The statement by the Intergovernmental Panel on Climate Change in late 1995 that "...the balance of evidence suggests that there is a discernible human influence on global climate," (IPCC, 1995) set off a firestorm of debate in the economic and political circles on the validity, impacts, and societal responses to climate change. Climate change and climate variability have been thrust further into the public debate with recent attention to El Nino and emerging projections of global warming, culminating with the recent discussions and accord in Kyoto Japan (COP-3, 1997). The process that led to the IPCC statement was a multi-national, multi-disciplinary compilation of climate change science by over 400 contributing authors from 26 countries, reviewed by over 500 scientists from 40 countries, and approved at a meeting in Madrid in November 1995 by 177 delegates from 96 countries. That consensus (although not universal) agreement was achieved is a remarkable result. However, a few climate scientists have raised a voice of opposition to the consensus that warming is occurring and that humans are partially responsible. As a result, the business community, policy makers, and the general public have been receiving conflicting messages about the level of uncertainty on various elements of the global warming debate.
The present paper seeks to clarify this issue by summarizing the facts, projections, and uncertainties relating to global climate change. A few myths that have arisen will be identified, and some impediments to further progress in narrowing uncertainties will be outlined. The goal of this paper is to clarify the confidence and uncertainty of current scientific understanding on global climate change so that the debate on this issue can focus on the economic, political, and ethical issues such as responsibility of developed countries for past emissions and the responsibility of the present generation to allow future generations equal access to earth resources to meet their societal needs.
In a recent article in Science magazine (Mahlman, 1997), Dr. J. D. Mahlman senior climate researcher at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration in Princeton, NJ outlined the facts and uncertainties of our understanding about the global climate system and the role of humans in global warming. The following statements are drawn largely from points made in his article.
Climate Change Facts and Uncertainties
What do we know with certainty about the climate system and its future?
The following items are based primarily on observations of the global climate and do not depend on models of the global climate. The scientific community is in broad agreement on these issues, even those who are skeptical about future global warming.
* Water vapor is the most abundant greenhouse gas, but its concentrations are closely related to global temperatures and are relatively constant. Atmospheric concentrations of other greenhouse gases are increasing due to human activity. Carbon dioxide concentration is rising due primarily to the burning of fossil fuels and deforestation. Atmospheric methane concentrations are increasing due at least in part to natural emissions from wetlands, termites, and oceans being augmented by emissions from ruminant animals, rice paddies, natural gas operations, and landfills (considerable uncertainty exists in quantifying sources of methane, but the rise in total atmospheric methane concentration is indisputable). Concentrations of nitrous oxide are increasing, in part due to continued use of nitrogen fertilizer. Most chlorofluorocarbons have no natural sources.
* Greenhouse gases keep a planet warmer than it otherwise would be. If the Earth had no greenhouse gases, the surface temperature would be about 50oF colder than at present.
* Periods of high concentrations of CO2 in past history of the Earth have been accompanied by high temperatures at the Earths surface, and periods of low carbon dioxide had relatively low surface temperatures.
* Greenhouse gases have long lifetimes, ranging from 10 years for methane to about 100 for carbon dioxide to 150 years for nitrous oxide. Anthropogenic increases in these gases will influence the earths climate for many centuries. The climate system has high inertia, primarily due to the long time scales of ocean dynamical processes.
* Sulfate particles that form from sulfur dioxide emitted primarily by the burning of coal contribute to local cooling although the magnitude is uncertain.
* Decreases of ozone in the lower stratosphere have contributed to cooling effects in that region.
* Anthropogenic CO2 increases together with decrease in stratospheric ozone have cooled the stratosphere by more than 1oC (1.8oF). This result is consistent with model projections.
* Over the past 100 years, the Earths surface has warmed by about 0.5oC (+/- 0.2oC).
* Natural variability of climate makes detection of anthropogenic contributions difficult. There may be interactions between human and natural contributions to warming.
* Significant reduction of uncertainties relating to effects of clouds, water vapor, ice, ocean circulation, and regional differences likely will require a decade or more.
What do we know from climate predictions with a confidence of at least 99%?
* The stratosphere will continue to cool as CO2 concentrations continue to rise. Ozone depletion will add to the cooling.
* Water vapor in the lower troposphere (0-3 km) will increase about 6% for every 1oC of warming. Relative humidities will stay approximately the same.
What do we know from climate predictions with a confidence of at least 90%?
* The warming of the last century is consistent with model projections of global warming due to CO2 modified by the regional cooling effect of sulfate particles.
* Doubling of CO2 over pre-industrial levels (likely to occur in the later half of the 21st century unless emissions are significantly reduced) is projected to lead to a global warming of 1.5 to 4.5oC (2 - 8oF).
* A quadrupling of CO2 , if it should occur, will lead to warming of twice this amount.
* By 2100, under reasonable assumption on CO2 increases, we can expect temperature increase of 1.5oC to 5oC.
* Sea-level rise is most likely to be 50 (+/-25) cm by year 2100 with continued rise beyond that time highly likely. Continued high (quadrupled) CO2 could lead to 2+/- m rise in sea level.
* Global mean precipitation will increase at 2 (+/-0.5)% per 1oC of warming.
