Atmospheric layers driving accelerated far North warming
Laura Nielsen for Frontier Scientists –
Recent research published in Nature Geoscience states that the largest contributors to warming in the Arctic are the region’s distinct surface temperatures coupled with the Arctic atmosphere’s prevailing vertical temperature structure. The research suggests that diminished snow and melting ice cover, previously thought to have the largest impact on Arctic warming by baring more dark surfaces which soak up heat from the Sun, in fact represents the second-largest contributor to Arctic warming.
Temperature feedbacks and atmospheric layers
The paper “Arctic amplification dominated by temperature feedbacks in contemporary climate models” was written by climate scientists Felix Pithan and Dr. Thorsten Mauritsen of the Max Planck Institute for Meteorology. They analyzed cutting-edge climate model simulations to uncover their findings.
The Arctic is warming two times faster on average than any other region of the world, a phenomenon called Arctic amplification. Amplified Arctic warming is contributing to more frequent and more extreme weather events in North America and Europe.
The Max Planck Institute for Meteorology introduced the paper, stating that “The main cause of the high Arctic climate sensitivity is a weaker temperature feedback, due to 1) the low temperatures that prevail and 2) the increasing temperatures with height trapping warming to remain near the surface.”
Over much of the world, the air in the atmosphere mixes vigorously. Heat that soaked into the ground (much like a hot car seat that’s been sitting in sunlight) can seep into the air, and the air can mix higher and higher into the atmosphere until the heat escapes into space. However, lead author Pithan noted that: "The Arctic atmosphere is much more inefficient than the tropics at getting rid of that extra energy." It is difficult for air from the surface to mix with layers of air that are higher in the atmosphere, and therefore difficult for heat to radiate back into space. Instead, “As the climate warms, most of the additional heat remains trapped in a shallow layer of the atmosphere close to the ground, not deeper than 1 or 2 kilometers [0.6 to 1.2 miles]."
The staid, still layer of air he describes is especially common in the winter. During the Arctic winter an event called a temperature inversion is more likely to dominate the structure of the atmosphere. Normally the layer of air near the ground is relatively warm and temperatures decrease as you move higher through the atmosphere. Yet during a temperature inversion, the layer of air closest to the ground is colder than the layers of air that sit higher in the sky. This can happen, for instance, because: a dusting of snow covering the tundra reflects sunlight away, keeping the air near the ground very cold, or as night falls the ground very swiftly loses the scant heat it absorbed during the day, or when cold ocean water wells up near a coast and chills the nearby air. Hot air is less dense and therefore lighter, while cold air is more dense and heavier. It is extremely difficult for the heavy cold air to mix higher, and so the air lingers near the ground. There is little convection, or mixing, and the layers of the atmosphere remain separated. That can cause big problems when wood smoke or smog get trapped in the heavy, still, cold layer of air sitting over a city during a temperature inversion.
Pithan and Mauritsen’s study found that the peculiar interaction of temperature and atmospheric layers in the Arctic actually contributes the most to amplified Arctic warming.
Causes of Arctic amplification
The Arctic Research Plan for 2013–2017, an initiative of the Interagency Arctic Research Policy Committee (IARPC), records:
“This Arctic amplification phenomenon is recognized as an inherent characteristic of the global climate system (Serreze and Barry 2011); the causes are believed to include complex interactions associated with heat exchange between the atmosphere and ocean (with its changing sea-ice extent), meridional heat transport, and radiative forcing from atmospheric constituents.”
Let’s look closer. The segment says first that Arctic amplification is real: in a warming world climate, the far North warms faster than elsewhere does. Dr. James Overland with the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory gave a talk this year at the Arctic Encounter Symposium. He noted that during present times, “Human forcing is already in the climate system [... and the] Arctic amplifies the changes.” You can see the effects of Arctic amplification: “Summer Arctic-wide sea ice loss is likely to occur within a decade or two.”
Next, the Research Plan mentions the interaction between the atmosphere and ocean. “Ultimately the ocean and the atmosphere have to be coupled,” Richard Alley explained during a presentation for the Chapman Conference on Communicating Climate Science. He said that: “In the last decade the system has dumped more of the heat into the oceans and less into the atmosphere.” The ocean is a mighty heat sink, and it has been able to sequester, or store, a lot of the heat entering Earth’s system with anthropogenic (human-impacted) climate change. “Yes, heat is still accumulating in the Earth's system with high confidence,” Alley said, despite this winter’s chilly temperatures, and “No, there really hasn't been a stop in global warming.” When we look at atmospheric layers in the warming Arctic, we are investigating an aspect of how the atmosphere transports heat.
