A measure of the reflectivity of a surface is called albedo, and when applied in Earth-sciences, albedo is a measure of the reflectivity of the Earth’s surface in relation to Sun’s energy. When some of the Sun’s energy is reflected back into space by surfaces on Earth, it has an overall cooling effect on the Earth (G.P. Wayne, 2016). This phenomenon is called the albedo effect.
High albedo of a surface implies that the surface reflects a lot of light, and low albedo implies that it absorbs a lot of light. The lighter the colour of a surface, the more of the Sun’s energy it tends to reflect, and the darker the colour of a surface, the more of the Sun’s energy it tends to absorb. Clouds, and snow and ice surfaces are examples of surfaces with high albedo in terms of the Sun’s energy as they tend to occur in lighter colours. An example of a low albedo surface on Earth is dirt, which tends to occur in dark colours such as brown and black, and do not reflect much of the Sun’s energy (Parzival S., 2016).
Fig: The Albedo Effect
Source: Biomimicry KTH
The Albedo Effect and Global Warming
The most significant impact of the albedo effect in the Earth-sciences is its projected present and future effects on global warming. With the consistent melting of ice and snow on the Earth’s surface due to a rise in global mean temperatures, white surfaces globally are decreasing in area. This is steadily decreasing the amount of the Sun’s energy reflected back into space, leading to even greater warming of the Earth due to the trapping of heat.
The albedo effect is thus creating a positive feedback loop, especially in the case of Arctic ice, where the ice is melting and the resultant increase in heating is aggravating even more melting of ice and so on. Even when the ocean water is exposed to sunlight, the warming of the water can cause the ice to melt from beneath. The melted ice produces more water, which adds to this melting effect. This doubles the action of melting in the Arctic regions where ice melts both due to action from below and due to increased surface temperatures due to increased greenhouse gas emissions. Even an increased amount of cloud cover would mean increased water vapour, which is a greenhouse gas, contributing to the positive feedback loop of global warming (G.P. Wayne, 2016).
The albedo of a surface is measured on a scale alternating between 0 to 1, with o being the ultimate dark surface with full absorption and 1 being the ultimate bright surface with full reflection of incident light energy. The albedo measurement for sea is o to 0.1, for forests is between 0.1 to 0.2, for soil is between 0.1 to 0.25, for grass is 0.15 to 0.25, for desert is between 0.25 to 0.4, and for dry snow is between 0.75 to 0.95 (National Learning Centre for Remote Sensing, undated).
Lawrence C. Nkemdirim from the University of Calgary, Canada accumulated data on measurements of albedo on various surfaces and published his study in the paper titled ‘A Note on the Albedo of Surfaces’. Nkemdirim carried out his measurements using devices called solari-albedometers in conjunction with a computerized data acquisition system. His inferences were based on average albedo out of variations in the inclination of the Sun and general cloudiness.
After taking various variables such as cloudiness and the inclination of the Sun into account, Nkemdirim choose his site as a partially cropped potato farm located in a flat praire. As such irrigation was also a factor as it is responsible for moisture. The location makes a difference and although measurements might vary slightly, certain relative values can be ascertained. The co-ordinates for the location were 51oN and 114oW, with an elevation of 1080 m from sea level.
Taking observations over many days, Nkemdirim found that in spite of the symmetry in global solar radiation, a great amount of diurnal variance was observed for albedo, which is incredibly relevant for energy budget studies.
In Nkemdirim’s experiment, non-irrigated potatoes displayed a greater albedo effect as the ability to reflect back the Sun’s energy, while the moist irrigated potatoes displayed a slightly lesser albedo effect, most probably due to the presence of excess water vapour, which is a greenhouse gas. Their albedo measurements varied also according to the inclination of the Sun and general cloudiness (L.C. Nkemdirim, 2000). Thus they were able to perceive how less solar heat is reflected back in the presence of greenhouse gases, which act to absorb solar heat, trapping them within the Earth’s atmosphere.
They also noted how the albedo effect is dependent on the location and its various specificities as also the inclination of the Sun at a particular location and general cloudiness, although the quality of global solar radiation was more or less the same. Besides the warming and cooling of the Earth, the albedo effect can also have an influence on the general climate of the Earth.
The effect of changes in albedo can influence rainfall, and in a study on semi-arid regions in North America, Africa and Asia carried out by Jule Charney, the patterns of evaporation were studied to see how an increase in the albedo effect led to a decrease in convective clouds and precipitation. Charney found that an increase in albedo led to a huge decrease in rainfall in the regions when it was accompanied by a lack of local evaporation.
Urbanization Impacts on Surface Albedo
Land use and surface albedo has been radically altered by the great increase in urbanization that has been happening since the industrial revolution, with impacts on local and even regional climate.
Global warming now is coupled with what are called urban heat islands, in which urban objects like concrete can absorb heat during the day and release this in the night, leading to warmer nights in densely constructed cities than in the countryside and warmer days overall. Evaporative cooling can also be suppressed by impermeable roofs and absorption by pavements, converting solar radiation into sensible heat rather than latent heat, thus adding to the urban heating effect with higher urban air temperatures generally.
The changes in the Earth’s radiative energy balance and climate due to the reduction in the albedo effect from urbanization can have direct and indirect impacts. Significant uncertainties exist over the complete climactic impacts of a change in the albedo effect, although sizeable numerical and research data along with computer simulations of possible future impacts are available. Similarly, uncertainties exist over the exact nature of change in Earth’s radiative energy budget due to the differential impacts of widespread and heterogeneous urban settlements globally.
The biggest impact of albedo change is on radiative forcing, which is a change in net irradiance at the top of the Earth’s atmosphere as a human-induced change. This can create imbalances in the radiative energy budget of Earth’s atmospheric system, that can have palpable effects on climactic patterns. An effect of radiative forcing is climate feedbacks, influencing temperature, moisture, sensible heat and latent heat, with extensive effects on the hydrological cycle including an increase in water vapour. This can tend to increase the effects of global warming as water vapour both is a greenhouse gas that traps heat as well as precipitates in the form of rainfall with a greater amount of accumulated moisture.
One solution that has been suggested is cool roofs and cool pavements strategies, which would be aimed at increasing the solar reflectance of roofs and pavements in urban agglomerations. Although there is wide acceptance of the idea of cool roofs, installing cool pavements is an idea that has not yet taken off.