Tuesday, November 23, 2010

Life expectancy

One of the interesting things - to me - about the climate change debate is how many people seem to think that the effects of climate change will not be experienced by them but rather by their children or grandchildren.

These statements are even made, perhaps rhetorically, perhaps not, by people who are authorities on the science. Storms of my grandchildren is a book written by James Hansen, the head of the Goddard Institute and the man who runs one of the five major global temperature data sets, GISSTemp. While I am sure that James Hansen is aware of what climate change is doing to the world now, the emphasis is on what will occur many decades into the future. (To be fair, Hansen is 69, so his grandchildren are likely around 10 or so).

However, even someone who is 69 and who lives in the wealthy west has a reasonable chance of living for another 20 years.

And that leads me to the point of this post: examining life expectancy.

Let us examine the life expectancy by age tables published by the ABS here:


What they show us is that an Australian male (and James Hansen is American, but the difference will not be all that great) aged 69 has a life expectancy of a further 15.7 years.

The tables towards the bottom of the page show something even more interesting. They show that as you get older the age at which you are expected to die increases quite signficantly.

For example, someone who was 40 in 1989 was expected to die at age 75.9. Those members of that demographic who reached the age of 60 in 2009 were expected to die at age 82.9, an increase of seven years in a 20-year period.

If you think about, this is at least partly to be expected. If you survive from age 40 to age 60, the most obvious conclusion is that you have not died. Thus, you have successfully avoided the dangers that have taken the lives of others in your demographic. Those who died were taken into account in working out the expected age of death of 75.9. They no longer exist, and so the expected age of death for the survivors must be higher than 75.9.

However, an increase of seven years seems quite large. Think about it this way: once we reach a point where our average age of death increases by one year for every year that goes by, we will have reached statistical immortality (in a way - there will still be deaths, but they will be compensated for, in the statistical sense, by faster and faster increases in life expectancy). Seven in 20 is a reasonable step towards that mark. And the figures for those aged 60 in 1989 who survived to 80 in 2009 are even more interesting: the expected age at death increased by 10 years in those 20 years.

What is going on here? Well, apart from death winnowing out people from the second set of statistics (ie, not everyone is making it to 80), medical technology is improving quite rapidly. This is expanding life expectancy, and it is particularly doing so for those aged 40 or above.

Having done some calculations based on these tables, it is my conclusion that someone who is aged approximately 40 today has a 25 per cent chance of living to 120. These calculations assume the continuation of the steady increase in life expectancies, with no spectacular breakthroughs.

It is also my calculation that 'statistical immortality' will be acheived in 100 years, with those aged around 20 today having about a 30 per cent chance of reaching that point.

And I will write a more detailed post about what I mean by 'statistical immortality' in the near future.

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