What is Carbon Footprint?
First, why "carbon"?
Carbon is the essential element common to almost all lifeforms on this planet. Its the thread running through, and stitching together, earth's biology.
So it's not surprising that carbon shows up in the gasses produced by virtually all biological processes. This is true of bovine belches, decomposition of flora/fauna and, most critically, combustion of the substances lifeforms leave behind. That is to say, burning of wood, coal, petroleum, etc. Its also not surprising then that carbon is the basic component of the most common greenhouse gasses.
Call it poetic license.
Each of us active earthlings enjoy a modern life that is, so far, made possible by (pardon the alliteration) carbon carcass combustion. Carbon stuff powers our lifestyle.
Each of us consumers can be associated with the amount of carbon burnt to support our activities and make our "stuff". In a way, we're each responsible for the amount of greenhouse gasses produced on our behalf... our footprint.
It's how much greenhouse gas emissions can be attributed to an individual or to anything else like a country or a factory whose "carbon footprint" we're calculating. This can be "primary" (your car burned the gas) or "secondary" (you bought something and its manufacture or distribution caused the gas emissions). Tracking secondary emissions will keep the carbon accountants busy.
It's not the carbon itself that's the problem, but the greenhouse gasses, the most common of which are carbonaceous in nature.
What Does Greenhouse Gas Mean?
Now comes the less well known part.
We have all held our hands out toward a campfire or woodstove to warm them. It was infra-red radiation coming from the fire or stove that was warming our hands. In this same way, the earth cools by sending some of its heat back into outer space.
That's not the only way things cool off, but its the way the earth cools to balance the sun's heating.
Here's the "greenhouse" trick part.
Thus the earth and its atmosphere trap some of the sun's energy but lose some back into space.
The amount of these gasses in the atmosphere determines the amount of the IR from the earth that gets out versus the amount trapped as heat. How much heat is trapped by "bouncing" back and forth between earth and atmosphere is determined by the atmosphere's greenhouse gas content.
The amount of energy (heat) trapped has kept the planet comfy or cold (ice age) due in large part to the rising or falling greenhouse gas levels in the atmosphere.
But now the excess greenhouse gasses we're creating are building to levels that haven't existed on earth since back near the dinosaur's reign and beyond when the planet reached much higher temperatures (as much as 12C or 21F higher on average).
Heating something as large as our planet takes a while, and in spite of the tremendous amounts of extra energy being trapped, the temperature hasn't moved much yet (not quite 2F). Picture heating a cast iron frying pan to cooking temperature with a heat lamp.
How Long Do We Have?
The main scientific discussion now is how long before we lose the ability to affect whether the temperature rises to a level that makes life as we know it difficult or impossible.
Once in the atmosphere, CO2 lasts a long time. Additionally, the rising temperatures will eventually release huge stores of methane which is now trapped in the frozen arctic tundra and under the ocean floors as frozen methane hydrate. Methane is about 23 times as effective at trapping heat from IR light as CO2 is. Once this self-sustaining methane release feed-back loop gets rolling, we will have little if any ability to affect the outcome.
There are strong global hints in the expansion of deserts, melting glaciers, rising oceans, increasing scarcity of fresh water, reduced productivity of oceans and soils, movement of habitation zones toward the poles, etc, that we have a very short time to act... maybe as little as 10 years.