Does the lifespan get shorter or longer ?

The statement, "Lifespan of human is growing and will become 100 years" needs extensive research and proof. Honestly, I didn't read it in anywhere so far yet. But how about what we see everyday; the condolence columns in newspapers, tombstones in cemeteries, all is saying that we lived only around 70-80 years. I rarely see someone lived for 90 years and over on them. Now people usually says "you are very long live" when we see someone who is over 90.

The idea of "Human lifespan is growing" might come from Mathematical estimation. We experienced wars in our history. Young people died in Europe and many other areas. Mathematically lifespan may be lower in past centuries due to these events. Comparatively, now the lifespan is higher than that but it will not be over 100 years.

Yes it increases from 10 years to 84000 years and then decreases to 10 years again repeatedly and gradually

It comes from the prediction in DN 26 Cakkavatti Sutta, but The Buddha doesn't give an exact specific amount of years when it would happen or say if it would happen to everyone.

The prophecy in DN 26 is that the children of the humans that have a lifespan of a hundred will eventually have lifespan of ten-years-old:

"Among those of a ten-year life-span no account will be taken of mother or aunt, of mother's sister-in-law, of teacher's wife or of one's father's wives and so on--all will be promiscuous in the world like goats and sheep, fowl and pigs, dogs and jackals. Among them, fierce enmity will prevail one for another, fierce hatred, fierce anger and thoughts of killing, mother against child and child against mother, father against child and child against father, brother against brother, brother against sister, just as the hunter feels hatred for the beast he stalks" (Cakkavatti Sutta, DN 26)

The prediction then states that eventually the lifespan would go up from there up to 80,000 years:

"The children of those whose life-span is twenty years will live to be forty, their children will live to be eighty, their children to be a hundred and sixty, their children to be three hundred and twenty, their children to be six hundred and forty; the children of those whose life-span is six hundred and forty years will live for two thousand years, their children for four thousand, their children for eight thousand, and their children for twenty thousand years will live to be forty thousand, and their children will attain to eighty thousand years." (Cakkavatti Sutta, DN 26)

Some people think that the lifespan of 10 years already happened in the past temporarily and now we are in the age when the lifespan will go up from 80 to 160 but others think that the lifespan will soon go down to 10 years briefly then back up eventually.

Of course there probably will be a global food crisis in the future (which means the lifespan would go down dramatically) but then it would probably go up from there.

"A scientific model has suggested that society will collapse in less than three decades due to catastrophic food shortages if policies do not change." (http://www.independent.co.uk/environment/climate-change/society-will-collapse-by-2040-due-to-catastrophic-food-shortages-says-study-10336406.html)

Without food people kind of die, so the life expectancy would go down dramatically if a food crisis breaks out.

According to modern scientists, there is no fixed theoretical limit on the lifespan (Gavrilov, L. A.; Gavrilova, N. S. (1991). The Biology of Life Span: A Quantitative Approach. New York City: Starwood Academic Publishers. In Gavrilov, Leonid A.; Gavrilova, Natalia S.; Center on Aging, NORC/University of Chicago (June 2000). "Book Reviews: Validation of Exceptional Longevity" (PDF). Population Dev Rev. 26 (2): 403–4).

Many researchers already believe that humans will eventually live to be thousands of years old in the future.

"We will be able to live not just a little bit longer, but a lot longer, and to look younger as well. We are on the verge of a number of separate medical breakthroughs that together will allow us to live to 150. These technologies will definitely be available" (Harvard Medical School genetics professor Dr. David Sinclair, http://www.newsmax.com/SciTech/aging-researchers-immortality-Aubrey-de-Grey/2015/04/16/id/638926/)

The prediction is that the lifespan will first go down to ten briefly then up to twenty to forty to eighty to one sixty, etc...until eighty-thousand.

My personal opinion is that the food crisis not simply possible but likely. So much evidence shows us that it's going to happen yet so few humans are prepared for it. Right now I envision the world population in the 2200s as 500 million or less.

I'm sure if you told someone in the year 2000 that the food price index would be more than double in ten years they wouldn't believe you.

The idea that lifespan was longer in ancient times is nonsense, and has been throughly debunked, disproven, and so on. Over and over again.

Any monk or nun who states something like this, that they somehow "know" that lifespan has "decreased" is bullsh*tt*ng you in order to seem "magical" or "wise". Don't fall for that nonsense.

At best, the average lifespan in ancient times was the same as now, but that isn't what we find when we investigate. Universally, Human beings are now, on average, living much longer than in ancient times, and this has been proven over and over again by the careful analysis of human remains.

It is true that we have found some surprising outliers in ancient bones.. ages that seem to be upwards of 75 and greater. But nothing whatsoever that shows that anyone has ever lived 160 years, and the averages are in the 40s and 50s. And lets not even discuss infant and child mortality, which was 5 in 6 back in the good old days.

There is simply no hard evidence for age expansion or contraction, and the insistence that modern man is somehow "degenerate" is a dangerous idea that seeks to place 2000 year old ideology ahead of current life in order to create power structures.

In other words, it is a con game. Please do not fall for it.

And yes, human average lifespan has gotten longer, although it seems to have plateaued.. aka stopped growing.

In Buddha's time the life span is said to have been 100 years and there were several theras who touched even 120 years e.g. Ananda and Maha Kassapa. In fact Bakkula thera is said to have lived 160 years, he had ordained when he was 80. But we rarely hear anyone passing 100 at present.

A well-respected monk in Sri Lanka (late Ven. Nauyane Ariyadhamma) has mentioned that the life span reduces by 10 years in each millennium. Judging from that, the 10 year period should occur roughly in 7500 years from now. Who knows what can happen within this period.

The causes of illnesses and therefore, the reduction in life span according to Buddhism is mentioned in Cakkavatthi sutta.

Thus, brethren, from goods not being bestowed on the destitute poverty grew rife; from poverty growing rife stealing increased, from the spread of stealing violence grew apace, from the growth of violence the destruction of life became common, from the frequency of murder lying grew common, from lying growing common, evil speaking grew abundant, from evil speaking growing abundant, adultery grew common, from adultery growing common abusive and idle talk grew common, from abusive and idle talk growing common, covetousness and ill-will grew common, from covetousness and ill-will growing common, false opinions grew common, from false opinions growing common, incest, wanton greed and perverted lust grew common, finally from incest, wanton greed and perverted lust growing common lack of filial and religious piety and lack of regard for the head of the clan grew great.

From these things growing, the life-span of those beings and the comeliness of them wasted, so that, of humans whose span of life was two and a half centuries, the sons lived but one century. DN 26

The essence of this is that for the life span to increase, human qualities also have to improve, advances in medication and increase in the standard of living alone will not be enough.