The evidence mounts
Holding the biggest piece of the pie in terms of greenhouse gases is carbon dioxide. It is fairly easy to measure because it mixes uniformly through the layers of the atmosphere on time scales of a year or two. So it is no surprise that researchers, looking to quantify evidence supporting global warming, checked to determine whether the amount of carbon dioxide has been increasing at any one location over time. Since 1958, these measurements have been made directly at a site in Hawaii; data for prior years is gathered by sampling bubbles of air trapped in ice cores [see figure].
The measurements indicate that while carbon dioxide levels have varied over hundreds of thousands of years, the upswing that began with the industrial age about 200 years ago shows an unprecedented rate of change.
The next bit of evidence is global mean temperature. This data is gathered from instrument records back to 1860, plus indicators that are sensitive to climate, such as tree rings, ice layers as measured in cores of ice from glaciers, ice caps, and ice sheets, and annual coral rings from cores in coral colonies. Over the last hundred years, the data shows an increase of at least 0.55 °C, a larger fluctuation than in any other past millennium.
“That increase,” said Kevin Trenberth, head of the climate analysis section for the National Center for Atmospheric Research [NCAR] in Boulder, Colo., “is one of the main reasons we believe we have detected climate change. The trends in the temperature record, particularly in the last 20 years, are now outside the realm of natural variability. They’re not caused by variations in solar radiation. They’re not caused by pollution from volcanic eruptions. The [increase in] global mean temperature is outside the realm of anything that can be accounted for except by the increases in greenhouse gases.”
Other evidence indicates the world is getting warmer. With a few exceptions, glaciers are melting. Oceans, measured consistently since the 1950s, are warming up. Sea ice in the Arctic Ocean, to cite recently declassified submarine data, has thinned by about 40 percent since the 1970s and diminished in extent. Sea level rose about 15 cm in the past 100 years, what with glaciers melting and oceans warming. The freezing season, or how long lakes and rivers around the world remain frozen in winter, has decreased by one to two weeks. Vegetation is creeping up mountains. And the list goes on.
Making the connection
The theory behind global warming starts with the Greenhouse Effect, first defined over 100 years ago. Greenhouse gases do not stop the sun’s radiation from penetrating the atmosphere and reaching the earth, where it is converted into heat. But once that happens, the gases act as a blanket, reducing the amount of heat that can escape. This natural effect makes the planet habitable.
If the amount of greenhouse gases increases, according to the next step in this theory, the earth gets warmer. Therefore, since human activities–including the burning of fossil fuels and wood, the cutting down of forests, and the intensification of agriculture–cause such an increase, then human activities are responsible for global warming.
Climate models link carbon dioxide and particulates to climate change. When they are omitted from a climate model, predictions [gray curve, left] diverge from observations [red curves]. Leaving them in [right] yields closer agreement.
Click to enlarge.
Climate models are used to test this theory. They grew out of efforts to get computers to predict the weather. They use information on what forces influence the weather and climate: for example, the amount of solar energy reaching the earth and its distribution; the earth’s surface characteristics; the composition of the atmosphere, including the amount of particles in it; and basic laws of physics expressed as mathematical equations, including those for the conservation of thermodynamic energy; the conservation of air mass and water, and the behavior of air and water as fluids.
A climate model visualization shows the increase between average temperatures in the 2090-2100 decade and those from 1990 to 2000, assuming 1 percent annual carbon dioxide increase. The difference is larger than expected but roughly accounts for other greenhouse gases.
All this data serves to relate temperatures, pressures, winds, humidity, clouds, and rainfall to one another in a physically consistent way to simulate the climate and make predictions. These models run on supercomputers: on vector machines that use a single or a small number of central processors to operate on arrays of numbers or on massively parallel computers.
Today, climate models typically consider the earth as a grid of 100-kilometer-square boxes, with about 20 layers in the atmosphere plus 20 more in the oceans [see figure, left]. (Using a higher resolution would add to the accuracy of models, but could bring even today’s fastest supercomputers to their knees.)