Natural Disasters- Meaning and nature of natural disasters, their types and effects

The word Disaster is from a French word Disaster meaning bad or evil star. However this is a very narrow conception of disaster and in our context, any disaster means a situation in which there is a sudden disruption of normalcy within society causing widespread damage to life and property.

Types

1.      Natural [rain, flood, cyclone, storm, landslides, earthquake, volcanoes]

2.      Man made [war including biological, arson, sabotage, riots, accident (train, air, ship), industrial accidents, fires (forest fires), bomb explosions, nuclear explosions and ecological disasters].

Natural disaster

A natural disaster is the effect of a natural hazard (e.g. flood, tornado, volcano eruption, earthquake, or landslide) that affects the environment, and leads to financial, environmental and/or human losses. The resulting loss depends on the capacity of the population to support or resist the disaster, and their resilience. This understanding is concentrated in the formulation: "disasters occur when hazards meet vulnerability.” A natural hazard will hence never result in a natural disaster in areas without vulnerability, e.g. strong earthquakes in uninhabited areas.

Types of natural disorders

1. Land slide

2. Earthquakes

3. Volcanic eruptions

4. Hydrological disasters (Floods, Limnic eruptions, Tsunami)

5. Meteorological disasters (Blizzards, Cyclonic storms, Droughts, Hailstorms, Heat   

   waves, Tornadoes)

6. Wildfires

7. Health disasters (Epidemics)

8. Space disasters (Impact events, Solar flares, Gamma-ray burst)

1.      Landslide

A landslide (landslip) is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.

Landslides occur when the stability of a slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone.

Causes of landslides

·         groundwater (porewater) pressure acting to destabilize the slope

·         Loss or absence of vertical vegetative structure, soil nutrients, and soil structure (e.g. after a forest fire, deforestation, cultivation and construction, which destabilize the already fragile slopes)

·         erosion of the toe of a slope by rivers or ocean waves

·         weakening of a slope through saturation by snowmelt, glaciers melting, or heavy rains or vibrations from machinery or traffic, blasting

·         earthquakes adding loads to barely stable slope

·         earthquake-caused liquefaction destabilizing slopes

·         volcanic eruptions

Types of land slide

Debris flow: Slope material that becomes saturated with water may develop into a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses and vehicles, thus blocking bridges and tributaries causing flooding along its path.

Earth flow

Earthflows are down slope, viscous flows of saturated, fine-grained materials, which move at any speed from slow to fast. Though these are a lot like mudflows, overall they are slower moving and are covered with solid material carried along by flow from within. They are different from fluid flows in that they are more rapid. Clay, fine sand and silt, and fine-grained, pyroclastic material are all susceptible to earthflows. The velocity of the earthflow is all dependent on how much water content is in the flow itself: if there is more water content in the flow, the higher the velocity will be. Earthflows occur much more during periods of high precipitation, which saturates the ground and adds water to the slope content. Fissures develop during the movement of clay-like material which creates the intrusion of water into the earthflows. Water then increases the pore-water pressure and reduces the shearing strength of the material.

Debris avalanche

A debris avalanche is a type of slide characterized by the chaotic movement of rocks soil and debris mixed with water or ice (or both). Debris avalanches differ from debris slides because their movement is much more rapid. This is usually a result of lower cohesion or higher water content and commonly steeper slopes.

Sturzstrom

A sturzstrom is a rare, poorly understood type of landslide, typically with a long run-out. Often very large, these slides are unusually mobile, flowing very far over a low angle, flat, or even slightly uphill terrain.

Shallow landslide

Landslide in which the sliding surface is located within the soil mantle or weathered bedrock (typically to a depth from few decimetres to some metres) is called a shallow landslide. They usually include debris slides, debris flow, and failures of road cut-slopes. Landslides occurring as single large blocks of rock moving slowly down slope are sometimes called block glides.

Deep-seated landslide

Landslides in which the sliding surface is mostly deeply located below the maximum rooting depth of trees (typically to depths greater than ten meters). Deep-seated landslides usually involve deep regolith, weathered rock, and/or bedrock and include large slope failure associated with translational, rotational, or complex movement. These typically move slowly, only several meters per year, but occasionally move faster. They tend to be larger than shallow landslides and form along a plane of weakness such as afault or bedding plane. They can be visually identified by concave scarps at the top and steep areas at the toe.

Earthquakes

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.

Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible or weak and magnitude 7 and over potentially cause serious damage over larger areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011, and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal. An earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter.

Effects of earthquakes

·                     Shaking and ground rupture

·                     Landslides and avalanches

·                     Fires damaging electrical power or gas lines (San Francisco earthquake,  1906)

·                     Soil liquefaction occurs water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid.

·                     Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.

·                     Earthquakes may cause landslips to dam rivers, which collapse and cause floods. (Ex. Usoi Dam in Tajikistan).

·                     Effect of an earthquake include injury and loss of life, road and bridge damage, general property damage, and collapse or destabilization of buildings. The aftermath may bring disease, lack of basic necessities.

