Making Sense of Our Changing Climate: 2. Weather and Climate

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I often find people using the terms weather and climate interchangeably - this creates much confusion, and makes any serious discussion of the important subject of climate change (CC) very difficult.  The media, political leaders, industries with vested interests have conveniently exploited this confusion. 

Both weather and climate describe certain properties of the atmosphere - like temperature, wind speed and direction, humidity, rainfall etc.  The difference between weather and climate is that 

weather describes the atmospheric conditions at a particular time and location, while 

climate is the averaged atmospheric conditions of a region over a period of time (normally 30 years or more).
  
Weather in London at say 3 pm tells us what the temperature is, whether it is sunny, cloudy, raining or snowing, what the wind speed is and what is the direction of air flow etc.  The weather in London after a little while may be vastly different - weather can change rapidly. Likewise, weather in Manchester at 3 pm might be completely different from that in London at that time.
Climate pertains to an average of atmospheric properties over a 30 year period and refers to an extended region.  The region may be a county or a country or even larger areas.  
Atmospheric gases are always moving causing weather to change continuously.   Their mixing over an extended period of time enables us to describe an average atmospheric condition for the region.  The set of such mean atmospheric conditions (temperature, rainfall, snowfall etc.) is the climate for that region.

Weather is the atmosphere that you feel while climate is what you expect the atmospheric conditions to be like - and the two can be very different.  For example, for UK climate, I expect that the average temperature in July will be about 17C.  In London the temperature in July may routinely change over a range from 10C to over 30C.  Even within the UK, we can have several different climatic zones - Scottish climate is colder and wetter than the English climate.

Following slides demonstrate the difference between weather and climate

SLIDE 1:  Data from

The data in slide 1 demonstrates the variability inherent in the weather at a location.  Red and blue bars represent larger variations from the more normal weather (green bars) that one encounters.  Even the green bars have significant range.  The grey curve shows the observed maximum temperatures since records were kept.  Interestingly, in many of the months in 2018, maximum monthly temperatures almost reached recorded maximum - an indication of global warming trends (more of this in later blogs).
Similar variability is observed in other atmospheric parameters like precipitation, wind speed etc.
The main take away from the figure is that weather does change rapidly over a large range - and it is highly unpredictable.  

Climate, on the other hand averages out short-term variations and gives us a very good idea about what atmospheric conditions to expect (on average) in the vicinity of a particular location.

Slide 2 shows monthly averaged temperatures in the UK over 5 years.  For a given month, there is still a spread but is typically less than 3C. Looking at slide 2, we could say that February in 2018 was 2C colder than January (but 3C warmer in 2019) - against an expectation of similar temperatures in both months. 

SLIDE 2:

Climate of a region is defined as average atmospheric condition over 30 years.  Atmospheric data are obtained from a large number of weather stations distributed in the region and a mean value calculated for each parameter.
One can then start to talk about annual mean temperatures (AMT) for a region or annual mean rainfall etc.  Staying with the UK,  Slide 3 shows how AMT has varied from 1910 to 2017.  The fluctuations in AMT are typically half a degree centigrade from one year to the next.
  
SLIDE 3:

We make some interesting observations from this figure. Between 1961 and 2010, the 30-year mean temperature has increased by 0.4 to 0.5C for the UK and England. We can state that the UK climate has warmed by half a degree during the second half of the 20th century.  The warming trend is particularly pronounced since 1990, although one can have (as in 2010 and 2011) a sharp drop in AMT for a brief period.  From the data in the slide, I also notice that the English climate is 2C warmer than the Scottish climate.

I have discussed the case of the UK climate to demonstrate the difference between weather and climate.  The warming trend over the last century or more is not particular to the UK only but the Earth's climate is showing a consistent warming trend - generally called Global Warming (GW) or Climate Change (CC). 

Of all the atmospheric parameters, the temperature is the most important and is universally used as a primary indicator of the climate.  Most input of energy to the Earth is from the Sun and majority of this energy is absorbed by the land and seas in the low to mid latitude regions lying on either side of the equator.  The resulting increase in temperature of the tropics warms the atmosphere causing circulation of atmospheric air that then distributes the energy to northern latitudes.  This drives wind currents which also carry the evaporated water from the oceans to create rain and snow.  Distribution of air currents and precipitation determines the climate in different parts of the globe. See my blog on Air Circulations etc.

I shall discuss the global situation in my next blog.

Making Sense of Our Changing Climate: 1. Introduction

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Much has been written about Climate Change (CC).  I have been involved in creating public understanding of this important subject for the past 15 years.  My general impression is that while there is more awareness of the term CC, there is not a great deal of enthusiasm/understanding among the general population about what may be done to alleviate the effects of CC.  I shall publish a series of blogs to explain the science of CC in a way that a layperson without a science background can appreciate the situation.  

