ScienceWeek

CLIMATOLOGY: ON MODERN GLOBAL CLIMATE CHANGE

The following points are made by T.R. Karl1 and K.E. Trenberth (Science 2003 302:1719):

1) The atmosphere is a global commons that responds to many types of emissions into it, as well as to changes in the surface beneath it. As human balloon flights around the world illustrate, the air over a specific location is typically halfway around the world a week later, making climate change a truly global issue.

2) Planet Earth is habitable because of its location relative to the sun and because of the natural greenhouse effect of its atmosphere. Various atmospheric gases contribute to the greenhouse effect, whose impact in clear skies is 60% from water vapor, 25% from carbon dioxide, 8% from ozone, and the rest from trace gases including methane and nitrous oxide (1). Clouds also have a greenhouse effect. On average, the energy from the Sun received at the top of the Earth's atmosphere amounts to 175 petawatts (PW) (or 175 quadrillion watts), of which 31% is reflected by clouds and from the surface. The rest (120 PW) is absorbed by the atmosphere, land, or ocean and ultimately emitted back to space as infrared radiation (1).

3) Over the past century, infrequent volcanic eruptions of gases and debris into the atmosphere have significantly perturbed these energy flows; however, the resulting cooling has lasted for only a few years (2). Inferred changes in total solar irradiance appear to have increased global mean temperatures by perhaps as much as 0.2 C in the first half of the 20th century, but measured changes in the past 25 years are small (2). Over the past 50 years, human influences have been the dominant detectable influence on climate change (2).

4) The main way in which humans alter global climate is by interference with the natural flows of energy through changes in atmospheric composition, not by the actual generation of heat in energy usage. On a global scale, even a 1% change in the energy flows, which is the order of the estimated change to date (2), dominates all other direct influences humans have on climate. For example, an energy output of just one PW is equivalent to that of a million power stations of 1000-MW capacity, among the largest in the world. Total human energy use is about a factor of 9000 less than the natural flow (3).

5) Global changes in atmospheric composition occur from anthropogenic emissions of greenhouse gases, such as carbon dioxide that results from the burning of fossil fuels and methane and nitrous oxide from multiple human activities. Because these gases have long (decades to centuries) atmospheric lifetimes, the result is an accumulation in the atmosphere and a buildup in concentrations that are clearly shown both by instrumental observations of air samples since 1958 and in bubbles of air trapped in ice cores before then. Moreover, these gases are well distributed in the atmosphere across the globe, simplifying a global monitoring strategy. Carbon dioxide has increased 31% since preindustrial times, from 280 parts per million by volume (ppmv) to more than 370 ppmv today, and half of the increase has been since 1965 (4). The greenhouse gases trap outgoing radiation from the Earth to space, creating a warming of the planet.(5)

6) In summary: Modern climate change is dominated by human influences, which are now large enough to exceed the bounds of natural variability. The main source of global climate change is human-induced changes in atmospheric composition. These perturbations primarily result from emissions associated with energy use, but on local and regional scales, urbanization and land use changes are also important. Although there has been progress in monitoring and understanding climate change, there remain many scientific, technical, and institutional impediments to precisely planning for, adapting to, and mitigating the effects of climate change. There is still considerable uncertainty about the rates of change that can be expected, but it is clear that these changes will be increasingly manifested in important and tangible ways, such as changes in extremes of temperature and precipitation, decreases in seasonal and perennial snow and ice extent, and sea level rise. Anthropogenic climate change is now likely to continue for many centuries. The authors suggest we are venturing into the unknown with climate, and its associated impacts could be quite disruptive.

References (abridged):

1. J. T. Kiehl, K. E. Trenberth, Bull. Am. Meteorol. Soc. 78, 197 (1997)

2. J. T. Houghton et al., Eds., Climate Change 2001: The Scientific Basis (Cambridge Univ. Press, Cambridge, 2001) Available at www.ipcc.ch/

3. R. J. Cicerone, Proc. Natl. Acad. Sci. U.S.A. 100, 10304 (2000)

4. Atmospheric CO2 concentrations from air samples and from ice cores are available at http://cdiac.esd.ornl.gov/trends/co2/sio-mlo.htm and http://cdiac.esd.ornl.gov/trends/co2/siple.htm respectively.

