The severe drought and famine ravaging Ethiopia and this year's massive flooding in Bangladesh, which has dispossessed 30 million people and killed more than 700, are not "natural disasters" in the usual sense but are largely the result of human activity.

That is the conclusion of a joint report issued here yesterday by the Swedish Red Cross and Earthscan, a branch of the U.N.-funded International Institute for Environment and Development.

In both countries, the report said, decades of poverty-driven misuse of the land have made the effects of natural processes far worse than would otherwise have been the case.

The two disasters, along with continuing drought in parts of Africa, threaten to make the 1980s the worst decade in modern history for so-called natural disasters, the report said. According to figures complied by the Swedish Red Cross, disasters killed an average of 22,570 people annually during the 1960s, a figure that shot up to 142,920 for the 1970s.

The annual number of floods, storms, earthquakes and droughts in the 1980s is running well ahead of the earlier figures.

Lloyd Timberlake, editorial director of Earthscan, said that while each of these events has a natural component, human factors determine how severe the effects will be. Storms and earthquakes afflict the world's poorest people most severely because they lack protective housing and early warning systems. Floods and droughts, which affect more people than any other disaster, can be brought on by human activity.

Timberlake said that the Ethiopian famine, which is affecting more than 5 million people, was started by too little rain but was vastly worsened because the rain that does fall is no longer absorbed by the soil. Deforestation for firewood has allowed rainwater to run off before it can soak in, washing away productive soils in the process. Even if rains come, the report says, the water may flow down bare slopes, leaving crops in "pseudo-drought" conditions.

Timberlake said Ethiopia is believed to have had a 40 percent vegetation cover around 1900. Satellite photos show it to have 4 percent today.

In Bangladesh the same cause has led to the opposite effect. The annual monsoon-season floods have been getting worse because of deforestation on the flanks of the Himalaya mountains. Only one-fifth of the water that began flooding the country in May fell from the sky, Timberlake said. The rest is runoff that used to keep the upland regions green.

Though the waters are receding, it will be a long time before farmers can plant and harvest new crops. "The next children we see dying on our television sets are going to be Bengali children in Bangladesh," Timberlake said.

Concern about vegetation loss and its effects has been prominent among a small band of global environmental experts for many years, but it has not greatly aroused governments' concern since 1977, when many agreed to take major steps during the U.N. Conference on Desertification.

The conference was prompted by a drought in Africa's Sahel region that peaked in 1974. The conference stressed the role of human activity in declining land productivity but, according to scientific consultants to the conference, almost none of the remedial measures that were adopted have been put into action.

Although governmental attention to the problem has nearly vanished, the scientists say, the Sahel drought has not. It is no longer as severe as it once was, but it continues into its 17th year, suggesting that a long-term shift in climate may have taken place.

The new report, written by Timberlake and Anders Wijkman, secretary general of the Swedish Red Cross, is entitled "Natural Disasters: Acts of God or Acts of Man?"