Head in a Cloud

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Wait, the Earth is cooling?

February 18th, 2007 by tanya · 1 Comment

The surface of the Earth, and those living on it, are directly affected by the troposphere. Thus, climate change in this layer from both anthropogenic and natural sources is a hot topic and the subject of much research. However, Earth’s atmosphere extends far beyond the troposphere and each layer is influenced by the other. As a result researchers have been striving to understand how changes in these lower layers propagate up and influence the upper atmosphere. One paper published on November 24th 2006 in Science Magazine, titled “Global Change in the Upper Atmosphere” and written by J. LaÅ¡tovička et al., gives an overview of research conducted in the upper atmosphere. Not to give the ending away, but the main point of the paper is to highlight how a decrease in density, due to a cooling of the upper atmosphere, will increase the orbital lifetime of satellites because of a decrease in drag.

First, a brief overview of the structure of the upper atmosphere. The mesosphere either is the start of or sits right below the beginning of the upper atmosphere (depending on who you talk to, I always thought the mesosphere was not part of the upper atmosphere). It ranges from approximately 50 km to 90 km and has been an area of much research in recent years. Above that is the thermosphere, which is the neutral part of what I call the upper atmosphere, again, depends on who you talk to. This part of the atmosphere runs from about 90 km to 800 km above Earth. Embedded in these two regions is the ionosphere, the charged/ionized part of the upper atmosphere, usually described as a plasma. This part of the atmosphere is used daily for radio waves and GPS tracking devices, as examples. These devices are becoming more and more important since society is accustomed to and dependent upon its electronic gadgets. This is the main motivation for understanding and predicting ‘weather’ in the upper atmosphere. Below is a figure describing the different regions.

Layers of the Earth’s Atmosphere

Many studies point toward a cooling of the upper atmosphere due to the radiative effects of greenhouse gases, such as carbon dioxide. This cooling results in a decrease in the density of the thermosphere, thus a contraction of the layer. Everyone knows a cooler object is densier than a hotter object, but I personally have not considered the specific repercussions of a densier thermosphere for life on Earth. As a result of this cooling/density increase, the layers in the ionosphere will be displaced downwards. According to the paper, the model cited, Rishbeth et al. 1992, simulates the cooling of the upper atmosphere accurately. Question, are there any models that do not predict such a cooling trend and do they factor in solar minimums to help obtain accurate trends? For example, the MSIS empirical model is well known and has been used in papers to highlight a cooling trend. However, there are ‘constants’, such as the O+ – O collision frequency, hence Burnside Factor, which is not yet determined and could change thermospheric trends. Currently the Burnside Factor is thought to be about 1.2, but has fluctuated between 0.8 to 1.6.

Basically the ionosphere can be measured directly using Incoherent Scatter Radar (ISR), satellites, etc …. , but the thermosphere has to be calculated using ionosphere data. Its parameters, like density and temperature profiles, can be found using two different methods that I know of, the energy equation and the momentum equation. The energy equation makes some assumptions and basically states that energy, hence temperature, transfers from free electron, to ionized particle, to neutral particle. This allows room for error and makes me question trends, from data or models, of the thermosphere. Thus I disagree with their statement that the trends between the ionosphere and thermosphere are “qualitatively consistent”. Is my stance of disbelief in their accuracy unfounded? Are the formulas used for the thermosphere accurate enough? Not by any means am I an expert in the field, but I always like to know what types of errors are floating around.

According to the paper, temperature trends between 50 km to 100 km are: decrease of 2 to 3 K per decade for 50 km to 70 km (with the largest amplitude in the tropics), slightly larger cooling trends between 70 km to 80 km in low to midlatitudes, essentially a zero cooling trend between 80 km to 100 km. Curious, what are possible physical reasons for no trend between 80 km to 100 km? Also, the authors found that overall the Earth’s surface temperature has increased by 0.2 to 0.4 degrees C and the middle to lower mesosphere temperature has decreased by 5 to 10 degrees C. As stated previously, temperature decreases result in a density increase. Thermal contraction follows which should result in a downward displacement of the ionospheric layers. This was verified by LaÅ¡tovička and Bremer, Surv. Geophys. 25, 69 (2004). Overall the thermospheric densities have decreased by 2 to 3% per decade, trends increase with height. Also, the maximum electron density of the E and F1-layer increased slightly, while the height of the maximum electron density decreased slightly in the E-region. According to the paper, all of this is in qualitative agreement with the model they are referencing. Below is a graph summarizing the above statements.

Structures and Trends in Earth’s Atmosphere

The basic conclusion in the paper is: The troposphere and anthropogenic influences on that layer can propagate up and affect the upper atmosphere. Thus affecting the space based technologies there, which society relies on and uses every day.

Tags: climate

1 response so far ↓

  • 1 Tanya Phillips // Jun 17, 2009 at 9:28 am

    Colette, thanks for the comment. I agree with all of your statements except the last one. I do not think that it is ‘completely unknown exactly what kind of reaction will occur’ due to carbon dioxide increases in the atmosphere. Scientists in their respective fields agree in the recorded temperature increases due to human influences on our atmosphere and the potential consequences, such has rainfall change and drought. Disagreement arises when deciding the how much the global temperature will increase over the next century and the degree to which this will change local/regional weather patterns. Maybe I have misunderstood what you mean by ‘completely unknown’. Again thanks for the comment and hopefully entertaining my response.

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