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Water Cycle - The Atmospheric Water Cycle On A Global Scale

In this article, we will discuss the water present on Earth and its function in the water cycle, which includes the movement of water vapor between oceans and continents as well as the return of water to the oceans through rivers.

An important factor in the energy balance of the Earth is water vapor. Nearly half of the solar energy that is absorbed at the surface is turned into heat that is released into the atmosphere and used to warm the atmosphere and cool the surface through evaporation.

Latent heat is the main reason why the atmosphere is getting warmer and why heat is moving from low latitudes to high latitudes.

The main greenhouse gas is also water vapor, which makes the climate system warm up by 24°C.

But because temperature is the only thing that affects water vapor, it is a passive part of the troposphere and should be thought of as part of the climate feedback system.

Even though water vapor is often well observed and studied, it is hard to notice when it rains, especially over the oceans.

The water cycle, which is also known as the hydrological cycle, responds in many ways to global warming.

Stages Of Water Cycle

In addition to the fundamental activities that are depicted in the diagram of the water cycle, there are many other processes that are involved in the transportation of water. The various steps of the water cycle are described below.

Evaporation

The sun is the most powerful source of energy there is, and the majority of the evaporation that takes place on Earth is driven by it. In most cases, evaporation takes place when the water molecules that are present at the surface of bodies of water become stimulated and ascend into the surrounding air. These molecules have the maximum kinetic energy, and they tend to congregate in clouds of water vapor. In most cases, evaporation takes place at temperatures lower than those at which water boils. Evaporation that takes place through the leaves of plants is part of a different process referred to as evapotranspiration. This mechanism is responsible for a significant amount of the water that is found in the atmosphere.

Sublimation

The process of sublimation takes place when solid snow or ice transforms directly into water vapor without melting into liquid water. It is typically brought on by dry winds and low levels of relative humidity. People have reported seeing sublimation in action on mountain peaks, where the air pressure is very low. Due to the low air pressure, less energy is required to sublimate the snow into water vapor than would be the case under normal conditions. The phase in which fog emerges from dry ice is another illustration of the process of sublimation. The ice sheets that cover the poles of the world are the greatest contributors to the process of sublimation that occurs on Earth.

Condensation

The low temperatures that might be encountered at higher altitudes cause the water vapor that has accumulated in the atmosphere to eventually become cooler. At some point, these vapors turn into tiny pieces of water and ice that stick together to form clouds.

Precipitation

When the temperature is higher than 0°C, the vapors will turn into droplets of water. However, it is impossible for it to condense in the absence of dust or other contaminants. Because of this, water vapor condenses onto the surface of the particle. When there are sufficient droplets that join together, they will fall from the sky and land on the ground below. The term for this procedure is "precipitation" (or rainfall). When the temperature is exceptionally low or the air pressure is extremely low, the water droplets freeze and fall to the ground as snow or hail.

Infiltration

The process of infiltration allows precipitation to be taken up by the ground and used by plants. The amount of water that has been absorbed by a substance is proportional to the amount of water that has seeped into it. A good comparison would be between rocks and soil in terms of their ability to hold water. The path that groundwater takes can be either that of streams or rivers. On the other hand, it could just sink deeper, thereby creating aquifers.

Runoff

If the water from rainfall does not collect in aquifers, it will take the path of least resistance, which is typically down the slopes of hills and mountains, eventually becoming rivers. Runoff is the term used to describe this phenomenon. When the amount of snowfall is greater than the rate of evaporation or sublimation, ice caps can form in areas with a lower average temperature. The polar regions of our planet are home to the world's largest ice caps.

Every one of the processes described above happens in a cycle that does not have a beginning or an end that is predetermined.

Implications Of Water Cycle

  • The air is known to be cleaned by the water cycle. For instance, in order for precipitation to occur, water vapors must adhere to dust particles. In heavily polluted cities, raindrops pick up water-soluble gases and pollutants as they fall from the sky, in addition to dust. Raindrops have a history of picking up biological agents like bacteria as well as smoke and soot from factories.
  • The water cycle has an effect on every aspect of life on earth.
  • A number of other biogeochemical cycles also need the water cycle to work.
  • The water cycle has a significant bearing on the weather patterns that prevail. To give one example, the greenhouse effect will bring about an increase in temperature. If the evaporative cooling effect of the water cycle were eliminated, the temperature of the earth's surface would increase by a significant amount.

Water On Planet Earth

The total amount of accessible water on Earth is approximately 1.5 x 109 km3. The oceans contain the most of this, 1.4 x 109 km3. Approximately 29 x 106 km3 is locked up in land ice and glaciers, with an additional 15 x 106 km3 thought to exist as groundwater. If all land ice and glaciers melted, sea levels would rise by 80 meters.

