who was the doomsday economist?

Answer 1

It was indeed Thomas Malthus, who stated that the world population was growing geometrically and food production was growing arithmetically. He, however, forgot to take into account innovation and population growth dynamics. The green revolution, when pesticides and synthetic fertilizers were developed, allowed food production to increase while developed economies have reduced their rate of population growth, thus allowing food production to outpace population growth.
Later, Jeremy Bentham, the father of utilitarianism and founder of the London School of Economics, realized that before the food problem became a reality, the human race was going to ran out of land because of all the buried corpses of that geometrically growing population. He suggested that people embalm their dead and display them as statues to save space. He himself was embalmed upon his death.

Answer 2

That would be Malthus, the Pernicious Parson, who theorized that population increases geometrically while food increases arithmetically (not quite true- the former is exponential while the latter has been, in fits and spurts, hyperexponential), thus guaranteeing strife, scarcity, starvation and suffering.

Answer 3

Thomas R. Malthus