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"Global" and "warming." Get it?
Just what is "global warming"? Well, a key idea is that the warming must be "global." Even warming over certain large areas of the earth may not be "global," if there are other large areas that are cooling. Another important idea, but often overlooked, is that many people intend to refer to "global warming caused by increased atmospheric concentrations of greenhouse gases," when they refer to "global warming." But "global warming" can (and has!) been caused by things other than concentrations of greenhouse gases. For example, the sun has been responsible for repeated ice ages and warm interglacial periods. So, as a listener, you should always be very careful when someone says, "global warming." When I write that global warming is occurring, I am definitely not referring to "global warming caused by increased greenhouse gas concentrations in the atmosphere." When Iwrite that "global warming is occurring," I simply mean that average temperatures around the globe are increasing...for some unknown reason.
We also need to discuss how "global warming" is measured. It turns out that there are many ways to measure or evaluate global temperatures:
1) Global temperature can be evaluated by measuring temperatures at 1000's of weather stations all over the world. These stations measure temperatures with standard thermometers, and report those temperatures to a central database. These measurements are "surface" temperature measurements. They measure temperatures right at ground level.
2) Surface temperatures around the world can also be evaluated indirectly, rather than by directly measuring temperatures. These indirect temperature evaluations can be made by recording the days on which lakes or rivers freeze over and thaw, measuring the retreat or advancing of glaciers, and other such methods.
3) Temperature in the troposphere (lower atmosphere) can be measured by weather balloons. If weather balloons are launched in many locations, for a long enough period of time, an estimate of the temperature change in the lower atmosphere can be made.
4) Satellites can measure temperatures in the atmosphere around the whole globe. These satellites use a Microwave Sounding Units (MSU) to determine temperature. Satellite temperature measurements have only been available since 1979.
In order to help laypeople who aren't too familiar with this who global warming issue, I've defined below some terms used on this site. Anyone who has a technical technical background can probably skip this.
Greenhouse gases (GHGs): These are gases that absorb heat that is radiating from the earth's surface, keeping the surface warm. Without these gases in the atmosphere, the earth would be a frozen wasteland, nearly 60 degrees colder. (That's colder than the coldest ice age.) The main greenhouse gas in the earth's is water vapor. But the main greenhouse gases emitted by humans that are causing concern about global warming are carbon dioxide (CO2) and methane (CH4). Various refrigerants, such as chlorofluorocarbons (CFCs) also cause some global warming. But for all practical purposes, we can focus on CO2 and methane without being terribly inaccurate.
Troposphere: The lower part of the atmosphere. (The upper part of the atmosphere is the stratosphere.) The troposphere definitely should be warming, if global warming is caused by greenhouse gas emissions.
Intergovernmental Panel on Climate Change (IPCC): This is a group of about 2500 scientists that get together to write very long reports on global warming.
Parts Per Million (ppm): A unit of concentration. The concentration of CO2 in the atmosphere is measured in ppm.
Parts Per Billion (ppb): Another unit of concentration, even smaller than parts per million. The concentration of methane in the atmosphere is usually reported in ppb.