toward the far colder North Pole. The coriolis effect deflects this northward wind eastward, creating a circular wind current that surrounds the pole, trapping the cold polar air inside it. This current is dependent on a strong temperature difference between the poles and the middle latitudes, so it is strongest during the summer when the middle latitudes are much warmer than the Arctic. During the winter, when the middle latitudes become cooler, the jet stream weakens. This causes the polar vortex to expand and cover more of the planet in cold arctic air and on some occasions, break into smaller vortices that release a lot of cold air across the northern hemisphere.
The Arctic is currently the fastest warming part of the planet, though it is still, and will remain, exceptionally cold. But as it warms, the temperature difference that drives the polar vortex lessens, especially in the winter. While these cyclical changes to the polar vortex have always existed, they are becoming more common and more severe. Many climatologists are concerned that as climate change worsens, it could make the severe winter storms caused by the oscillation of the jet stream more common. Now, this is not to say that the recent snowstorm that devastated Texas is definitely the result of climate change. Given how complex the climate is, it’s impossible to know what the weather would look like were it not for human-made climate change. But it is to say that these types of storms could become more common as the Arctic continues to warm.
I should also note that this is another example of how a changing climate is deadly in large part because of the specificity of human infrastructure. For example, houses in Texas aren’t designed to hold on to heat while electrical infrastructure isn’t designed to weather these types of storms.* A major source of devastation right now is pipes freezing and bursting due to them not being designed for extremely cold weather. And the long-term solutions to these problems now might require a retrofit of many of the state’s buildings and parts of its infrastructure.
And this is not the only way in which this mechanism could cause more extreme weather. In addition to containing the polar vortex, the polar jet stream, and the planet’s other jet streams, plays a role in moving weather systems. Rain and different pressure zones are blown by jet streams from west to east, allowing weather systems to only last a few days before they move somewhere else. When these streams weaken, weather systems can become stuck in the same place for extended periods of time. A weakened jet stream could result in both flooding and droughts as rain becomes stuck over some regions and can’t move to others. Going back to Texas, some have speculated that this was part of what made Hurricane Harvey in 2017 so devastating. Harvey stalled over the Gulf Coast for about four days, deluging the region and causing record-breaking flooding in the Houston area. This was because of weak prevailing winds caused by a high pressure area that stalled over the central United States. Again, this is not to say that climate change definitely caused this storm, merely that Harvey is an example of a phenomenon that could become common if climate change is allowed to continue.
Ocean Conveyor
A recent study (detailed here) has found that the Ocean Conveyor, another climate impacting mechanism, is the weakest it has been in over a millennium. What this means is...complicated.
The Ocean Conveyor Belt, or Thermohaline Circulation, is a series of deep ocean currents that redistribute heat across the planet. The exact paths of these deep currents is the result of continent placement and coriolis forces, but the basic physics is simple. In the tropics, ocean water is heated up by the sun and rises to the surface. This warm water is driven toward the Arctic where cold air pulls heat out of the water and causes it to sink to the ocean floor. This cold deep water then flows back south where it can be heated again, continuing the cycle. This system of currents distributes heat more evenly across the planet, slightly cooling the tropics and warming the poles. One of the major currents in the Ocean Conveyor is the Atlantic Meridional Overturning Circulation (AMOC), a system of currents running throughout the North Atlantic Ocean. AMOC begins with the Gulf Stream, which follows prevailing winds westward through the Caribbean Sea and the Gulf of Mexico, then eastward back into the Atlantic.This circuitous route to the North Atlantic allows this water to absorb a lot of heat. The Gulf Stream then travels up the East Coast of North America before crossing the Atlantic to become the North Atlantic Current. This current branches out next to Northwestern Europe, releasing its heat into the colder air. It’s because of the Gulf Stream that Western Europe is a temperate climate similar to the United States in spite of being at the same range of latitudes as Canada.
The problem is that this current is dependent on salt concentration (the haline in thermohaline circulation is Greek for salt). The difference between the density of warm and cold salt water is significantly greater than the difference in density between warm and cold fresh water. It’s this difference in density that allows warm water to rise to the surface and cold water to sink to the ocean floor, thus driving the current. But as ice sheets melt due to climate change, they release thousands of tons of fresh water into the ocean, making the seas less salty. The melting of the Greenland Ice Sheet is the single largest contributor to global sea rise, so its position right next to the AMOC currents poses a big risk of slowing down or even stopping this circulation system.
You might be familiar with the idea that climate change could cause another ice age, which was the plot of the 2004 film The Day After Tomorrow (though the effects of AMOC stopping in the film are wildly unrealistic in their speed and severity). The actual version of this hypothesis is that if AMOC stops due to ice melt, it would cut Europe off from this heat source and cause temperatures to drop to that of Northern Canada, meaning mild summers and extremely cold winters that the continent is not prepared for. Now, the hypothesis that a halting AMOC would freeze Europe isn’t universally supported and has actually lost some support in recent years. Many scientists now believe that the temperature rises caused by climate change would be enough to offset the temperature drops caused by AMOC stopping. But again, this is a prediction for future decades based on trends being calculated with less-than-ideal data. All we can really know for sure is that Europe’s climate will be different from what it is now.
Regardless of what happens to Europe, AMOC stopping would be a big problem. All the heat that is currently being distributed to the North Atlantic would become stuck in the tropics, causing sea level rise and larger, more numerous storms in the Gulf of Mexico and the rest of the Atlantic. The Ocean Conveyor also plays a role in evenly distributing dissolved oxygen and nutrients throughout the ocean, which is necessary for aquatic life. A shutdown could lead to the Atlantic becoming depleted of oxygen and nutrients, causing a local mass extinction. And all this excess heat could wind up elsewhere in the Global Ocean Conveyor, potentially affecting temperature, rainfall, and fish stocks throughout the world.
One of the scarier aspects of this slowdown is how quickly it can happen. Studies of ice cores from the last ice age have shown that starting 20,000 years ago, the climate began warming as the ice age was coming to an end. But around 12,900 years ago, the climate snapped back to extreme glacial conditions, with average temperatures in Greenland dropping about 7°C in only a few decades. It would be another 1,200 years before temperatures would return to what they were before this sudden drop. Based on concentrations of radioisotopes in ocean floor sediment (a topic for another post), it is believed that this sudden drop in temperature was caused by meltwater from receding ice sheets shutting down the AMOC. And this period of cooling (called the Younger Dryas) was just the most recent of several sudden temperature drops as the last ice age ended in fits and starts. If AMOC slows down again, its effects could be seen within a single human lifetime, whatever those effects may be.
Why Should We Care
The study mentioned earlier confirmed that the AMOC currents are about 15% slower than they were only a few decades ago, making them the weakest they’ve been in over a millennium. As I said previously, the risk that climate change poses comes from how dependent our civilization is on the dependability of our climate. Whatever changes occur, if we do nothing now our infrastructure will not be able to be adapted fast enough to prevent extreme economic depression, mass loss of life, and potentially even political instability to the point of collapse. If we can learn anything from the Covid pandemic, it’s that world-changing disasters can be insidious to the point of mundanity, uneven in their impact, and cause damage by straining our existing societal infrastructure to the point that it starts to struggle. Foresight and preparation will be far less costly than living with the aftermath of a significant climate change and its societal repercussions.
For More Details
*I would be remiss if I didn’t mention that many if not most of the problems the state of Texas faced during this storm were political instead of meteorological. I won’t go into this here since it doesn’t really fit the topic of this blog, but I feel it’s important to understand how climate related problems can be compounded by poor planning and mediocre governance. This video from Vox says just about everything I would say on this topic.
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