Strong winds of at least 119km/h and a rotary circulation
Requires very warm, moist air – most occur in late summer when ocean temperatures reach 27C or higher, so they often do not occur in colder waters of the S Atlantic or E S Pacific, or beyond 20 degrees of latitude
Sufficient spin or twist provided by the Coriolis effect, so usually do not form within 5 degrees of Equator
Warm sea heats air above, warm moist air rises creating low pressure
Wind rushes into low pressure zone spiralling upwards, releasing heat and moisture
Rising air cools, creating cumulonimbus clouds which release latent heat upon condensation, rising higher. Self-sustaining heat energy increases speed of the system
Characteristics of Tropical Cyclones
Location of Cyclone Occurrences
Most occur between 5 and 20 latitude over tropical oceans except South Atlantic and Eastern South Pacific due to cold currents
N hemisphere – occur between June and December in Atlantic Pacific and Indian Oceans.
S hemisphere – occur between January and March in Pacific and Indian oceans
Cyclone Paths
Initially move west due to trade winds, but easterly influence diminishes after intensity increases
Upper level winds and spatial distribution of water temperature are key factors determining their speed and direction. Tend to move towards warmer seas
Once developed, they are likely to move poleward and move into influence of westerlies, thus moving eastward
Irregular paths make movement unpredictable
Features of a Tropical Cyclone
Average about 600km across, the eye about 25km across
The eye is a zone where precipitation stops and winds subside
Air within eye descends and heats by compression, making it warm. However, the subsidence is often not strong enough to inhibit cloud formation
Beyond the eye is the eye wall, a zone of intense convective activity where the greatest wind speeds, thickest cloud cover and heaviest rainfall occur
Consists of a large number of thunderstorms arranged in a pinwheel formation
Dissipation of Tropical Cyclones
When move over ocean waters that cannot support warm moist tropical air
When reaching a location where large-scale flow aloft is unfavourable
When moving onto land, due to depletion of water source, cooler air and increased surface roughness and friction
Impacts of Tropical Cyclones
Storm Surge
In coastal zone, 90% of deaths and majority of damage is caused by storm surge
A dome of water 65-80km long which sweeps across the coast
Due to piling up of water when heavy winds drag surface waters landward, bringing heavy surf. Also, low atmospheric pressure causes water level to rise by 1cm every millibar
Bangladesh – most land less than 2m above sea level. Nov 13 1970, official death toll 200k, but unofficial about 500k
Wind Damage
Crop destruction, flying debris, spreading of fires in urban and forested areas
US$20billion of damage in southern Florida and Louisiana due to Hurricane Andrew
Heavy Rainfall
Can affect places 100s of km from the coast for several days after the storm’s winds have slowed
Loss of life, property damage, crop destruction from flooding. Contamination of water can lead to disease outbreaks. Landslides or mudflows. Saline intrusion.
Hurricane Agnes in 1972 – US$2 billion in damage and 122 lives due to flooding
Cyclone Damages – A Comparison
In coastal zones, storm surge is usually the cause of extensive damage and loss of lives
Wind damage not as catastrophic, but affects a larger area, especially when building codes are inadequate. Also, most wind damage occurs 200km of coast since cyclones weaken inland
Far from coast, rainfall and flooding can still cause damages, sometimes greater than storm surge
Management of Cyclonic Hazards
No atmospheric conditions can be measured to predict cyclone development, only forecasting its path is possible
Use of the Saffir-Simpson Scale
Ranks relative intensities of tropical cyclones
Advantage of being related to property damage, can be used to predict
Disaster potential can be monitored and precautions can be planned and implemented
Predicting and Monitoring Tropical Cyclones
The Atlantic Tropical Cyclone Prediction Model
Forecasters use atmospheric models to predict tracks of tropical cyclones to provide storm warnings or evacuations. Colorado State University attempt to predict degree of activity up to eleven months in advance of hurricane season in North Atlantic
Forecasts based on series of indices which show statistical relationships with cyclonic activity, such as the El Nino Southern Oscillation which reduces hurricane activity, as well as ocean surface temperatures
The Role of Satellites
Detect and track cyclones even before cyclonic flow is developed
However, by itself is not entirely accurate. Errors of up to 10s of km in position or wind speed estimates
Aircraft Reconnaissance
Accurately measure details of position and state of development. Higher accuracy than remote satellites
Radar
Since 1960s, radar system in place covering Gulf of Mexico and Atlantic regions.