Volcanos

Volcanic eruptions

·         Direct harm of the volcano or the fall of rock during eruption.

·         Lava be produced during the eruption of a volcano destroys many buildings and plants it encounters.

·                     Volcanic ash (cooled ash) may form a cloud, and settle thickly in nearby locations and forms a concrete-like material when mixed with water. In sufficient quantity ash may cause roofs to collapse under its weight but even small quantities will harm humans if inhaled. Since the ash has the consistency of ground glass it causes abrasion damage to moving parts such as engines. The main killer of humans in the immediate surroundings of a volcanic eruption is the pyroclastic flows, which consist of a cloud of hot volcanic ash which builds up in the air above the volcano and rushes down the slopes when the eruption no longer supports the lifting of the gases. It is believed that Pompeii was destroyed by a pyroclastic flow. A lahar is a volcanic mudflow or landslide. The 1953 Tangiwai disaster was caused by a lahar, as was the 1985 Armero tragedy in which the town of Armero was buried and an estimated 23,000 people were killed.

·                     A specific type of volcano is the supervolcano. According to the Toba catastrophe theory 75,000 to 80,000 years ago a super volcanic event at Lake Toba reduced the human population to 10,000 or even 1,000 breeding pairs creating a bottleneck in human evolution. It also killed three quarters of all plant life in the northern hemisphere. The main danger from a supervolcano is the immense cloud of ash which has a disastrous global effect on climate and temperature for many years.

Hydrological disasters

It is a violent, sudden and destructive change either in quality of earth's water or in distribution or movement of water on land below the surface or in atmosphere.

Floods

Principal types and causes

Areal (rainfall related)

Floods can happen on flat or low-lying areas when the ground is saturated or impermeable and water either cannot run off or cannot run off quickly enough to stop accumulating. Localised heavy rain from a series of storms moving over the same area can cause areal flash flooding when the rate of rainfall exceeds the drainage capacity of the area. When this occurs on tilled fields, it can result in a muddy flood where sediments are picked up by runoff and carried as suspended matter or bed load.

Riverine

River flows may rise to floods levels at different rates, from a few minutes to several weeks, depending on the type of river and the source of the increased flow. The increase in flow may be the result of sustained rainfall, rapid snow melt, monsoons, or tropical cyclones. Localised flooding may be caused or exacerbated by drainage obstructions such as landslides, ice, or debris.

Rapid flooding events, including flash floods, more often occur on smaller rivers, rivers with steep valleys or rivers that flow for much of their length over impermeable terrain. The cause may be localised convective precipitation (intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, or glacier.

Estuarine and coastal

Flooding in estuaries is commonly caused by a combination of sea tidal surges caused by winds and low barometric pressure, and they may be exacerbated by high upstream river flow.

Coastal areas may be flooded by storm events at sea, resulting in waves over-topping defences or in severe cases by tsunami or tropical cyclones. A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

Catastrophic

Catastrophic flooding is usually associated with major infrastructure failures such as the collapse of a dam, but they may also be caused by damage sustained in an earthquake or volcanic eruption.

Effects

·         The primary effects of flooding include loss of life, damage to buildings and other structures, including bridges, sewerage systems, roadways, and canals.

·                     Disease spread (typhoid, giardia, cryptosporidium, cholera and many other diseases depending upon the location of the flood) due to contaminated water  Lack of clean water combined with human sewage in the flood waters raises the risk of waterborne diseases.

·                     Damage to roads and transport infrastructure may make it difficult to mobilise aid to those affected or to provide emergency health treatment.

·                     Flood waters typically inundate farm land, making the land unworkable and preventing crops from being planted or harvested, which can lead to shortages of food both for humans and farm animals. Entire harvests for a country can be lost in extreme flood circumstances. Some tree species may not survive prolonged flooding of their root systems 

Secondary and long-term effects

·         Temporary decline in tourism, rebuilding costs, or food shortages leading to price increases is a common after-effect of severe flooding.

Some of the most notable floods include:

·         The Huang He (Yellow River) in China floods particularly often. The Great Flood of 1931 caused between 800,000 and 4,000,000 deaths.

·         The 1998 Yangtze River Floods, in China, left 14 million people homeless.

·         The 2000 Mozambique flood covered much of the country for three weeks, resulting in thousands of deaths, and leaving the country devastated for years afterward.

Cyclone

Severe tropical cyclones are responsible for large number of causalities and considerable damage to property and agricultural crop. The destruction is confined to the coastal districts and the maximum destruction being within 100 km from the centre of the cyclone and on the right side of the storm track.

Principal dangers from cyclones are: (i) very strong winds, (ii) torrential rain, and (iii) high storm tides.

Most casualties are caused by coastal inundation by storm surge. Maximum penetration of storm surges varies from 10 to 20 km inland from the coast. Heavy rainfall and floods come next in order of devastation. They are often responsible for much loss of life and damage to property. Death and destruction directly due to winds are relatively less. The collapse of buildings, falling trees, flying debris, electrocution, aircraft and ship accidents and disease from contaminated food and water in the post-cyclone period also contribute to loss of life and destruction of property.

