When the history of the last half century is written, will we think that governments, scientists and environmental leaders identified the right global problems and got to grips with  them, as the human population doubled, and looked like doing so again?

Or will we think that huge problems emerged on our watch while reason slept?

The inconvenient truth about the sea, which covers 70 per cent of the Earth, is that arguably the worst impact upon it so far – if you study the latest scientific assessments - has been caused by the mundane pursuit of human food and not  by global warming or acidification, major threats though these are to our common future.

When I interviewed Al Gore over a year ago, I noticed that he, too, had begun to talk about the oceans. And in the lecture which was the origin of the film, An Inconvenient Truth –  a lecture which he still gives - he talked  about the destruction of natural resources, whether rainforests or fish.

He sees the growing human population as being the big problem of our century but remains convinced that climate change occupies the leading position in the hierarchy of the world’s environmental problems.  I am not so sure.

I have always instinctively believed that the destruction of species and habitats on land and at sea was as great a threat, because they are both a crime against nature itself and they also drastically reduce the human race’s options in a warmer world.

At least as important as tackling climate change must surely be to ensure a secure supply of food:  what uncontrolled modern fishing technology is doing to the oceans today is to threaten the food security of billions.

In the end, the climate change argument is all about timescales and in the case of the world’s oceans, the time scale is pressing: if carbon emissions get much more out of control the impact on the chemical composition of the oceans might alter dramatically - leading to a mass extinctions in the marine environment irrespective of fishing.

The fact is , overfishing is already here: changing evolution, removing options for the future, changing menus, making nature less productive, less interesting.

In some areas of the world, over-fishing may have a long history, but it is the newest threat on the block in scientific terms and we have still to adapt to its realities.

The world learned for the first time only in 2002 that global wild fish catches had peaked and were in decline.

For reasons the film, The End of the Line, explains, this milestone had actually been passed without anyone knowing about it back in 1989.

Knowing then what we know now, would we have thought it right for celebrity restaurants, serving endangered species such as bluefin tuna and caviar, to enjoy such a boom in the last two decades?

Would we advise people to eat two portions of fish a week?

Would we have thought it right for environmental leaders to concentrate wholly, in Europe anyway, on  the scares of genetic modification, or waste from the nuclear industry (which now looks more like something we will have to live with if we are to tackle the greater threat of carbon emissions), while our management of the seas, which was supposed until very recently to be rational and scientific, had run so wildly out of control?

To choose a more particular example, again with hindsight: was it right, given what we know now, for environmental groups in the 1990s to campaign about local pollution issues, such as Shell’s dumping of the Brent Spar oil platform - rather than over-fishing?

In 1995, for example, the “999″ factory in Esbjerg, Denmark, was responsible for processing a million tons of sand eels and other small fish into food for fish farms.

The sand eel population, the base of the food chain for marine mammals and sea birds as well as fish in the North Sea, has since collapsed, probably because of over-fishing - and with it the “999″ factory, which at that time, to add insult to injury, was selling fish oil to be burned in power stations.

When I revealed that fact, Greenpeace International wouldn’t touch the issue because it was at the height of its campaign against Shell’s dumping of the Brent Spar.

It was left to Greenpeace UK, which did “get it” to unfurl a banner down the “999″ factory on its own. With the benefit of 20/20 hindsight, we can see that the environmental movement has had its priorities at least partially wrong.

It is only now beginning to catch up with the extent to which modern fishing technology has removed future options.

If you over-fish the cod, as they have on the Grand Banks and in the Firth of Clyde, you get shellfish.  Now, in the Clyde anyway, the shellfish are riddled with disease.  What happens after the shellfish collapse? Plankton and jellyfish, probably.  Do we really want to go there?

If we end up living in a world without fish, today’s mismanagement of the cod, the bluefin tuna and once-plentiful fish resources off West Africa and  the Indian Ocean will seem enormous crimes. Today they seem like business as usual.

Al Gore has chosen to place climate change at the top of the hierarchy of the world’s great environmental problems. In the long term, he may be right, but, as our film shows, the destruction of wildlife in the sea is an even more immediate problem that will affect our resilience to global warming.

The science that tells us so is relatively uncomplicated and resistant to challenge. Rampant, uncontrolled fishing is already pushing whole species, such as the magnificent bluefin tuna, that mankind has depended on for thousands of years towards extinction.

What we have done to the oceans in the pursuit of food is the world’s most overlooked environmental truth, a pressing problem in need of re-evaluation.

It is also, happily, the problem that this generation stands the best chance of doing something about - if we can create the political will.

So, please, do see our film and join our campaign to save the seas.