Captain Imperio Answers Your Climate Questions

Q: How does the current temperature compare with past temperatures?

A: It’s hot compared to the last five hundred years, and maybe even compared to the last several thousand years. It’s cold compared to most of the last 500 million years.

Q: How about CO2? How do current CO2 values compare with those in the past?

A: Current values are high compared to most of the last several hundred thousand years (see: here) but much higher concentrations of CO2 occurred in the past. Before life originated, much of the atmosphere may have been CO2. Values for most of the last 500 million years appear to have been much higher than at present, but considerable uncertainty exists here

Q: If it was hotter in the past, and there was a lot more CO2, why should we worry? Is mass extinction a threat?

A: Mass extinction is already happening, due to habitat destruction, mostly but not entirely independently of warming. Clearly life can adapt to many temperatures and CO2 concentrations, but we, and all our close relatives, evolved in conditions much like the present (cool, low CO2), and there is some evidence that primates almost disappeared in the last big CO2 spike.

Most don’t consider current global warming trends a threat to human existence though. What it does threaten is massive economic and ecological disruption, with large parts of the Earth, including some highly populous portions, likely to be come much less habitable.

Q: How do we know there is a greenhouse effect?

A: An object is heated when it absorbs energy. One of the ways this can happen is through absorption of solar radiation (light). In addition to absorbing radiant energy, all objects also radiate (lose energy by emitting radiation). The amount of radiation emitted increases rapidly with its temperature (as the fourth power of temperature). We are all pretty familiar with that phenomenon from our experience with hot objects (like glowing coals) and even warm objects, like a big stretch of asphalt just after dark.

The Captain lives in a very sunny part of the country, which is often hot. For this reason, he likes to buy white cars. While wandering through a dealership, he notices that while a black or dark red car will be hot enough to burn his hand, a silver car will be only fairly warm, and a white car might even be cool to the touch. This, he knows, is because while all the cars are about equally efficient at cooling themselves by infrared radiation or conduction into the air, the darker colored cars are much more efficient at absorbing sunlight. Thus it is with all sorts of objects left in the sunlight.

White car or black, though, there has to be an approximate balance between energy incoming and energy outgoing (otherwise the temperature would keep changing). This balance comes about by the temperature of the darker car increasing until the amount of energy it radiates and has conducted away is equal to the amount it absorbs.

The same principles apply to planets, in some what simplified form, since they need to shed nearly all their heat by radiation (conduction not working well in the near vacuum of space). We can measure rather accurately how much heat the planets absorb from the Sun. We can also measure the emissive efficiency of the Earth’s surface. Putting those numbers into equations of radiative balance, it’s fairly easy to compute (it’s a typical problem assigned in beginning physical climatology or atmospheric radiation courses) that the temperature of the Earth should average out to be -18 C (0 F). In fact, the planet is a lot warmer, about 15 C (59 F). The difference comes from what we left out – the atmosphere. It acts as an insulating blanket that makes the Earth less efficient at radiating its heat – and insulating blanket that accounts for the extra 33 C (59 F) that makes life possible on the planet.

Similar effects are at work on Mars and Venus, dramatically so in the case of Venus, which is about 500 C warmer than the simple calculation suggests. Mercury and the Moon, lacking atmospheres, don’t experience this effect.

Most of the greenhouse effect for Earth comes from water vapor, but CO2 also contributes 10 C or so, and a few other gases also contribute. On Venus, which has 300,000 times as much CO2 as Earth, CO2 is the big player, but other minor constituents play a key role in plugging some “leaks” in the CO2 blanket.

I would add that such denialists as Lindzen and Motl admit that there is a real greenhouse effect - they just claim that it's unimportant compared to other effects, and that global warming probably isn't really that bad for you anyway.