Global Warming – Causes and Consequences



Global Warming – Causes and Consequences

What is the Greenhouse Effect?

The earth’s atmosphere is like the roof of a greenhouse; it allows the sun’s energy to enter, but prevents some of the energy from radiating back into space. The atmosphere absorbs much of the sun’s energy and controls the temperature of our planet. Without the greenhouse gases, the Earth’s surface temperature would be about 0°F. The “greenhouse effect” refers to the fact that this blanket is getting “thicker” because we are increasing the concentration of gases in the atmosphere. As a consequence, more of the sun’s energy is being absorbed by the atmosphere and re-radiating back to Earth, thus increasing the temperature of our planet.

What are the Greenhouse Gases?

Water Vapor – the #1 greenhouse gas. This is a surprise to most people, because we keep hearing about CO2. Global warming is likely to cause more water to evaporate from the surface and more water to be absorbed by the warmer atmosphere. However, we don’t really know what the consequences will be of an increase in atmospheric water vapor. Will the increase in water vapor increase temperatures (a positive feedback loop)? Or, will the increase in water vapor lead to more clouds to reflect incoming solar radiation, thus preventing it from reaching the surface?

Carbon Dioxide – the #2 greenhouse gas. CO2 is naturally released into the atmosphere as a result of respiration by living organisms, and by geological events such as volcanic eruptions. However, these processes have always gone on and are not the cause of the current level of global warming. What has changed recently is the production of CO2 by humans through burning coal, oil, natural gas, and wood. CO2 is responsible for about 62% of human-made greenhouse gases.

The increase in CO2 is unequivocal. The figure below shows the CO2 concentration measured at Mauna Loa, Hawaii, and represents the globally mixed concentration of CO2.

Other greenhouse gases:

• Methane (CH4)

o an extremely effective absorber of radiation

o lasts only about 10-12 years in the atmosphere

o released as part of biological processes in low oxygen environments (e.g., swamps, rice paddies)

o growing rice, raising cattle, using natural gas, and mining coal have caused an increase in methane from 1.52 ppmv in 1978 to the current concentration of about 1.77 ppmv; it has not risen much since around 1990

o Is responsible for about 19% of human-produced greenhouse gases

• Tropospheric Ozone (O3)

o UV radiation and oxygen interact to form ozone in the stratosphere, forming the ozone layer

o has increased due to emissions from automobiles, pollution from factories, and burning of vegetation.

o concentrations have risen by around 30% since the pre-industrial era

• Nitrous Oxide (N2O)

o produced by microbial processes in soil and water, fossil fuel burning, nylon production, nitric acid production, and car emissions

o produced by interactions between microbes and nitrogenous fertilizers

• Chlorofluorocarbons (CFCs)

o CFCs are produced exclusively by humans as refrigerants, aerosol propellants and cleaning solvents

o destroy stratospheric ozone

o because of regulations, levels of CFCs are now remaining level or declining

Is our Planet Getting Warmer?

The annual mean global temperature

anomalies over the period 1880-2001. The

zero line represents the long term mean

temperature from 1880-2001, and the red and

blue bars are showing annual departures from

that mean.

• Global surface temperatures have increased by about 0.6°C (plus or minus 0.2°C) since the late 19th century.

• Global surface temperatures have increased by about 0.4°F (0.2 to 0.3°C) over the past 25 years.

• 2001 was second only to 1998 in terms of global temperature.

• The trend has been toward increasing temperatures at least since the beginning of the 20th century.

• Land temperatures have greater anomalies than the ocean, which is to be expected since land heats up and cools down faster than water

•

• global warming in not spatially uniform. Some areas (including parts of the southeastern U.S.) have cooled over the last century. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N

• indirect indicators of warming such as borehole temperatures, snow cover, and glacier recession data, are in substantial agreement with the more direct indicators of recent warmth

• See: Changes in glacier length

Arctic sea ice has decreased

Sea-ice in the Antarctic

Where are the Greenhouse Gases Coming From?

In 1987, the base year for many CO2 reduction plans, 20.5 billion tons of carbon dioxide were pumped into the atmosphere. The industrial world, where only one-third of the population lives, produced 80% of that amount. The United States leads all countries in total and per capita CO2 emissions.

[pic]

What are the Consequences of Global Warming?

Enhanced El-Nino Events. It has been hypothesized that warmer global sea surface temperatures can enhance the El Niño phenomenon. El Niños have been more frequent and intense in recent decades. The true connection between global warming and El-Nino events is unknown.

• Projections for changes in El Nino events are still controversial since most models can’t distinguish El Nino events.

• Results from the German Coupled Climate Model (ECHAM) suggest that, under warmer climates, the number of weak El Nino/La Nina events are likely to decrease while the number of stronger events increase.

• The changes in projected intensity of La Ninas is much greater than for El Ninos.

Change in the Water Cycle. There have been many recent changes in the water cycle (e.g., evaporation and precipitation). However, the exact connection between global warming and these changes in not know.

• Overall, land precipitation for the globe has increased by about 2% since 1900.

• Precipitation is expected to increase over the 21st century, particularly at northern mid-high latitudes, though the trends may be more variable in the tropics.

• There has been a general increase in precipitation of about 0.5-1.0% per decade over land in northern mid-high latitudes (except in parts of eastern Russia).

• There has been a decrease of about -0.3% per decade in precipitation during the 20th century over land in sub-tropical latitudes.