I start by introducing the subject in a very general way:

During a cold spell, one hears comments like - 
'So, where is Global Warming now?', 
'Climate Change is not really happening', 
'OK, Earth might have warmed a little, but it is getting colder again' etc.

Most of the time, these are genuinely felt comments from people not directly involved in meteorological investigations, and represent normal (defensive) reaction to the much talked about dire/threatening consequences of climate change.  Climate Change (CC) is variously described to bring misery to the lives of billions of people - by causing extreme weather events, sea level rise, forced migration, droughts, famines and much else.  Some people even call CC as an existential threat to the human race.  But, this is all going to happen by the end of this century - most bad thing will properly be felt in 50 or 100 years time when a good fraction of people alive today will no longer be around.   

That is the difficulty; CC is a problem of the distant future - we are okay today. It is easy to think that CC is not something that we need to pay too much attention just now - it is a few scientists who are making predictions on impending doom that may or may not happen but we are being asked to make some serious sacrifices in terms of changing the way we live, to give up our extravagant lifestyles - not eat meat products, reduce road and air travel and much more - in fact abstain from almost everything we enjoy.  We are even asked to buy used clothes to save the planet.  

It is not surprising that politicians and authorities in power do not take CC seriously - they have to win elections, safeguard their power base - one can't do that by asking voters to make serious sacrifices for something that is in the future and might not even happen. They pay lip service to mitigating, even reversing, CC but do little in practical terms.  

Against such prevailing background of climate change scepticism, one has to work much harder to convey the scientific case for climate change to the general public.  Scientists are particularly poor in their communication with communities and lay people whose lives will be impacted so severely by CC and who can put real pressure on politicians and authorities to take action.
Also, CC is a global phenomenon and must be addressed by all nations of the world in an effective way. So far, consensus on a global level has been specially difficult to achieve because vested interests - mostly the fossil fuel industry - have sabotaged/confused the science and it is not possible to agree on some fundamental causes of CC. Committing huge sums of money is not easy when the target has been made so fuzzy.

I list some obvious question with short answers:  

• Who is responsible for CC? -  Mostly developed nations in OECD so far, but recently China too.
• What has caused CC? -  Population increase and unbridled, wasteful consumption habits are the primary reasons.
• What should be done to stop further damage by CC? -  Control population and reduce consumption
• How does one achieve this? - Developed countries point finger at developing countries for recent increase in emissions of green house gases (GHG) leading to CC.  Developing countries blame the rich countries to have caused the problem over the past 200 years.  It does seem unreasonable for rich countries to ask others to stay poor while not wanting to sacrifice their high consumption lifestyles.
• Will renewable energy (RE) solve CC? - Hardly. The main problem is that too many people are consuming and wasting too many resources. RE is a mere diversion - it is too little and too late.
• Who will pay for CC mitigation/reversal? - Difficult question to answer. Nations have been making commitments since 1997 Kyoto protocol but have rarely implemented these.  
• Is it too late to solve the CC conundrum?  -  Probably. Most scientific opinion is beginning to converge on a mean global temperature rise of about 3 degree centigrade (C) provided nations start to take some serious action.  On the current trajectory, a rise of 4C or even greater is likely.  
• Should one just give up and hope for the best?  - Not advisable.  To continue as now (business as usual) will create many bad situations that will be irreversible - some of them will cause runaway uncontrolled changes whose consequences cannot be predicted at present.  In this scenario, CC might indeed become an existential threat for the human race.
• Who will be affected most by CC? - Majority of people in the developing countries and the poor in rich countries.   

I have indicated here that overpopulation and overconsumption are the driving forces of CC.  Until about 1990, OECD countries overconsumed resources and set CC in motion.  With the emergence of China and to some extent India, a much larger population has joined the binge - resulting CC trajectory is looking much worse.  Ironically, developed countries continue to consume several times greater resources per capita than people in developing countries do. The trajectory of resource consumption and hence global warming is very worrying.

Superficially, it appears that the developed countries in Europe and USA have controlled their emissions - but this is misleading.  Developed countries have moved on from manufacturing to service industry and buy thier manufactured goods in great quantities from Asia - mainly China.  While the emissions from China appear to be rising, a lot of these products are consumed in Europe and America.  

Please click on the link below to understand the pressures that humans have put the Earth under, and how it is damaging our planet and destroying the welfare of future generations.

Over-population is 'Elephant in the Room'; Why Do We Not Address the Real Problem?  

In the next blog, I shall explain the difference between weather and climate.

