The greenhouse effect is not controversial. In fact, according to Stephen H. Schneider of the National Center for Atmospheric Research in Boulder, Colorado, the phenomenon is "one of the best-established theories in atmospheric science." At least a century ago, Schneider says, scientists knew that while carbon dioxide (CO2) and other "greenhouse gases" freely allow solar radiation to enter a planet's atmosphere, these gases permit only a portion of the infrared radiation produced to escape back to space. The greenhouse effect explains, for example, why Venus, which has a dense CO2 atmosphere, is very hot; why Mars, with only a thin CO2 layer, is ice-cold; and why temperatures on Earth have been ideally suited for plant and animal life.
Scientists have also long known that human activities, particularly the burning of fossil fuels, are artificially increasing the volume of greenhouse gases in the earth's atmosphere and that this increase eventually will make the planet the hottest it has been in human history. What remains controversial about the greenhouse effect, says Schneider, is the rate of this global warming, its regional distribution, and most of all, what to do about the problem.
Schneider spoke at a recent conference, the First North American Conference on Preparing for Climate Change: A Cooperative Approach. The meeting addressed two problems - the greenhouse effect and stratospheric ozone depletion.  John C. Topping, Jr., president of the Washington, DC - based Climate Institute, which organized the conference, said that the meeting was the largest gathering ever on the subject of adapting to climate change. More importantly, he said, it was the first time that climatologists and "climate impact scholars" (those who study the effects of climate change) had met with a broad array of policy makers, including representatives of state and federal government; electric utilities; chemical, oil, and gas industries; forestry and agriculture; automobile makers; and leading environmental organizations.
Most participants left the meeting feeling optimistic, Topping said, because the program focused on specific actions to address a problem far too often viewed fatalistically. He said, "People came away seeing that they could divide the issue into chewable pieces and start working on those pieces right now."
As far back as the turn of the century, a few scientists had already begun to worry that massive amounts of CO2 being released as a result of the Industrial Revolution would change the world's climate.  In the 1950s, these concerns were supported by measurements demonstrating that atmospheric CO2 was in fact increasing. In recent years, both data and worries have proliferated, coming to a head in 1985 when a distinguished international group of scientists issued dire warnings following a conference sponsored by the International Council of Scientific Unions, the World Meteorological Organization, and the United Nations Environment Programme.
At a hearing the following year before the Senate Committee on Environment and Public Works, Wallace S. Broecker of Columbia University in New York City echoed the concerns of many scientists. "The inhabitants of planet earth are quietly conducting a gigantic environmental experiment," Broecker said. "So vast and so sweeping will be the impacts of this experiment that, were it brought before any responsible council for approval, it would be firmly rejected as having potentially dangerous consequences." 
Levels of CO2 have already increased approximately 25% since 1900. Today most atmospheric scientists agree that, even if fossil
fuel emissions are reduced somewhat, CO2 levels will double by the second half of the 21st century. This doubling will powerfully affect not only global temperatures, but also other physical phenomena, including rainfall, winds, ocean currents, sea level, and storm patterns.
Because the effects of doubled CO2 cannot be studied directly, researchers have relied on global climate models - mathematical representations of the atmosphere that simulate, on computers, climate change under various scenarios. These models generally predict that a doubling of CO2 levels will cause global mean temperatures to rise between 2 and 5 deg C.
One of the most widely used climate models was developed at the Goddard Institute for Space Studies (GISS) in New York City. According to James E. Hanson, one of the creators of the GISS model, support for the predicted 2-5 deg C warming comes from empirical as well as theoretical evidence. For example, he said, recently developed paleoclimate records - which show fluctuations in atmospheric CO2 over the past 100,000 years and correlate these fluctuations with various climate parameters - have provided "remarkable confirmation" of the models' predictions.
Still, many uncertainties remain. Oceans, for example, have a tremendous capacity to store heat. Their ability to delay global warming, while considered in models, remains somewhat uncertain. Clouds are another important yet unpredictable variable. Changes in global temperature and precipitation patterns will certainly alter the number, distribution, and kinds of clouds, yet the precise changes and how they will affect climate are unknown. There also are many uncertainties concerning human behavior, said Schneider. These include population growth, per capita fossil fuel consumption, deforestation and reforestation rates, and the possibility of new technology to mitigate CO2 buildup.
