Energy Balance • Energy from the Sun = energy returned to space by Earth’s radiative emission • The absorption of solar radiation takes place mostly at the surface of the Earth. • The emission of solar radiation to space takes place mostly in the atmosphere. • Because its atmosphere efficiently absorbs and emits IR radiation, the surface of the Earth is much warmer than it would be in the absence of the atmosphere (greenhouse effect)
The global energy balance is the balance between incoming energy from the Sun and outgoing heat from the Earth. The global energy balance regulates the state of the Earth's climate and modifications to it as a result of natural and man-made climate forcing cause the global climate to change. • Energy released from the Sun has a temperature of approximately 6000°C. When it reaches the Earth's surface, some is reflected back to space by clouds, some is absorbed by the atmosphere, and some is absorbed at the Earth's surface.
The energy received by the Earth from the Sun balances the energy lost by the Earth back into space. In this way, the Earth maintains a stable average temperature and therefore a stable climate (although of course differences in climate exist at different locations around the world).
The Earth atmosphere contains a number of greenhouse gases, which affect the Sun-Earth energy balance. The average global temperature is in fact 33°C higher than it should be. • The atmosphere is mostly transparent in the visible light (which is why we can see the Sun), but significant blocking (through absorption) of ultraviolet radiation by the ozone layer and infrared radiation by greenhouse gases occurs.
The absorption of infrared radiation trying to escape from the Earth back to space is particularly important for the global energy balance. Such energy absorption by the greenhouse gases heats the atmosphere, and so the Earth stores more energy near its surface than it would if there was no atmosphere. The average surface temperature of the moon, about the same distance as the Earth from the Sun, is -18°C. The moon, of course, has no atmosphere. By contrast, the average surface temperature of the Earth is 15°C. This heating effect is called the naturalgreenhouse effect
Greenhouse Effect • The Sun, which is the Earth's only external form of heat, emits solar radiation mainly in the form of shortwave visible and ultraviolet (UV) energy. As this radiation travels toward the Earth, 25% of it is absorbed by the atmosphere and 25% is reflected by the clouds back into space. The remaining radiation travels unimpeded to the Earth and heats its surface. The Earth releases a lot of energy it has received from the Sun back to space.
Greenhouse Effect • Greenhouse gases like water vapour, carbon dioxide, methane and nitrous oxide trap the infrared radiation released by the Earth's surface. The atmosphere acts like the glass in a greenhouse, allowing much of the shortwave solar radiation to travel through unimpeded, but trapping a lot of the longwave heat energy trying to escape back to space. This process makes the temperature rise in the atmosphere just as it does in the greenhouse. This is the Earth's natural greenhouse effect and keeps the Earth 33°C warmer than it would be without an atmosphere, at an average . In contrast, the moon, which has no atmosphere, has an average surface temperature of . 15°C -18°C
Te = 255 K = -18 C Observed: 288 K = +15 C !
The greenhouse effect also limits the amplitude of the diurnal variation in surface T over land greenhouse effect is very important
Consequences • Hole in the ozone layer • Ice melting and glacier retreat • Sea-level rise • Floods • Drought • Hurricanes, Typhoons, Tornadoes • Earthquakes?
Ozone Hole For nearly a billion years, ozone molecules in the atmosphere have protected life on Earth from the effects of ultraviolet rays
Ozone Hole The ozone layer resides in the stratosphere and surrounds the entire Earth. UV-B radiation (280- to 315- nanometer (nm) wavelength) from the Sun is partially absorbed in this layer. As a result, the amount of UV-B reaching Earth’s surface is greatly reduced. UV-A (315- to 400-nm wavelength) and other solar radiation are not strongly absorbed by the ozone layer. Human exposure to UV-B increases the risk of skin cancer, cataracts, and a suppressed immune system. UV-B exposure can also damage terrestrial plant life, single cell organisms, and aquatic ecosystems.
Ozone Hole In the past 60 years or so human activity has contributed to the deterioration of the ozone layer.