• Northern Hemisphere annual snow cover extent has consistently remained below average since 1987, and has decreased by about 10% since 1966 (mostly due to a decrease in spring and summer snowfall over both the Eurasian and North American continents since the mid-1980s). Winter and autumn snow cover extent has shown no significant trend for the northern hemisphere over the same period.

• The general trend of increasing cloud amount over both land and ocean since the early 1980s seems to have reversed in the early 1990s, and total cloud amount over land and oceans now appears to be decreasing.

• More frequent droughts will deplete surface water that supplies cities in many regions. Fresh water reserves in some western states of the USA are now disappearing after many dry years. Winter snowfalls are already 40 to 70 percent lower than normal in some temperate regions. Climate change may aggravate these trends.

• According to the Intergovernmental Panel on Climate Change, up to “3 billion additional people will experience significant decreases in water resources by 2080.”

More Extreme and Variable Climate. On a global scale there is little evidence of sustained trends in climate variability or extremes. However, on a regional scale, there is clear evidence of climate changes (see: Observed Variability and Trends in Extreme Climate Events: A Brief Review).

• In areas where a drought or excessive wetness usually accompanies an El Niño, dry or wet spells have been more intense in recent years. Other than these areas, little evidence is available of changes in drought frequency or intensity.

• In some areas where overall precipitation has increased (i.e., the mid-high northern latitudes), there is evidence of increases in heavy and extreme precipitation events.

• Extra-tropical cyclone activity seems to have increased over the last half of the 20th century in the northern hemisphere, but decreased in the southern hemisphere. It is not clear whether these trends are fluctuations or part of a longer-term trend.

• Tropical storm frequency and intensity show no significant long-term trend.

• Global temperature extremes have been found to exhibit no significant trend in inter-annual variability, but several studies suggest a significant decrease in intra-annual variability. There has been a clear trend to fewer extremely low minimum temperatures in several widely-separated areas in recent decades. Widespread significant changes in extreme high temperature events have not been observed.

• There is some indication of a decrease in day-to-day temperature variability in recent decades.

• There may be more frequent and more violent storms. Hurricanes and cyclones get their energy from the heat in sea water, and warmer oceans might expand storm regions. On the other hand, global warming might reduce the number of mid-latitude storms. Warming of the air will likely be greatest towards the poles. This may reduce the difference in air temperature between the poles and the equator, a primary cause of such storms.

Change in Sea Level. Global mean sea level has been rising at an average rate of 1 to 2 mm/year over the past 100 years, which is significantly larger than the rate averaged over the last several thousand years. Projected increases from 1990-2100 are anywhere from 0.09 to 0.88 meters. A rise in 20 cm predicted by 2030 will affect coastal towns and cities, wetlands, and lowland.

Decrease in Agricultural Productivity. Although some countries may enjoy increased agricultural productivity, overall a worsening of production inside the continents is predicted. Small changes in local and regional crop production are likely to affect global food supplies. The effects of global warming on agriculture are likely to increase as the world’s population continues to grow.

• Fertile delta and coastal regions will lose agricultural land.

• In Egypt, Bangladesh, India, and Chine, about 15% of farmable land will be flooded.

• Grain-producing areas, such as the Great Plains of the United States, may experience more frequent droughts and heat waves.

• In mid-latitude regions, agricultural zones may shift towards the poles by 200 to 300 km for every degree Celsius of warming.

• Irrigation will be difficult in central and southern Europe.

• Long term drought already threatens water supplies in some European and North American cities.

• Wheat, corn, and rice yields may increase in temperate regions, and decline in tropical and subtropical regions.

Fisheries Impact. Many fish species are very sensitive to changes in temperature; thus global warming can be expected to severely impact fish populations and fishery industries throughout the word. For example, warmer water temperatures off of British Columbia are expected to increase migration of salmon northward, leading to a decrease in the British Columbia salmon population.

Increase in Health Problems. An increase in temperature can be expected to increase health problems of urban dwellers. In many cities, an increase in temperature means an increase in pollution and associated problems.

• Malaria transmission is likely to increase worldwide; people in central Asia, North

America, and northern Europe will be at greater risk as the distribution of the malarial parasite and its vector shifts. However, transmission will be limited to a shorter time period.

• Reductions in crop yields due to global warming may increase the number of malnourished people; the Hadley Centre model shows an increase of 70-125 million people at risk of malnourishment by the end of the century.

Salinization Problems. A rise in sea level may cause salt water intrusion into aquifers and streams. A 30 cm rise in sea level would cause salt water to flow 5-15 km further inland.

Erosion Problems. A rise in sea level will cause erosion of coastal areas. In the United States, 20,000 km of coast are likely to be affected.

Forest Impacts. In general, trees won’t be able to adapt quickly to global warming and there will be a significant shift in forest types as global ecozones change.

• Those species with limited distributions will be most affected.

• In the mountains of Europe, over half the trees are already diseased, and climate change is hastening the death of forests.

• High temperatures will increase population levels of many insect pests and pathogens.

• The composition of tropical and boreal forests are likely to change because of changes in rainfall patterns, pest populations, and fires.

• As forests change, uptake of carbon by trees may decrease.

Demographic Changes. As climate changes, there will be major shifts in the distributions of people throughout the world as people look to new sources of water, land, and food.

• There is likely to be a shift in population towards northern cities; if just 1% of the global population in 2025 are affected, 80,000,000 people will move to northern cities.

• Of 18 million refugees in the world today, at least 10 million are environmental refugees.

• A third of the world's population today lives within 60 km of coastlines; rising sea levels will cause millions of people to lose their homes and sources of food.

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