Outdoor Air Pollution in Delhi - an Update

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'the pollution levels entered the "severe plus" or "emergency" category late Thursday night.  Delhi Chief Minister ... described the city as a "gas chamber" while distributing      5 million breathing masks'      .......... NDTV (1/Nov/2019) & BBC

Exactly a year ago, I had published an article detailing the science of outdoor air-pollution.   It is responsible for more than 4 million premature deaths, largely from vehicles and industry, throughout the world.  People in developing countries suffer the most serious pollution levels; people living in less affluent areas are the worst affected.  Air pollution (indoors and outdoors) deaths in India are estimated at 2.5 million per year.

About 75% of outdoor air pollution is due to human activities.  Most sources of air pollution are well beyond the control of individuals and demand/require concerted action by local and national level policy-makers working in sectors like transport, energy, waste management, urbal planning, and agriculture. 

Historically, big cities in China and India have had the worst outdoor air pollution.  China has taken steps to control pollution, reducing it by 17% between 2010 and 2015 but the situation in India has not improved at all.  It is same story year after year.  The slide shows the PM2.5 levels on 7th November 2017 in Delhi:

The outdoor air pollution is a problem not only in Delhi but in most of India.  According to WHO, india has 14 out of the 15 top most polluted cities in the world in terms of PM2.5 concentrations.  These are:
Delhi (153), Patna (149), Gwaliar (144), Raipur (134), Ahemdabad (100), Lucknow (96), Firozabad (96), Kanpur (93), Amritsar (92), Ludhiana (91), Allahabad (88), Agra (88) and Khanna (88)

In the end, the problem really boils down to the number of people living on the Earth. As more and more people move to western lifestyles with unbridled consumption; the human footprint, that is already too big, will increase further and our planet will just not be able to cope with the demands put on it. 
Air pollution is just one of the ways that the Earth is responding to our irresponsibility. 

PS:  4th November 2019:  The Delhi pollution continues with authorities unable to provide a solution.  

https://ektalks.blogspot.com/2018/11/air-pollution-invisible-killer.html

2023 Report:  (Outdoor Air Pollution Levels are getting worse)  All but one of the 100 cities with the world’s worst air pollution last year were in Asia, according to a new report, with the climate crisis playing a pivotal role in bad air quality that is risking the health of billions of people worldwide.  The vast majority of these cities — 83 — were in India and all exceeded the World Health Organization’s air quality guidelines by more than 10 times, according to the report by IQAir, which tracks air quality worldwide.

Common Sense and Superstitions - Personal, Cultural, Spiritual and Scientific Dimensions

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As member of a social group, I regularly meet my friend who holds deeply religious views and a doctorate in chemistry. Irrespective of the subject, we end up talking about how religious rituals/superstitions are fundamental to explaining everything that 21st century life throws at you. It can be difficult to keep a straight face when one has to argue against the solar system being constructed and operated according to some divine order described in the Indian mythology, etc. 

As with flat-earthers and climate sceptics, evidence based arguments are ineffective if your strongly held beliefs tell you otherwise. It is as if the brain has a bipolar or may be multipolar structure to it -  one could be totally rational about some things, but at the same time incapable of understanding the reality of many others.

One could say that the mind shows good common sense most of the time but can also be doggedly irrational/superstitious. The psychology is fascinating and I wish to explore this in more detail in this article.   
First we look at how common sense and superstition are defined - how we acquire them and why they serve important purpose in our lives.

Common Sense:  A quick search of some dictionaries tells us that common sense is 

1.  Basic level of practical knowledge and judgement that we all need to help us live in a reasonable and safe way.

2. Your natural ability to make good judgements and to behave in a practical and sensible way.

3.  Sound practical judgement that is independent of specialised knowledge, training, or the like; normal native intelligence.

                     -------------------------------------------------------

Have you read?     Laws of Nature, Common Sense and Religion           

                             Physics of Rainbows; Moonbows; Fogbows 

                             Why Do Humans Have two Front-Facing Eyes?

                    --------------------------------------------------------

Common Sense (CS) is generally attributed to a person's individual behaviour/judgement.  But, how does one acquire good common sense? - It develops with age through interaction with your environment and others in the society - it is like wisdom accumulated through experience.  However, there is much more to common sense than just on an individual level.

Cultural Common Sense:  While CS refers to something that an individual naturally possesses (or is expected to posses), there is a wider, collective undertone to the concept of CS. For example, when we say - 'no doubt that the court's decision is a victory for common sense' - there is an implied/accepted code of behaviour that common sense refers to.  CS is embedded in the culture of the society; an individual's CS is moulded by the cultural norms (customs, beliefs, traditions etc.) of the society he/she is living in. A rational belief, that is widely accepted and practised in a population over an extended period of time, becomes common sense - accepted by the majority of population without questioning.  For example, common sense tells us that the child should be vaccinated at the correct age to prevent the danger of catching infections. 