Ozone Hole • Each spring in the stratosphere over Antarctica (Spring in the southern hemisphere is from September through November.), atmospheric ozone is rapidly destroyed by chemical processes. • As winter arrives, a vortex of winds develops around the pole and isolates the polar stratosphere. When temperatures drop below -78°C (-109°F), thin clouds form of ice, nitric acid, and sulphuric acid mixtures. Chemical reactions on the surfaces of ice crystals in the clouds release active forms of CFCs. Ozone depletion begins, and the ozone “hole” appears. • The ozone "hole" is really a reduction in concentrations of ozone high above the earth in the stratosphere
Ozone Hole • Man-made chlorines, primarily chloroflourobcarbons (CFCs), contribute to the thinning of the ozone layer and allow larger quantities of harmful ultraviolet rays to reach the earth. • The Montreal Protocol (16 Sept. 1987) stipulated that the production and consumption of compounds that deplete ozone in the stratosphere--chlorofluorocarbons (CFCs), halons, carbon tetrachloride, and methyl chloroform--were to be phased out by 2000 (2005 for methyl chloroform). Scientific theory and evidence suggest that, once emitted to the atmosphere, these compounds could significantly deplete the stratospheric ozone layer that shields the planet from damaging UV-B radiation.
Ice Melting & Glacier Retreat The retreat of glaciers since 1850, worldwide and rapid, affects the availability of fresh water for irrigation and domestic use, mountain recreation, animals and plants that depend on glacier-melting, and in the longer term, the level of the oceans. Studied by glaciologists, the temporal coincidence of glacier retreat with the measured increase of atmospheric greenhouse gases is often cited as an evidentiary underpinning of anthropogenic (human-caused) global warming. Mid-latitude mountain ranges such as the Himalayas, Alps, Rock Mountains, Cascade Range, and the southern Andes, as well as isolated tropical summits such as Mount Kilimanjaro in Africa, are showing some of the largest proportionate glacial loss
Ice Melting & Glacier Retreat • Until about 1940, glaciers around the world retreated as the climate warmed. Glacial retreat slowed and even reversed, in many cases, between 1950 and 1980 as a slight global cooling occurred. However, since 1980 a significant global warming has led to glacier retreat becoming increasingly rapid, so much so that some glaciers have disappeared altogether, and the existence of a great number of the remaining glaciers of the world is threatened. (eg. In locations such as the Andes of South America and Himalayas in Asia, the demise of glaciers in these regions will have potential impact on water supplies. The retreat of mountain glaciers, notably in western North America, Asia, the Alps, Indonesia and Africa, and tropical and subtropical regions of South America, has been used to provide qualitative evidence for the rise in global temperatures since the late 19th century.) The recent substantial retreat and an acceleration of the rate of retreat since 1995 of a number of key outlet glaciers of the Greenland and West Antarctic ice sheets, may foreshadow a rise in sea level, having a potentially dramatic effect on coastal regions worldwide.
Ice Melting & Glacier Retreat • The World Glacier Monitoring Service reports on changes in the terminus, or lower-elevation end, of glaciers from around the world every five years. In their 1995–2000 edition, they noted the terminal point variations of glaciers across the Alps. Over the five-year period from 1995 to 2000, 103 of 110 glaciers examined in Switzerland, 95 of 99 glaciers in Austria, all 69 glaciers in Italy, and all 6 glaciers in France were in retreat. French glaciers experienced a sharp retreat in the years 1942–53 followed by advances up to 1980, and then further retreat beginning in 1982. As an example, since 1870 the Argentière Glacier and Mont Blanc Glacier have receded by 1,150 (3,800 ft) and 1,400 m (4,600 ft), respectively. The largest glacier in France, the Mer de Glace, which is 11 km (7 miles) long and 400 m (1,300 ft) thick, has lost 8.3% of its length, or 1 km (0.6 miles), in 130 years, and thinned by 27%, or 150 m (500 ft), in the midsection of the glacier since 1907. The Bossons Glacier in Chamonix, France, has retreated 1,200 m (3,900 ft) from extents observed in the early 20th century. In 2005, of 91 Swiss glaciers studied, 84 retreated from where their terminal points had been in 2004 and the remaining 7 showed no change
Ice Melting & Glacier Retreat • Crucial to the survival of a glacier is its mass balance, the difference between accumulation and ablation (melting and sublimation). Climate change may cause variations in both temperature and snowfall, causing changes in mass balance. A glacier with a sustained negative balance is out of equilibrium and will retreat. A glacier with sustained positive balance is also out of equilibrium, and will advance to reestablish equilibrium. Currently, there are a few advancing glaciers. • Glacier retreat results in the loss of the low-elevation region of the glacier. Since higher elevations are cooler, the disappearance of the lowest portion of the glacier reduces overall ablation, thereby increasing mass balance and potentially reestablishing equilibrium. However, if the mass balance of a significant portion of the accumulation zone of the glacier is negative, it is in disequilibrium with the climate and will melt away without a colder climate and or an increase in frozen precipitation. • The key symptom of a glacier in disequilibrium is thinning along the entire length of the glacier.