Despite these uncertainties, Schneider says, "there is no disagreement that large climatic changes are highly probable and at rates that are fast relative to rates that have caused significant ecological changes in the past." These changes will dramatically affect both human and natural systems, threatening such crucial activities as agriculture, forestry, fisheries, pollution control, and the protection of parks and preserves. At the conference, scientific panels considered the effects of climate change on these and more than a dozen other activities. It was clear that researchers are just beginning to understand the effects of greenhouse climate changes; it will be many years at least before they are able to tell society how to prepare for those changes.
Perhaps more than any other human activity, agriculture depends on and is extremely vulnerable to climate. Food crops respond strongly to changes in not only temperature, but also precipitation amounts and patterns, winds, and storm frequency. CO2 by itself directly affects crops by increasing their photosynthetic rate. Most experts agree, however, that increased yields resulting from this phenomenon would be insufficient to offset decreases from CO2-created climate changes, particularly higher temperatures (which increase plant respiration rates) and reduced water availability. Members of a conference panel looking into the effects of greenhouse climate change on North American agriculture did not agree on how damaging those effects would be.
Using the GISS global climate model, R. B. Stewart of Agriculture Canada in Ottawa has studied the effects of climate change on spring wheat production in Saskatchewan. He reported that climate changes predicted by the model would reduce wheat production by 6% - 28% annually. More importantly, Stewart said, the greenhouse effect is likely to increase both the frequency and severity of droughts. Although farmers may be able to switch to less drought-sensitive crops, the implications for the Canadian economy are serious. Spring wheat is Canada's most important grain crop, bringing $2.96 billion in cash receipts to the country's prairie regions. Global warming "could have major repercussions for both prairie agriculture and the Canadian economy in general," said Stewart.
Basing her studies on the GISS model also, Louise Arthur of the University of Manitoba painted a more optimistic picture. Doubling the CO2 levels would bring about climate changes, Arthur agreed, but "even if those changes were high right now, we have the technology to adapt."
Daniel Dudek of the Environmental Defense Fund in New York was most concerned about water availability. "The issue is not whether we're going to starve," he said. Yet increased summer dryness in the western United States - predicted by some of the models - could boost irrigation water demand by 30%. "It would not be possible to meet this demand without new technology or new water management policies," Dudek said.
Despite a lack of consensus on the effects of climate change, panelists took a first pass at adaptation strategies. They predictably recommended more research, with a focus on variables such as heat waves, frost, drought, and violent storms that are expected to be more significant than average temperatures in determining agricultural yields. Some panelists urged plant breeding for tolerance to heat and drought stress. "If climate change comes rapidly, and it might, we'll need a wide range of new cultivars," said Norman Rosenberg of Resources for the Future in Washington, DC.
Greenhouse warming also will affect commercially important tree species. "Trees, being both stationary and longlived, would be particularly vulnerable to climate changes," said Carl H. Winget of the Canadian Forestry Service in Quebec. Moreover, because many commercially valuable species occur in unmanaged ecosystems, "we'll be forced to accept the changes imposed on us," he said.
Winget expressed particular concern about secondary effects of global warming such as changes in fire incidence and the hardiness of pests, including the spruce budworm and gypsy moth, whose destructive effects are now limited in part by temperature and moisture. Pests and disease-causing microorganisms will adapt to climate change faster than trees; other stresses such as air pollution and acid precipitation could exacerbate the damage. Winget deplored the uncertainties that make today's management strategies "purely speculative." Nevertheless, he said, "the need to plan is urgent even at minimal rates of climate change." Winget proposed as a first step that forest managers consider climate change when preparing management plans and that genetic improvement programs focus more on "selecting stock having reasonably rapid growth over a broad range of conditions" than on maximal growth under ideal conditions.
The effects of greenhouse climate change on fish are more uncertain.  All physical changes predicted for oceans in the models have the potential to dramatically influence fish and thus commercial fisheries. These potential physical changes include, in addition to surface warming, alterations in global circulation patterns, storms, upwelling, salinity, pH, turbulence, and the amount and distribution of sea ice. Shifts in species ranges resulting from such changes could create new community associations and either enhance or limit access by commercial fishermen. Moreover, fish, even more than trees, inhabit basically unmanageable ecosystems. "We'll be mere spectators in the adjustments to climate change," said Jim Titus of the US Environmental Protection Agency (EPA) in Washington, DC. "People will have to adapt to whatever the fish decide to do."
