This fact doesn't make the local weather forecast undependable. It doesn't stop the thermostat on the wall of your living room from keeping the room at a pleasant temperature. Nor will food spoil at random places in your refrigerator, while the same food stays fresh in other places, as long as the fridge is working properly.

But your fridge needs to remove the heat that keeps leaking in from outside. It does this with a radiator, usually on the back, sometimes on the bottom. If that gets choked up with dust-bunnies, it doesn't remove heat as fast. Then, the inside of the fridge warms up. Your food might spoil.

It's the same with Earth. When something cuts down the flow of heat out into space, but sunlight doesn't change, every part of the planet slowly gets warmer. The parts don't see the same temperature rise. Nights are still cooler than days; winter cooler than summer. This autumn might be cooler than last autumn. But, over longer time periods, the temperatures trend upwards — just as we measure them doing now.

Plots of changes in temperature since a baseline year (what's called "temperature anomaly") differ on this. Plots from Britain's Hadley Centre show 1998 hottest, because they omit measurements from the Arctic, which has warmed fastest. The plots from NASA's GISS, which do include the Arctic, show 2005 as the hottest year.

A complicating factor is the roughly 30-year El Niño/La Niña cycle. El Niño made 1998 hotter; then a La Niña cooling phase kicked in, sucking heat from North America's land surface. The heat removed from the land went into the sea. The total heat content of the planet is still rising, but measurements on land don't always reveal this.

The important thing to note is the long-term temperature trend, which is still upward.

Think of it this way: You walk outside in your shirtsleeves into cold conditions like those found in February at International Falls, North Dakota. You will feel like you're freezing, and you'll be right. But to a camera that records your infra-red emissions, as the sensors on satellites do, you'd look very warm indeed.

But if a friend runs up and hands you an overcoat, and you put it on, you'd suddenly look a lot colder to the distant IR sensor. The outside of the heavy coat would indeed be colder. Despite that, you would be retaining body heat a lot better. Your skin would get warmer.

This is exactly how the greenhouse effect operates. The total heat energy held in Earth's climate system goes up, while the heat it radiates into space goes down. This is just what we've observed since 1978.

Another factor is the immense heat capacity of the oceans. If they absorb heat from land areas, the temperature of those areas could go down even though the total heat held by the planet goes up.

Also notice how this, like any claim that the Earth as a whole is warming or cooling, contradicts the first argument.

During most of the last 10,000 years, the concentration of CO₂ was also essentially constant. But once CO₂ began to rise with industrial activity, it too became a positive feedback effect.

The atmosphere now holds 40 percent more CO₂ than it did before the start of the industrial revolution. Temperatures have been rising in concert — not smoothly and continuously, but always trending upwards.

Computer models do not spring full-grown from the forehead of Zeus. Like most other large-scale software, they are developed incrementally, their performance is continually tested against reality, and the modelers improve their fidelity by incorporating new physical phenomena (e.g. the effects of clouds and aerosols.) But because mainstream scientists cannot precisely predict the state of climate in any future year (e.g. 2100), Denialists are able to sow doubt about their entire story of rising seas and other hazards.

It also reflects a serious misunderstanding of scientific uncertainty. Scientists know their knowledge is limited. This is why they present a range of results, and often qualify it by describing the assumptions that underlie their work. In a world where trust demands a simple answer delivered with absolute confidence, reliable scientific results may seem untrustworthy. But science is inherently uncertain; that is its strength. Therefore, generally, the scientist one should mistrust is the dogmatic one.

Another point worth noting is that if it's impossible to predict future climate, things might change less than the scientists say they will (as many deniers assume.) But equally likely is the prospect that they might change even more, with correspondingly more drastic impacts.

Finally, despite all these qualifications, the latest computer models track the past fairly well, and James Hansen's 1988 GISS model closely matches the actual temperature of the subsequent 20 years.

CO₂ may not be the triggering event, but once warming begins, CO₂ sinks turn into sources. So it's plausible that the CO₂ released by a small temperature spike (due to changes in Earth's orientation or distance to the Sun) acts to amplify that spike, leading ultimately to the release of massive amounts of CO₂ trapped in soils, permafrost, and ocean waters.

This also seems to be the reason that cooling takes so much longer than warming. Once released, the CO₂ stays in the atmosphere for hundreds or thousands of years.

It is true that, as they warm up, CO₂ sinks turn into sources. So it's plausible that the oceans, as well as soils and permafrost, could be releasing CO₂. However, four lines of evidence show that this is not happening.

