RIO DE JANEIRO — Calling Aldo Rebelo a climate-change skeptic would be putting it mildly. In his days as a fiery legislator in the Communist Party of Brazil, he railed against those who say human activity is warming the globe and called the international environmental movement “nothing less, in its geopolitical essence, than the bridgehead of imperialism.”

Though many Brazilians have grown used to such pronouncements from Mr. Rebelo, 58, his appointment this month as minister of science by President Dilma Rousseff is causing alarm among climate scientists and environmentalists here, a country that has been seeking to assert leadership in global climate talks.

“At first I thought this was some sort of mistake, that he was playing musical chairs and landed in the wrong chair,” said Márcio Santilli, a founder of Instituto Socioambiental, one of Brazil’s leading environmental groups. “Unfortunately, there he is, overseeing Brazilian science at a very delicate juncture when Brazil’s carbon emissions are on the rise again.”

Brazil won plaudits for lowering its annual emissions from 2004 to 2012, largely by slowing the rate of deforestation in the Amazon. But emissions jumped 7.8 percent in 2013, according to the Climate Observatory, a network of environmental organizations. Several factors were to blame, the observatory said: deforestation on the rise again, growing use of power plants that burn fossil fuels, and increased consumption of gasoline and diesel.