You might be asking yourself: "Is it just me or are summers hotter than ever lately?" Or, "What happened to all those snowy winters we had growing up?" So, is global warming real? The answer is "yes."

Global warming is a scientific consensus that earth's atmosphere is warming in a fundamental and potentially serious way. While the implications of global warming are not precisely clear, the risks of inaction are now too great to ignore.

How do we know the planet is warming?
There is direct and indirect evidence that the earth is getting hotter. We know from direct temperature readings that the average temperature near the earth's surface has risen about 1.5 degrees F since the middle of the 1800s, when measuring and recording temperature became common. By looking at evidence such as tree rings and ancient records of when crops were harvested, scientists can estimate earth's temperature way back in time. The 1.5-degree increase in the past 150 years doesn't sound like much until you compare it with the 850-year period from about 1000 AD to 1850; during that time, the average temperature actually dropped about half a degree, leading to what climate scientists call the "Little Ice Age" — just one of the factors that made life particularly tough for America's first European settlers.

Since then, it has gotten a lot warmer, and the temperature has increased incredibly fast. If the numbers don't convince you, just ask a farmer or someone who spends a lot of time outdoors. They'll tell you that spring is coming earlier, and the snow and ice at high altitudes are melting faster than ever before. In fact, the warmest years ever measured were 2005, 1998, 2002, 2003 and 2006.

So, earth is getting warmer, but why?
The atmosphere of our planet has a special property known as the "greenhouse effect" that helps maintain a temperature that supports life. The atmosphere lets sunlight pass through and warm the surface of the planet, but it also traps some of the heat that radiates back toward the sun. Without the greenhouse effect, earth would be too cold for life to exist.

One of the most important gasses in the atmosphere is carbon dioxide, or CO2. It's odorless and colorless, but its molecules are key to the heat-trapping effect of the atmosphere. We need just the right amount of it to maintain a nice, steady temperature on earth. Unfortunately, when we burn fuels in our cars, homes, power plants and factories, we release extra CO2 into the atmosphere. And when we cut down trees to make room for houses or cattle, we're removing one of the ways the earth regulates CO2.

Since the Industrial Revolution (around the middle of the 1800s) we've both burned more fuel and cut down more forest than ever before, and as a result, CO2 concentrations have risen about 30 percent. The correlation between the big swing we've seen in earth's temperature (from a downward trend pre-1850 to a rapid rise since then) and the human-caused increase in CO2 is no coincidence. That's what the Intergovernmental Panel on Climate Change — a non-partisan, international panel of leading scientists — concluded earlier this year, calling the connection "very likely."

1.5 degrees doesn't sound like much, and I like warm weather. Why should I care?
Even though the warming over the past 150 years might seem moderate, we've already seen major effects. The number of Category 4 and 5 hurricanes has almost doubled in the past 30 years, species are losing their habitats and diseases like malaria are on the rise. But what's worse is that the warming trend shows no signs of slowing down, and we're experimenting with the only home we have. Estimates for warming over the next century range from 2 to 11 degrees F, and with that kind of warming comes a lot of uncertainty:

• Sea levels could rise, putting places like Florida (and Bangladesh, with nearly 150 million inhabitants) in grave danger
• Storms may become more frequent and intense
• Agriculture could be disrupted
• Drought and wildfire could be more common
• Species might go extinct

Finally, there is the possibility that the earth's atmosphere will react in a dramatic, as-yet-unforeseen way that could accelerate warming beyond the predictions. We don't want to go there. What we know is that the earth is warming, humans almost certainly have something to do with it, and the risks of not doing anything about it are high — for us and especially for our children.

This is pretty depressing. Is there anything we can do?
The paradoxical good news about global warming is that it's a problem caused by humans, so it can be fixed by humans, and a lot of the changes we should make aren't that difficult, or even that expensive. The most important thing we can do is to start to understand our personal contribution to global warming and take steps to reduce it.

I don't produce CO2 — or do I?
We're all responsible in some form for emitting CO2. The average American produces between 7 and 20 tons of it each year, depending on how you count things like emissions from office buildings and manufacturing that can't easily be attributed on a person-by-person basis.

In general, our CO2 emissions split into three roughly equal pieces of the pie: home, transportation and commerce. At home, we produce CO2 directly when we turn on the furnace to keep warm and indirectly when we use electricity to power lights and air conditioning. (Despite the growing popularity of wind power and other "renewable" electricity sources, most of the electric grid is still powered by burning coal or natural gas.)

When traveling, we produce CO2 by burning fuel in our cars, trucks, buses and airplanes. And almost anything we buy has some CO2 emissions associated with it — from things produced on electric assembly lines to the food we eat, which is harvested by machines that run on diesel fuel.

If CO2 comes from burning fuel, doesn't reducing my emissions save me money?
Yes! Since CO2 emissions come from burning fuel, the best thing you can do to reduce your emissions is to improve your energy efficiency. Small changes add up, and you'll find many good tips throughout this site, but here are three good starters for home, travel and commerce:

• Home: Change that always-on porch light to a compact fluorescent, or CFL). CFLs last longer, reduce emissions by about 65 percent as compared to a regular bulb and can save you about $30 over the life of the bulb.
• Travel: Slow down. Air resistance (and the potential severity of an accident) goes way up with speed. If you drive a lot on the highway, try slowing down by 5 mph for a tank, and watch your fuel economy go up.
• Commerce: Avoid buying items with excessive packaging. Packaging is an environmental triple-threat. It often comes from cutting down trees, requires fuel to transport, then decays in landfills to produce methane, a global warming gas that's 20 times more potent than CO2.

A Note on Carbon Offsets
Some carbon dioxide emissions are easier to reduce than others. It's easier to switch your lights to CFLs, for example, than it is to skip that yearly flight to visit family back home. That's where carbon offsets come in.

When you buy a carbon offset, you're paying someone else to reduce their emissions because it's not feasible or too expensive to reduce your own. Remember:

• Offsets are no substitute for reducing your direct emissions, when possible.
• Not all offsets are the same. Look for ones that have third-party verification, sold by companies that "open their books," so people understand how the offsets are made.
• Offsets are voluntary — for now. It's likely that we'll see some sort of carbon-trading law in the United States by 2010, under which businesses will have limits on the amount of CO2 they can produce — and will have to buy offsets if they can't meet the limits, or be able to sell them if they are ahead of the curve.