THE INCREASE OF greenhouse gas concentrations in the Earth’s atmosphere, above natural levels, is causing anthropogenic changes to Earth’s climate. The temperature at the Earth’s surface has shown a significant and rapid rise since the 1990s, compared to the past two millennia. It is challenge for scientists to attribute these known changes to specific mechanisms. Some of the complex feedback processes that add to these changes are not fully understood.
The energy balance, the distribution of energy in space (or Earth’s atmosphere), and temporal energy variations characterize the Earth’s climate system. The Earth’s radiative energy balance is governed by the balance between solar radiation and absorption by the Earth and subsequent radiation from the Earth to outer space. The Earth absorbs solar shortwave radiation in the daytime, and emits thermal infrared or longwave radiation back to outer space to maintain the heat energy balance. However, this energy balance is more imbalanced recently, because of the many components of the Earth-atmosphere system, such as clouds or aerosols and other radiation scattering particles present in the atmosphere.
The National Aeronautics and Space Administration (NASA) CERES (Clouds and the Earth’s Radiant Energy System) instrument on the Tropical Rainfall Measuring Mission (TRMM) and Terra satellite mission provide a new set of Earth radiation balance data. Along with these data and a 16-year record of the Earth Radiation Budget Satellite (ERBS), NASA has compiled the 22 years of accurate satellite observed broadband radiative fluxes, showing the huge energy imbalance or long-wave anomalies since 1995.
The hydrologic cycle
The hydrologic cycle is part of this heat energy imbalance process. This feedback process complicates the global warming phenomena and involves water vapor, clouds, and aerosol particles. Evaporation is the basis of this important process. Water is evaporated by Sun, incorporated into clouds as water vapor, falls to the land and water bodies as rain, and enters water bodies to complete the cycle. Conversely, to make matters worse, water vapor acts as a prominent greenhouse gas as it is involved in an important climate feedback loop. Due to ascending evaporation from water bodies, the water vapor amount in the atmosphere increases.
The additional water vapor absorbs radiated thermal infrared energy that would otherwise escape from Earth’s surface to outer space, and radiates the heat back to the Earth again. Thus, it makes the Earth’s surface even warmer. This somber picture is further complicated by important interactions among water vapor, clouds, atmospheric motion, and radiation from both the Sun and the Earth.
Moreover, continuous increases of the Earth’s surface temperature augments the sea or ocean surface temperature. Therefore, increased evaporation and higher annual rainfall amounts should occur. Computer simulation models found that a global warming of 7.2 degrees F (4 degrees C) is expected to increase global evaporation and precipitation by about 10 percent. Scientists found through models that the upper tropospheric water vapor amount will increase by 15 percent with each degree of atmospheric temperature rise, because of increased evaporation.
Another evaporation feedback mechanism involves the atmospheric wind-evaporation-convection feedback in the tropics. The feedback is shown to be very effective in channeling perturbations from one component of the climate system to other components, such as from evaporation to surface wind, and from atmospheric convective activity to evaporation. Through this feedback process, it is also observed that surface evaporation over tropical oceans is connected with cloud absorption of shortwave radiation. Diminished shortwave absorption by clouds causes excessive shortwave radiation at the Earth’s surface, leading to excessive evaporation. As the loop continues, the Earth’s surface gets warmer and warmer.
This same wind-evaporation-convection feedback in the water is also a concern. This is a huge contributor toward spatial variability of evaporation and precipitation on the Earth. Recent rises in the Earth’s surface temperature may lead to a near-term collapse of the ocean’s thermohaline circulation. Thermohaline circulation is a global ocean circulation pattern or convection mechanism. It distributes water and heats both vertically, through the water column, and horizontally across the globe. Due to this collapse of thermohaline circulation, warm surface waters move from the tropics to the North Atlantic and extra-warm water surfaces in the Pacific Ocean surface surrounding the equator.
Therefore, spatial variation in ocean temperature is observed. This spatial change in ocean surface temperature is changing usual wind direction, creating unusual evaporation rate, and subsequent spatial variability in rainfall. This pattern has been evident for the past two decades. Therefore, more precipitation is occurring in northern Europe, Canada, and northern Russia, but less to swathes of sub-Saharan Africa, southern India, and Southeast Asia. El Nino and La Nina are examples of the recently observed changed thermohaline circulation process. Scientists working specifically on polar warming amplification observation found that much stronger evaporation feedback leads to a final warming in low latitudes that is stronger in the atmosphere than at the surface.
Atmospheric water vapor or the evaporation feedback is supported by many studies and researchers concluded that this feedback process is positive, continually increasing the Earth’s surface temperature. Scientists proved this observation with profound evidence using new satellite-generated water vapor data and concluded that the feedback is not overestimated in models. It is also observed that the global water vapor amount will increase by 7 percent with each degree of atmospheric temperature rise, thus complicating the global warming and climate change mechanism.
