Earth’s atmosphere – From a hot cloud of dust and gas to life nurturing environment

 

The early atmosphere –

History of earth – 4.6 billion years. 

Evidences - Oldest occurring rock – Faux Amphibolites of the Nuvvuagittuq greenstone belt - found in Quebec Canada with age of 4.28 billion years. Isotopic analysis of meteorites, soil, rock samples of moon.

Faux Ambibolites (Nuvvuagittuq Greenstone Belt, Eoarchean, 4.28 Ga; western Ungava Peninsula, eastern side of Hudson Bay, Quebec, Canada) James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons

 The continuous collisions between primordial solar nebula particles (interstellar gas and dust) is mainly considered responsible for the most rudimentary form of earth. This continued for some 150 million years until eventually clumped into a rock form. The mass of earth had very high temperature due to terrestrial accretion heat generated through –

o   Decay of radioactive isotopes

o   Gravitational energy of sinking metals

o   Impact of small planetary bodies bombarding into earth.

 ·         Eventually, rock material started melting bringing about sinking of heavier material rich in iron and nickel, to core and rising of lighter elements to top. The maximum amount of lighter primordial gases like hydrogen and helium escaped during this phase.

 ·      The light molten material crystallized to build initial thin crust. This thin and unstable crust collapsed again formed many times to ultimately form a thicker crust as a result of large scale convection current inside earth. At the same time, outgassing of gases from surface volcanic activity led to oceanic evolution and secondary atmosphere.

Oceanic evolution -

H2O particles were already present in the Earth mass during the planetary accretion phase of formation of earth. During outgassing temporary steam atmosphere was created which precipitated during the subsequent cooling phases of the earth.

Secondary atmosphere –

·         Volcanism was the major route through which degassing of volatile material from the inner Earth took place. Volcanic activity produces water vapor, Carbon dioxide, Carbon monoxide methane gas, nitrogen in substantial quantities but no oxygen.

Decrease in CO2 –

·         Carbon dioxide was the main gas providing heat in the early atmosphere as the early solar system was illuminated by a weak, young Sun that only delivered ∼75% of the present day energy.

According to a climate model, at 2.75 Ga, CO2 level was 500 times present atmospheric levels. However, by the Cambrian, CO2 levels were close to present atmospheric levels.

The main reason for the extraction of CO2 out of the atmosphere was the development of life forms in the ocean that seized carbon in organic and later inorganic (calcium carbonate) forms and buried it in sedimentary formations on the sea floor.

Evidence -  presence of early carbonated sediments,

        Increase in O2 –

·         The secondary atmosphere formed by outgassing was anoxygenic. Amount of oxygen in the atmosphere increased gradually through

• Solar radiation - UV rays break H2O into hydrogen and oxygen
• Organic photosynthesis by earliest form of life - Blue-green algae (Cyanobacteria) uses CO2 and H2O for photosynthesis and release free oxygen.

 ·         Oxygen production continued for 2.2 billion years and did not achieve considerable levels until 2 Ga and only approached present-day levels by 1.5 Ga. Due to chemical action with the material deposited through lava eruptions, such as iron and evaporite formations.

Evidence - Iron rock formation in earliest sediments at Isua in western Greenland aged 3.8 billion years ago. Barite gypsum bearing evaporite found in Pilbara region of western Australia. 3.5 billion years ago.

·         O2 level rose in atmosphere occurred with decrease in the iron deposition.

Evidence – Earliest form of life i.e. Eukaryotes which were present 1.4 billion years ago, require O2 content of about 0.02 present atmospheric level. Similarly, soft life forms like jelly fish, worms developed about 650 million years ago, which require the minimum oxygen level of 0.1 present atmospheric level. Plants first appeared 400 million years ago.

·         The evolution of more complex form of plants and animals extract more CO2 from the atmosphere and provide an additional source of O2.

·         Once O2 started to accumulate in the atmosphere, the production of ozone (O3) could occur, shielding the Earth’s surface from ultra violet (UV) radiation. 

