Why is it so difficult for scientists to model potential climate change that would result from increased CO2?

Question

Why is it so difficult for scientists to model potential climate change that would result from increased CO2?

Answer 1

because all green plants use CO2.

Answer 2

Good question.  To model climate change scientists have to make assumptions for how much of each of the major greenhouse gases (such as CO2, methane, CFC-12, and nitrous oxide) will be emitted per year, add in the influence from some assumed quantity of black carbon, subtract out a cooling influence from some quantity of sulfur dioxide (aerosols).  

Some portion of the released gases will be taken out of the atmosphere by forests, add some amount due to that year's growth, but subtract out some amount for that year's clearcutting.

Then the additional feedback mechanisms must be factored in.  The direct warming from the factors above will cause a certain amount of warming, but that will cause the ocean to heat at some rate (depending upon currents, mixing, and cycles such as the ENSO and AO oscillations, it may not be constant).  Ocean heating will cause CO2 to be released and increase water vapor in the atmosphere (another greenhouse gas), feedback mechanisms that lead to more warming.

Warming will cause reduced ice pack and glacial coverage, decreasing albedo (reflectivity of the earth's surface, causing more energy to be absorbed (another positive feedback mechanism that leads to more warming).  

As the earth heats, melting tundra can release methane, a very potent greenhouse gas, another positive feedback causing more warming.  Methane may also melt out of frozen ocean sediments ("clathrates").

As the water content of the atmosphere increases, certain types of clouds can increase temperatures (act as a barrie to outgoing radiation) or decrease temperatures (raise the earth's albedo and reflect incoming radiation back out).

Then major unpredictable events such as volcanoes have to be factored in (they first cool through sulfate aerosols, then heat due to CO2 release.

So there are a lot of variables that need to have assumptions filled in.  Any factor that is not accurately forecasted in the first few years (including the ones that can't be forecasted, such as volcanoes) may have an increasing effect on the accuracy of the forecast as time passes by.  This is why forecasts must be updated as real events such as volcanoes happen.  The model has to be re-run to reflect the "real" event, so we can determine how accurate the model is and to make future models and forecasts more accurate.  Other factors such as solar cycles are also factored in.

All the while weather creates a large "noise" component on top of the gradual underlying "signal" of warming, so any one year's forecast or resulting temperature will always be fairly unimportant.  

Other than these minor challenges, climate forecasting is pretty easy!  It's amazing that in such a complicated system the models have been as accurate as they have.

Answer 3

J.S. said:

" It's amazing that in such a complicated system the models have been as accurate as they have."

It depends on what you mean by accurate--if you mean a particular scenario of a particular model is fairly close to observations for a period of time, then yeah, they have been accurate.

Answer 4

Making a mathematical model for global warming from CO2 is fairly easy.  Lots of researchers have done it and concluded that CO2 would not result in enough warming to measure.  The difficult part is making a plausible model that shows that significant warming is caused by increasing the CO2 concentration of the atmosphere.  

Climate researchers have to include all kinds of speculative feedback factors and other theories to construct a model which predicts significant warming.

Answer 5

Climate models rely of lots of guesses.  For instance: A cloud blocks the sun so some of the sun's heat is lost to space (net cooling).  It also holds in some of the heat because IR energy from a warm ground can not radiate to space (net heating).  Obviously these are conflicting and no one really knows the total effect.  Someone guesses based on the best measurements around which can be messed up by day to day temp changes due to other causes.  Same for soot in the atmosphere.  Same for lots of things.  The early climate models which started global warming predicted the same rain levels for the Sahara desert and Ireland.

Answer 6

Because there are so many variables involved, no scientist is sure what the variables are, and no scientist knows the values of all the variables of which they are aware.  To compound matters, many unscrupulous researchers deliberately exclude variables in order to produce a desired outcome; most models do not account for solar variability or the height of the tropopause.

Answer 7

Because it doesn't

Answer 8

Not enough data to back the model theories.

Where data exists there are a lot of anomolies that cast doubt on the validity of the theory.

The scientists still cannot predict accurately what the weather will be like this afternoon let alone cause and effect of changes in variables such as CO2 in the atmosphere.