Cultural norms are different in different cultures leading to the corollary that an individual's CS in one culture will not be the same as that of somebody in a different culture - there is a culture divide. For example, on entering a house in tropical countries like India, it is common sense to take your shoes off to prevent dust and infections; and expect to be offered a glass of water. This is not so in the West.
Or, it is common sense not to violate somebody's personal space - the personal space in Romania may be 1.4 meters while in Argentina it is only 0.76 meters.
For another good example see this.
  

Cultural norms also change with time.  In the past, this was a slow process but in the modern  knowledge-based societies, travel and communications have become enormously more efficient with the result that there is significant intermixing of ideas and beliefs leading to greater cultural homogenisation.  There is also a greater evolving understanding of natural events and psychology.      

Intrinsic Common Sense: These concern with personal safety and physical welfare  - it is common sense not to touch a very hot surface or jump out of a high window or point a sharp knife away from your hands when chopping vegetables.  These are common to all and we follow them without debating what we should do.  Such CS are evolutionary - we are naturally safety conscious - and it takes extra effort to go against them.  

Common Sense and Laws of Nature:  Humans are distinguished from other animals by their curiosity to understand and make sense of our origins and surroundings.  Centuries of such enquiries/experience brought common sense explanations of many phenomena in spiritual, philosophical and scientific disciplines.  However, we are seriously limited by our capability in fully studying our surroundings in spatial and temporal domains. For example, we only see objects of size greater than about 0.01 mm, hear sounds in the range up to 20 kiloHertz and can not resolve events separated in time by less than about 0.001 sec.  It is not surprising that common sense theories built on such limited observational base spectacularly fail to make proper sense of the world and construct suitable laws of nature which can determine/predict what is happening in the physical universe.  Common sense does not sit at all comfortably with the counter-intuitive aspects of the most successful modern theories of quantum mechanics and relativity.  For a detailed discussion see 1 and 2. 

The inherent inability of common sense to properly explain natural phenomena and other fundamental questions has lead to much confusion. The development of personal as well as organised religions with many unsatisfactory aspects led people to search elsewhere for some sort of order like

Mysticism (search for truth, knowledge and closeness to God through meditation and prayer; ill-defined religious beliefs)
Superstitions (a belief or practice resulting from ignorance, fear of the unknown, trust in magic or a false conception of causation),
Spirituality (a sense of connection to something bigger than ourselves - typically involves a search for meaning in life)

and many others.  Here, I shall look at superstitions as these are observed by a great majority, if not all, of the populations throughout the world.

Superstitions:  are beliefs that are not based on human reason or scientific knowledge but are connected with old ideas about magic etc.
For example, Walking under a ladder brings bad luck or number 13 is unlucky - the street I live also does not have a house no. 13!  Superstitions are very different from common sense - CS is mostly based on rational analysis and past experience of members in a society.  Without common sense, life will be impossible.
Superstitions are by and large irrational  - they arise when something cannot be explained by rational thought.  Historically, before science could make sense of natural phenomena like thunder and lightning, people thought that these represent God's  displeasure and are sent there to punish them.  Many cultures consider rainbows as a warning of doom.  

Wanting more control/certainty is the driving force behind superstitions.  We tend to look for some kind of rule or explanation why things happen.  One believes in superstitions because sometimes the creation of a false certainty is better than no certainty at all.  Frequently, we express our superstitions through rituals believing they have some magical significance and will bring us good luck or ward off evil outcomes.  Keeping your fingers crossed, tapping on wood are rituals most people have followed.  People who are afraid of Friday the 13th, might change travel plans or cancel an appointment because of unnecessary anxiety such action will create. 
Similarly, superstitious thinking or behaviour can give us a sense of security and confidence; like carrying an object or wearing an item of clothing that you think had brought luck in the past. There may be an element of placebo affect here - if you think something will help you then it may just do that  - there is tremendous power in belief.  
Interestingly, intelligence seems to have little to do with whether we are superstitious or not - Harvard students rub the foot of John Harvard statue for good luck.  

I leave with some thoughts on the way things are going these days...

Obituary of Common Sense

(http://orthodoxengland.org.uk/obitcs.htm)

Today we mourn the passing of a beloved old friend, Common Sense, who has been with us for many years. No one knows for sure how old he was, since his birth records were long ago lost in bureaucratic red tape. He will be remembered as having cultivated such valuable lessons as: -Knowing when to come in out of the rain; - Why the early bird gets the worm; - Life isn't always fair; and -maybe it was my fault. 

Common Sense lived by simple, sound financial policies, don't spend more than you can earn and adults, not children, are in charge. 

