ScienceWeek

SCIENCE POLICY: ON THE REALITY OF CLIMATE CHANGE

The following points are made by David A. King (Science 2004 302:176):

1) Climate change is real, and the causal link to increased greenhouse emissions is now well established. Globally, the ten hottest years on record have occurred since 1991, and in the past century, temperatures have risen by about 0.6 C (1). In that same period, global sea level has risen by about 20 cm -- partly from melting of land ice and partly from thermal expansion of the oceans. Ice caps are disappearing from many mountain peaks (2,3), and summer and autumn Arctic sea ice has thinned by up to 40% in recent decades, although there is some evidence for stabilization (4,5).

2) In Britain, usage of the Thames Barrier, which protects London from flooding down the Thames Estuary, has increased from less than once a year in the 1980s to an average of more than six times a year. This is a clear measure of increased frequency of high storm surges around North Sea coasts, combined with high flood levels in the River Thames. Last year, Europe experienced an unprecedented heat wave, France alone bearing around 15,000 excess or premature fatalities as a consequence. Although this was clearly an extreme event, when average temperatures are rising, extreme temperature events become more frequent and more serious. The author suggests that climate change is the most severe problem that we are facing today -- more serious even than the threat of terrorism.

3) Some climate change can always be attributed to natural cycles and disturbances in the Earth's climate system, but we cannot explain the general warming trend over the last century without invoking human-induced effects. For instance, researchers from the United Kingdom's Hadley Centre modeled the effects on climate of such factors as volcanic eruptions and changes in solar output and compared these with the effects of additional greenhouse gases emitted through the burning of fossil fuels, land-use change, and industrial processes. Only the forcing from increasing greenhouse gas and aerosol concentrations could explain the general upward trend in temperature over the past 150 years.

4) In less than 200 years, human activity has increased the atmospheric concentration of greenhouse gases by some 50% relative to preindustrial levels (1). At about 372 ppm, today's atmospheric carbon dioxide level is higher than at any time in at least the past 420,000 years. Owing to the inertia of the climate system, it is already too late to stop any further warming from occurring. However, if we could stabilize the atmosphere's carbon dioxide concentration at some realistically achievable and relatively low level, there is still a good chance of mitigating the worst effects of climate change. For instance, current models suggest that stabilizing carbon dioxide levels at around 550 ppm by 2100 could reduce flooding frequency by some 80 to 90% along the most vulnerable parts of the Indian and Bangladesh coastlines, as compared with a scenario of continuing growth in consumption of fossil fuels.

5) Climate change is no respecter of national boundaries. The UK is attempting to show leadership, and many other countries, including some of European partners, are also in the vanguard. But they cannot solve the problem in isolation. The UK is responsible for only approximately 2% of world's emissions, the US for more than 20% (although it contains only 4% of world's population). The US is already in the forefront of the science and technology of global change, and the next step is surely to tackle emissions control too. We can only overcome this challenge by facing it together, shoulder to shoulder. The countries in the rest of the world are now looking to the US to play its leading part.

References (abridged):

1. C. K. Folland et al., in Climate Change 2001: The Scientific Basis, J. T. Houghton et al., Eds. (Contribution of Working Group I to the IPCC Third Assessment Report, Cambridge Univ. Press, Cambridge, 2001), pp. 99-181

2. L. G. Thompson, et al., Science 298, 589 (2003)

3. E. Righot et al., Science 302, 434 (2003)

4. Wadhams, Ice in the Ocean. Gordon & Breach (now Taylor & Francis), London, 1997.

5. S. Laxon et al., Nature 425, 947 (2003)

Science http://www.sciencemag.org

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CLIMATOLOGY: ON MODERN GLOBAL CLIMATE CHANGE

The following points are made by T.R. Karl 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)

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