Mitigating the Effects of Tropical Cyclones
Early Warning Systems
Hurricane watch: announcement for coastal areas that are potentially threatened within 36 hours
Hurricane warning: when sustained winds of 119km/h or higher are expected within 24 hours
So preparations can be taken
Adequate lead time is required to protect life and property
Over-warning must be minimised because it is costly and diminishes credibility. Delicate balance to issue warning
Reduced deaths, but increased property damage due to rapid population growth and development in cyclone prone areas
Evacuation Exercises and First Aid Strategies
Reduce casualties – DCs are more capable of doing this than LDCs
Case Studies for Comparison
Effects depend in development of economy, transport network and infrastructure
United States: focus on early warning and monitoring though satellites. Coastal protection works, cloud seeding. However, evacuation takes too long, and warning time may not be sufficient. Furthermore, complacency in population has set in due to no serious cyclones so far. No evacuation experience.
Bangladesh: ineffective warning from Pakistani Meteorological Bureau – detected 3 days in advance but not issued until the evening it hit, and even then most people were sleeping. Looting occurs. Evacuation impossible because of illiterate population with low education who do not understand risks of hazard.
Past Climate Change
Climatic Changes in the Past
Ice Ages
Most of the time, planet has usually been warmer, only punctuated by about 7 short ice ages
Most recent is the Pleistocene Ice Age
Glacials and Interglacials within the Pleistocene Ice Age
Climate not uniform within ice age, it oscillates in cycles between glacials and interglacials
Glaciation involves a general cooling of temperature, and ample snowfall must have persisted
Deglaciation involves shrinkage of ice sheets in depth and volume
Currently we are in a warm interglacial
The Last Glacial Maximum – the Wisconsinian Glaciation
18000 years ago, thick ice sheets covered North America and Europe and Northern Asia and Southern South America.
Evidence of Climatic Change
Glacier Evidence
Glacial advance and retreat leave erosional and depositional features
Alpine glaciers erode valleys, transforming V-shaped valleys into U-shaped valleys
They also leave striae on rocks, smoothing them out
Ice sheets are also capable of moving very large sized sediment, depositing these tillites when they melt, which contain a wide assortment of sediment sizes
Poorly sorted deposits indicate past ice sheets and glaciations
Most glaciers in the N have been decreasing in size in the 20th century, probably as a result of rising temperatures. However, exceptions like the Hubbard Glacier in Alaska have been increasing, which shows that they respond to local climatic conditions as well.
Sediment Evidence
Ocean drilling programs collect cores of deep sea sediments
Remains of organisms settle on floor, such as shells. The numbers and types of such organisms change with climate changes.
Also, oxygen isotope analysis can be used to analyse calcium carbonate in shells of organic life, such as foraminifera
O16 is lighter than O18, making it evaporate more readily, so when glaciers are expanding, more O16 is removed from ocean and deposited as snow, so O18:16 ratios will be higher in the ocean and also in the shells
Ice Core Evidence
Oxygen isotope analysis can also be applied to ice cores taken from ice sheets. The ratio of O18:16 in ice cores is influenced by temperature since more O18 is evaporated with higher temperatures
Also, air bubbles trapped in ice give an indication of the components of past atmosphere. Higher temperature coincided with higher concentrations of carbon dioxide
Also, dust from volcanic activity can settle on glaciers, which show up in core analysis. Can be valuable in determining the importance of volcanic activity in climate change
Pollen Analysis
Climate influences distribution of vegetation communities
Spores and pollen can be preserved in lake beds or bogs, indicating the type of vegetation which occupied that region, which can then be subject to radiocarbon dating to figure out the kind of climate at the time
Not all pollen can be identified to species level, and not all species produce the same amount of pollen, resulting in underrepresentation of some species in palynology
Tree Ring Analysis
Each year trees increase width of trunk by growth of concentric rings
Width depends on climate – if favourable, grow more. If climatic stress conditions from lack of moisture or too much warmth, growth is less
Natural Causes of Climatic Change
Astronomical Theory
Variation in Eccentricity
