Global warming, as it has come to be referred to in the popular and scientific press, refers to an increase in the temperature of the Earth that is expected to result from the injection of greenhouse gases into the atmosphere by human activities.
Greenhouse gases are molecules like carbon dioxide, methane, and water that absorb infrared light leaving the Earth. Although the molecules do re-radiate the light at other other wavelengths, half of this light is radiated back toward the Earth and is absorbed. Increasing the amount of greenhouse gases in the atmosphere causes less total radiation to leave the Earth. At the same time, if the amount of radiation arriving at Earth from the Sun remains approximately the same, then thermodynamics requires that these two quantities come back into balance. Therefore, if more energy arrives at the Earth than leaves, then the Earth must heat up until it is able to radiate an energy to that arriving at Earth. In this way, increasing the concentration of greenhouse gases in the atmosphere should cause Earth's temperature to rise.
Several human activities can release greenhouse gases into the atmosphere, including making heaps of trash, raising livestock, and clearing land (both forests and grasslands) for agriculture. However, the largest contributor to human greenhouse gas emissions is the burning of coal, oil, and natural gas, which releases carbon dioxide. Although natural chemical processes allow carbon dioxide to be exchanged between the atmosphere, oceans, and plants, the additional emissions produced by humans have not been canceled by these processes (about half of carbon dioxide humans have produced has remained in the atmosphere), and the concentration of carbon dioxide in the atmosphere has increased by roughly 40% since James Watt patented the steam engine.
Carbon dioxide is of particular concern in climate change, because it is the longest-lived of the major greenhouse gases. Chemical reactions allow water vapor and methane to be removed from the atmosphere relatively quickly, so that if the artificial sources for them are removed, their concentrations will return to the original equilibrium level relatively quickly. In contrast, carbon dioxide takes a thousand of years to be removed from the atmosphere (this is set by the circulation of deep ocean currents), so its effects on climate are long-lived. Moreover, the increase in temperature caused by additional carbon dioxide in the atmosphere should cause the amount of water vapor from natural sources to increase, which results in a positive feedback that causes further heating (although this is partially canceled if more clouds form).
As a result, it is expected that if current trends continue, the concentration of carbon dioxide will double by mid-century, which will raise the Earth's average temperature by 2° Celsius. This basic conclusion has been supported by models for 30 years, because although the climate system is complex, the direct effect of carbon dioxide on atmospheric temperature is well understood.
The notion of a changing climate emerged in the 19th century, when scientists developed a theory for ice ages in order to explain, among other things, massive boulders that were deposited widely at high latitudes. In the 20th century, with better understandings of geologic history, atmospheric chemistry, and the interaction between the Earth and the Sun, researchers began to model what might have caused shifts in the the Earth's climate. Scientists also began to consider Earth's future climate By the late 1970s, two developments led global warming to emerge as the dominant theory for how Earth's climate would evolve over the next few hundred years.
First, it became widely recognized that burning fossil fuels and clearing land for agriculture was causing the carbon dioxide concentration in the Earth's atmosphere to increase. Calculations and laboratory experiments demonstrated that doubling the concentration of carbon dioxide in the atmosphere, without considering any other effects, would cause the Earth's temperature to rise by 1° Celsius. The warmer Earth would also alter the balance of the water cycle slightly, causing the concentration of water vapor in the atmosphere to increase. Water vapor is also a greenhouse gas, so the Earth's climate could warm yet further. Scientists then considered whether other effects, such as increased cloud formation, would counteract this heating. The consensus was that no effects would counteract the heating caused by increasing the carbon dioxide concentration.
The second set of influences on the theory for global warming
were studies of the atmosphere of
Venus.
Satellite missions had revealed that Venus is the hottest planet the
solar system, despite the fact that it was farther from the Sun than
Mercury. The explanation was that Venus's atmosphere was almost
completely carbon
dioxide, which caused a runaway greenhouse effect that headed the
surface to over 460° Celsius (860° Fahrenheit). These
studies (and related ones of the climate of Mars) solidified the
notion that changes in the atmosphere of the Earth could have significant
effects on the climate. Indeed, Earth itself would be
At the time that global warming first emerged as a widely-accepted
theory in the late 1970s, scientists were not convinced that they had
actually measured the climate getting warmer. However, with 30 years'
worth of additional data, it is now clear that
the
Earth has warmed by about 0.6° C over the past few decades.
With this, the simplest prediction of global warming theory appears to have
been verified.
The fact that the Earth is warming is supported by a wide variety of
other indicators that are indirectly related to temperature, such as
the fewer days during winter on which lakes and rivers freeze over, the
earlier arrival times of migrating birds, the increasing humidity in the
lower atmosphere, and the increasing mean sea level.
Global warming theory also makes more subtle predictions. Since the late
1970s, climate models have included many factors that
force the climate, including changes in the Sun's radiative output
(these are observed to have been slight in the last century), the production
of sulfate particles that reflect sunlight back to space, carbon
particulates that absorb light. They have also included feedback effects, such
as changes in the amount of clouds and surface ice that can reflect sunlight.
These effects are calibrated using data from previous decades, and then used
to forecast future effects. Even in the late 1970s, these models predicted
that the poles of the Earth should warm fastest. This has now been
confirmed to be the case.
Finally, if the greenhouse effect is operating, one would expect that at
the same time as the lower atmosphere (troposphere) is warming, the upper
atmosphere (stratosphere) should be cooling. This is observed.
Predictions
Although the theories underlying global warming theory &mdash chemistry and radiative transfer &mdash have resulted in many technological advances, global warming theory itself has not. This is because people are rightfully unwilling to let scientists modify the climate of the planet on which we are currently constrained to live.
There is much that remains to be understood about the effects of climate change. Although all models predict warming, there is a significant wide variance between models as to how much warming will occur. Two of the largest sources of uncertainty are (1) the amount of negative feedback that will be produced by increased cloud cover, which could reduce global warming, and (2) whether the ocean will continue to absorb a large fraction of the carbon dioxide, which, if it shuts off, could greatly exacerbate global warming.
Likewise, it is uncertain how much sea levels will increase, and it is not yet possible to predict the change in the average temperature of the Earth will effect local climates. Some places on Earth may well develop more favorable climates, with more rainfall or milder temperatures. People one house away from the beach may well find themselves with beach front property. However, the bulk of the changes are likely to be detrimental, especially in poorer countries where people have chosen the places they live based on previous patterns of rainfall, temperature. Therefore, models are being improved in an attempt to predict the economic cost of global warming, so that citizens and politicians can plan for how to cope with them.
James E. Hansen has done a lot of excellent work on climate change, and is politically active about the subject. His personal site contains a lot of good information.
Of course, the 2007 IPCC Report provides a detailed and exhaustive summary of the current scientific consensus on climate change.
Like science itself, these pages are under construction. OK, so they are in a lot worse shape than science. I welcome your comments.