Perhaps you didn't read the article, because its main point is that the official UN forecast is for world population to essentially flatten out at 10.9B around 2100, which is 40% more than today, not 60%.
Correction: we don't want to manage now. The world has far more than enough wealth and resources to hold climate change to 2 degrees and to stop clear cutting rain forests.
We can and do manage now, thank you very much. Poverty is decreasing at an incredible rate around the globe; education is increasing at an incredible rate. We've made the world's knowledge available to virtually everyone via in the internet just in the time I've been an adult. We've eradicated diseases that once killed millions every year.
Two thirds of the global extreme poverty (defined by the World Bank as about $1.80/day in current dollars) has been eradicated in just the past two decades.
We also have about ten years of carbon budget left if we want to avoid the worst climate change yet our emission rate is still increasing. Permafrost is already melting much faster than the pessimistic models assumed, so it might already be too late to prevent the release of the absurd amount of carbon locked in the ice.
I expect to see fossil fuel use as a major energy source eradicated within my lifetime. The planet is just fine; it's our own living environment we need to worry about. Earth has survived worse things than mere humans.
As a point of comparison, the global population has almost quadrupled in the previous 80 years. So even in the worst case scenario, population growth is dropping spectacularly.
The important point is that this is a large increase when we already cannot manage now.