"Global warming" is something that scientists prefer to call the problem of climate change. "Climate change" is not a polemical label, like "ancient forests," but a broad term recognizing that the climate, whether too cold or too warm in human terms, can go either way. The same factors behind global warming today account for global cooling at other times. "Climate change" is a general scientific problem that encompasses more than today's global warming trend. This article covers the basics of that general scientific problem.
Climate always changes from natural causes. What makes today special is that human causes have entered the mix, and scientists are challenged to untangle human causes from the rest and assess their role. Climate specialists juggle a large number of factors, most of which are imperfectly known, and use them as input for computer simulations, none of which are yet reliable for policy purposes.
The Earth's climate system can be regarded as a machine that converts radiation from the sun into the heat and motion of the lower atmosphere and ocean. The major elements that add or subtract energy inputs into this machine are called forcings, and climatologists divide them into natural and anthropogenic (human). In addition to forcings are internal variations within the climate system, such as El Niño in the oceans and the North Atlantic Oscillation in the atmosphere, and others that involve both sea and air.
For purposes of global-warming research, we study factors that are important for the last 2000 years, so this discussion will ignore orbital cycles and the geotectonic carbon cycle, which dominate climate over longer periods. This page is about the forcings, the next page is about the internal variations, and the page after that is about how we measure these factors. The final page summarizes what we can say with confidence about modern climate change.
Natural Forcings
Solar forcing is the energy the sun sends to Earth, warming the atmosphere. This is accurately known only since 1978, from satellite measurements made in space. Observations of stars similar to the sun suggest that solar forcing could fluctuate significantly.
Natural greenhouse gases act to warm the Earth's surface because they block long-wave (thermal) radiation from exiting to space. The two main gases of interest are carbon dioxide and methane. Many natural events affect these gases, but natural fluctuations are usually of regional scale and don't perturb the world average much.
Volcanic eruptions put sulfate aerosols into the atmosphere, cooling the surface and warming the upper atmosphere. The largest eruptions can affect the entire planet; for instance the 1991 eruption of Pinatubo cooled global average temperature by nearly a degree the following year.
Human Forcings
The oldest human forcing is changing land use that affects how the soil absorbs sunlight. For thousands of years, human activities have reduced forests, drained wetlands, dammed watercourses and disturbed the soil. The history of this factor is very poorly known, but it is growing in significance as human population and industry grow. One estimate is that recent land-use changes have acted to cool the northern hemisphere slightly as a whole.
Human greenhouse gases, again, tend to warm the Earth's surface. These include:
- Carbon dioxide from burning of fossil fuels
- Carbon dioxide from controlled agricultural burning
- Carbon dioxide from uncontrolled burning of coal seams
- Carbon dioxide from production of concrete
- Methane from disturbed and flooded land, including rice fields
- Methane from uncontrolled leakage of natural gas
Just as volcanoes do, human activities also emit sulfate aerosols, primarily from burning of coal. Sulfate aerosols tend to cool the surface.
Human activities produce large quantities of soot (fine particulates). Soot tends to warm the surface.
Most of these forcings can be measured or estimated fairly accurately today, although uncertainties need to be reduced further. But to look backward past the satellite era, about 1980, we must rely on substitute indicators, or proxies for the forcings. More about those later. Next we'll look at the internal variations in climate.
