Human Limits

Exploring performance and health with Michael J. Joyner, M.D.

Photo of Michael J. Joyner, M.D.

Exercise vs. Global Warming: Gradually Then Suddenly?

In early May atmospheric CO2 reached 400 parts per million (PPM).   This was seen as a major milestone in global warming and a harbinger of more bad news to come.   The figures below are for atmospheric CO2 since the late 1950s and the relationship between atmospheric CO2 and global temperature since 1880.








Climate Change Skeptics

There is skepticism about the reality of climate change and the extent to which it is being caused by human activity.   Some of this is highly political and driven by all sorts of other considerations, notably money and power.  However, on balance the weight of scientific evidence favors a major role for human activity in global warming and I find the arguments for this position much more convincing than the arguments against it.  I am a physiologist/physician not an ecologist, economist, or atmospheric physicist, so what do I have to add?


Gradually Then Suddenly!

With that question as a background, here is what a physiologist has to add along with the relevance of Hemingway’s famous lines about bankruptcy in “The Sun Also Rises”:

“How did you go bankrupt?’ Bill asked. ‘Two ways,’ Mike said. ‘Gradually and then suddenly.’


Gradually then suddenly describes a lot of things in physiology and biology.   For example when people stand up the blood pools in their legs away from the heart and this tends to make blood pressure fall.  In response the nervous system constricts the blood vessels and increases heart rate to keep blood pressure normal.  This works well, but only for a while.  If people stand up for too long eventually the responses noted above “fail” and most of us will faint suddenly.  The video clip shows some examples of just how fast this can happen.


click here for video

Another good example of gradually then suddenly comes from exercise physiology.  As people go from rest to mild or moderate exercise the lactic acid levels in their blood do not increase much and then the levels start to rise a lot as they go harder.  The figure below shows an example of what is seen during an exercise test in an elite athlete.





Exercise Metabolism vs. Earth’s Metabolism?

The exercise example is perhaps a good analogy for climate change.  Our ability to sustain exercise for prolonged periods of time starts to drop at exercise levels above the lactate threshold.  Metabolism ramps up and not much happens for a while and then a lot happens above the “lactate threshold” that can limit exercise duration.  In healthy young people metabolism can increase 5-6 times above rest without too much difficulty.  In elite endurance athletes, metabolic rate can increase perhaps 15 times above rest before things get too out of balance.  If we think of global CO2 production as a form of metabolism then perhaps the earth as an organism can adapt to pretty dramatic increases without much happening.    The figure below shows what has happened to global CO2 production since industrialization.  Who knows exactly where the Earth’s CO2 climate threshold might be, but if we take the value at around 1900 as “baseline” we are above the 5-6 fold increase in metabolism that can be tolerated by untrained people.  It also looks to me like we are getting close to the 15 fold increase that can be tolerated by elite endurance athletes.  How much farther do we dare push these curves before something really dramatic happens to the environment over a short period of time?





The Price of Denial

In my last post I pointed out that as the developing world gets richer and older there will be even more pressure on resource consumption and energy use.   In rich countries like the U.S. denial about the implications of aging for things like Medicare and Social Security is leading to political gridlock, a lack of creative solutions, and long term financial choices that are not pretty.   The same is true for Earth as a whole when it comes to denial about climate change and the human factors driving it.   When has denial ever been a good policy option?


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