The nation's unemployment rate jumped back up to a seasonally adjusted 8.8 percent last month, while another 5.1 percent of the work force was employed only part time because of the recession, the Labor Department reported yesterday.

Altogether, some 14.9 million workers had no job or only part-time work last month. Janet L. Norwood, commissioner of the Bureau of Labor Statistics, told a congressional committee that, even if current forecasts of an economic recovery later this year are correct, as many as 30 million people could be unemployed sometime in 1982.

The Reagan administration expects unemployment this year to average 8.9 percent, but officials have warned that it could reach 10 percent before the predicted recovery gains enough momentum to begin to reduce the rate.

Unemployment rates for blacks and teen-agers were at record levels last month, the department said. Joblessness among blacks reached 17.3 percent, tying the record set in December, while teen-age unemployment climbed 0.6 percentage points to 22.3 percent, the highest in history.

The 5.6 million people forced to work part time was also a record. Norwood said that the "involuntary" part-time workers are "generally people who have a harder time in the labor market," such as women and minorities.

The overall unemployment rate had fallen from 8.8 percent in December to 8.5 percent in January. Analysts say they believe this resulted largely from problems with seasonal adjustments rather than from any improvement in the economy.

Even though the unemployment rate rose in February, some economists saw hints in the numbers that the economy's plunge into recession has slowed significantly.

During November and December payroll employment fell by an average of about 350,000 each month. In January and February that average dropped to less than 100,000 a month.

Another brighter note, too, was a tiny 0.1 percent increase in the Labor Department's hourly earnings index. In the 12 months ended in February, the index, which covers production and non-supervisory workers, rose only 7.5 percent.

That was the smallest rise for a 12-month period in more than four years, and it was down more than two percentage points from the rates of early last year.

Adult women accounted for most of the rise in unemployment last month, as their rate climbed to 7.6 percent, the same as for adult men. The rate for whites was 7.7 percent compared with the 17.3 percent rate for blacks and a 12.6 percent rate for workers of Hispanic origin.

Meanwhile, the Federal Reserve reported that the nation's money supply fell by $3 billion to a level of $444.8 billion in the week ended Feb. 24. The decline, one of several in recent weeks, wipes out much of the early-January bulge that pushed the money supply far above current Fed targets. The decline should pave the way for a continuation of a drop in interest rates.

Among industries, employment in manufacturing continued to fall last month, particularly at companies making machinery, primary and fabricated metals and rubber and plastics.

Employment in construction rose, but the severe winter weather the month before, which complicated the seasonal-adjustment problems, was responsible for this apparent improvement. February's construction employment was down nearly 50,000 jobs from December's.

Jobs in service-producing industries rose by 70,000 last month despite a drop in federal, state and local government employment. Normally during recessions, government employment rises as anti-recession programs go into effect.