Floods generated by cyclone rainfall are more destructive than winds. Rainfall of the order of 20 to 30 cm per day is common.

As mentioned, the worst danger emanates from the storm surge. In the storm centre, the ocean surface is drawn upward by 30 cm or so above normal due to the reduced atmospheric pressure in the centre. As the storm crosses the continental shelf and moves coast ward, the mean water level increases.

This abnormal rise in sea level caused by cyclone is known as storm surge. The surge is generated due to interaction of air, sea and land. The cyclone provides the driving force in the form of very high horizontal atmospheric pressure gradient and very strong surface winds. As a result, the sea level rises and continues to rise as cyclone moves over increasingly shallower water as it approaches coast, and reaches a maximum on the coast near the point of landfall (Point of crossing coast). Surge is maximum in the right forward sector of the cyclone and about 50-100 Km from the centre coinciding with the zone of maximum wind. Winds in this sector are from ocean to land.

Tropical cyclones can result in extensive flooding and storm surge, as happened with:

·         Bhola Cyclone, which struck East Pakistan (now Bangladesh) in 1970,

·         Typhoon Nina, which struck China in 1975,

·         Hurricane Katrina, which struck New Orleans, Louisiana in 2005, and

·         Cyclone Yasi, which struck Australia in 2011

Droughts

Drought is an extended period of months or years when a region notes a deficiency in its water supply whether surface or underground water. 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.

 Human activity can directly trigger exacerbating factors such as over farming, excessive irrigation, deforestation, and erosion adversely impact the ability of the land to capture and hold water. Overall, global warming will result in increased world rainfall. Along with drought in some areas, flooding and erosion will increase in others.

Types

As a drought persists, the conditions surrounding it gradually worsen and its impact on the local population gradually increases. People tend to define droughts in three main ways:

1.     Meteorological drought is brought about when there is a prolonged period with less than average precipitation. Meteorological drought usually precedes the other kinds of drought.

2.     Agricultural droughts are droughts that affect crop production or the ecology of the range. This condition can also arise independently from any change in precipitation levels when soil conditions and erosion triggered by poorly planned agricultural endeavors cause a shortfall in water available to the crops. However, in a traditional drought, it is caused by an extended period of below average precipitation.

3.     Hydrological drought is brought about when the water reserves available in sources such as aquifers, lakes and reservoirs fall below the statistical average. Hydrological drought tends to show up more slowly because it involves stored water that is used but not replenished. Like an agricultural drought, this can be triggered by more than just a loss of rainfall.

Effects

·         Diminished crop growth or yield productions and carrying capacity for livestock

·         Dust bowls, themselves a sign of erosion, which further erode the landscape

·         Dust storms, when drought hits an area suffering from desertification and erosion

·         Famine due to lack of water for irrigation

·         Habitat damage, affecting both terrestrial and aquatic wildlife

·         Hunger, drought provides too little water to support food crops.

·         Malnutrition, dehydration and related diseases

·         Mass migration, resulting in internal displacement and international refugees

·         Reduced electricity production due to reduced water flow through hydroelectric dams

·         Shortages of water for industrial users

·         Snake migration and increases in snakebites

·         Social unrest

·         War over natural resources, including water and food

·         Wildfires, such as Australian bushfires, are more common during times of drought.

·         Reduce dilution of pollutants and increase contamination of remaining water sources.

Well-known historical droughts include:

·         1900 India killing between 250,000 to 3.25 million.

·         1921-22 Soviet Union in which over 5 million perished from starvation due to drought

·         1928-30 Northwest China resulting in over 3 million deaths by famine.

·         1936 and 1941 Sichuan Province China resulting in 5 million and 2.5 million deaths respectively.

·         In 2006, states of Australia including South Australia, Western Australia, New South Wales, Northern Territory and Queensland had been under drought conditions for five to ten years. The drought is beginning to affect urban area populations for the first time. With the majority of the country under water restrictions.

·         In 2006, Sichuan Province China experienced its worst drought in modern times with nearly 8 million people and over 7 million cattle facing water shortages.

·         12-year drought that was devastating southwest Western Australia, southeast South Australia, Victoria and northern Tasmania was "very severe and without historical precedent".

·         In 2011, the State of Texas lived under a drought emergency declaration for the entire calendar year. The drought caused the Bastrop fires.

Heat waves and cold waves

A cold wave is a weather phenomenon that is distinguished by a cooling of the air. Specifically, as used by the U.S. National Weather Service, a cold wave is a rapid fall in temperature within a 24 hour period requiring substantially increased protection to agriculture, industry, commerce, and social activities. The precise criterion for a cold wave is determined by the rate at which the temperature falls, and the minimum to which it falls. This minimum temperature is dependent on the geographical region and time of year.

