SkS Analogy 9 - The greenhouse effect is a stack of blankets

Tag Line

The greenhouse effect is like a stack of blankets on a winter night.

Elevator Statements

  1. More blankets = more warmth: The greenhouse effect is like blankets warming the Earth. If it is -18°C (0°F) in your bedroom, you need a few blankets to keep yourself warm. More blankets = more warming. Too many blankets and you sweat. So the point is that the greenhouse effect is a good thing, up to a point.
  2. More blankets means warmer inside, cooler outside: With an increasing number of blankets, the temperature above the blankets gets cooler, because more energy is trapped below the blankets. With increasing greenhouse gases (GHGs) in Earth’s atmosphere, the upper atmosphere gets progressively cooler, because more energy is trapped in the lower layers of the atmosphere. This is one way that scientists know that the recent warming is due to greenhouse gases and not due to increasing solar output. In the sleeping analogy, if you turned up the temperature in the room, it would get warmer both above and below the blankets. If the recent warming was due to a hotter sun, then both the upper and lower atmosphere would warm. But the upper atmosphere is getting colder, just as the top of the outer blanket covering you gets colder when you add more blankets but leave the room temperature the same. See the SkS article "Is the CO2 effect saturated?"
  3. It is not the rate at which you put blankets on, but the total number of blankets that determines your final warmth. CO2 emission rates don’t affect the final temperature due to global warming, except that if we slow the emission rates it buys us more time. It is the total CO2 emitted that determines the final temperature we will reach, just like it is only the total number of blankets over you that matters, and not the rate at which you put them on.1 Note: the emission rates are important for determining how rapidly the climate changes and increased emission rates will make it more difficult for animals and humans to adapt to the increased warming. But just like when filling a bathtub, to keep the bathtub from overflowing (i.e., keeping global warming to under 2°C), it is not the rate at which the bathtub is filled that matters, but the total amount of water put into the tub. Carbon budgets refer to the total amount of CO2 we can emit before we exceed a dangerous level of warming, just as a blanket budget represents the total number of blankets we can tolerate before we start to sweat and overheat. Some skeptics refer to a time about 100's of millions of years ago, during the late Ordovician when CO2 levels were higher, but earth was the same temperature as now, or cooler. They point to this time to imply that CO2 levels do not correlate to temperature. But during the Ordovician the sun was cooler (like a colder bedroom), so that the colder bedroom combined with more blankets = similar temperature as today. If you turn down the heat in your room, you need an extra blanket or two. Thus, with a colder room, your blanket budget is higher.

Climate Science

On the topic of the blanket budget, assuming that we warm 3°C for every doubling of CO2 (this is the average climate sensitivity used by the IPCC [Intergovernmental Panel on Climate Change]), this corresponds to the following temperature increase for given CO2 atmospheric concentrations. Each of these atmospheric concentrations roughly corresponds to a particular CO2 budget.
•    350 ppm CO2 = 1°C warming
•    445 ppm CO2 = 2°C warming (2°C is the target agreed to by the Paris Agreement)
•    560 ppm CO2 = 3°C warming
•    700 ppm CO2 = 4°C warming (considered by many Climate Scientists to be unbearable)
We are currently at about 406 ppm, increasing at about 2 ppm/year. This means that at the current emission rates we will have reached our budget for 2°C by the year 2035 and crossed into really dangerous territory. This is why Climate Scientists are saying that there is no time to waste for cutting our carbon emissions.

The budgets used by the IPCC  are based on scenarios more complex than the simple math above, but IPCC budget estimates also often assume that we will be able to suck CO2 out of the air and bury it in the ground … at some time in the future. My simple estimate uses a climate sensitivity of 3°C/doubling of CO2, and assumes that we will not be successful at sucking CO2 out of the atmosphere and storing it underground. After all, to bring CO2 concentrations down means that we have to suck all of the CO2 emitted in a given year + an extra amount. Is that feasible? Great if we succeed, but at current emission rates we will be at our budget limit by 2035, and the planet will be warming while we are trying to bring these massive negative emissions technologies online. A good read on the subject is Kevin Anderson, or if you can watch him as well.

1. The rapid rates of increase in emissions we are seeing will create huge difficulties in adaptation, but simply slowing rates of growth will not be enough. What counts is total emissions load in the atmosphere that we eventually get to, as this will drive up temperatures for millenia, because the Earth's system will take considerable time to re-absorb CO2.

Posted by Evan on Thursday, 29 June, 2017

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