5. M. Sato et al., Proc. Natl. Acad. Sci. U.S.A. 100, 6319 (2003)

Science http://www.sciencemag.org

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ON GLOBAL CLIMATE CHANGE AND HEALTH

The following points are made by J.A. Patz and M. Khaliq (J. Am. Med. Assoc. 2002 287:2283):

1) Global climate change is expected to have broad health impacts.[1] If current warming trends continue, heat waves, floods, and droughts and their attendant physical effects are likely to become more frequent and severe. Warmer air temperatures can influence the concentration of regional air pollutants and aeroallergens. Less direct health impacts may result from the disruption of ecosystems and of water and food supplies, which in turn could affect infectious disease incidence and nutritional status. Finally, sea-level rise could lead to major population displacement and economic disruption.

2) Human activities related primarily to the burning of fossil fuels and changes in land cover such as deforestation are changing the concentration of atmospheric constituents or properties of the earth's surface that help to absorb or scatter radiant energy.[2] Since the preindustrial mid-1800s, increases in concentrations of three major greenhouse gases, carbon dioxide, methane, and nitrous oxide, have exceeded past changes that occurred over the last 10 000 years; carbon dioxide alone has increased by 30% since the late 1800s.[1] Warmer air, such as that resulting from the greenhouse effect, can hold more moisture and more quickly evaporate surface water, thereby increasing the frequency of severe storms, floods, and droughts.[1]

3) According to the United Nations Intergovernmental Panel on Climate Change (IPCC), "An increasing body of observations gives a collective picture of a warming world and other changes in the climate system.[3] During the 20th century, global average surface temperature increased about 0.6 degrees C, global average sea level rose 10 cm to 20 cm, and snow and ice cover decreased.[2] The latest IPCC report predicts that if current trends continue, sea level rise will rise 45 cm and global temperatures will increase by 3 degrees C by the year 2100.[3]

4) Small changes in global mean temperatures can produce relatively large changes in the frequency of extreme temperatures.[2] Mortality rates increase at both hot and cold extremes of temperature.[4] Increases in temperature have a direct and substantial impact on excess mortality for elderly individuals and individuals with pre-existing illnesses. Much of the mortality attributable to heat waves is a result of cardiovascular, cerebrovascular, and respiratory disease.[5] A 1995 heat wave in Chicago that caused 514 heat-related deaths (12 per 100 000 population) may be part of a recent trend of longer, more frequent heat waves and record-setting temperatures. Long-term global warming trends are further exacerbated by the "heat island" effect, whereby high concentrations of heat-retaining surfaces such as asphalt and tar roofs sustain higher temperatures through the night. Heat waves also have the secondary effect of worsening urban air pollution. Ozone, which forms chemically from precursor pollutants, is the most temperature-dependent air pollutant and may contribute to the development of asthma in children.

References (abridged):

1. Patz JA, Engelberg D, Last J. The effects of changing weather on public health. Ann Rev Public Health. 2000;21:271-307.

2. Intergovernmental Panel on Climate Change (IPCC). Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the IPCC. Houghton J, Ding Y, Griggs M, et al, eds. Cambridge, England: Cambridge University Press; 2001.

3. McMichael A. Human health. In: IPCC Working Group II, ed. Climate Change 2001: Impacts, Adaptation, and Vulnerability. Cambridge, England: Cambridge University Press; 2001:453-485.

4. Curriero FC, Heiner KS, Samet JM, et al. Temperature and mortality in 11 cities of the eastern United States. Am J Epidemiol. 2002;155:80-87.

5. Kilbourne E. Heat waves. In: Noji E, ed. The Public Health Consequences of Disasters. New York, NY: Oxford University Press; 1997:51-61.

J. Am. Med. Assoc. http://www.jama.com

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