Effects

A cold wave can cause death and injury to livestock and wildlife. Exposure to cold mandates greater caloric intake for all animals, including humans, and if a cold wave is accompanied by heavy and persistent snow, grazing animals may be unable to reach needed food and die of hypothermia or starvation. They often necessitate the purchase of foodstuffs at considerable cost to farmers to feed livestock.

The belief that more deaths are caused by cold weather in comparison to hot weather is true as a result of the after affects of these temperatures (i.e. cold, flu, pneumonia, etc.) all contributing factors to hypothermia. However statistics have shown that more deaths occur during a heat wave than in a cold snap in developed regions of the world. Studies have shown that these numbers are significantly higher in undeveloped regions.

Extreme winter cold often causes poorly insulated water pipelines and mains to freeze. Even some poorly protected indoor plumbing ruptures as water expands within them, causing much damage to property and costly insurance claims. Demand for electrical power and fuels rises dramatically during such times, even though the generation of electrical power may fail due to the freezing of water necessary for the generation of hydroelectricity. Some metals may become brittle at low temperatures. Motor vehicles may fail as antifreeze fails and motor oil gels, resulting even in the failure of the transportation system. To be sure, such is more likely in places like Siberia and much of Canada that customarily get very cold weather.

Fires become even more of a hazard during extreme cold. Water mains may break and water supplies may become unreliable, making fire fighting more difficult. The air during a cold wave is typically denser and any cold air that a fire draws in is likely to cause a more intense fire because the colder, denser air contains more oxygen.

Winter cold waves that aren't considered cold in some areas, but cause temperatures significantly below average for an area, are also destructive. Areas with subtropical climates may recognize unusual cold, perhaps barely freezing, temperatures, as a cold wave. In such places, plant and animal life is less tolerant of such cold as may appear rarely.

Cold waves that bring unexpected freezes and frosts during the growing season in mid-latitude zones can kill plants during the early and most vulnerable stages of growth, resulting in crop failure as plants are killed before they can be harvested economically. Such cold waves have caused famines. At times as deadly to plants as drought, cold waves can leave a land in danger of later brush and forest fires that consume dead biomass. One extreme was the so-called Year Without a Summer of 1816, one of several years during the 1810s in which numerous crops failed during freakish summer cold snaps after volcanic eruptions that reduced incoming sunlight.

A heat wave is a prolonged period of excessively hot weather, which may be accompanied by high humidity. While definitions vary, a heat wave is measured relative to the usual weather in the area and relative to normal temperatures for the season. Temperatures that people from a hotter climate consider normal can be termed a heat wave in a cooler area if they are outside the normal climate pattern for that area. The term is applied both to routine weather variations and to extraordinary spells of heat which may occur only once a century. Severe heat waves have caused catastrophic crop failures, thousands of deaths from hyperthermia, and widespread power outages due to increased use of air conditioning.

The definition recommended by the World Meteorological Organization is when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C (9 °F), the normal period being 1961–1990.

Heat waves form when high pressure aloft (from 10,000–25,000 feet (3,000–7,600 metres)) strengthens and remains over a region for several days up to several weeks. This is common in summer (in both Northern and Southern Hemispheres) as the jet stream 'follows the sun'. On the equator side of the jet stream, in the middle layers of the atmosphere, is the high pressure area.

Summertime weather patterns are generally slower to change than in winter. As a result, this mid-level high pressure also moves slowly. Under high pressure, the air subsides (sinks) toward the surface. This sinking air acts as a dome capping the atmosphere.

This cap helps to trap heat instead of allowing it to lift. Without the lift there is little or no convection and therefore little or no convective clouds (cumulus clouds) with minimal chances for rain. The end result is a continual build-up of heat at the surface that we experience as a heat wave.

Effects

·         Health hazards like heat edema (presents as a transient swelling of the hands, feet, and ankles and is generally secondary to increased aldosterone secretion, which enhances water retention), heat rash, (prickly heat), heat cramps (painful, often severe), heat syncope (heat exposure that produces orthostatic hypotension), heat exhaustion (forerunner of heat stroke, hyperthermia).

·                     Mortality occurs due to exposure to heat waves are the most lethal type of weather phenomenon, overall.

·                     Heat build-up causes air conditioners to turn on earlier and to stay on later in the day. As a result, available electricity supplies are challenged during a higher, wider, peak electricity consumption period. Heat waves often lead to electricity spikes due to increased air conditioning use, which can create power outages, exacerbating the problem.

·                    If a heat wave occurs during a drought, which dries out vegetation, it can contribute to bushfires and wildfires.

·                     Heat waves can and do cause roads and highways to buckle, water lines to burst, power transformers to detonate, causing fires.

Climate change

Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions, or in the distribution of weather around the average conditions (i.e., more or fewer extreme weather events).

Global climate change has already had observable effects on the environment. Glaciers have shrunk, ice on rivers and lakes is breaking up earlier, plant and animal ranges have shifted and trees are flowering sooner.

Effects that scientists had predicted in the past would result from global climate change are now occuring: loss of sea ice, accelerated sea level rise and longer, more intense heat waves.