Most marine ecosystems are less well understood than terrestrial systems. Skip Livingston of Florida State University in Tallahassee asked, "How can we predict climate-change effects on ecosystems we know so little about now?" Thomas Sibley of the University of Washington in Seattle agreed. Scheduled to speak on the "Impact of climate change on North Pacific and Alaskan fisheries," Sibley said he was "surprised to see the title of my talk. We're nowhere near being able to say what those effects are." Nonetheless, he ventured several "guesses," including geographic shifts of many commercially valuable species toward the poles. These shifts could benefit some northern fishing economies at the expense of southern regions, Sibley said.
A few researchers have attempted laboratory studies of the impact of particular climate change predictions on fish. Robert C. Worrest of Oregon State University in Corvallis, for example, has studied effects of increased ultraviolet radiation (UVB), a consequence of the current thinning of the earth's ozone layer. Worrest reported that larval forms of fish and shellfish will be most vulnerable because they inhabit surface layers where the impact of higher UVB will be greatest. In his research, Worrest has found direct UVB damage to phytoplankton, copepods and other primary consumers, and larvae of several fin and shellfish species. The indirect results, he said, could be "severe economic losses" to fishing economies.
One of the most troubling likely consequences of global warming is sealevel rise, which would result from the expansion of upper layers of the ocean and from the melting of mountain and polar glaciers. EPA's Titus said that sea level will probably rise 50 - 200 centimeters globally by sometime in the 21st century. This rise could produce inundation of wetlands, which are nurseries for most commercially important fish species; erosion; increased flooding; and higher salinity and pollution levels. Titus says that if sea level rises 200 centimeters, 80% of US coastal wetlands could be lost.
Edward LaRoe of the US Fish and Wildlife Service in Washington, DC, disputed Titus's predictions. "The issue of sea-level rise and wetlands loss has been marked by conjecture and anecdotal evidence," he said. Most scenarios have been based on wetlands losses in coastal Louisiana, which has lost about half of its freshwater wetlands over the past two decades. Although sea-level rise is one explanation for the loss, said LaRoe, the most important causes have been human interference, including oil and gas development, which has caused land to subside, and an extensive system of levees along the Mississippi River that have prevented sediments from replacing those naturally lost at the river's mouth. Louisiana's marshes are different from those found in most US coastal areas, said LaRoe, "and there is a great deal contributing to their loss other than sea-level rise."
Predictions about how natural ecosystems might respond to greenhouse climate changes have come primarily from observations of present plant and animal distributions - and how they correspond to temperature and moisture patterns - and from studies relating species distributions determined from fossil pollen grains and animal bones to climatic conditions of the past. "These kinds of observations tell us that plants and animals are very sensitive to climate," said Robert L. Peters of the Washington, DC based World Wildlife Fund and The Conservation Foundation. Peters, who led a conference panel on the "Likely effect of climate change on biological diversity," said that "if global warming occurs during the next 50 years, it is likely to change the ranges of many species, disrupt natural communities, and possibly cause species extinctions."
From an ecological viewpoint, an average change of just a few degrees is "tremendous," said Peters. "It would create conditions that the natural biota has not had to contend with in 100,000 years." For example, during the last ice age, when much of North America was completely covered with ice, the earth was only about 5 deg C colder than it is today. Future climate changes will be more stressful to species than in the past, said Peters, because the changes will be large and will occur very quickly, and because human development has fragmented species into small, vulnerable populations. Species will be facing - in addition to temperature increases - changes in rainfall patterns, soil chemistry, sea level, and community composition. "The most optimistic thing that could be said about the future of natural systems under a regime of warming climate," said Peters, "is that a great deal of rearrangement would occur."
Because most species will have a tendency to migrate northward, eventually squeezing out those at the top, and because global warming will be most extreme at higher latitudes, plants and animals living near the poles are in more jeopardy than those living closer to the equator. Calling arctic ecosystems "surprisingly diverse," Sylvia Edlund of the Geological Survey of Canada in Ottawa, Ontario, said "the response to even small changes would be dramatic." Arctic species tend to be poor competitors, and as tree species migrate northward in response to global warming, arctic ecosystems could be reduced from one-third to one-quarter of Canada's land area, she said.