His health began to deteriorate rapidly when well-intentioned but overbearing regulations were set in place. Reports of a 6-year-old boy charged with sexual harassment for kissing a classmate; teens suspended from school for using mouthwash after lunch; and a teacher fired for reprimanding an unruly student, only worsened his condition. Common Sense lost ground when parents attacked teachers for doing the job that they themselves had failed to do in disciplining their unruly children. It declined even further when schools were required to get parental consent to administer sun lotion or an aspirin to a student; but could not inform parents when a student became pregnant and wanted to have an abortion. 

Common Sense lost the will to live as the churches became businesses and criminals received better treatment than their victims. Common Sense took a beating when you couldn't defend yourself from a burglar in your own home and the burglar could sue you for assault. Common Sense finally gave up the will to live after a woman failed to realise that a steaming cup of coffee was hot. She spilled a little in her lap, and was promptly awarded a huge settlement. 

Common Sense was preceded in death, by his parents, Truth and Trust, by his wife Discretion, his daughter Responsibility, and his son, Reason. He is survived by his 4 stepbrothers; I Know My Rights, I Want It Now, Someone Else Is To Blame, and I'm A Victim. Not many attended his funeral because so few realised he was gone. If you still remember him, pass this on. If not, don't worry..join the majority and do nothing.

Limits to Sustainable Human Energy Production - A Community Outreach Feature

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Humans and animals produce energy to fund their daily activity. How much energy we produce at any time depends on what we are doing; our bodies have evolved various pathways to supply the correct amount of energy -- but there are limits.
  
For example, when we rest or do gentle exercise, aerobic (in presence of oxygen) energy production is sufficient, but in situations where the need to supply lot of energy appears suddenly - as in a 100 metre sprint - the body can produce the extra energy very quickly using anaerobic metabolism without requiring oxygen. While the resting metabolic rate (aka basal metabolic rate or BMR) is of the order of 75 W (or 1550 Calories per day; 1 Calorie = 1000 calories used in physics) for a typical adult male, peak demand in a 100 meter sprint may rise to 2500 W or more - an increase of about 35 times over the BMR value.  Energy production by the body at such high levels over an extended period is totally unsustainable (you quickly run out of reserves).  Similar situations happen in many other activities, e.g. in long distance endurance running or cycling events, where sustained energy production at a much higher level than BMR is ideally desired but has been found to be limited to only a few times BMR.

BMR is the energy expenditure of a person at rest not having eaten for 12 hours.  BMR accounts for more than half of daily energy expenditure and includes normal body cellular homeostatis, cardiac function, brain and other nervous functions etc.  The table shows how BMR budget is used by vital organs - meeting this energy demand is an absolute priority for the body.

It is apparent that almost three quarter of the energy budget of BMR is used by four or five vital organs.

The aim of this publication is to examine the evidence regarding the limits of energy production in humans during high-endurance/high-demand situations.
World record times in running/walking events are a  good place to start:

Notice that for longer distances (hence longer completion times) the athlete's average speed gets progressively lower.  The body's energy production rate is much reduced when extra energy demand is sustained over extended periods.
Swimming records follow a similar trend - swimming requires much greater expenditure of energy - for 100 m freestyle, the world record time is 46.91 seconds.

I shall look at two activities at extreme ends of the endurance spectrum: 
(1) One hundred meter sprint is probably the most energy demanding race where an explosive release of energy over an extremely short period of time, of the order of 10 seconds, is required.  It will be instructive to see how the runner manages the demand.
(2) Race across the USA (RAUSA) in which athletes run a marathon (42.2 km per day), 6 days per week for 20 weeks to cross the USA from the Pacific to the Atlantic coast.  In RAUSA, the runners must sustain a high level of energy production over an extended period. 
The slide summarises the observed situation: 

100 Meter Sprint: The world record of 9.58 sec. was set by Usain Bolt in 2009 in Berlin.  His remarkable sprint has been the subject of many analyses.  I discuss the details in the following.

Usain Bolt at, 90 kg and 6'5" or 1.956m is a tall person and during the sprint, his stride averages 2.45 meters.

Even over the short duration of the sprint, Bolt was beginning to slow down after 70m when he  reached a peak speed of 12.78 m/s.  During the first 30m of the race, Bolt accelerates strongly producing about 2500W (= 33 BMR) but then he runs steadily for the next 40 meters. The estimated theoretical limit of power output of an Olympic level sprinter is ~ 4400 W. 
The 100m sprint is almost wholly anaerobic with aerobic respiration contributing only about 5% of the energy.  

Have You Read?