* By 2050, the higher latitudes of the Northern Hemisphere will experience temperature increases well above the global average. Significant precipitation increases are likely in the higher latitudes of the Northern Hemisphere.
What additional projections can be made with a confidence of at least 2 out of 3?
* Mid-latitude continents of the Northern Hemisphere will experience decreased soil moisture
* Little change in temperature is expected in the region of the South Pole and Antarctica
* Precipitation increases at high latitudes will reduce salinity and slow global ocean circulation
* Tropical storms will tend to be more intense
* Variance of temperature is likely to be similar under global warming compared to today. This means that higher averages coupled with similar variability will lead to higher incidence of heat waves and periods of plant stress and lower incidence of cold episodes.
What popular assertions abut global warming have no credible basis?
* Is it likely there will be more frequent storms, hurricanes, typhoons? This may seem physically plausible, but there is no model evidence to support it.
* Will mid-latitude cyclones become stronger leading to stronger winds? There is no research evidence to support this.
* Will higher surface temperatures lead to melting of the South Polar ice cap and contribute to sea-level rise? Higher temperatures in the polar region lead to higher saturation vapor pressures and therefore higher absolute humidity. Because temperatures over the South Polar land mass is so far below the melting point, a few degrees of warming will not increase melting but will increase snowfall. Even if the projections of little temperature rise over Antarctica are wrong, warming likely will increase, not decrease, ice accumulation over the South Polar region.
What scientific studies call into question the concept of global warming?
While the overwhelming body of evidence (from instrumental surface temperature records, retreat of mountain glaciers, borehole temperature trends, bleaching of coral reefs) suggest a warming, there are two studies that raise some doubts. The first is the satellite temperature measurements since 1979 reported by Spencer and Christy (see Christy et al, 1995), which show a warming but only half the value of surface measurements over this time. A statistical study (Friis-Christensen, E. and K. Lassen, 1991) suggests changes in the cycle of seasons are responsible for the global temperature changes over the last 100 years, although no link to radiative forcing is presented to identify the physical processes. These studies require further investigation, but they do not at present offer substantial challenge to the evidence that global warming is a reality (Houghton et al, 1996).
Some who doubt that recent warming is of anthropogenic origin contend that much of the warming over the last 130 years was early in the present century before the level of atmospheric carbon dioxide began its rapid rise, and therefore must be due to natural variability. Modelers counter that when the effects of sulfates (which are formed from sulfur dioxide emitted in coal combustion and lead to cloud brightening and hence cooling effects) are included, the observed warming is very likely anthropogenic.
What are the major impediments to reducing uncertainty?
Reconciliation between observations of the changes in climate and model results of change in climate rest on advances in the following areas:
* the role of clouds in the climate system
* the role of the biosphere in the climate system
* regional effects of global warming
* lack of computing power to resolve these uncertainties.
Reducing Uncertainty in Projections of Future Regional Climate
Economic, political, and social impacts of climate change are felt most acutely at a regional (continental or subcontinental) scale, rather than the global scale. Under any scenario of global climate change, some particular regions will experience extreme excursions from global means. A program at Iowa State University initiated under the International Institute of Theoretical and Applied Physics and involving faculty from the Departments of Agronomy and Geological and Atmospheric Sciences seeks is addressing regional implications of global climate change, with initial emphasis on the US Midwest.
* Under funding from the Electric Power Research Institute (EPRI, 1998), we are carrying out a systematic intercomparison of major regional climate models used throughout the world for climate assessment (Takle, 1995; PIRCS, 1998). The first experiments are exploring capabilities of models for prediction of summer climate for a drought year (1988) and a flood year (1993).
* We also are preparing long-term, high-resolution simulations of climate for the Central US for present greenhouse gas concentrations and future concentrations of both greenhouse gases (that lead to warming) and sulfate aerosols (that lead to regional cooling). These simulations will be used for follow-on studies of potential impacts to agriculture, water resources, human health, and extreme events resulting from global climate change.
Controversial issues in the Kyoto Protocol
The UN Framework Convention on Climate Change Third Conference of the Parties (COP-3, 1997), held in Kyoto Japan in early December 1997 and attended by ministers and other high-level officials from 160 countries ended with an agreement on reductions of greenhouse gas emissions:
* Industrialized countries are required to reduce their collective emissions of greenhouse gases by 5.2% from 1990 levels. Compared to the emissions levels that would be expected by 2010 without emissions-control measures, the Protocol target represents a 30% cut (COP-3, 1997).
* The US is called on to reduce its emissions 7% below 1990 levels. But since emissions have risen since 1990, actual cuts from current (1998) levels are much higher.
* Developing countries are exempt from having to implement emissions reductions.
The accord encourages countries to reduce emissions by improving energy efficiency, reforming energy and transportation sectors, protecting forests and other biomes that sequester carbon, promoting renewable forms of energy, limiting methane emissions from waste management and energy systems, and phasing out fiscal measures and market incentives that are counter to reducing emissions.
This paper has presented a summary of facts and uncertainties relating to global climate change. As the debate on climate change intensifies, it will be necessary to continue to clarify the level of uncertainty of the various scientific issues so that the economic, political, and ethical alternatives can be evaluated in the most rational manner.
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Friis-Christensen, E. and K. Lassen, 1991: Length of the solar cycle: an indicator of solar activity closely associated with climate. Science 254, 698-700.
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