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Climate Change

Climate change is the statistical distribution of weather spanning from decades to millions of years. Generally, it is a change in the average weather or a change in the average distribution of weather events. While there are a number of environmental stressors that can affect global warming, human activity has become the single most important driving force behind rising global temperatures.

The most detrimental human activities that are changing the planet’s climate are:

  • CO2emissions
    • Fossil fuel burning, aerosols and cement manufacturing
  • Ozone depletion
  • Farming
  • Deforestation

Charles Keeling’s groundbreaking observations, which began in the 1950s at the Mauna Loa observatory, brought attention to the negative impact of human activities on the world’s climate. He revealed a long-term trend of increasing CO2 levels in the atmosphere. CO2, a potent greenhouse gas, is only one of a number of man-made molecules that is affecting the Earth’s climate. Others include methane, nitrous oxide, black carbon aerosols, ozone, and the now banned chlorofluorocarbons. However, CO2 is the most detrimental to the world’s climate due to the length of time it remains (many thousands of years) in the atmosphere.

The Science of Climate Change

The Earth’s climate has experienced many periods of extended warming and cooling in the past1. Paleoclimate data, uncovered from deep-ocean sediments and ice cores, have allowed us to examine our atmosphere in detail over the last 420,000 years.

This data confirms that there have been atmospheric warming and cooling periods2 (Fig.1), but also reveal that the causes are subtly but importantly different from those driving climate change today.

Figure 1
Paleoclimate data generated from an Antarctic ice core10 at the Russian Vostok research station reveals cyclical (~100,000 year) oscillations in the global atmosphere for previous 420,000.

On prehistoric Earth, temperature rises preceded both increases in greenhouse gases and sea level rises unlike today where greenhouse gases are now known to be the forcing agent. It is now widely accepted that prior to the industrial era, the most significant climate forcing agents were tectonic forces, the precession or “wobble” of the planet on its axis and eccentricity of the Earth’s orbit about the sun1 (Boxed Text). Changes in insolation (amount of radiation that reaches the Earth’s surface) vary3 as a factor of the two latter forcing agents4. In the past, small increases in temperature resulting from increased insolation, raised atmospheric temperatures slightly. The slightly elevated temperatures caused two slow but positive feedback mechanisms to come into play:

  • Slow melting of high latitude ice, which reduced the Earth’s albedo5 (or “reflectiveness”) and caused the planet to reflect less radiation
  • Warming of the ocean, which led to increases in atmospheric CO2 (as CO2 is less soluble in warmer water). Higher levels of atmospheric CO2 reinforce temperature change by trapping more radiated heat.

It is now known that human greenhouse gas generation and alterations in land use are overwhelming our planet’s natural climate cycles6. Currently, under normal circumstances, Earth should be moving from an interglacial period into a period of cooling. What we are actually seeing, however, are rises in atmospheric temperature at rates that are unprecedented in the history of our planet7. Despite awareness of the problems posed by unchecked production of carbon dioxide, methane, and other greenhouse gases, trends in the amount added to the atmosphere each year are on the increase (Fig.2). So not only are the levels in the atmosphere increasing, but the amount we generate each year too!

Figure 2
Decadal variation (parts per million) in atmospheric CO2 levels calculated from data collected at the Mauna Loa Observatory11 and from air bubbles trapped in Antarctic ice12.

Most is known about carbon dioxide. Accurate direct measurement began in the early 1960s at the Mauna Loa Observatory in Hawaii. Methods for indirect, retrospective atmospheric assessment (such as the analysis of air trapped in Antarctic ice core bubbles) have since been developed to allow us to determine pre-1960 atmospheric CO2 levels2. These data reveal a long pre-industrial period of variation within a narrow range, going back at least 400,000 years with levels of CO2 in the atmosphere oscillating between 200 and 280 parts per million (ppm) by volume.

Current CO2 levels are approaching 390 ppm (Fig.3), are predicted to reach 450 ppm by 2040 and will continue to rise to 750-1000 ppm by the end of this century7. These levels have not been seen for many millions of years and are considered sufficiently dangerous to threaten even human life.

Figure 3
Annual levels of atmospheric CO2 calculated from data collected at the Mauna Loa observatory and from air bubbles trapped in Antarctic ice.