Minor significance – elliptical orbit of Earth around sun
Earth is 3% closer to sun during perihelion than aphelion, meaning Earth receives about 6% more solar energy during perihelion
This depends on eccentricity – when longer ellipse, radiation received at perihelion may be 23-30% more than at aphelion
Changes in Obliquity
Change in tilt of Earth’s axis – between 22.1 and 24.5. Currently 23.5
Smaller tilt = smaller temperature difference between summer and winter
Reduced seasonal contrast promotes growth of ice sheets
Warmer winter and cooler summer allow for more precipitation to fall and lesser melt
When tilting towards Vega, the times when winter and summer solstices occur will be reversed, resulting in warmer summers at perihelion and colder winters at aphelion
Credibility of the Astronomical Theory
Climatic changes occur because they change the degree of contrast between seasons, not because of distance from sun
Statistical correlation concludes that astronomical theory is the fundamental cause behind the Quaternary Ice Ages
Solar Variability and Climate
Hypothesis – Sun is a variable star which changes output over time
However, no major long term variations in total intensity of solar radiation have been measured outside atmosphere
Cycle of sunspots, places where huge magnetic storms occur, of 11 years, increasing and decreasing regularly
Prolonged periods of absent sunspots corresponded with cold periods in Europe and N America. Also, plentiful sunspots correlated with warmer times.
However, global data does not show significant correlation, and no actual physical mechanism exists to explain the effect
Volcanic Activity and Climate
Eruptions emit gases and debris into atmosphere, which remain in the stratosphere for long periods of time
Sulphur in the form of sulphur dioxide and hydrogen sulphide reforms into an aerosol of sulphuric acid droplets, which absorb and scatter solar radiation, cooling
Also, they serve as condensation nuclei, increasing surface area of clouds to reflect
More extensive, longer-lived, brighter clouds
However, impact is small and short-lived – significant changes can only happen with many eruptions within a short time
Plate Tectonics and Climatic Changes
Glacial features in Africa, Australia and India indicate the movement of plates
Continental drift accounted for many dramatic climate changes as landmasses shifted to different latitudes
However, plate movement is very slow – significant changes can only occur over a great period of time. Not useful for explaining climatic variations on a shorter time scale.
Climate of the Cities
Changes in Temperature
The Urban Heat Island Effect
Urbanised areas have higher temperatures than countrysides
Determinant of the Magnitude and Effect
Size of the city and density of population. Greater size and density have larger hear island effect
Spatial Variation of the Effect
Highest temperatures occur at greatest building densities and industrial areas
Urban-rural differences greatest in winter months due to heat generation
Tropical heat island less pronounced than temperate heat islands due to lesser human-generated heat sources, but in NICs quite close to DCs
Causes of the Urban Heat Island Effect
Modifications to the energy balance when natural surfaces are paved and built upon and as a result of human activity
Changes in Thermal Capacity
Materials used in buildings (concrete, asphalt) have a far higher heat capacity than natural surfaces, resulting in more stored heat available for transfer to atmosphere at night
Surface of city cools at slower rate
Sensible and Latent Heat Transfer
Urbanisation favours sensible heat transfer over latent heat transfer, since concrete is impervious to water, and runoff is diverted by drains and canals
Reduction in water available for evaporation reduces latent heat and increases sensible heat
The Urban Structure
Buildings change the surface albedo, as urban surfaces have lower albedos than natural ones
Also, 3D structure of city increases amount of heat absorbed via multiple reflections
Vertical walls do not allow radiation to escape as easily as flat surfaces – diffuse radiation is easily absorbed by adjacent walls, increasing total absorption
Similar processes occur at night, slowing rate of cooling and increasing minimum temperatures
Heat Production by Combustion
Waste heat from home heating, power generation, industry and transport
Magnitude of human energy very significant – 1/3 of solar radiation in Sheffield, England in a year
Most pronounced in winter due to heating – 2.5 times greater than solar energy in Manhattan in winter
Presence of Pollutants
