The prediction of climate change due to human activities began with a prediction made by the Swedish chemist, Svante Arrhenius, in 1896. Arrhenius took note of the industrial revolution then getting underway and realized that the amount of carbon dioxide being released into the atmosphere was increasing. Moreover, he believed carbon dioxide concentrations would continue to increase as the world's consumption of fossil fuels, particularly coal, increased ever more rapidly. His understanding of the role of carbon dioxide in heating Earth, even at that early date, led him to predict that if atmospheric carbon dioxide doubled, Earth would become several degrees warmer. However, little attention was paid to what must have been seen to be a rather far-out prediction that had no apparent consequence for people living at that time.
Arrhenius was referring to a potential modification of what we now call the greenhouse effect. A simplified explanation of this is as follows (see the diagram). Shortwave solar radiation can pass through the clear atmosphere relatively unimpeded, but longwave infrared radiation emitted by the warm surface of the Earth is absorbed partially and then re-emitted by a number of trace gases--particularly water vapor and carbon dioxide--in the cooler atmosphere above. Because, on average, the outgoing infrared radiation balances the incoming solar radiation, both the atmosphere and the surface will be warmer than they would be without the greenhouse gases. One should distinguish between the "natural" and a possible "enhanced" greenhouse effect. The natural greenhouse effect causes the mean temperature of the Earth's surface to be about 33 degrees C warmer than it would be if natural greenhouse gases were not present. This is fortunate for the natural greenhouse effect creates a climate in which life can thrive and man can live under relatively benign conditions. Otherwise, the Earth would be a very frigid and inhospitable place. On the other hand, an enhanced greenhouse effect refers to the possible raising of the mean temperature of the Earth's surface above that occurring due to the natural greenhouse effect because of an increase in the concentrations of greenhouse gases due to human activities. Such a global warming would probably bring other, sometimes deleterious, changes in climate; for example, changes in precipitation, storm patterns, and the level of the oceans. The word "enhanced" is usually omitted, but it should not be forgotten in discussions of the greenhouse effect.
Nearly 100 years after the Arrhenius prediction, we are now aware that carbon dioxide in the atmosphere is increasing, with the likelihood that it will double by the middle of the next century from the levels at the time of Arrhenius. Post-World War II industrialization has caused a dramatic jump in the amount of carbon dioxide in the atmosphere. As the prospect of considerable change in the atmosphere becomes more real and threatening, new computer models are being applied to the problem. These models take into account the natural processes that must be part of the whole picture to understand what could happen to Earth's climate as carbon dioxide increases. An important aspect of the newer models is their treatment of the "amplifier" or feedback effect, in which further changes in the atmosphere occur in response to the warming initiated by the change in carbon dioxide.
In addition to moisture and cloud processes, the newer models are beginning to take into account the role of vegetation, forests, grasslands, and crops in controlling the amount of carbon dioxide that actually will be in the atmosphere. Along with their role as "sinks" for carbon dioxide, the various types of vegetation in the biosphere have further effects on climate. Plants heat or cool the air around them (through the reflection and absorption of solar radiation and the evaporation process), remove momentum from surface winds, and take up and release moisture into the air (thus contributing to alterations in the hydrologic cycle). In turn, changes in climate will affect the patterns of vegetation growth. For instance, forest stands that require relatively cool conditions may not be able to adjust to the relatively rapid warming that is being predicted for the interiors of climates. With slow warming, scientists expect that the northern edges of North American forests would creep slowly forward to more-favorable conditions, while the southern edges would give way to grasslands that are better suited to the warmer conditions. With overly rapid warming rates, however, the loss at the southern edge would be more extreme, and the migration at the northern edges would not be able to make up for the loss at the southern edge.
Other feedback effects at work also must be considered. In normal conditions, plant leaves take in carbon dioxide from the air and release moisture to the air as part of the photosynthesis process. The release of moisture through evapotranspiration causes the air to cool. With increasing atmospheric carbon dioxide, one can expect to see a change in plant carbon exchange rates and water relations. This may result in reduced evaporation rates, thus amplifying the summer continental warming. Without plants, the ground and air would become warmer, exacerbating the problem.
Global Climate Change
Climate Change - Effect On Water Resources
Fresh water, both on the surface of the land and in the ground, is an extremely valuable resource. We drink it, bathe in it, depend on it for transportation and recreation, water our lawns and crops with it, and eat the life that swims in it. Too much or too little water can have disasterous effects on our lives - floods, droughts, erosion, sinkholes, pests, and diseases are all related to the presence of more or less water than we usually experience.