" Taken as a whole, the range of published evidence indicates that the net damage costs of climate change are likely to be significant and to increase over time. "

- Intergovernmental Panel on Climate Change

Scientists have high confidence that global temperatures will continue to rise for decades to come, largely due to greenhouse gasses produced by human activities. The Intergovernmental Panel on Climate Change (IPCC), which includes more than 1,300 scientists from the United States and other countries, forecasts a temperature rise of 2.5 to 10 degrees Fahrenheit over the next century.

According to the IPCC, the extent of climate change effects on individual regions will vary over time and with the ability of different societal and environmental systems to mitigate or adapt to change.

The IPCC predicts that increases in global mean temperature of less than 1.8 to 5.4 degrees Fahrenheit (1 to 3 degrees Celsius) above 1990 levels will produce beneficial impacts in some regions and harmful ones in others. Net annual costs will increase over time as global temperatures increase.

"Taken as a whole," the IPCC states, "the range of published evidence indicates that the net damage costs of climate change are likely to be significant and to increase over time." 

Below are some of the regional impacts of global change forecast by the IPCC:

§  North America: Decreasing snowpack in the western mountains; 5-20 percent increase in yields of rain-fed agriculture in some regions; increased frequency, intensity and duration of heat waves in cities that currently experience them.²

§  Latin America: Gradual replacement of tropical forest by savannah in eastern Amazonia; risk of significant biodiversity loss through species extinction in many tropical areas; significant changes in water availability for human consumption, agriculture and energy generation.³

§  Europe: Increased risk of inland flash floods; more frequent coastal flooding and increased erosion from storms and sea level rise; glacial retreat in mountainous areas; reduced snow cover and winter tourism; extensive species losses; reductions of crop productivity in southern Europe.⁴

§  Africa: By 2020, between 75 and 250 million people are projected to be exposed to increased water stress; yields from rain-fed agriculture could be reduced by up to 50 percent in some regions by 2020; agricultural production, including access to food, may be severely compromised.⁵

§  Asia: Freshwater availability projected to decrease in Central, South, East and Southeast Asia by the 2050s; coastal areas will be at risk due to increased flooding; death rate from disease associated with floods and droughts expected to rise in some regions.⁶

Causes

Volcanism

Volcanic eruptions release gases and particulates into the atmosphere. Particulate matter released cause cooling (by partially blocking the transmission of solar radiation to the Earth's surface) for a period of a few years. The eruption of Mount Pinatubo in 1991, the second largest terrestrial eruption of the 20th century (after the 1912 eruption of Novarupta) affected the climate substantially. Global temperatures decreased by about 0.5 °C (0.9 °F). The eruption of Mount Tambora in 1815 caused the Year without a Summer. Much larger eruptions, known as large igneous provinces, occur only a few times every hundred million years, but may cause global warming and mass extinctions.

Volcanoes are also part of the extended carbon cycle. Over very long (geological) time periods, they release carbon dioxide from the Earth's crust and mantle, counteracting the uptake by sedimentary rocks and other geological carbon dioxide sinks.

Human influences

In the context of climate variation, anthropogenic factors are human activities which affect the climate. The scientific consensus on climate change is "that climate is changing and that these changes are in large part caused by human activities," and it "is largely irreversible."

Of most concern in these anthropogenic factors is the increase in CO₂ levels due to emissions from fossil fuel combustion, followed by aerosols (particulate matter in the atmosphere) and cement manufacture. Other factors, including land use, ozone depletion, animal agriculture and deforestation, are also of concern in the roles they play - both separately and in conjunction with other factors - in affecting climate, microclimate, and measures of climate variables.

Global warming

Global warming is the rise in the average temperature of Earth's atmosphere and oceans since the late 19th century and its projected continuation. Since the early 20th century, Earth's mean surface temperature has increased by about 0.8 °C (1.4 °F), with about two-thirds of the increase occurring since 1980. Warming of the climate system is unequivocal, and scientists are more than 90% certain that it is primarily caused by increasing concentrations of greenhouse gases produced by human activities such as the burning of fossil fuels and deforestation. These findings are recognized by the national science academies of all major industrialized nations.

Climate model projections were summarized in the 2007 Fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC). They indicated that during the 21st century the global surface temperature is likely to rise a further 1.1 to 2.9 °C (2 to 5.2 °F) for their lowest emissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) for their highest. The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations.

Future warming and related changes will vary from region to region around the globe. The effects of an increase in global temperature include a rise in sea levels and a change in the amount and pattern of precipitation, as well a probable expansion of subtropical deserts. Warming is expected to be strongest in the Arctic and would be associated with the continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include a more frequent occurrence of extreme-weather events including heat waves, droughts and heavy rainfall, ocean acidification and species extinctions due to shifting temperature regimes. Effects significant to humans include the threat to food security from decreasing crop yields and the loss of habitat from inundation.