Particulate matter, water vapour, carbon dioxide absorbs long-wave radiation and reemits it, leading to a localised greenhouse effect
Alteration of Wind Speed
Tall buildings retard the flow of air due to rougher surfaces
Decrease ventilation by inhibiting movement of cooler air into centre
If regional winds are strong enough, structures are not enough to block ventilation, and the heat island cannot be detected
Adding of condensation nuclei and freezing nuclei from industrial discharge
Rougher city surface leads to low-level convergence and upward air motion
Rain producing processes may linger over urban area due to impeding progress of weather systems, increasing precipitation
Condensation can set in at 78% relative humidity due to hygroscopic nuclei
Case Studies – St. Louis and Paris
Precipitation over city is about 10% greater than rural
St. Louis – increases in precipitation downwind that grew along with industrial
Precipitation was significantly greater on weekdays than weekends in both places
Changes in Other Aspects of the Urban Climate
Reduction in Solar Radiation
Urban pollution adds aerosols like carbon, lead and aluminium, causing a pollution dome to occur, reducing insolation depending on ventilation and angle of sun (lower angle reflects more)
c.p., solar energy will be reduced more at higher-latitude cities during winter
The Lowered Relative Humidity
Temperatures are higher and evaporation is reduced due to lack of large water bodies and vegetation for evapotranspiration
Fogs and clouds are greater mainly because of increased condensation nuclei
The Generation of Country Breeze
Heat of city creates upward air motion, low pressure, causing air to blow into city from countryside
The Enhanced Greenhouse Effect and Global Warming
The Enhanced Greenhouse Effect
The Phenomenon
Greenhouse effect is natural – long wave terrestrial radiation absorbed by greenhouse gases, reemitting and reabsorbing in a cycle
Keeps Earth 35C warmer than it would be without greenhouse gases
Greenhouse Gases
Human activities lead to the enhanced greenhouse effect by emitting more gases
Collectively important since they absorb different spectrums of energy
Carbon Dioxide
Burning of fossil fuels and clearing forested areas disturb the carbon cycle by adding more into atmosphere
Exponential increase since mid 19th century, by 1980 about 20kmil tonnes annually from fossil fuels, with most coming from DCs
Deforestation – lesser photosynthesis, burning of plants. About 20% of Co2 anthropogenically added each year from deforestation
Methane
Byproduct of energy consumption and agricultural activity
Other Trace Gases
Include nitrous oxide and CFCs, absorbing different wavelengths from the other 2
Global temperature is increasing, with warming occurring since late 1970s
Long Term Carbon Dioxide and Temperature Variations
Fluctuations in CO2 content and temperature over the last 160000 years appear to be correlated
Solar energy variations does not seem to account for this since temperatures have been steadily going up without a corresponding increase in solar activity
Recent Measurement of Carbon Dioxide Content
At Mauna Loa Observatory in Hawaii, CO2 concentration has been measured to have increased from 315ppm to 370ppm
The Relationship between Carbon Dioxide Level and Temperature
Most believe effects of CO2 increase are not yet large enough to be clearly measured
Also, although there is a correlation, it does not imply causation, nor show which one causes which
Difficulties in Future Climate Change Projection
Various climatic feedback mechanisms make it difficult and complicate modelling efforts for climate change
Positive feedback mechanisms magnify and reinforce temperature rise
Negative feedback mechanisms produce opposite results and tend to offset rises
Positive Feedback Mechanism – Evaporation Rates
Greater greenhouse gases -> higher surface temperatures -> greater evaporation rates -> more water vapour -> more terrestrial radiation absorbed
Positive Feedback Mechanism – Albedo Changes
Increasing temperatures -> melting ice sheets -> reduced albedo -> increased insolation absorbed -> higher temperatures especially at higher latitudes
Negative Feedback Mechanism – Cloud Cover
Higher moisture -> increased cloud cover -> reflect more insolation (higher albedo more dominant than reflecting terrestrial radiation) -> lower temperature. However, magnitude is less great than positive feedbacks, ultimately still temperature rise
Negative Feedback Mechanism – Ocean Sink
Increase temperature -> increased marine life -> removal of carbon dioxide via photosynthesis -> carbon sink -> lesser carbon -> cooler. However, warm water absorbs less carbon dioxide than cold water.