Our water supply is directly tied to climate. The figure below shows annual mean runoff in the Upper Midwest as simulated using weather analyses from 1963 through 1995. Scientists now understand that the climate we experience today is not constant. Climate has been very different in the past and may be quite different from today's climate in our not-too-distant future.
Researchers at the Upper Midwest RESAC are studying how our regional water supply has changed in the past, and how those changes are related to climate. Our researchers are also developing techniques to investigate how future climate changes, such as global warming, may effect the supply of water in our area of the world. This research is intended to alert us regarding potential water resource problems in the future, and guide us in preventing those problems as much as possible.
Climate forcings, sensitivity, response time and feedbacks
Climate sensitivity is the response to a specified forcing, after climate has had time to reach a new equilibrium, including effects of fast feedbacks. A common measure of climate sensitivity is the global warming caused by a doubling in atmospheric CO2 concentration. Climate models suggest that doubled CO2 would cause 3 °C global warming, with an uncertainty of at least 50%. Doubled CO2 is a forcing of about 4 W/m2, implying that global climate sensitivity is about 3/4 °C per W/m2 of forcing.
Climate response time is the time needed to achieve most of the climate response to an imposed forcing, including the effects of fast feedbacks. The response time of the Earth's climate is long, at least several decades, because of the thermal inertia of the ocean and the rapid mixing of waters within the upper few hundred meters of the ocean. Climate sensitivity and response time depend upon climate feedbacks, which are changes in the planetary energy balance induced by the climate change that can magnify or diminish climate response. Feedbacks do not occur immediately in response to a climate forcing; rather, they develop as the climate changes.
Fast feedbacks come into play quickly as temperature changes. For example, the air holds more water vapor as temperature rises, which is a positive feedback magnifying the climate response, because water vapor is a greenhouse gas. Other fast feedbacks include changes of clouds, snow cover, and sea ice. It is uncertain whether the cloud feedback is positive or negative, because clouds can increase or decrease in response to climate change. Snow and ice are positive feedbacks because, as they melt, the darker ocean and land absorb more sunlight.
Slow feedbacks, such as ice sheet growth and decay, amplify millennial climate changes. Ice sheet changes can be treated as forcings in evaluating climate sensitivity on time scales of decades to centuries.
What can Paleoclimatology tell us about climate change relevant to society in the future?
To understand and predict changes in the climate system, we need a more complete understanding of seasonal to century scale climate variability than can be obtained from the instrumental climate record. The instrumental temperature record indicates that the Earth has warmed by 0.5°C (0.9°F) from 1860 to the present. However, this record is not long enough to determine if this warming should be expected under a naturally varying climate, or if it is unusual and perhaps due to human activities. Paleoclimatic proxy data can be used to extend climate records and provide a longer time frame (hundreds to tens of thousands of years) for evaluating the warming of the last 140 years. The cause of global warming over the last century remains a heated debate with significant economic and societal implications. Many scientists attribute the current global warming to the enhancement of the greenhouse effect by human activities. Other scientists have suggested that other factors not affected by humans, such as changes in the number and size of volcanic eruptions or an increase in the sun's output (such phenomena are referred to as climate forcings), are responsible. A paleoclimate perspective provides information about long term changes in different climate forcings that may be the underlying cause of the observed climate change. An analogy of how paleoclimatic data improves our understanding of climate can be explained in terms of the stock market. Stock market analysts use longer term trends (one, two, three, or six months) in the stock market indexes (DOW, NASDAQ, etc.) rather than depending on changes from one day to the next or over a week to predict what the market will do next (i.e., Bull or Bear Market). In much the same way, the paleoclimate perspective allows us to evaluate climate change many decades and centuries into the past, in order to develop a more reliable estimate of how climate may change in the future.
The paleoclimate perspective can help us answer many questions, including...
- Is the last century of climate change unprecedented relative to the last 500, 2000, and 20,000 years?
- Do recent global temperatures represent new highs, or just part of a longer cycle of natural variability?
- Is the recent rate of climate change unique or commonplace in the past?
- What does it mean if the last century is unprecedented in terms of warming?
- Can we find evidence in the paleoclimate record for mechanisms or climate forcings that could be causing recent climate change?
What is Climate?
Climate is the weather pattern we expect over the period of a month, a season, a decade, or a century. More technically, climate is defined as the weather conditions resulting from the mean, or average, state of the atmosphere-ocean-land system, often described in terms of "climate normals" or average weather conditions. Climate Change is a departure from the expected average weather or climate normals.