 References –

• Neil C. Wells - The atmosphere and ocean: a physical introduction, 3rd Edition, Wiley Blackwell
• Essentials of Meteorology – An Invitation to the Atmosphere, Eighth Edition 2018, C. Donald Ahrens and Robert Henson, Cengage Learning.
• Brian Frederick Windley (2016) Geologic history of Earth, Encyclopædia Britannica. Available at https://www.britannica.com/science/geologic-history-of-Earth.  Accessed on November 23, 2020

 

 

 

Scale of Climatic Events

Formation of Thunderclouds

Climatic phenomenon or events are global feature which occur all over the Earth’s surface. However, their nature, intensity and occurrence may be diverse. The Indian subcontinent may experience monsoonal rainfall while the same may not occur in other parts of the world. Similarly, snowfall may not occur in tropical islands whereas it is quite a normal occurrence in temperate and tundra region. These events occurring in these region are different from each other but are a global happening.

A distinguishing aspect about these climatic events is that they occur along different spatial and temporal scales. To understand this concept, let’s take the example of a climatic event like Amphan Cyclone of May 2020. Super Cyclonic Storm Amphan was a powerful and catastrophic tropical cyclone that caused widespread damage in Eastern India, specifically West Bengal, and also Bangladesh. Amphan super cyclone covered an area of about 700 sq. km., which refers to the spatial scale of the climatic event.

Now Amphan cyclone originated from development of a low pressure area, east of Sri Lanka on 13th May 2020. It intensified and extended over the warm ocean and moved northwards. On 20the May, 2020 the cyclone made landfall in West Bengal. The whole climatic event from origination to landfall took around 7-8 days which is the temporal scale of the climatic event.

Space and time scales of dynamical processes in the atmosphere. Source - UCAR

If we consider the temporal and spatial scale of climatic events occurring in different parts of the world, it is observed that an event with large spatial extent usually has a longer temporal range and vice versa. To understand this let’s look at the following graph by University Corporation for Atmospheric Research (UCAR), where the relationship between spatial and temporal scale of climatic events has been shown. Events like minor turbulence or heat wave covering small area occurs for few hours. Whereas events like Cyclones, planetary waves have a larger spatial extent and hence occur for longer period. So, there is usually a direct relationship between the areal coverage of individual atmospheric event and the time scale in which that occurs.

So considering the spatial and temporal scale, climatic events can be divided into following classes:

• Planetary scale -

• Synoptic scale

• Meso-climate

• Local scale

• Micro-climate

Let’s understand them with their examples -
 

• Planetary scale – These are the climatic events/phenomenon which occurs over an entire hemisphere or even the entire globe. This is the largest spatial scale possible because it encompasses atmospheric phenomena on the order of 10,000 to 40,000 km. The largest spatial systems mostly operate over the longest time scales, so planetary-scale systems operate over temporal scales that span weeks to months.

Example - Major latitudinal pressure and wind belts that encircle the planet.

• Synoptic scale - Climatic phenomenon of spatial scale covering about 100 to 10,000 km are synoptic scale events. The systems of this size typically operate over periods of days to weeks.

Examples - Cyclone systems with their associated fronts.

Synoptic scale climatic event Hurricane Irma on Sept. 07, 2017. Source - NOAA's GOES satellite image

• Mesoscale - Climatic events that operate over areas of about 5 and 100 km. They typically last from a few hours to a few days. It is possible that there exists mesoscale climatic event within a synoptic climatic event. For example, in a cyclone affected area there may be some pocket which does not experience severe activity. These difference may be created by elevation differences, slope aspect, bodies of water, differences in vegetation cover, urban areas, etc.

 Example - Mountain/valley breeze and land/sea breeze circulation systems.

• Local scale - Local scale climatic events operates over areas between about 0.5 and 5 km — about the size of a small town. The time duration of such an occurrence is few hours or a day at maximum. The local scale activity may be natural as well as caused by human activities. Minor weather changes caused by human activities mostly in urban areas is also a local scale climatic activity.

 Example - A typical thunderstorm also falls into the natural local scale activity. Whereas, occurrence of Urban heat island effect in cities where the city temperature is higher than the surrounding suburban/rural areas, is an example of human induced local scale activity.

• Micro scale – Climatic events that operate along this spatial scale are smaller than 0.5 km and typically last from a few minutes to a few hours.

 Example - A tree canopy with difference in temperature with surrounding area, wind movement in a forested and deserted area; micro climate of residential buildings as compared to factories are some of the example.

References

•      Climatology by Robert V. Rohli and Anthony J. Vega.

•      Weather and Climatology by A.K. Sil.

Image by jplenio from Pixabay