Roger Marsh is a sandy-haired, solid looking 34-year-old steelworker, head of his union local, the sort of fellow whose hard work was instrumental in creating the great democracies of the industrial West. Today, for all his natural confidence, Marsh is a troubled man.

"If I lose this job," he said just loudly enough to be heard above the din of steel making, "the chances are good I'll never get another."

It seems an incredible thought, yet in the Britain of 1982 it is grimly realistic. There may well be no similar work to be had. One cost of advancing technologies, higher productivity and a seemingly interminable recession has been the permanent loss of vast numbers of manufacturing jobs. But nowhere have more jobs disappeared than in steel.

The current economic crisis is global, the worst in many ways since the 1930s. As countries seek to protect as much as possible in an endangered, fiercely competitive area like steel, international trade threatens to become a scramble for advantage. Beginning Wednesday in Geneva, top trade officials from 88 countries will be struggling to hold the line in a meeting of the General Agreement on Tariffs and Trade, the first session of its kind in a decade.

The best the GATT negotiators can hope for is limited action to slow a steady erosion of free trade. An analysis on Page C7.

Their labors will be couched in the complex jargon of the field, setting terms of trade intended to give each nation the opportunity to sell its wares. But at bottom, they are talking about whether -- and how many -- jobs like Roger Marsh's can be preserved.

In the late 1970s, about 300,000 people were working in the British steel industry. By early 1983, the figure will be barely more than 110,000. And even at that size, business is so poor that "capacity," as managers call it, will have to be reduced further, creating more unemployment.

The state-owned British Steel Corp., which dominates the industry, is losing about $1.6 million a day. The smaller private sector, including Sheerness Steel in which Marsh works, is similarly depressed.

"We don't expect any improvement in demand as far as the eye can see," said Ian Blakey, a director of the British Independent Steel Producers Association.

Given the personal distress involved in each lost job and the cumulative impact of so deep a decline in manufacturing output, it is hardly surprising that a strong, emotional impulse exists to protect what is left, to insulate, for instance, British steel from the pressures of competition with the United States, Japan, its partners in the European Community and so-called third country producers like Brazil.

In the United States the problem is equally serious. According to industry figures, about 100,000 steelworkers have been laid off during the past 12 months, about 40 percent of the total work force. As a result, last summer the Reagan administration, accusing European governments of subsidizing their steel exports to keep prices below those in the United States, moved to impose large tariffs on imports.

After prolonged bargaining, the Europeans agreed to limit their sales in the United States to 5 percent of the market, a painful cut that was preferable to to the prospect of paying duties so high that virtually no EC steel would be sold. But for Britain, the resolution of that problem left many others, including the underlying one of cutting now unacceptable losses to a minimum in hopes that when the recession here finally ends there will still be a viable British steel industry.

As Blakey explained it in an interview, even though steel output is running about half of what it was in the mid-1970s, there is still too much steel to go around and the situation would be the same even if the economies of the world were to improve substantially. Lighter cars, tighter engineering and construction techniques and heavy investment in equipment when times were flush add up to a fundamental economic stalemate -- more supply than the possible demand.

The solution has two main elements that sound simple enough but, in fact are extremely difficult to achieve: continuing to make production more efficient, with the aim of selling more British steel at lower cost; and working out arrangements within the EC that apportion the hardships of a severe depression.

On the first point, the British Steel Corp. run by Ian McGregor, an American, has embarked on a major "rationalization," program, another euphemism for cutting jobs and costs. This could eventually lead to the closure of one of the five remaining large, integrated steel plants, which would have a devastating impact on the region. Ravenscraig in Scotland, the most likely plant to be shut, employs 10,000 people.

The alternative is to make selective cuts in elements of the 60 plants that British Steel operates. That is what appears to be happening now with almost daily reports of a 150 or 200 jobs trimmed. In addition, the state-owned company could turn to the government for financial assistance, but this accomplishes nothing toward profitability.

For the private steel companies, the government is about to put up about $55 million to help defray the cost of scaling back between now and November 1984, enabling companies to combine assets, rearrange equipment and pay laid-off workers. Sheerness, which is Canadian-owned, is determined to be one of the survivors, according to its managing director, Clancy Schuppert, an American who has been with the company since it started in 1971.

Within the European Community, one main objective is to reduce steel imports from the "third countries" that are underselling the Europeans. Britain is importing twice as much from Brazil and other non-European suppliers as a year ago. Slamming the door -- as some politicians and many steelworkers would like to do -- is impossible because Britain is so dependent on what it exports.

The EC yesterday did agree to cut such imports by 12.5 percent of their total in 1980. The partners are also committed to fixed minimum prices for their own steel, quotas for individual producers in each country and a phase-out of government steel subsidies by 1985.

Making those measures stick, however, is proving very hard because conditions are so bad. Controls have to be made "as watertight as can be," the London Financial Times said in an editorial last week, "Cheating has to be eliminated."

What all this means to Roger Marsh and the hundreds of thousands of steelworkers like him, is that their futures are to a large extent out of their hands. Hard work is not enough to save their jobs or help them find new ones. International agreements and negotiations will protect some people, but not others.

Expectations of job security are gone, shattering expectations of a steadily improving standard of living.

"We all want too much," said Marsh. "If we all limited what we want it might be better." But that, he acknowledges sadly, is not possible either.