The atmosphere contains 13 x 103 km3 of water vapor, or 26 kilograms per m2 of water per column of air on Earth. Low and high latitudes are geographically distinct. Annually, the ocean and land exchange 103 km3 of water.

Net transit from the ocean to land is 38 units, with the same amount returned by rivers. However, precipitation over continents is three times as high, demonstrating water recirculation over land. According to Trenberth et al., recirculation has a yearly cycle and huge continental differences. Summer and tropical areas have more recirculation.

The water cycle of the oceans differs from continent to continent. Most Pacific Ocean water recirculates within the ocean, with little net movement to land. The Atlantic and Indian Oceans exchange water differently. Two-thirds of the net water transport to continents comes from the Atlantic, with the balance from the Indian Ocean. Most of the continent's water comes from the Atlantic and is returned through rivers.

Water Vapor As A Greenhouse Gas

Water vapor accounts for 75% of Earth's greenhouse effect. Circulation controls atmospheric water vapor. Water vapor in the atmosphere experiences a complex series of vertical and horizontal motions that can include condensation. The latest condensation temperature determines the saturation mixing ratio. The Clausius–Clapeyron relation predicts that the mixing ratio will grow with temperature.

Water vapor stays in the atmosphere for about a week, but carbon dioxide (CO2) stays for centuries. Evaporation from oceans and land replenishes water vapor, but temperature regulates it.

Water vapor is part of the climate system's response to external stimuli. Changes in the concentration of greenhouse gases like CO2 can cause a shift in climate forcing. Solar radiation, volcanic aerosols, or anthropogenic emissions can also cause it.

Water vapor absorption bands are close to saturation, similar to those of CO2, with absorption largely in the spectral wings. A logarithmic function of the mixing ratio approximates the radiation balance. This is how atmospheric water vapor affects long-wave radiation calculations in weather and climate models.

In contrast to well-mixed greenhouse gases, water vapor varies dramatically in time and space. Strong vertical mobility of signals shows significant moisture gradients.

Coarse-resolution climate models have trouble capturing abrupt gradients and underestimate outgoing longwave radiation (OLR) in dry air.

The logarithmic mixing ratio suggests that OLR is understated. Hagemann et al. found that reducing horizontal spectral resolution from T159 to T21 reduced OLR by 7 W m-2 in a clear sky. The amount of water vapor in the higher troposphere is proportionally more important, so where it is in the atmosphere is very important.

The strength of water vapor's positive feedback is debatable. Observational and modeling studies imply that relative humidity is maintained and water vapor follows Clausius–Clapeyron.

The feedback effect can be approximated from models by ignoring the component of the long-wave radiation code that evaluates the effect of rising water vapor. Water vapor raises the effect of doubling CO2 from 1.05°C to 3.38°C, implying positive feedback.

Soden and Held say that water vapor alone has an amplification factor of 1.9 to 3.2, but most of this range is due to changes in the lapse rate. When water vapor and lapse rate are looked at together, the warming is amplified by 1.3 to 1.6.

How Will A Warmer Climate Affect Precipitation??

Observations and computational models demonstrate that atmospheric water vapor follows temperature according to the Clausius–Clapeyron relation. An increase of 1°C in the lower troposphere increases water vapor by 6–7%.

Model integration shows a 1–2% rise in the hydrological cycle (global mean precipitation and evaporation) after 5 years. In a warmer climate, atmospheric water vapor is determined by atmospheric circulation, but global precipitation is limited by surface evaporation.

This is caused by the uneven surface radiation and is controlled by surface winds, the stability of the boundary layer, and the absorption of short-wave radiation by the atmosphere.

According to Takahashi, the sum of latent heat flux and short-wave absorption at the surface is governed by the long-wave flux above the boundary layer. Models that are inaccurate by 100% are unlikely. Hence, line-by-line integration reveals a higher absorption by water vapor than what is employed in climate models. Hence, models may overestimate latent heat flux.

Rising water vapor has far-reaching repercussions. The worldwide atmospheric water cycle can't keep up with the rapid growth in atmospheric water vapor because of modest variations in latent heat flux.

This suggests that water vapor's residence period in the atmosphere is growing, which reduces mass exchange between the boundary layer and convective mass flux in the tropics. This result contradicts some climate change beliefs, which hold that as the atmosphere warms, it becomes more energetic.

The average vertical exchange of mass between the boundary layer and the free atmosphere slows large-scale atmospheric circulation. It's unclear how much this would affect tropical cyclones, although it could limit their number in a warmer climate.

The Clausius–Clapeyron relation affects poleward water vapor transfer and evaporation–precipitation (E–P) patterns.

Wet regions get wetter as lower tropospheric water vapor accumulates, and dry regions get drier. This conclusion is essential for predicting future precipitation patterns in a warming environment.