Why Do We Need to Sleep Eight Hours per Day? Malaise caused by Impaired, Insufficient Sleep

The Science of Sleep & Circadian Rhythm

Biochemical Systems of Energy Production in Humans:  Muscles need energy to do work.  Our bodies have three main pathways for producing energy - in all of them, ATP (adenosine triphosphate - an adenosine molecule with three phosphate groups) is converted to ADP (adenosine diphosphate) in an exothermic (energy generating) hydrolysis (breakdown with water) reaction. In order to meet sudden energy demands, the body keeps a reserve of ATP by converting ADP to ATP with energy from available nutrients. The slide gives a summary: 

There is some ATP stored in the muscle cells - about 100 gm - this ATP is available for energy production right away but it lasts for only 2 to 3 seconds. 
Muscles also have about 120 gm of the chemical creatine phosphate (PCr) which is converted to ATP by an enzyme present.  This can produce energy very quickly but the supply of PCr lasts only about 10 seconds. With available ATP and PCr, the body is capable of producing a burst of energy that can be 60 or 70 times higher than BMR - particularly useful in sprints and weight lifting. One could say that evolution provided this facility to enable a fight or flight syndrome when facing a life-threatening situation.

The next stage is for muscles to use the much larger store of glycogen which is a complex carbohydrate made of many branches, each branch contains 8 to 10 glucose molecular units.  The body can break down glycogen into glucose (glycolysis) and produce ATP without using oxygen (anaerobic metabolism). However, in this process lactic acid is produced and causes muscle soreness and fatigue.  After 2 minutes, glycolysis is no longer able to meet high energy demands due to the build up of lactic acid in muscle fibre. 
Aerobic energy production is always present and uses glucose (in the blood stream or from the breakdown of glycogen) to produce ATP in presence of oxygen. It is a much slower process and not able to provide the burst of energy required in, for example, 100 meter dash.  Supply of oxygen to muscles is a complex process involving breathing, heart rate and flow of haemoglobin in the blood - a faster rate of breathing, an increased heart rate, higher pumping force for the blood are required for increasing supply of oxygen.  It is estimated that aerobic energy production rate is limited to about 15 times BMR.  More on this later...

Aerobic energy production supports long distance runners as in a marathon where a continuous steady rate of energy production over a long period of time is sought.  For exercise to continue, the exercise level needs to drop to a level that allows the aerobic metabolism (aka respiration) to be able to produce enough energy to meet the demands made.

This is summarised in the next two slides.

Aerobic Respiration:  For majority of sports and human activities, aerobic respiration (AERESP) is the main energy production pathway and warrants a more detailed discussion - particularly also due to its relevance for long duration endurance exercise.  

In AERESP, glucose is the fuel that 'burns' in oxygen to produce carbon di-oxide (CO2) and water as the end waste products with release of energy.  Oxygen is taken up by lungs from the atmospheric air, diffuses to the alveoli and binds to haemoglobin (HG) of the red blood cells (RBC) in the capillaries. The heart pumps the oxygen-rich blood to the muscles, where oxygen diffuses to the mitochondria of the muscle cells.  In the mitochondria, oxygen is metabolised to form ATP and produce energy. CO2 is cleared into the atmosphere following a reverse transport path. In the muti-step gas transport system, the capacity of each step is important in determining the maximum energy production possible in AERESP.  Factors like lung capacity, breathing rate, oxygen diffusion rate in the region of the alveoli, haemoglobin concentration in the blood, blood volume, and heart pumping rate determine the amount of oxygen that can be delivered to the mitochondria in muscle cells.  If any of the above factors are below par then the overall efficiency of oxygen transport will be reduced with corresponding effect on the amount of energy produced. 

Blood, Haemoglobin & Red Blood Cells:  Human blood is 45% RBC and 55% blood plasma and optimally contains 150 gram haemoglobin per litre (humans have 65 mL of blood per kilogram of body mass; ~6.5% of body volume).  Higher concentrations of haemoglobin would increase the viscosity of the blood and can be problematic in causing additional burden on the heart. 
How much and how strongly oxygen may be bound to haemoglobin is an interesting question.  How strongly oxygen is bound to the haemoglobin is a compromise due to the need to release oxygen from  haemoglobin to the mitochondria in the cells. In humans, haemoglobin is completely saturated at an oxygen partial pressure of 100 mmHg.  In atmospheric air, the partial pressure of oxygen is 21/100 * 760 = 276 mmHg and blood haemoglobin is fully saturated in the lungs.  Haemoglobin releases half of its oxygen in muscle cells where the partial pressure of oxygen is about 25 mmHg.  Dissociation of oxygen also depends on the temperature of the tissue - active muscles are warmer and more oxygen is released from the haemoglobin. 
A final interesting observation in the gas transport system is the size of the red blood cells which is 0.0075 mm (7.5 microns) in humans (RBC is of constant size of about 5 to 8 microns in most mammals).  This determines the lower limit of the blood capillary diameter in the lungs as 0.01 mm (or 10 microns)  to allow efficient blood flow in these constricted spaces.  

