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Science & Technology

A Guide for Skeptics of Anthropogenic Climate Change

Author: Larry Molnar, Professor of Physics and Astronomy
Minds exclusive
Posted on: Dec 18, 2009

The comment that has been in my head for some time, even before the recent controversy over the e-mails of some climate change researchers, is that while climate policy is a complex subject, one on which different people will have different viewpoints based on differing personal advantages and disadvantages as well as differing moral codes, the basis of anthropogenic climate change is simply not that controversial.  Apparent controversy results from talking around the core issues.

My contribution to the discussion then is to state the core logic of anthropogenic climate change in three steps.  I call this a guide for skeptics in that any genuine skepticism should be based in doubting one of these steps. Simply name the step you doubt and give a source of reliable contrary information, and then we have a genuine challenge.  On the other hand, if you buy these steps, you have bought anthropogenic climate change.  Everything else is a detail of who will benefit, who will suffer, and when.

I will also add a postscript on the nature of the IPCC, and respond to the previously-stated concern about how reliable their forecasts are.

1) Humans emit climate changing gases.  (The most important of these turns out to be CO2, but there are many others.  It is not difficult to document the rate of emission, as business reports document the rate at which fossil fuels are extracted from the Earth.)

2) These gases accumulate in the Earth’s atmosphere and climate.  (The accumulation is directly measured from many monitoring stations around the Earth.  The identification that the accumulation is due to the anthropogenic gases in particular is established by looking at the isotopic abundances.)

3) The “radiative forcing” of these accumulating gases can be directly calculated.  (That is to say, the degree to which the anthropogenic gases enhance the warming effect of sunlight is the product of the absorption or scattering properties of the gas times the quantity of the gas.  The IPCC reports list the radiative forcing for each contributor individually, along with the uncertainties.  The total radiative forcing is simply the sum.  They include every known contributor, with references for how much there is as well as what the absorption properties are.  Note that they also include anthropogenic emissions that have negative radiative forcing-that cool the Earth-such as aerosols.)

The IPCC reports add these results to natural phenomena (volcanic eruptions and solar variations) and find they reproduce the Earth’s past climate very well.

Note the word “climate” refers to timescales of decades to centuries. These are the relevant time scales.  Much shorter time scales are called “weather”, and show great fluctuations.  Much longer time scales just are not relevant to human planning. (This is because there is plenty of time to prepare for changes and because other natural factors bring about changes on those time scales.)  Much of the irrelevant verbiage about climate change comes from confusing weather for climate.

Back to the models.  With different inputs, these same physical models are used by planetary scientists to successfully understand the climates of other planets (including Venus which has a much greater greenhouse radiative forcing).  It is the agreements both on Earth and elsewhere that give confidence in the models.

As a computer model, they can also rerun the past century without anthropogenic changes.  This allows an estimate of whether the change is significant.  The result is that anthropogenic changes only begin to be significant over the past 30 years or so.  There just wasn’t enough CO2 emission before that to matter much.

I assert that what I have written so far is not controversial, because I do not see anyone challenging these core items.  There are also not any computer models that put the observations for the last century together and do not find a significant change over the last thirty years.

To model the future,  you need to know how much fossil fuel will be burned.  You also need to know how rapidly the system will respond to the changes.  There are therefore increasing uncertainties in just how fast climate will change.  But while the timing may be uncertain, the outcome is not.  If we burn all the fossil fuel available, enormous changes will occur that will be devastating for large populations.  Those not directly affected will be affected indirectly by the migration of those that are affected.

Some ask about the reliability of IPCC predictions.  The IPCC is by its organization a conservative organization.  When it makes predictions (like those I paraphrase in the preceding paragraph), it clearly labels everything with the appropriate level of uncertainty.  When an effect is not yet well understood, they note this and leave it out of their calculations.  This has led to poor predictions in the past.  A relevant example is sea level rise.  In their third report (2001), they did not foresee rapid melting of the Greenland ice sheet.  It was not melting rapidly at that time, and the direct effects of increased heat at the surface were not enough to make it melt.  However, in their fourth report (2007), they found the rate of sea level rise has doubled in the last decade.  It is also now understood that Greenland contributes to this greatly because it is not necessary to directly melt the sheet.  A small amount of meltwater sinks to the base of the ice sheet, and the sheet as a whole can simply slide downhill into the ocean.

Note that the prospect of melting the Greenland ice sheet is one of the enormous changes I was referring to.  It would flood land currently occupied by hundreds of millions of people.  And once melted, there is no prospect for rebuilding the ice sheet until the next ice age.  (This is because the average temperatures in Greenland will rise as the elevation of Greenland goes down.)

I could give other examples, but the sobering point is that IPCC predictions are conservative, so the error tends to underestimate significant problems.

I would like to end by giving my viewpoint in terms of a comparison.  If we were introducing a new heart medicine that would affect millions of people, we would insist that we knew for sure that making the change was safe before allowing it to happen.  If we were making a fundamental change to the food supply, we should insist on the same standard.  (I believe the spread of mad cow disease came about because of a change in animal feed that was not properly thought through in advance, leading to expensive and dangerous consequences.)  Along the same lines, before we make a fundamental change to the chemistry of our atmosphere and oceans, we should be certain it is not a mistake.  Yet we are making just such a change even though our best atmosphere models say this is not safe.  (To my knowledge we don’t even have reliable models to say what the CO2-induced changing acidity will do to the food chain in the world’s oceans.)  Those that insist we go forward offer only the hope that the models might be wrong.  In my view, this is the wrong standard to use.