Initial causes of temperature changes (external forcings)

The climate system can respond to changes in external forcings. External forcings can "push" the climate in the direction of warming or cooling.  Examples of external forcings include changes in atmospheric composition (e.g., increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun. Orbital cycles vary slowly over tens of thousands of years and at present are in an overall cooling trend which would be expected to lead towards an ice age, but the 20th century instrumental temperature record shows a sudden rise in global temperatures.

Greenhouse gases

The greenhouse effect is the process by which  absorption  and emission  of infrared  radiation  by gases in the atmosphere warm aplanet's lower atmosphere and surface. It was proposed by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.

Naturally occurring amounts of greenhouse gases have a mean warming effect of about33 °C (59 °F). The major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect; carbon dioxide (CO₂), which causes 9–26%; methane (CH₄), which causes 4–9%; and ozone (O₃), which causes 3–7%. Clouds also affect the radiation balance through cloud forcings similar to greenhouse gases.

Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO₂, methane, tropospheric ozone, CFCs and nitrous oxide. The concentrations of CO₂ and methane have increased by 36% and 148% respectively since 1750. These levels are much higher than at any time during the last 800,000 years, the period for which reliable data has been extracted from ice cores. Less direct geological evidence indicates that CO₂ values higher than this were last seen about 20 million years ago. Fossil fuel burning has produced about three-quarters of the increase in CO₂ from human activity over the past 20 years. The rest of this increase is caused mostly by changes in land-use, particularly deforestation.

Over the last three decades of the 20th century, gross domestic product per capita and population growth were the main drivers of increases in greenhouse gas emissions. CO₂ emissions are continuing to rise due to the burning of fossil fuels and land-use change. 

Particulates and soot

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, was observed from 1961 until at least 1990. The main cause of this dimming is particulates produced by volcanoes and human made pollutants, which exerts a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion – CO₂ and aerosols – have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO₂ greenhouse gases such as methane. Radiative forcing due to particulates is temporally limited due to wet deposition which causes them to have an atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in particulate concentrations will only delay climate changes due to carbon dioxide.

In addition to their direct effect by scattering and absorbing solar radiation, particulates have indirect effects on the radiation budget. Sulfates act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets, known as the Twomey effect. This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect. Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Indirect effects of particulates represent the largest uncertainty in radiative forcing.

Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds. When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface. The influences of particulates, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.^[87]

Solar activity

Observed and expected environmental effects

"Detection" is the process of demonstrating that climate has changed in some defined statistical sense, without providing a reason for that change. Detection does not imply attribution of the detected change to a particular cause. "Attribution" of causes of climate change is the process of establishing the most likely causes for the detected change with some defined level of confidence. Detection and attribution may also be applied to observed changes in physical, ecological and social systems.

Natural systems

Global warming has been detected in a number of natural systems. Some of these changes are described in the section on observed temperature changes, e.g., sea level rise and widespread decreases in snow and ice extent. Most of the increase in global average temperature since the mid-20th century is, with high probability, attributable to human-induced changes in greenhouse gas concentrations. On the timescale of centuries to millennia, the melting of ice sheets could result in even higher sea level rise. Partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic Ice Sheet, could contribute 4–6 metres (13 to 20 ft) or more to sea level rise.

Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean. Snow cover area and sea ice extent are expected to decrease, with the Arctic expected to be largely ice-free in 2037.

Ecological systems

In terrestrial ecosystems, the earlier timing of spring events, and poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming. Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs. It is expected that most ecosystems will be affected by higher atmospheric CO₂ levels, combined with higher global temperatures. Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.

Dissolved CO₂ increases ocean acidity. This process is known as ocean acidification and has been called the "equally evil twin" of global climate change. Increased ocean acidity decreases the amount of carbonate ions, which organisms at the base of the marine food chain, such as foraminifera, use to make structures they need to survive. The current rate of ocean acidification is many times faster than at least the past 300 million years, which included four mass extinctions that involved rising ocean acidity, such as the Permian mass extinction, which killed 95% of marine species. By the end of the century, acidity changes since the industrial revolution would match the Palaeocene-Eocene Thermal Maximum, which occurred over 5000 years and killed 35–50% of benthic foraminifera.

Large-scale and abrupt impacts

Climate change could result in global, large-scale changes in natural and social systems. Two examples are ocean acidification caused by increased atmospheric concentrations of carbon dioxide, and the long-term melting of ice sheets, which contributes to sea level rise.

Some large-scale changes could occur abruptly, i.e., over a short time period, and might also be irreversible. An example of abrupt climate change is the rapid release of methane from permafrost, which would lead to amplified global warming. Scientific understanding of abrupt climate change is generally poor. However, the probability of abrupt changes appears to be very low. Factors that may increase the probability of abrupt climate change include higher magnitudes of global warming, warming that occurs more rapidly, and warming that is sustained over longer time periods.

Observed and expected effects on social systems

Vulnerability of human societies to climate change mainly lies in the effects of extreme-weather events rather than gradual climate change. Impacts of climate change so far include adverse effects on small islands, adverse effects on indigenous populations in high-latitude areas, and small but discernable effects on human health. Over the 21st century, climate change is likely to adversely affect hundreds of millions of people through increased coastal flooding, reductions in water supplies, increased malnutrition and increased health impacts. 