Carbon dioxide production is the most critical to address. This is because once this greenhouse gas is in the atmosphere, it remains there for many tens of thousands of years8. Efforts to develop techniques for carbon sequestration (Boxed Text) have proved fruitless due to an inability to demonstrate that they are sufficiently scaleable or because they are not economically viable. Other greenhouse gases such as methane do not last for very long in the atmosphere. Reduction in methane production levels will therefore lead to a rapid reduction in atmospheric concentrations. Similarly, moves to more efficient, or reduced levels of fossil fuel burning will result in a reduction in black carbon aerosols.

Of primary concern are increases in average global temperatures that result from increases in greenhouse gas levels. These elevated temperatures will lead to increasing sea surface temperatures and accelerated melting of polar ice.

Additionally, increases in the levels of carbon dioxide in the world’s oceans lead to a direct change in ocean chemistry, termed ocean acidification. Ocean acidification has a number of potentially serious consequences for marine organisms, particularly coral reefs9.


References:
  1. Zachos, J., Pagani, M., Sloan, L., Thomas, E. & Billups, K. Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present. Science 292, 686-693 (2001).
  2. Petit, J. R. et al. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399, 429-436 (1999).
  3. Yin, Q. Z. & Berger, A. Insolation and CO2 contribution to the interglacial climate before and after the Mid-Brunhes Event. Nature Geosci. advance online publication.
  4. Imbrie, J. et al. On the Structure and Origin of Major Glaciation Cycles 2. The 100,000-Year Cycle. Paleoceanography 8, 699-735 (1993).
  5. Birchfield, G. E. & Wertman, J. Topography, Albedo-Temperature Feedback, and Climate Sensitivity. Science 219, 284-285 (1983).
  6. Lockwood, M. Recent changes in solar outputs and the global mean surface temperature. III. Analysis of contributions to global mean air surface temperature rise. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science 464, 1387-1404 (2008).
  7. Watson, R. T. et al. Climate Change 2001: Summary for Policymakers. IPCC Third Assessment Report Geneva (2001).
  8. Archer, D. Fate of fossil fuel CO2 in geologic time. J. Geophys. Res. 110, C09S05 (2005).
  9. Orr, J. C. et al. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437, 681-686 (2005).
  10. Petit, J.R., et al., 2001, Vostok Ice Core Data for 420,000 Years, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series 2001-076. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
  11. Dr. Pieter Tans, NOAA/ESRL (www.esrl.noaa.gov/gmd/ccgg/trends/)
  12. D.M. Etheridge, L.P. Steele, R.L. Langenfields, R.J. Francey, J.-M. Barnola & V.I. Morgan (1998). Historical CO2 records from the Law Dome DE08, DE08-2 and DSS ice cores in Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory – DOE, Oak Ridge, Tenn., U.S.A. (http://cdiac.ornl.gov/ftp/trends/co2/ )
Links
• El Niño Southern Oscillation
• Global Impact of Carbon Dioxide
• Ocean Acidification

















Precession and Orbital Eccentricity
Precession is the change in the angle of rotation of the Earth relative to the stars. This gyroscopic ‘wobble’ is caused by the tidal forces exerted upon it by the sun and the moon. These forces vary depending upon the proximity of the earth to the sun and moon. This precession has a periodicity of about 26,000 years.

In addition, the Earth does not follow a circular path around the sun. Therefore at certain times the Earth is closer to the sun than at other times. In addition, this elliptical path is affected by Saturn and Jupiter, and varies very slightly. The measure of this variation from a perfect elliptical path is known as the ‘Eccentricity’. Therefore there are periods when the Earth is much closer to the sun than normal, and other periods when it is further away than is the norm. An approximate period for this effect is 100,000 years.

















Carbon Sequestration
This is the process of capturing or reducing atmospheric carbon dioxide levels. Examples of this include capture of carbon dioxide emissions from power stations. This gas is forced into a liquid under pressure and reduced temperature. This liquid is then injected into rock deep down in the Earth’s crust beneath an impermeable rock layer. The costs of this process have so far proved to be a barrier to implementation. The public is also concerned about the possible side effects of injecting large quantities of carbon dioxide into the Earth’s crust.

Another example is iron fertilization. This involves fertilizing high latitude oceans with iron filings. Iron is a trace element which is essential for biological life. High latitude oceans, particularly, the Southern Ocean, has very low levels but all other components for high level biological productivity. The addition of iron, therefore rapidly stimulates high level plankton growth. Photosynthetic phytoplankton consume carbon dioxide in order to grow. This should therefore theoretically reduce the amount of carbon dioxide in the ocean and in turn the levels in the atmosphere. To date this process has failed to deliver the expected results when scaled up from laboratory experiments.

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