Sea-level rise • Most scientists agree that global warming presents the greatest threat to the environment. • There is little doubt that the Earth is heating up. In the last century the average temperature has climbed about 0.6 degrees Celsius (about 1 degree Fahrenheit) around the world. • From the melting of the ice cap on Mount Kilimanjaro, Africa's tallest peak, to the loss of coral reefs as oceans become warmer, the effects of global warming are often clear. • However, the biggest danger, many experts warn, is that global warming will cause sea levels to rise dramatically. Thermal expansion has already raised the oceans 4 to 8 inches (10 to 20 centimeters). But that's nothing compared to what would happen if, for example, Greenland's massive ice sheet were to melt.
Sea-level rise • The sea level has been rising at a rate of around 1.8 mm per year for the past century, mainly as a result of human-induced global warming. This rate is increasing; measurements from the period 1993–2003 indicated a mean rate of 3.1 mm/year. Global warming will continue to increase sea level over at least the coming century. The contribution from thermal expansion is well understood; substantial changes to the rate and magnitude of increase are largely dependent on how rapidly ice caps disintegrate with increasing temperatures—which is very difficult to model. The thermal expansion of sea water is currently the dominant contributor to sea level rise, and to the predicted rise over the next century, which is 90 to 880 mm (with a central value of 480 mm). Only if glacial melt substantially increases will it become the larger term. Ice can have a huge effect; the melting of the ice caps during the end of the last ice age resulted in a 120 meters rise in sea level.
Sea-level rise • If small glaciers and polar ice caps on the margins of Greenland and the Antarctic Peninsula melt, the projected rise in sea level will be around 0.5 m. • Melting of the Greenland ice sheet would produce 7.2 m of sea-level rise, and melting of the Antarctic ice sheet would produce 61.1 m of sea level rise.
Floods • A flood is an overflow of an expanse of water that submerges land, a deluge. In the sense of "flowing water", the word may also be applied to the inflow of the tide. • Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows, with the result that some of the water escapes its normal boundaries. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endangers land areas used by man like a village, city or other inhabited area.
Floods: typical effects Primary effects • Physical damage - Can range anywhere from bridges, cars, buildings, sewer systems, roadways, canals and any other type of structure. • Casualties - People and livestock die due to drowning. It can also lead to epidemics and diseases. Secondary effects • Water supplies - Contamination of water. Clean drinking water becomes scarce. • Diseases - Unhygienic conditions. Spread of water-borne diseases • Crops and food supplies - Shortage of food crops can be caused due to loss of entire harvest. However, lowlands near rivers depend upon river silt deposited by floods in order to add nutrients to the local soil. • Trees - Non-tolerant species can die from suffocation. Tertiary/long-term effects • Economic - Economic hardship, due to: temporary decline in tourism, rebuilding costs, food shortage leading to price increase etc
Drought • A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region. Although droughts can persist for several years, even a short, intense drought can cause significant damage and harm the local economy. This global phenomenon has a widespread impact on agriculture. The United Nations estimates that an area of fertile soil the size of Ukraine is lost every year because of drought, deforestation, and climate instability. Lengthy periods of drought have triggered mass migration in Africa in this last decade and in various other parts of the world for thousands of years.