Glycogen:  Glucose and glycogen are the primary fuels for AERESP.  

Human blood stream contains about 4 gram glucose that is available for immediate use by all organs. The hormones insulin and glucagon maintain blood glucose homeostatis (normal state) to satisfy the energy needs of various organs in the body. 

Liver stores about 120 gm of glycogen which can be broken down into glucose and introduced into the blood stream. Muscle cells store about 400 to 500 gram of glycogen solely for use to produce energy in the muscles.  It is not shared with other cells in the body.  Small amounts of glycogen are also present in other tissues and cells, including kidneys, red and white blood cells and gilia cells in the brain.  The uterus also stores glycogen during pregnancy to nourish the embryo.

An observation:  Anaerobic energy production is quite inefficient - producing only 2 ATP molecules from each glucose molecule that is metabolised.  Aerobic respiration, on the other hand, produces 35 ATP molecules.  Our planet's atmosphere only became oxygen rich (21%) about 1.5 billion years ago.  Prior to that for 2 billion years life was still abundant, supported by anaerobic energy production.  Availability of oxygen (which was toxic to the then-existing life forms and caused the so-called oxygen catastrophe until organisms learnt to metabolise oxygen for energy generation) gave various life forms much enhanced capabilities to grow in size with far superior mechanical performance - evolution of more complex life forms became possible.

Three Stages of Cellular Metabolism:  Humans mainly use two macronutrients - carbohydrates and fats for energy production.  Proteins are primarily used to build & repair tissue, make enzymes, hormones, muscles, skin, blood, bones etc. As a last resort, proteins can also be metabolised to produce energy.  Ths subject of cellular metabolism is highly complex; in the following, I present a brief and uncomplicated description as to how macronutrients are metabolised by the body to produce energy. Detailed discussion of the subject is available in 1, 2, 3, 4.
The metabolism of macronutrients proceeds in 3 main stages. I have prepared the following three slides to explain the process.

Stage 1:  In the digestive system, the large macromolecules of carohydrates, fats and proteins are broken down to their simple subunits of glucose and fructose; fatty acids and glycerol; and amino acids respectively.  These are stored in the liver and muscle cells.  Fructose can only be processed in the liver where it is converted to glucose and stored as glycogen.  Extra fructose is efficiently converted to fatty triglycerides and stored in fat and muscle cells (a potent route for non-alcoholic fatty liver disease!).

Stage 2:  The simple subunits produced in the digestive system are transported to the muscle cells by the blood stream. In a 10 step process, called glycolysis,  initially enzymes use 2 molecules of ATP to produce 4 units of ATP and 2 pyruvate molecules.  The net gain of ATP makes this process energy generating and is the basis of anaerobic (without oxygen) energy production as in the case when a sudden demand for energy is made. Pyruvate is excreted from the muscle cell in the form of lactates.  Build up of lactic acid causes muscle soreness and fatigue.

Stage 3: Pyruvate molecules formed in the cytosol are transported into the mitochondria where enzymes convert pyruvate, fatty acids and amino acids into acetyl CoA.  Carbon di-oxide is also released and is transported to the lungs for expiration.

Now it gets complicated: 

Acetyl CoA enters the citric acid cycle - also called the Krebs Cycle (Hans Krebs was awarded the 1953 Nobel Prize for discovering the citric acid cycle). In this process, which is cyclic, Acetyl CoA is converted to other molecules (NADH and FADH2) with large amount of chemical energy. Carbon di-oxide is also produced.  Figure 3 in Kreb's Nobel Lecture describes the citric acid cycle and is worth reading. 

The citric acid cycle products are fed into the oxidative phosphorylation or electron transport chain. Oxygen is required for this process to work (aerobic respiration).  Peter Mitchell was awarded the 1978 Nobel Prize for elucidating the many steps of the process.  I must say that his Nobel lecture is quite difficult to read. 

Essentially, electrons from NADH and FADH2 enter the electron transport chain (a complicated series of chemical reactions mediated by enzymes), and in several steps they are passed on to acceptors with release of energy.  In the final step, electrons from the last receptor are passed on to oxygen (most electro-negative of all acceptors) which then forms water - a waste product of the reaction.  

A small percentage of electrons do not complete the whole series and instead directly leak to oxygen, resulting in the formation of the free-radical superoxide, a highly reactive molecule that contributes to oxidative stress and has been implicated in a number of diseases and aging.