Food security

Under present trends, by 2030, maize production in Southern Africa could decrease by up to 30% while rice, millet and maize in South Asia could decrease by up to 10%.By 2080, yields in developing countries could decrease by 10% to 25% on average while India could see a drop of 30% to 40%.  By 2100, while the population of three billion is expected to double, rice and maize yields in the tropics are expected to decrease by 20–40% because of higher temperatures without accounting for the decrease in yields as a result of soil moisture and water supplies stressed by rising temperatures.

Future warming of around 3 °C (by 2100, relative to 1990–2000) could result in increased crop yields in mid- and high-latitude areas, but in low-latitude areas, yields could decline, increasing the risk of malnutrition. A similar regional pattern of net benefits and costs could occur for economic (market-sector) effects. Warming above 3 °C could result in crop yields falling in temperate regions, leading to a reduction in global food production.

Habitat inundation

In small islands and megadeltas, inundation as a result of sea level rise is expected to threaten vital infrastructure and human settlements. This could lead to issues of statelessness for populations in countries such as the Maldives and Tuvalu and homelessness in countries with low lying areas such as Bangladesh.

Sea level rise

Sea levels around the world are rising. Current sea-level rise potentially affects human populations (e.g., those living in coastal regions and on islands)^[2] and the natural environment (e.g., marine ecosystems). Between 1870 and 2004, global average sea levels rose 195 mm (7.7 in). From 1950 to 2009, measurements show an average annual rise in sea level of 1.7 ± 0.3 mm per year, with satellite data showing a rise of 3.3 ± 0.4 mm per year from 1993 to 2009, a faster rate of increase than previously estimated. It is unclear whether the increased rate reflects an increase in the underlying long-term trend.

Two main factors contributed to observed sea level rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the contribution of land-based ice due to increased melting. The major store of water on land is found in glaciers and ice sheets.

Sea level rise is one of several lines of evidence that support the view that the climate has recently warmed. It is very likely that human-induced (anthropogenic) warming contributed to the sea level rise observed in the latter half of the 20th century.

Sea level rise is expected to continue for centuries. In 2007, the Intergovernmental Panel on Climate Change (IPCC) projected that during the 21st century, sea level will rise another 18 to 59 cm (7.1 to 23 in), but these numbers do not include "uncertainties in climate-carbon cycle feedbacks nor do they include the full effects of changes in ice sheet flow".^[13] More recent projections assessed by the US National Research Council (2010)^[14] suggest possible sea level rise over the 21st century of between 56 and 200 cm (22 and 79 in).

On the timescale of centuries to millennia, the melting of ice sheets could result in even higher sea level rise. Partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, could contribute 4 to 6 m (13 to 20 ft) or more to sea level rise.

Overview of sea-level change

Local and eustatic sea level

Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL.

“Eustatic” change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.

Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the density of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers, polar ice caps, and sea ice. Over much longer geological timescales, changes in the shape of oceanic basins and in land–sea distribution affect sea level. Observational and modelling studies of mass loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2–0.4 mm/yr, averaged over the 20th century.

Glaciers and ice caps

Each year about 8 mm of ocean water falls on the Antarctica and Greenland ice sheets as snowfall. If no ice returned to the oceans, sea level would drop 8 mm every year. To a first approximation, the same amount of water appeared to return to the ocean in icebergs and from ice melting at the edges. Scientists previously had estimated which is greater, ice going in or coming out, called the mass balance, important because a non-zero balance causes changes in global sea level. High-precision gravimetry from satellites in low-noise flight determined that Greenland was losing more than 200 billion tons of ice per year, in accord with loss estimates from ground measurement. The rate of ice loss was accelerating, having grown from 137 gigatons in 2002–2003.

Ice shelves float on the surface of the sea and, if they melt, to a first order they do not change sea level. Likewise, shrinkage/expansion of the northern polar ice cap which is composed of floating pack ice do not significantly affect sea level. Because ice shelf water is fresh, however, melting would cause a very small increase in sea levels, so small that it is generally neglected.

·         The melting of small glaciers and polar ice caps on the margins of Greenland and the Antarctic Peninsula melt, would increase sea level around 0.5 m. Melting of the Greenland ice sheet or the Antarctic ice sheet would produce 7.2 m and 61.1 m of sea-level rise, respectively. The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5–6 m.

·         The interior of the Greenland and Antarctic ice sheets, as of 2009, was sufficiently high (and therefore cold) that direct melt would require several millennia. They could do so through acceleration in flow and enhanced iceberg calving. Also, melt of the fringes of the ice caps could be significant, as could be sub-ice-shelf melting in Antarctica.

·         Climate changes during the 20th century were estimated from modelling studies to have led to contributions of between −0.2 and 0.0 mm/yr from Antarctica (the results of increasing precipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation and runoff).