Global Dimming
Pollution caused by humans cause pollutant layer and forms condensation nuclei, which cuts insolation, producing a cooling effect
Models predicting global warming may have been affected by this cooling effect, being under sensitive to temperature changes
Reducing insolation has also caused droughts due to reducing heat changing the positions of the ITCZ
Solving global dimming can cause global warming to worsen
Predicted Future Temperature Changes
Consensus is that increasing CO2 and trace gases will lead to warmer planet, but more so in polar regions, due to stable air preventing heat from rising, and melt in sea ice increasing temperatures
Depends on how much humans emit in future years, affected by economic conditions, population growth and alternative energy sources
Even if no gas was added, temperatures would still increase due to heat being released from oceans
Impossible to conclude for sure either way, but generally accepted that currently largely the result of increases in greenhouse gases
Consequences of Global Warming
Sea Level Changes
Thermal Expansion
Warmer atmosphere cause increase in ocean volume due to expansion, causing water level to rise
Any rise in gently sloping shorelines will result in significant erosion and shoreline retreat
Glacial Melt
On a large scale, lead to much greater rise in sea level. However, significant melting not expected this century
Different Extent of Sea Rise
Shoreline shift is minor when coastal land is steep. Major when gentle.
Impact of Sea Level Rise
Drastic, especially in tropics and warm temperate regions. Low-lying areas are also susceptible
Maldives – 1m rise would cover 85% of capital Malé
Delta environments, such as Egypt and Nile, losing prime agricultural land, infrastructure and tourist revenue
Bangladesh and India
Economical solution – seawater diverted into depressions such as Dead Sea
Changes in Agricultural Productivity
Some regions experience significant changes in precipitation and runoff, affecting distribution of world’s water resources and productivity of agricultural regions
May be offset by gains elsewhere – expansion of agriculture into areas currently not suited to crop production
However, it improves in wealthy countries like Canada but declines in poorer tropical countries that feed majority of the world
Effects on Plant Migration
Plant communities migrate north and south in the past due to climate change
The rate at which they migrate is important. Rapid changes in climate may affect it, but not by much since they migrate rather fast
However, human intervention may have fragmented plant communities, hindering their ability to migrate
Other Possible Changes
Higher frequency of tropical storms
Shift in paths of cyclonic storms, affecting distribution of precipitation and storms
Increases in intensity of heat waves and droughts
Actual impact is largely unknown due to many factors and lack of understanding of the complex interplay of feedback mechanisms. Unlikely to accurately predict climate change
Mitigation of Climate Change
Reduction of Energy Consumption
Reduce per capita consumption of energy through conservation and efficiency
Inexpensive and simple e.g. insulation for buildings to minimise heat loss, mass public transportation
Key issue is cost – currently coal and natural gas are the cheapest fuels, while alternative energy sources are still quite expensive, but the prices are reducing
Wind Power
Cost of wind power has dropped since 1990s. Once built, cost of providing energy is zero. No pollutants.
However, can kill animals, unsightly, and cost of cables to transport energy is quite high.
Solar Energy
Effective but expensive to manufacture silicon solar panels
New thin-film solar materials are much cheaper to manufacture
Nuclear Energy
Japan, western European countries generate significant nuclear energy, about 16% of worldwide energy supply
However, limited deposits of uranium fuel can only last about 50 more years
Power plant disasters cause bad reputation, such as Chernobyl, Three Mile Island and Fukushima Dai Ichi Plant
Carbon Trading
Cap-and-trade law to limit carbon emissions. Encourage companies to reduce greenhouse gases without hindering growth
However, does not distinguish between different levels of green technology being used
Global Cooperation
The Kyoto Protocol
Meaningful reductions in greenhouse gas emissions can only be achieved with global cooperation
111 countries signed in 1997, with Russia signing in 2004
Plant forests, change agricultural practices, clean technology, buy carbon credits to keep to the limit
Problems Associated with the Kyoto Protocol
Bush argued that it would hurt US economically, despite being the largest contributor at the time
LDCs argue that DCs should reduce their emissions because they can afford the costs, while LDCs are poorer and cannot purchase more efficient fuels
Other International Cooperation Efforts
July 2009, the G8 committed to reducing carbon emission by 80% by 2050
India and China pledged to reduce the rate of increase, not absolute level
