If you take a boat up the Dalyan River in Turkey you will pass the ruins of the Carian city of Kaunos. Kaunos used to be a nice place to live, with a bustling market, fantastic views of the river and the Mediterranean Sea, an amphitheater, a large church, baths and villa-lined streets. But farming and wood collecting led to erosion and the harbor silted up. Things got so bad that Kaunosites were called "yellow-eyes" because of the malaria their swampy surroundings fostered. Eventually everyone left, and Kaunos passed into history.

It is easy to imagine the city elders debating during the city's long decline.

Some would argue that the hill sides should be preserved with various ordinances, others would say that the silting problem is overblown, and how do we know it comes from erosion anyway? And an ex-elder would be awarded laurels for producing a play called "Silting: an Inconvenient Truth."

I think of Kaunos whenever climate-change deniers spread their nonsense, which, unfortunately is having an effect on the public: Pew surveys consistently show that 40 percent of Americans do not believe in man-made climate change.And world leaders, who gathered last week in Pittsburgh at the G-20 summit and in New York at the U.N. Climate Change Summit, are under pressure to soft-pedal the issue.

Climate change is real and it is certainly caused by us. The detailed argument can get complicated but the general picture is easy to understand.

Let's consider a familiar situation, namely your car on a sunny day.

You know it can get very hot in there. Why is that? The sunlight entering your car carries energy and when it impacts pleather and vinyl (I'm assuming you have a 12-year-old Dodge like me) it stirs up the atoms making up those substances. This stirring up is registered as heat by our senses.

So far so good, but we are missing something important: We know the air in your car is warmer than the outside air. What's going on?

The answer lies in some tricky properties of glass.

Glass lets light through (it is, after all, transparent), some of the light that gets into your car is reflected by the interior (that is how you can see it) and the rest goes into heating vinyl. It turns out that stirred-up vinyl atoms emit their own energy, called thermal radiation.

Thermal radiation is a low-frequency version of light that is invisible to our eyes, but can be detected by special cameras (maybe you've seen thermal images before). Emitting thermal radiation is nature's way of keeping a lid on the temperature -- pump more light energy in and more thermal energy will be emitted back.

Here's the catch: Windshield glass is transparent to visible light but does not permit thermal radiation to pass through easily. Thus the thermal radiation is trapped in your car, causing the interior to heat up more than the exterior. The net result is a steering wheel too hot to hold.

I am afraid that your car has a greenhouse windshield problem.

Your car is a pretty good facsimile of what is happening to the Earth.

Every day the Earth receives a certain amount of light energy from the sun (equivalent to putting a 100-watt light bulb on every square foot of the surface of the Earth).

Some of this light energy is reflected back to space. About 50 percent of it gets through to the surface, where it heats the planet and powers life. And just like in your car, the heated ground and oceans give off thermal radiation, and just like your car, much of this cannot get back out to space.

In this case the role of your windshield is played by the atmosphere. More specifically, it is played by certain gases in the atmosphere: water vapor, carbon dioxide (CO2) and methane. These are the "greenhouse gases." Incidentally, the first measurements of the insulating properties of greenhouse gases in the atmosphere were made in the 1880s on the North Side by Samuel Pierpont Langley, the first director of the Allegheny Observatory.

Make no mistake, the atmospheric greenhouse effect is very important. By measuring energy flow, we can deduce that it raises the average temperature of the Earth some 60 degrees, which is a good thing. The atmosphere is Earth's warm puffy quilt.

But what happens if you suddenly increase the amount of greenhouse gases? Unless something else changes, more thermal energy is trapped in the Earth's atmosphere and the average global temperature goes up. This is a consequence of energy balance and there is no way around it.

Those are the ground rules, now for some facts.

The measured amount of CO2 in the atmosphere has gone up from 280 ppm (parts per million measured by volume) to more than 380 ppm in the past two centuries. The extra 100 ppm represents about 760 billion tons of additional CO2 in our atmosphere.

Not by coincidence this is the same as the amount we have put into the atmosphere by burning fossil fuels. There can be no question: generating the energy that drives our standard of living has raised atmospheric C02 levels. This additional CO2 makes for a cozier atmospheric quilt and is equivalent to turning up the heat one degree.

Direct measurements confirm this expectation: The Earth's average temperature has increased about 1.5 degrees Fahrenheit since 1900. That doesn't sound so bad, but things will get worse.

The world has approximately 900 billion tons of recoverable coal left. Burning all of this (as recent experience with water shortages show, it would take an act of unprecedented courage and foresight for humanity not to burn everything) will add an extra 3,000 billion tons (or 400 ppm) of CO2 to the atmosphere. This is expected to happen by 2065 with current consumption patterns. Yes, all the coal will be gone in your children's lifetime.

We might guess that if 100 ppm raised the temperature 1.5 degrees, 400 ppm should correspond to 6 degrees. It turns out this estimate is low because warmer temperatures cause other changes (like less snow cover) that also raise the temperature. The experts take everything into account and predict that another 400 ppm of CO2 will raise the Earth's average temperature 9 to 11 degrees over preindustrial averages.

Almost everywhere will be warmer, with northern latitudes experiencing the largest effect. Rain patterns will shift, with dry places like the southwest becoming dryer. Sea levels will rise by at least 12 feet and the oceans will become more acidic. The net result will be huge migrations of plants, animals and people.

If it happens slowly enough, things may not be so bad. If it happens quickly, humanity will be placed under tremendous stresses and the results will not be pretty.

Darfur, Rwanda and Bangladesh may be providing glimpses of our future.

If we decide that climate change is bad, some serious decisions will need to be made. Half-hearted measures are going to accomplish little. We essentially must stop burning coal and oil right now.

It is clear that breaking the fossil-fuel habit will be next to impossible so we must shoot for the next best option: developing viable large-scale renewable energy sources as quickly as possible. About the only good news is that coal and oil will be gone soon anyway, so we may as well get to work now.

Energy will be expensive, but at least we will have it, and most of our standard of living.

The other option is that our grandchildren return to a world powered by donkeys and water wheels.

The Kaunosites could always move up the coast; we have nowhere to run. The sooner we start facing facts, the better.

Cartoonist Rob Rogers does "Rob's Rough," an early look at his work and his creative process, exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.