Energy Production in Endurance Exercise:  Endurance events demand energy availability at the highest level that the body can maintain over an extended period of time of weeks to months.  Aerobic respiration is the way body meets this demand.   Along with the supply of nutrients (glucose and fatty acids), the health of the cardiorespiratory (CR) system determines the level that energy may be produced.  As I have discussed in the section on aerobic respiration, the CR system has several units (lungs, blood, heart, etc.) that are involved in the supply of oxygen from the atmosphere to mitochondria in muscle cells.  There are biological limitations that the maximum capacity each unit may have - although a well designed training programme can greatly help in optimising the efficiency of the CR system; athletes do perform much better than average population.  
A sustained energy production rate of about 5 times the resting metabolic rate (generally called sustained metabolic scope or SusMS) is considered the limit in human endurance activities. This limitation is generally assumed to be due to the CR system - the nutrients (glucose and fatty acids) are assumed to be available in sufficient quantity.  

The availability of nutrients is obviously not a problem in short duration exercise as the body stores glycogen in the muscles and fat in the adipose tissue in sufficient quantities. However, these are quickly reduced in endurance events lasting several weeks.  The body also has to prioritise nutrient availability to ensure that vital functions like brain, heart, breathing, thermoregulation etc. have sufficient resources to operate effectively.  A recent study finds that in endurance events like race across the USA (RAUSA) lasting 14 to 20 weeks where athletes run one marathon per day, six days per week, SusMS is limited to about 2.5 times BMR.  An endurance event, completely different from athletics, is a mother's high energy demand during pregnancy and lactation where metabolic scope peaks at about 2.2 times BMR.  

The RAUSA study attributes the limiting value of SusMS to the inability of body's digestive system to metabolise food at a rate fast enough to match the cardio-respiratory system. I find this a surprising, somewhat counterintuitive finding as one would think that there are always enough nutrients available and the limitation is the CR system's ability to transport and deliver sufficient oxygen to the mitochondria. The situation is obviously more complex. 
The idea in the RAUSA study was quite straightforward.  Twenty three athletes received food as and when they needed, but six of the athletes were overfed.  The study measured their metabolic scope (how much energy they are generating in units of BMR) and also monitored any weight gain or loss.  The energy intake for an athlete is just the sum of the measured metabolic scope and the energy equivalent of weight loss (a negative number) or weight gain (a positive number).
Weight loss studies give 7650 Calories as the energy equivalent to one kilogram of weight change (usually quoted in popular literature as 3500 Calories needed to make a difference in weight by 1 pound). 
The slide shows the results obtained:

  Essentially, any energy expenditure greater than 2.5 BMR cannot be met by additional nutrient intake and must be supplied by drawing down energy stores to supplement alimentary supply. The digestive system appears to create a metabolic bottleneck and restricts the endurance activity to a level below 2.5 BMR.  
Discussion:  Evidence that most human endurance activities are capped at 2.5 BMR appears counterintitive and does not immediately make good sense.  However, our physiological functions have been optimised over many million of years and it is interesting to speculate why we might have evolved this way. This is what I think:

First, let us look at the survey of physical activity levels (PAL) of populations distributed on our planet.  People in developing countries (low human development index - HDI) work hard in farming and other physically demanding chores.  In the rich countries (high HDI), people have sedentary life style and many labour saving gadgets - they do not move about as much.  One would think that PAL in developing countries will be much higher than in developed countries.  That is not the case at all - as I show in the following table: The data is for people with a body mass index in the range from 23 to 27 (healthy active people - not obese).

Mean PAL Values in units of BMR

Men      in Low HDI  countries   PAL = 1.88 ± 0.06  
               High HDI countries             1.79 ± 0.02  
Women in Low HDI  countries            1.70 ± 0.03  
               High HDI countries             1.71 ± 0.02

These numbers suggest that energy demand is pretty constant for most populations and this is what our bodies evolved to supply in a secure way.  There  might be situations, like confronting a lion, when we need to boost energy almost instantly - this is no different than the situation in a 100 meter sprint - and the body has mechanism to look after that. Taking the mean PAL as 1.85 x BMR, it seems that we are capable of sustained work output at about 35% more intensely than the average demand. Of course, short term extra demands over a few days may be met by drawing on nutrient reserves in the body.  It appears that evolution has designed the human system optimally.
While the RAUSA study points to the digestive system's failure to support higher SusMS, it is not clear if other factors like the waste disposal organs  work as efficiently after many weeks of operation at an elevated level.  The cardio-respiratory system involves many enzymes and muscles and when called upon to function at an elevated level for long periods - will it show some sort of fatigue affecting overall metabolic efficiency? 
The body also needs a minimum level of vitamins and minerals to synthesize essential hormones, enzymes  and other chemicals.  It is not clear, if during periods of sustained elevated demand, the nutrition requirements were fully met in the RAUSA study.  There are many unanswered questions that I am sure will be addressed in future studies.

We look forward to much interesting research in this context.