First, the oceans are gaining CO₂, not losing it. This is measured by direct sampling, and is also the reason why ocean acidification is becoming a concern.

Second, getting the twentieth century's increase in CO₂ concentration from the oceans would take a temperature rise of 5°C (9°F) over that period. That is 8.3 times the rise we measure.

Third, carbon isotope ratios of the CO₂ are shifting toward those of fossil fuels. Carbon-14 is constantly replenished in Earth's atmosphere by cosmic rays, so CO₂ above ground has a fixed percentage of this radioactive carbon isotope. Fossil fuels, however, have been buried long enough that most of their C-14 has decayed away. Of the two stable carbon isotopes, C-12 and C-13, plants prefer the lighter C-12 over C-13, so plant CO₂ has less C-13 than the atmosphere. Thus, as more fossil fuels are burned, both ratios (C-13/C-12 and C-14/C-12) go down.

Fourth, oxygen leaves the atmosphere at a rate consistent with the fuel combustion.

Even though we now have spot coverage of a few places on the surface of Mars, and can measure temperature accurately at those places, we are far from being able to measure general warming on its surface. Distant Pluto is even harder to measure. We do have indications, based on monitoring the IR light they emit, that both planets have warmed since we began to examine them.

However, our satellites don't measure increased solar output. All satellite measurements of the Sun since 1978 (as long as we've had satellites to do that) find a slight decrease in its output. So there must be some other cause for this warming. Dust storms may account for Mars' warming. As for Pluto, the explanation seems to be that methane was vaporized at its closest approach to the Sun in 1989 and has since been acting as a thermal blanket. Yes, it looks like Pluto is in the grip of greenhouse gas.

And forgive me for beating a dead horse, but if Earth has no mean temperature, Mars and Pluto cannot either.

This is actually the most plausible objection to AGW. However, it is not conclusive; global data for the period are too fragmentary. Most important, the globe's temperature during the last fifty years has risen faster than previously, and the rate of increase is itself increasing. And that rise tracks the concentration of CO₂; both appear to be rising exponentially.

Studies have shown that human activities produce over 100 times as much CO₂ every year as volcanoes do.

Even absent those studies, if CO₂ from a recent volcanic eruption affected temperatures worldwide, there should be a spike in the CO₂ concentration around that time. Instead, the measurements show a fairly smooth upward curve.

And another thing. If it were true, we should see a spike in temperature, at least locally, with each eruption, like Mount Saint Helens in 1980 or Mount Pinatubo in 1991. Instead, we see the opposite because the dust and sulfur compounds expelled by the eruption make more clouds, causing a temporary cooling.

Basically, the situation is that parts of Antarctica are cooling and other parts are warming. There are few monitoring stations on the ice-covered continent, and most of these are on the coasts. Even so, the temperature records go back only a few decades. But it is clear that the West Antarctic Peninsula has warmed by several degrees. So, while the interior does appear to be cooling, the blanket statement that the entire continent is cooling is false.

There is a permanent circular wind pattern surrounding Antarctica. This carries air warmed by the oceans across the West Antarctic Peninsula, but blocks it from the interior. Scientists think that the "ozone hole" — caused by CFCs — contributes to the cooling of the interior. So one effect of civilization may be partly compensating for another. But if this is true, the cooling of Antarctica's interior should stop as the ozone hole closes.

The main thing to take away is that even if all of Antarctica were cooling, it would say nothing about the planet as a whole. A related point is the increased snowfall observed in the interior. This fits with the predictions of global warming theory. More water evaporates from warmer oceans. When that extra water vapor hits the cold near the poles, it freezes, leading to more snowfall. This is true in Greenland's interior too. Still, measurements indicate both land masses are losing more ice than they are gaining.

What happens is that cities, with their large areas of asphalt and concrete, absorb the Sun's heat during the day and release it at night, causing islands of unusual warmth. This is known as the Urban Heat Island Effect (UHIE). Since most temperature-measuring stations are located in or near cities, according to the argument, global mean temperatures are artificially skewed upwards.

There are four things wrong with this argument.

First, it's not true that most stations are located in or near cities.

Second, satellites also measure Earth's surface temperature, and they show the greatest warming trend in remote areas near the poles.

Third, even the record from just the city stations shows an upward trend that cannot be explained by growth in city energy use or population.

Fourth, it's easy to correct for the bias caused by urban stations, and this is being done.