Man-caused emissions are a fraction of a fraction of Earth's atmosphere, and the atmosphere is just one of the drivers of the weather that we experience every day.

"The Physical Elements of Geography," a reference book, puts climate in perspective. Solar radiation from the sun powers our complex weather, piercing through several hundred miles of gaseous atmosphere to warm the Earth's land and water.

"Greenhouse Gases" in the atmosphere, like the roof of an earth-bound greenhouse, trap radiant heat from the sun. Without the greenhouse, this world would be a chilly place, but if the gas mixture gets out of whack, Earth gets warmer. Or much cooler.

An Internet discussion group, www.realclimate.org, says current thinking is that water vapor makes up the lion's share of our greenhouse gas mix. Carbon dioxide is believed to be the largest component of the remaining trace gasses. Together, these gasses absorb infrared radiation. That traps heat in the atmosphere from which it is transferred to air just above the Earth's surface.

The process isn't simple.

Many plants collect CO2, acting as carbon sinks. Others emit CO2. Depending on which scientific study you reference, natural CO2 and water vapor account for up to 95 percent of the atmosphere's volume of heat-trapping gases.

Since 1830, when the industrial revolution turned to burning large quantities of coal to fire factory boilers, there are indications CO2 has been on the increase. Human-caused emissions rose sharply after World War II (see "hockey stick" graph on Page A1).

Products of burning carbon-based fuels are tagged as primary causes of increased carbon emissions: They combine with the atmosphere's oxygen to form CO2. Some estimates say current CO2 levels are about 30 percent greater than they were in 1830 when the industrial revolution torqued up combustion. That's not all of what's going on in the atmosphere: Some particulate emissions form the nucleus around which water droplets gather or ice crystals grow, also shaping the weather we see on Earth. And that leads to the difference between weather - what's happening in your neighborhood - and climate, the long term trend.

From sprawling cities with asphalt streets and masonry buildings that absorb solar radiation, to growing crop plants and forests that soak up CO2 and tuck it back into the soil, from shifts in warm or cold waters on the ocean's surface, the climate takes shape. Elevation of mountains and distance from the ocean influence air movement and the release of precipitation in somewhat predictable fashion. Together, these factors make climate. It's the long-term look.

In the United States, the National Weather Service describes climate in a rolling 30-year average. When you read the average temperature for this day it's based on a data set that started in 1975. Next year, it will be a data set that began in 1976. Weather is the condition of the atmosphere above a particular portion of the Earth. Factors that make weather include temperature, air moisture content and winds or changes in air pressure.

- Tam Moore

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