Third, the quote above mentions meridional heat transport. A meridion is one of the vertical lines you can see traced on a map of the Earth. They follow the north-south longitude direction. Earth gains a great deal of heat in the regions near the equator (the horizontal midline of the globe) because those tropical places are exposed to more direct sunlight over the year than the poles are. Meanwhile, the Arctic and Antarctic experience long, dark winters, during which little sunlight reaches them. The hot-and-cold difference creates an imbalance. Heat must flow toward cold, and so atmospheric and oceanic currents act to circulate heat from the equator towards the poles. That motion is meridional heat transport.
Last is radiative forcing. Energy from the Sun enters the Earth’s atmosphere. That warm sunlight might be absorbed by the planet, or it might be radiated back into cold space. Radiative forcing describes the balance of energy and heat the Earth gains or loses during this process. Especially important to this process is the idea of albedo, or reflectiveness. Pithan and Mauritsen’s study challenges the previously held theory which held that changes in albedo were the most influential alterations leading to Arctic amplification. Fields, forests, or open ocean water have a low albedo, and reflect only about 30% of sunlight away from Earth’s surface. In contrast, bright surfaces like snow, sea-ice, or even fluffy white clouds have a high albedo and can reflect up to 85% of sunlight. When there is no sea ice in the Arctic during summer, Overland explained: the change will “Increase Alaskan open water duration by 2-3 months,” exposing more dark ocean water and absorbing more heat, raising temperatures.
There are other things in the atmosphere –atmospheric constituents– which absorb heat. Water vapor is a powerful greenhouse gas, and as Earth’s temperatures rise more water is evaporated into the atmosphere. Water vapor joins other greenhouse gasses like carbon dioxide and methane in trapping and holding heat inside Earth’s atmosphere. Dark particles also absorb heat. Some of the atmospheric warming observed since 1976 in the Arctic and worldwide can be attributed to aerosols, tiny particles that float around in the atmosphere and slowly drift to the ground. These minuscule particles come from many sources: fossil fuel combustion, ash from trees burned after deforestation or during wildfires, particles from volcanic explosions, soil turned to dust after overgrazing or unsustainable farming practices, and pollution from factories.
The complexities of global warming
When you look at climate change, “You see the human influence,” Richard Alley noted. He asked: What is really going on with modern-day global warming? “Humans are forcing warming. Warming is happening. The human fingerprint is still clear in the data. ...And then there are other things that are also in the game.” Papers like Felix Pithan’s published this month in Nature Geoscience help us understand the complex temperature feedbacks that are governing influential warming on our planet.
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
Pithan, F. and T. Mauritsen, 2014: Arctic amplification dominated by temperature feedbacks in contemporary climate models. Nature Geoscience, doi: 10.1038/ngeo2071
- ‘Arctic amplification dominated by temperature feedbacks in contemporary climate models’ F. Pithan & T. Mauritsen, Nature Geoscience (February 2014)
- ‘Arctic Research Plan: FY2013–2017’ IARPC: Interagency Arctic Research Policy Committee (February 2013)
- ‘Arctic's 'Layer Cake' Atmosphere Blamed for Rapid Warming’ Becky Oskin, LiveScience (February 2014)
- ‘Chapman Conference on Communicating Climate Science’ Richard Alley, American Geophysical Union (AGU) Youtube Channel (June 2013)
- ‘Explained: Radiative forcing‘ David L. Chandler, MITnews (March 2010)
- ‘Temperature feedback plays larger role for increased climate change in the Arctic than melting’ Max Planck Institute for Meteorology (February 2014)
- ‘Temperature feedbacks amplify Arctic warming’ Nature Geoscience Press Release (February 2014)
- ‘Temperature Inversion Layers’ Amanda Briney, About.com (November 2013)
- ‘The Oceanic Heat Budget’ Robert H. Stewart, Department of Oceanography, Texas A&M University (September 2008)
- ‘The Warming Arctic: Faster than Expected’ James Overland, NOAA Pacific Marine Environmental Laboratory, at the Arctic Encounter Symposium (February 2014)