This includes increased precipitation in the Intertropical Convergence Zone (ITCZ) and middle and high-latitude storm tracks and less precipitation in the Mediterranean, California, Texas, and southern Africa, and southern Australia.

A black boy drinking water from the kitchen tap
A black boy drinking water from the kitchen tap

Extreme Precipitation Changes

A gamma function best describes the distribution of precipitation as a function of time since most precipitation occurs at short intervals of time. Small-scale convective systems with large vertical movements cause the most intense precipitation.

Bengtsson says convective processes are embedded in almost all precipitation systems, including warm fronts at high latitudes in the winter. Convection-driven storms like tropical cyclones have intense precipitation. In such occurrences, 1000 mm or more of rain fell in less than 12 hours.

Hours-to-days-long strong precipitation does the biggest damage to society. How might strong precipitation change in a warming climate?

In warmer climate models, extreme precipitation increases. Extreme 6-hr precipitation increases by 40–50% in the storm track region (at the 99 percentile or higher). As average precipitation decreases, the precipitation spectrum shifts.

As climate warming has been moderate so far, just around 25% relative to Bengtsson et al's experiment, we think it will be difficult to discern such changes in precipitation time spectrum.

Allan and Soden found natural variability signals. This requires long-term, high-quality observations. Karl and Knight reported a rise in 90th percentile or greater precipitation over the central USA, which agreed with Semenov and Bengtsson's model.

The model-predicted increase in intense precipitation could have serious effects. Changes in precipitation intensity will lessen the likelihood of extreme events.

Using the same model experiment as Semenov and Bengtsson, the 50-year return flow in the Yangtse river was altered to a 5-year return flow for a climate projected at the end of the century using the IPCC A1B (moderate) scenario. If such changes occur, flood protection measures must be upgraded immediately.

People Also Ask

How Do You Explain The Water Cycle?

Water vapor is created when liquid water evaporates, and this water vapor then condenses to form clouds and falls back to earth as rain and snow. Different stages of water flow through the atmosphere (transportation).

What Is The Importance Of Water Cycle?

The water cycle is a critical process that controls our planet's weather patterns and ensures that water is available for all living things. Clean water, which is necessary for life, would run out if water didn't naturally recycle itself.

Where Does The Water Cycle Begin?

The water cycle doesn't have a beginning. However, since the seas contain the majority of the water on Earth, we'll start there. Ocean water is heated by the sun, which powers the water cycle. A portion of it releases vapor into the atmosphere.

What Affects The Water Cycle?

This cycle becomes more intense as a result of climate change because more water evaporates into the atmosphere as air temperatures rise. Greater water vapor absorption by warmer air can result in more powerful rainstorms, which can cause severe issues like excessive flooding in coastal cities all over the world.

Conclusion

Water vapor is crucial for the climate. Water from Earth's surface cools the surface and warms the atmosphere by 80 W m-2. Water vapor transports energy between low and high latitudes.

The water cycle affects ocean circulation as net evaporation increases salinity, causing upper ocean convection and net precipitation areas to be opposite. Water vapor is the main greenhouse gas, but it's controlled by temperature, so it's part of the climate feedback loop.

All signs point to water vapor being a positive climate feedback, but its strength is model-dependent.

This is largely owing to low and high-latitude temperature and lapse rate shifts, since coupled water vapor lapse rate feedback is robust. In a warmer climate, water vapor increases rapidly, but global precipitation may rise more slowly. In a warmer climate, locations with high precipitation rates will see increases and areas with low rates will see decreases, making the hydrological cycle more intense.

About The Authors

Suleman Shah

Suleman Shah - Suleman Shah is a researcher and freelance writer. As a researcher, he has worked with MNS University of Agriculture, Multan (Pakistan) and Texas A & M University (USA). He regularly writes science articles and blogs for science news website immersse.com and open access publishers OA Publishing London and Scientific Times. He loves to keep himself updated on scientific developments and convert these developments into everyday language to update the readers about the developments in the scientific era. His primary research focus is Plant sciences, and he contributed to this field by publishing his research in scientific journals and presenting his work at many Conferences. Shah graduated from the University of Agriculture Faisalabad (Pakistan) and started his professional carrier with Jaffer Agro Services and later with the Agriculture Department of the Government of Pakistan. His research interest compelled and attracted him to proceed with his carrier in Plant sciences research. So, he started his Ph.D. in Soil Science at MNS University of Agriculture Multan (Pakistan). Later, he started working as a visiting scholar with Texas A&M University (USA). Shah’s experience with big Open Excess publishers like Springers, Frontiers, MDPI, etc., testified to his belief in Open Access as a barrier-removing mechanism between researchers and the readers of their research. Shah believes that Open Access is revolutionizing the publication process and benefitting research in all fields.

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