Also at special risk are rare species living in parks and reserves. Such species are confined to habitats that many biologists feel are already too small to ensure long-term survival. Climate change would probably force reserve species to migrate, yet few would find suitable habitat because of barriers such as cities, roads, reservoirs, and farm land. "Few animals or plants would be able to cross Los Angeles on the way to the promised land," said Peters.  Herman Cole of the Adirondack Park Agency in Ray Brook, New York, fears that parks will receive the lowest priority when people struggle to adapt agricultural and other "essential" ecosystems to global climate change.
Some atmospheric scientists say that past emissions of greenhouse gases have already committed the earth to heat up 0.5 - 1.5 deg C. "If we can't stop the greenhouse effect," said Schneider, "maybe we can slow it. We shouldn't minimize the importance of this, if it's all we can do." Slowing climate change, he added, "would buy time" to better understand the effects and devise strategies to adapt.
Slowing greenhouse warming will be difficult, however. Unlike many environmental problems, "there is no single villian" in the climate change issue, said Topping. Countless human activities, from agriculture to energy to energy use, emit compounds that are contributing to the greenhouse effect. Similarly, no single law or set of regulations can solve the problem.
Policy makers are already active in many arenas. Several US senators and representatives, for example, have taken a special interest in the issue and have held hearings over the past few years; two senators, George Mitchell (D-ME) and John Chafee (R-RI), served as honorary cochairmen of the climate conference. In 1986, eight senators, including Mitchell and Chafee, asked EPA to conduct two comprehensive studies - one focusing on the climate change effects on various human and natural systems and the other examining policy options for stabilizing greenhouse gas emissions. The reports are expected by October 1988. Other federal agencies, including the US Department of Energy and the National Oceanic and Atmospheric Administration's US National Climate Program Office, are also studying the climate change problem.
Many scientists say that, despite the uncertainties, it is time to move beyond the research stage. Howard Ferguson, head of Canada's Atmospheric Environment Service in Ottawa, noted the tremendous lag time between planning of any significant environmental action and that action's implementation. "I am convinced," he said, "that we will eventually need a 'Global Law of the Atmosphere' to solve this problem." Toward that end, several US senators, including Mitchell and Chafee, plan to introduce a resolution calling for an international convention on the global climate change issue. A much-heralded September 1987 international agreement to limit chlorofluorocarbon emissions to protect the earth's stratospheric ozone may catalyze such efforts.
Short of an international agreement, there are many smaller steps both public and private policy makers could take to slow greenhouse warming. To decide among these possibilities, Schneider recommends using the "tie-in strategy" - choosing actions that also have other obvious benefits. Shifting away from fossil fuel energy sources, for example, would reduce our dependence on foreign oil and lessen the acid rain problem even in the unlikely event that the greenhouse effect "turns out to be an 'infrared herring'." Similarly, there are many compelling reasons to slow tropical deforestation, which also contributes significant amounts of CO2 to the atmosphere.
The dilemma of deciding when to act on the climate change problem, Schneider said, boils down to "our need to gaze into a very dirty crystal ball ... the tough judgment to be made is precisely how long to clean the glass before acting on what we think we see inside."
- The conference was held 27-29 October 1987 in Washington, DC. Although it addressed both the greenhouse effect and stratospheric ozone depletion, this story will focus only on the former. For a review of the ozone issue, see Bioscience 37: 647-650.
- Scientists now know that there are many greenhouse gases in addition to CO2; these include methane, nitrogen oxides, and chlorofluorocarbons. Although levels of these gases also are increasing and, taken together, may be as important as CO2 in accelerating the greenhouse effect, they often are omitted from discussions of the problem because they have complicated biogeochemical interactions about which little is known.
- The conference and Senate hearing were described in A Matter of Degrees: The Potential for Controlling the Greenhouse Effect, a report by Irving M. Mintzer of the World Resources Institute (Washington, DC, 1987).
- A one-and-a-half-day symposium on the impact of climate change on fisheries immediately followed the conference. The symposium was the first in a series that will explore the effects of climate change on various human activities. It was organized by the Climate Institute and sponsored by the US Environmental Protection Agency.
- For an in-depth discussion of this problem, see Bioscience 35: 707-717.
All material (except for some code, external links and Optional Readings) © Jeffery A. Schneider, 2003