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Greenhouse Gases and Global Warming

The Earth is undergoing an unprecedented period of warming, driven by increasing levels of atmospheric greenhouse gases (Boxed Text). It is now known that this build-up in atmospheric greenhouse gases is largely being driven by human activities such as the burning of fossil fuels, deforestation and agriculture. There are many gases that contribute to the greenhouse effect. Of these the main protagonists are carbon dioxide (CO2) and methane (CH4). Human carbon dioxide production is of particular concern due to the long life (many thousands of years) that this gas has in the atmosphere.

Increases in global temperatures cause a host of environmental problems including melting of high latitude ice, increasing sea surface temperatures (SSTs) and changes to global & local weather patterns.

Global Warming

Melting of high latitude ice

Global warming has led to atmospheric heating of polar regions. The warming of polar regions is higher than in regions close to the equator. Recent studies have demonstrated acceleration in the melting rates of ice in the Arctic, Antarctic, and Greenland1-3. This reduction of ice reduces the earth’s albedo4 (“reflectiveness”), accelerating the process of global warming. In addition, an increase in the area of darker, radiation-absorbing, seawater exacerbates the problem. The formation of ice at the poles drives oceanic thermohaline circulation (Boxed Text). As ice is formed, the remaining water, which is now more saline and dense, sinks. As this dense water sinks, warmer water and air is drawn from the tropics to replace it. Melting ice dilutes the salinity of surface water and may interfere with thermohaline circulation5.

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Thermohaline Circulation
In higher latitudes, closer to the polar ice caps, ice is constantly forming. This is principally driven by snowfall and by freezing of surface waters. When these freeze, salts in the seawater that would normally interfere with the freezing process remain in the unfrozen component. This means that ‘pure’ non-salty water is frozen and the remaining water becomes more saline (salty). The more salty the water is, the denser it is. This denser water descends through the water column towards the seafloor. As it does so, surface water is drawn from lower latitudes to replace it. This water is in turn partially frozen and so the process continues. This is known as ‘thermohaline circulation’ and is the force that drives the Gulf Stream in the North Atlantic drawing warmer water from the Caribbean Sea up to Northern Europe.

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Melting glaciers in higher latitudes result in increases in sea-levels that, coupled with thermal expansion of the oceans6, will increasingly threaten low-lying coastal regions and near-surface organisms such as those that build or exist upon coral reefs.

Increasing sea surface temperatures

The oceans’ long-term exposure to increasing atmospheric temperatures has increased sea surface temperatures (SSTs) and will continue to do so. Increased sea surface temperatures are having, and will continue to have, a number of effects.

  • The ocean will become increasingly stratified (Boxed Text). Higher levels of stratification reduce vertical mixing processes, interfere with natural upwelling that brings nutrients from deeper water and sediments, and results in relatively oligotrophic (low nutrient) surface waters, especially during summer months. The vast majority of primary ocean productivity occurs (Boxed Text, next page) in the upper 100 meters of the ocean - within the photic (light penetrating) zone. Continued reductions in the availability of nutrients required by organisms such as phytoplankton which lie at the base of most marine food webs will impact the entire marine ecosystem.
  • Marine organisms that exist near the sea surface will suffer from a relatively rapid temperature rise in their environment coupled to increasing periods of extreme temperature. Coral bleaching events, for example, are the result of prolonged periods of increased mean SSTs7. These events are increasing in frequency and severity. Pigmented zooxanthellae (coral symbionts), unable to function correctly at these temperatures are expelled by their coral hosts. This results in corals losing their colour (hence the term “bleaching”). More importantly, the loss of zooxanthellae, leads to a loss of an important supply of primary organic nutrients. Corals exposed to long term bleaching will eventually die.


Increasing SSTs and Weather

The most well known global weather pattern is the periodic El Niño Southern Oscillation (ENSO). The ENSO has an oceanic (El Niño/La Niña) and atmospheric (Southern Oscillation) component.

El Niño events are the result of sustained rises in SSTs in the Pacific. The Pacific basin drives global precipitation patterns. During El Niño events, rainfall that normally falls as monsoons in India and Indonesia, now falls in the Americas. Sub-surface waves carry these warmer surface waters towards the Eastern Pacific where they overwhelm the nutrient-rich Humboldt Current and prevent cooler, nutrient-rich, waters from rising to the surface off the western coast of South America. As a result, primary productivity is seriously reduced in this region during El Niño years. A direct effect of this is the crashing of fish stocks. La Niña events occur when SSTs drop below average for a significant period. La Niña events produce broadly opposite effects to El Niño events.

