How We Know Recent Global Warming Is Not Natural

Dr. Roy Spencer, like Dr. Richard Lindzen (the subject of a few recent articles), is one of very few climate scientists who remain unconvinced that most of the the recent global warming has been caused by humans (anthropogenic).  Dr. Spencer has grown frustrated with the fact that most of his climate scientist colleagues conduct research under the premise that the recent warming is anthropogenic, and in an article on his blog, has thrown down the gauntlet:

"Show me one peer-reviewed paper that has ruled out natural, internal climate cycles as the cause of most of the recent warming in the thermometer record." 

This challenge is problematic for a few reasons.  Firstly, the fact that research has not ruled out a hypothesis does not mean the hypothesis necessarily has any validity.  For example, there have been no peer-reviewed papers ruling out leprechauns as the cause of most of the recent global warming, either.  But perhaps more importantly, our understanding that humans are causing global warming is not based on just one scientific study, but rather a very wide range of scientific evidence.

For example, scientists have measured the amount of heat being re-directed back towards the Earth's surface due to the increased greenhouse effect.  Quantifying the amount of global warming that this will cause simply involves multiplying the increased downward energy by the climate sensitivity.  As the name suggests, climate sensitivity is a measure of how sensitive the climate is to this build-up in heat - how much the planet will warm in response to an increase in the amount of greenhouse gases in the atmosphere.

Therefore, in order to argue that humans are not the driving force behind the current global warming, skeptics like Spencer and Lindzen require that the climate sensitivity to increasing greenhouse gases is low.  The problem with this position is that there are many lines of evidence that the planet will warm between 2 and 4.5 degrees Celsius (°C) if the amount of carbon dioxide (CO2) in the atmosphere doubles. 

For example, some scientists have studied the climate response to recent large volcanic eruptions, which can have a measurable impact on global temperatures.  Other studies have examined how the global temperature has changed in response to changes in solar activity.  Some other research has compared CO2 and global temperature changes over the past thousand years, and tens of thousands of years, and hundreds of thousands of years, and even millions of years ago.  We can even compare how the temperature has changed over the past century to human-caused atmospheric CO2 changes.  In every case we arrive at this same climate sensitivity range of 2 to 4.5°C, and the most likely value is 3°C for a doubling of atmospheric CO2.

If we take the lower end of this range, even a 2°C climate sensitivity would mean that humans have been responsible for more than half of the global warming over the past century.  So in order for Spencer and Lindzen to be right, all of these different lines of evidence which are in agreement with the likely range of climate sensitivity would all have to be somehow wrong, and all biased high.  Not an impossibility, but certainly not a likely scenario, either.

There are also many "fingerprints" of human-caused global warming.  For example, as the Earth's surface and lower atmosphere have been warming, the upper atmosphere has been cooling.  There are not many mechanisms which can explain these observations, but they are precisely what we would expect to see from human-caused global warming.  As the concentration of greenhouse gases in the lower atmosphere increases, they effectively trap more and more heat in this lower layer, causing it to warm and causing the layers above to cool.  Another human "fingerprint" is the higher rate of warming at night than during the day.   This is because at night, when the surface is cooler and not being bombarded by solar energy, the increased amount of greenhouse gases are able to make more of a difference in the surface temperature.

Dr. Spencer has proposed an alternative to the anthropogenic global warming theory.  He suggests that some unknown mechanism has caused global cloud cover to decrease over the past century.  Low-level clouds cause a cooling effect by reflecting sunlight, so if these types of clouds become less prevalent, it can cause the surface to warm.  However, this hypothesis cannot explain the "fingerprints" describe above.  A decrease in cloud cover would not cause the upper atmosphere to cool.  Nor would it cause nights to warm faster than days - quite the opposite.  Cloud reflectivity only plays a significant role during the day when being bombarded by sunlight.

Dr. Spencer also suggested in his blog post that the "null hypothesis" should be that global warming is caused by natural factors.  A null hypothesis is basically the default assumption which a scientific study sets out to disprove.  It's true that until recently, global warming (and cooling) has been caused by natural factors.  However, even natural climate changes must have a physical mechanism causing them.  Scientists have investigated these natural mechanisms (the Sun, volcanoes, the Earth's orbital cycles, etc.), and they simply cannot explain the global warming over the past century.  Spencer's new hypothesis - that some unknown mechanism is causing cloud cover to change, which in turn is driving global temperatures - is a new idea with very little supporting evidence.  Conversely, our understanding that human greenhouse gas emissions are driving global temperatures has a proverbial mountain of supporting evidence.

Skeptics like Spencer and Lindzen believe that the default assumption should be one which requires that a very large body of scientific evidence is wrong.  The only alternative hypothesis they have put forth cannot explain the many empirically-observed "fingerprints" which are consistent with human-caused global warming.  Although Spencer's unspecified "natural internal cycle" hypothesis has not been explicitly disproved, there is a very low likelihood that it is correct.  For this reason, we should operate under the assumption that humans are causing dangerous global warming - an assumption which is supported by a very large body of evidence - until the skeptics can provide solid reason to believe that this scientific theory is wrong.

Posted by dana1981 on Monday, 7 February, 2011

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