Technovelgy reader Winchell Chung found an earlier source: a Poul Anderson short story, published in 1955, suggesting that Sagan was preceded by an sf writer. Here is an excerpt from Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, published in 1975:

Perhaps inspired by Poul Anderson's short story entitled “The Big Rain,“ published in 1955, Dr. Carl Sagan in 1961 proposed a terraforming project to modify the environment of Venus. Our sister planet has a hellish climate, with temperatures upwards of 750 °C and pressures of 90 atm at the surface. To prepare it for human habitation it will be necessary to lower the surface temperature and pressure, and to elevate by at least two orders of magnitude the fraction of molecular oxygen present in the atmosphere. Most of the air is carbon dioxide, and this must be eliminated as well.

Sagan suggests the injection of blue-green algae into the Venusian atmosphere at high altitudes where it is relatively cool. These tiny organisms would consume the CO2 by growing more algae cells with water and aerial nutrients. Molecular oxygen would be expired as a waste product. Over a period of several years the carbon dioxide level begins to drop, thus reducing the green-house effect and cooling the planet overall. When the ground was sufficiently cool, cargo landers armed with fusion bombs could be de-orbited and set down on the surface. These machines, able to burrow like moles and detonate beneath the surface, may be used to trigger new volcanic chains in order to help percolate more water into the dry atmosphere. Eventually the first “big rain“ will fall. Says Sagan: “The heat-retaining clouds will partly clear away, leaving an oxygen-rich atmosphere and a temperature cool enough to sustain hardy plants and animals from Earth.“

How reasonable is the astronomer's proposal?

Actually, as it turns out, this was mostly Poul Anderson's proposal, not Sagan's. The following quote is from The Big Rain:

(The Big Rain, by Poul Anderson)

There was oxygen everywhere, locked into rocks and ores, enough for the needs of man if it could be gotten out. Specially mutated bacteria were doing that job, living off carbon and silicon, releasing more gas than their own metabolisms took up; their basic energy source was the sun...

Meanwhile giant pulverizers were reducing barren stone and sand to fine particles which could be mixed with fertilizers to yield soil; and the genetic engineers were evolving still other strains of life which could provide a balanced ecology; and the water units were under construction.

These would be the key to the whole operation. There was plenty of water on Venus, trapped down in the body of the planet, and the volcanoes brought it up as they had done long ago on earth...

At the right time, hydrogen bombs were to be touched off in places the geologists had already selected, and the volcanoes would all wake up. They would spume forth plenty of carbon dioxide,,, there would be water too, unthinkable tons of water. And simultaneously aircraft would be sowing platinum catalyst in the skies, and with its help Venus' own lightning would attack the remaining poisons in the air. They would come down as carbohydrates and other compounds, washed out by the rain and leached from sterile ground.

That would be the Big Rain. It would last an estimated ten Earth-years, and at the end there would be rivers and lakes and seas on a planet which had never known them... Venus would still be mostly desert, the rains would slacken off but remain heavy for centuries...

A hundred years after the airmen had finished their work, the reclaimed sections might be close to Earth conditions. In five hundred years, all of Venus might be paradise.

Anderson further proposed that ground-based airmakers would be useful in the work of terraforming Venus. By the way, the word terraforming was coined by science fiction writer Jack Williamson in his 1941 story Collision Orbit.

So, was Carl Sagan preceded by science fiction writers by six years? Actually, no; I found an earlier source.

In his 1930 classic Last and First Men, science fiction writer Olaf Stapledon suggested that Venus should be terraformed first! He writes:

On the other hand, Mars could not be made habitable without first being stocked with air and water; and such an undertaking seemed impossible. There was nothing for it, then, but to attack Venus. The polar surfaces of that planet, shielded by impenetrable depths of cloud, proved after all not unendurably hot. Subsequent generations might perhaps be modified so as to withstand even the sub-arctic and "temperate" climates. Oxygen was plentiful, but it was all tied up in chemical combination. Inevitably so, since oxygen combines very readily, and on Venus there was no vegetable life to exhale the free gas and replenish the ever-vanishing supply. It was necessary, then, to equip Venus with an appropriate vegetation, which in the course of ages should render the planet's atmosphere hospitable to man.

It appears that science fiction writers (and science fiction readers) were well out ahead of the scientists this time.