The Southern Oscillation is driven by changes in SSTs, which cause changes in atmospheric pressure. Typically, as warm air rises above heated water, surface areas of low pressure are created (air masses, over cooler waters, descend and increase surface pressures). SSTs of the Eastern and Western Pacific vary during a Southern Oscillation, impacting the direction and strength of trade winds and the location and intensity of rainfall. Rising SSTs clearly have the potential to significantly affect ‘global’ weather patterns8.

Increasing SSTs also have ‘local’ weather effects. One of these effects is the intensification of open-water storm systems that are themselves the result of rising warm air over warm seas. It is predicted for example that the numbers, durations and intensities of hurricanes in the North Atlantic are likely to increase as a result of increased SSTs9- 11. Storm surge and waves that result from open-water storm systems will increase in intensity thereby threatening fragile coral reefs and coastal communities.

References:

  1. Pritchard, H. D. & Vaughan, D. G. Widespread acceleration of tidewater glaciers on the Antarctic Peninsula. J. Geophys. Res. 112, F03S29 (2007).
  2. Pritchard, H. D., Arthern, R. J., Vaughan, D. G. & Edwards, L. A. Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature 461, 971-975 (2009).
  3. Rignot, E. & Kanagaratnam, P. Changes in the Velocity Structure of the Greenland Ice Sheet. Science 311, 986-990 (2006).
  4. Birchfield, G. E. & Wertman, J. Topography, Albedo-Temperature Feedback, and Climate Sensitivity. Science 219, 284-285 (1983).
  5. Stocker, T. F. & Wright, D. G. Rapid transitions of the ocean's deep circulation induced by changes in surface water fluxes. Nature 351, 729-732 (1991).
  6. Clark, P. U. & Huybers, P. Global change: Interglacial and future sea level. Nature 462, 856-857 (2009).
  7. Warner, M. E., Fitt, W. K. & Schmidt, G. W. Damage to photosystem II in symbiotic dinoflagellates: a determinant of coral bleaching. PNAS USA 96, 8007- 8012 (1999).
  8. Ashok, K. & Yamagata, T. Climate change: The El Nino with a difference. Nature 461, 481-484 (2009).
Links
• El Niño Southern Oscillation
• Global Impact of Carbon Dioxide
• Ocean Acidification


The Greenhouse Effect
Greenhouse gases are transparent to high-energy ultra violet radiation from the sun and do not prevent this radiation from reaching the planet’s surface. Radiated heat from the planet, however is in the form of lower energy infra-red radiation. Greenhouse gases are not transparent to this type of energy and readily absorb it. This leads to an energy imbalance with more energy captured by the planet than released (Fig. 1). The result is a build-up of atmospheric temperatures. The higher the concentration of greenhouse gases, the greater the energy imbalance and the quicker the atmosphere heats up.

There are a number of gases and compounds that affect the Earth’s atmosphere. Some of the most important include carbon dioxide, black carbon aerosols (soot), methane, ozone and nitrous oxide. There are also some that act to oppose the greenhouse effect such as sulphates produced by phytoplankton and by volcanoes. These reflect solar radiation back into space by stimulating cloud formation.






























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Ocean Stratification
A thermocline in the ocean is a relatively thin layer across which temperatures change much more rapidly than in the layers above and below. The upper surface of the ocean is in constant contact with the atmosphere and is constantly being heated. Wave and wind action then mixes this surface water to varying depths. This mixing effect varies according to the strength of the wind and waves but is typically no deeper than 100 metres. The temperature of water below this layer remains relatively unaffected by surface activity and so is much colder. As the surface continues to heat, the difference becomes greater. This is known as ‘ocean stratification’. This surface thermocline acts as a barrier to normal mixing processes preventing nutrients from greater depth being brought to the more biologically productive surface waters, leading to relative oligotrophy (lack of nutrients) and a reduction in primary productivity (See Boxed Text).

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Primary Productivity
At the base of the marine food web are the photosynthetic organisms known as phytoplankton. These organisms use sunlight to drive the incorporation of dissolved nutrients in the ocean into complex, long- chained organic molecules. These long-chained molecules are energy-rich and are used by the phytoplankton to carry out many biological processes. This production of complex organic molecules by phytoplankton is known as ‘primary productivity’. They in turn are consumed by organisms higher up the food chain that need these energy- rich molecules. This process continues to the top of the food tree. These complex organic molecules, produced by countless single-celled phytoplankton, therefore underpin the entire marine food web.

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