·         Estimates suggest that Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr over the 20th century as a result of long-term adjustment to the end of the last ice age.

The current rise in sea level observed from tide gauges, of about 1.8 mm/yr, is within the estimate range from the combination of factors above but active research continues in this field. The terrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quite large.

Ozone depletion

Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon is referred to as the ozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polartropospheric ozone depletion events.

The details of polar ozone hole formation differ from that of mid-latitude thinning, but the most important process in both is catalytic destruction of ozone by atomic halogens. The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants (CFCs,freons, halons). These compounds are transported into the stratosphere after being emitted at the surface. Both types of ozone depletion were observed to increase as emissions of halo-carbons increased.

CFCs and other contributory substances are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol that bans the production of CFCs, halons, and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in skin cancer,cataracts, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion.

Consequences of ozone layer depletion

Since the ozone layer absorbs UVB ultraviolet light from the sun, ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increase in skin cancer. This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer and eye damage in human beings. This is partly because UVA, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for lifestyle changes in the populace.

Increased UV

Ozone, while a minority constituent in Earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness and density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface. Ozone-driven phenolic formation in tree rings has dated the start of ozone depletion in northern latitudes to the late 1700s.

Increases in surface UVB due to the ozone hole can be partially inferred by radiative transfer model calculations, but cannot be calculated from direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, New Zealand).

UV-215 and more energetic radiation is responsible for creation ozone in the ozone layer from O₂ (regular oxygen). UV-215 through UV-280 increases as a result of reduction in stratospheric ozone, but this is insufficient to do more than dissociate the single oxygen bond of ozone, and of course disrupt DNA bonding.

Biological effects

The main public concern regarding the ozone hole has been the effects of increased surface UV radiation on human health. So far, ozone depletion in most locations has been typically a few percent and, as noted above, no direct evidence of health damage is available in most latitudes. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic. As the ozone hole over Antarctica has in some instances grown so large as to reach southern parts of Australia, New Zealand, Chile, Argentina, and South Africa, environmentalists have been concerned that the increase in surface UV could be significant.

Ozone depletion would change all of the effects of UVB on human health, both positive and negative.

UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to skin cancer and to produce Vitamin D. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.

Basal and squamous cell carcinomas

The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood—absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates. These cancers are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery. By combining epidemiological data with results of animal studies, scientists have estimated that a one percent decrease in stratospheric ozone would increase the incidence of these cancers by 2%.

Malignant melanoma

Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15–20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet well understood, but it appears that both UVB and UVA are involved. Experiments on fish suggest that 90 to 95% of malignant melanomas may be due to UVA and visible radiation whereas experiments on opossums suggest a larger role for UVB. Because of this uncertainty, it is difficult to estimate the impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in UVB radiation was associated with a 19% increase in melanomas for men and 16% for women. A study of people in Punta Arenas, at the southern tip of Chile, showed a 56% increase in melanoma and a 46% increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.

Cortical cataracts

Studies are suggestive of an association between ocular cortical cataracts and UV-B exposure, using crude approximations of exposure and various cataract assessment techniques. A detailed assessment of ocular exposure to UV-B was carried out in a study on Chesapeake Bay Watermen, where increases in average annual ocular exposure were associated with increasing risk of cortical opacity. In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date. However, subsequent data from a population-based study in Beaver Dam, WI suggested the risk may be confined to men. In the Beaver Dam study, the exposures among women were lower than exposures among men, and no association was seen. Moreover, there were no data linking sunlight exposure to risk of cataract in African Americans, although other eye diseases have different prevalences among the different racial groups, and cortical opacity appears to be higher in African Americans compared with whites.

Increased tropospheric ozone

Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts.

Increased production of vitamin D

Vitamin D is produced in the skin by ultraviolet light. Thus, higher UV-B exposure raises human vitamin D in those deficient in it. Recent research (primarily since the Montreal protocol), shows that many humans have less than optimal vitamin D levels. In particular, the lowest quartile of vitamin D (<17.8 ng/ml), in the US population were found using information from the National Health and Nutrition Examination Survey to be associated with an increase in all cause mortality in the general population. While higher level of Vitamin D are associated with higher mortality, the body has mechanisms that prevent sunlight from producing too much Vitamin D.

Effects on non-human animals

Scientists at the Institute of Zoology in London found that whales off the coast of California have shown a sharp rise in sun damage, and these scientists "fear that the thinning ozone layer is to blame".

The study photographed and took skin biopsies from over 150 whales in the Gulf of California and found "widespread evidence of epidermal damage commonly associated with acute and severe sunburn", having cells that form when the DNA is damaged by UV radiation. The findings suggest "rising UV levels as a result of ozone depletion are to blame for the observed skin damage, in the same way that human skin cancer rates have been on the increase in recent decades."

Effects on crops

An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as rice, depend on cyanobacteria residing on their roots for the retention of nitrogen. Cyanobacteria are sensitive to UV radiation and would be affected by its increase.